Baroreflex control of muscle sympathetic nerve activity: a nonpharmacological measure of baroreflex sensitivity

Frequency-Domain Features and Low-Frequency Synchronization of Photoplethysmographic Waveform Variability and Heart Rate Variability with Increasing Severity of Cardiovascular Diseases

Objective-Heart rate variability (HRV) and photoplethysmographic waveform variability (PPGV) are available approaches for assessing the state of cardiovascular autonomic regulation. The goal of our study was to compare the frequency-domain features and low-frequency (LF) synchronization of the PPGV and HRV with increasing severity of cardiovascular diseases. Methods-Our study included 998 electrocardiogram (ECG) and finger photoplethysmogram (PPG) recordings from subjects, classified into five categories: 53 recordings from healthy subjects, aged 28.1 ± 6.2 years, 536 recordings from patients with hypertension (HTN), 49.0 ± 8.8 years old, 185 recordings from individuals with stable coronary artery disease (CAD) (63.9 ± 9.3 years old), 104 recordings from patients with myocardial infarction (MI) that occurred three months prior to the recordings (PMI) (65.1 ± 11.0 years old), and 120 recordings from study subjects with acute myocardial infarction (AMI) (64.7 ± 11.5 years old). Spectral analyses of the HRV and PPGV were carried out, along with an assessment of the synchronization strength between LF oscillations of the HRV and of PPGV (synchronization index). Results-Changes in all frequency-domain indices and the synchronization index were observed along the following gradient: healthy subjects → patients with HTN → patients with CAD → patients with PMI → patients with AMI. Similar frequency-domain indices of the PPGV and HRV show little relationship with each other. Conclusions-The frequency-domain indices of the PPGV are highly sensitive to the development of any cardiovascular disease and, therefore, are superior to the HRV indices in this regard. The S index is an independent parameter from the frequency-domain indices.

Angiotensin Receptor-Neprilysin Inhibitor Is Associated With Improved Cardiac Autonomic Function in Heart Failure

BACKGROUND

Heart failure with reduced ejection fraction is associated with potentially deleterious imbalance of the cardiac autonomic nervous system. Sacubitril/valsartan (angiotensin receptor-neprilysin inhibitor [ARNI]) reduces cardiovascular mortality and hospitalization for heart failure with reduced ejection fraction. Whether ARNI affects the cardiac autonomic nervous system has not been studied.

METHODS AND RESULTS

This investigator-initiated, prospective, single-center cohort study compared heart rate (HR) variability, HR, deceleration capacity, and periodic repolarization dynamics as noninvasive measures of the cardiac autonomic nervous system before and after initiation of ARNI therapy. Patients underwent standardized 12-lead Holter-ECG, echocardiography and laboratory testing before and 3 months after start of therapy. End points were changes in HR variability (SD of normal-to-normal intervals, mean square of differences between consecutive R-R intervals), HR, deceleration capacity, and periodic repolarization dynamics as well as ventricular function and NT-proBNP (N-terminal pro-B-type natriuretic peptide). Of 63 patients with heart failure with reduced ejection fraction enrolled, 48 (76.2%) patients were still on ARNI at follow-up. SD of normal-to-normal intervals increased from 25 to 36 milliseconds (P<0.001), mean square of differences between consecutive R-R intervals increased from 12 to 19 milliseconds (P<0.001), HR decreased from 73±9 bpm to 67±4 bpm, (P<0.001), and deceleration capacity increased from 2.1 to 4.4 milliseconds (P<0.001). A trend for periodic repolarization dynamics reduction was observed (5.6 deg2 versus 4.7 deg2, P=0.09). Autonomic changes were accompanied by increased left ventricular ejection fraction (29±6% versus 40±8%, P<0.001) and reduced NT-proBNP (3548 versus 685 ng/L, P<0.001). Correlation analysis showed a significant relationship between volume-unloading (as evidenced by NT-proBNP reduction) and autonomic improvement.

CONCLUSIONS

Three months of ARNI therapy resulted in a significant increase in cardiac parasympathetic tone. The improvement in autonomic properties may be mediated by "volume unloading" and likely contributes to the beneficial effects of ARNI in heart failure with reduced ejection fraction.

REGISTRATION

URL: https://www.clinicaltrials.gov; Unique Identifier: NCT04587947.

Sympathetic baroreflex sensitivity is enhanced in postmenopausal women

The sympathetic nervous system is critical for regulating blood pressure (BP) via the arterial baroreflex and sympathetic transduction in the peripheral vasculature. These mechanisms interact, and both may be altered with aging and impacted by menopause. Although age-related decreases in sympathetic transduction have been demonstrated in women, it remains unclear whether sympathetic baroreflex sensitivity (BRS) is impaired in postmenopausal women (POST). We tested the hypothesis that sympathetic BRS would be enhanced in POST compared with premenopausal women (PRE). We examined beat-by-beat BP and muscle sympathetic nerve activity (MSNA) in 19 PRE (22 ± 2 yr, 22 ± 3 kg/m2) and 12 POST (57 ± 5 yr, 24 ± 2 kg/m2) during 10 min of rest. Spontaneous sympathetic BRS was quantified as the slope of a linear regression between MSNA burst incidence and diastolic BP. Sympathetic transduction to mean arterial pressure (MAP) for the 10 cardiac cycles following spontaneous MSNA bursts was assessed via signal averaging method. Resting MAP was similar (PRE: 82 ± 8 vs. POST: 85 ± 8 mmHg, P = 0.43), whereas resting MSNA was elevated in POST (PRE: 10 ± 6 vs. POST: 45 ± 16 bursts/100 heart beats, P < 0.0001). Spontaneous sympathetic BRS was enhanced in POST (PRE: -2.0 ± 1.2 vs. POST: -5.2 ± 1.9 bursts/beat/mmHg, P < 0.0005). Sympathetic transduction to MAP was attenuated in POST (time: P < 0.001, group: P < 0.001, interaction: P < 0.01). These data suggest that sympathetic BRS may be enhanced in POST. Consistent with recent hypotheses, enhanced sensitivity of the arterial baroreflex's neural arc may signify a compensatory response to reduced efficiency of the peripheral arterial baroreflex arc (i.e., sympathetic transduction) to preserve BP buffering capacity.NEW & NOTEWORTHY Studies examining sympathetic baroreflex function with aging remain equivocal, with some studies showing an increase, decrease, or no change in sympathetic baroreflex sensitivity (BRS) in older adults compared with younger adults. With aging, women experience unique physiological changes due to menopause that influence autonomic function. For the first time, we show that postmenopausal women exhibit a greater sympathetic BRS compared with young premenopausal women.

The interactive effects of posture and biological sex on the control of muscle sympathetic nerve activity during rhythmic handgrip exercise

Body posture and biological sex exhibit independent effects on the sympathetic neural responses to dynamic exercise. However, the neural mechanisms (e.g., baroreflex) by which posture impacts sympathetic outflow during rhythmic muscular contractions, and whether biological sex affects posture-mediated changes in efferent sympathetic nerve traffic during exercise, remain unknown. Thus, we tested the hypotheses that increases in muscle sympathetic nerve activity (MSNA) would be greater during upright compared with supine rhythmic handgrip (RHG) exercise, and that females would demonstrate smaller increases in MSNA during upright RHG exercise than males. Twenty young (30 [6] yr; means [SD]) individuals (9 males, 11 females) underwent 6 min of supine and upright (head-up tilt 45°) RHG exercise at 40% maximal voluntary contraction with continuous measurements of MSNA (microneurography), blood pressure (photoplethysmography), and heart rate (electrocardiogram). In the pooled group, absolute MSNA burst frequency (P < 0.001), amplitude (P = 0.009), and total MSNA (P < 0.001) were higher during upright compared with supine RHG exercise. However, body posture did not impact the peak change in MSNA during RHG exercise (range: P = 0.063-0.495). Spontaneous sympathetic baroreflex gain decreased from rest to RHG exercise (P = 0.006) and was not impacted by posture (P = 0.347). During upright RHG exercise, males demonstrated larger increases in MSNA burst amplitude (P = 0.002) and total MSNA (P = 0.001) compared with females, which coincided with greater reductions in sympathetic baroreflex gain among males (P = 0.004). Collectively, these data indicate that acute attenuation of baroreflex-mediated sympathoinhibition permits increases in MSNA during RHG exercise and that males exhibit a greater reserve for efferent sympathetic neural recruitment during orthostasis than females.NEW & NOTEWORTHY The impact of posture and sex on cardiovascular control during rhythmic handgrip (RHG) exercise is unknown. We show that increases in muscle sympathetic nerve activity (MSNA) during RHG are partly mediated by a reduction in sympathetic baroreflex gain. In addition, males demonstrate larger increases in total MSNA during upright RHG than females. These data indicate that the baroreflex partly mediates increases in MSNA during RHG and that males have a greater sympathetic vasoconstrictor reserve than females.

Evening but not morning aerobic training improves sympathetic activity and baroreflex sensitivity in elderly patients with treated hypertension

The blood pressure-lowering effect of aerobic training is preceded by improving cardiovascular autonomic control. We previously demonstrated that aerobic training conducted in the evening (ET) induces a greater decrease in blood pressure than morning training (MT). To study whether the greater blood pressure decrease after ET occurs through better cardiovascular autonomic regulation, this study aimed to compare MT versus ET on muscle sympathetic nerve activity (MSNA) and baroreflex sensitivity (BRS) in treated patients with hypertension. Elderly patients treated for hypertension were randomly allocated into MT (n = 12, 07.00-10.00 h) or ET (n = 11, 17.00-20.00 h) groups. Both groups trained for 10 weeks, 3 times/week, cycling for 45 min at moderate intensity. Beat-to-beat blood pressure (finger photoplethysmography), heart rate (electrocardiography) and MSNA (microneurography) were assessed at the initial and final phases of the study at baseline and during sequential bolus infusions of sodium nitroprusside and phenylephrine (modified-Oxford technique) to evaluate cardiac and sympathetic BRS. Mean blood pressure decreased significantly after ET but not after MT (-9 ± 11 vs. -1 ± 8 mmHg, P = 0.042). MSNA decreased significantly only after ET with no change after MT (-12 ± 5 vs. -3 ± 7 bursts/100 heart beats, P = 0.013). Sympathetic BRS improved after ET but not after MT (-0.8 ± 0.7 vs. 0.0 ± 0.8 bursts/100 heart beats/mmHg, P = 0.052). Cardiac BRS improved similarly in both groups (ET: +1.7 ± 1.8 vs. MT: +1.4 ± 1.9 ms/mmHg, Pphase  ≤ 0.001). In elderly patients treated for hypertension, only ET decreased mean blood pressure and MSNA and improved sympathetic BRS. These findings revealed that the sympathetic nervous system has a key role in ET's superiority to MT in blood pressure-lowering effect. KEY POINTS: Reducing muscle nerve sympathetic activity and increasing sympathetic baroreflex sensitivity plays a key role in promoting the greater blood pressure reduction observed with evening training. These findings indicated that simply changing the timing of exercise training may offer additional benefits beyond antihypertensive medications, such as protection against sympathetic overdrive and loss of baroreflex sensitivity, independent markers of mortality. Our new findings also suggest new avenues of investigation, such as the possibility that evening aerobic training may be beneficial in other clinical conditions with sympathetic overdrive, such as congestive heart failure and hypertrophic cardiomyopathy.

Sympathetic determinants of resting blood pressure in males and females

Discharge of postganglionic muscle sympathetic nerve activity (MSNA) is related poorly to blood pressure (BP) in adults. Whether neural measurements beyond the prevailing level of MSNA can account for interindividual differences in BP remains unclear. The current study sought to evaluate the relative contributions of sympathetic-BP transduction and sympathetic baroreflex gain on resting BP in young adults. Data were analyzed from 191 (77 females) young adults (18-39 years) who underwent continuous measurement of beat-to-beat BP (finger photoplethysmography), heart rate (electrocardiography), and fibular nerve MSNA (microneurography). Linear regression analyses were computed to determine associations between sympathetic-BP transduction (signal-averaging) or sympathetic baroreflex gain (threshold technique) and resting BP, before and after controlling for age, body mass index, and MSNA burst frequency. K-mean clustering was used to explore sympathetic phenotypes of BP control and consequential influence on resting BP. Sympathetic-BP transduction was unrelated to BP in males or females (both R2 < 0.01; P > 0.67). Sympathetic baroreflex gain was positively associated with BP in males (R2 = 0.09, P < 0.01), but not in females (R2 < 0.01; P = 0.80), before and after controlling for age, body mass index, and MSNA burst frequency. K-means clustering identified a subset of participants with average resting MSNA, yet lower sympathetic-BP transduction and lower sympathetic baroreflex gain. This distinct subgroup presented with elevated BP in males (P < 0.02), but not in females (P = 0.10). Sympathetic-BP transduction is unrelated to resting BP, while the association between sympathetic baroreflex gain and resting BP in males reveals important sex differences in the sympathetic determination of resting BP.NEW & NOTEWORTHY In a sample of 191 normotensive young adults, we confirm that resting muscle sympathetic nerve activity is a poor predictor of resting blood pressure and now demonstrate that sympathetic baroreflex gain is associated with resting blood pressure in males but not females. In contrast, signal-averaged measures of sympathetic-blood pressure transduction are unrelated to resting blood pressure. These findings highlight sex differences in the neural regulation of blood pressure.

The degree of engagement of cardiac and sympathetic arms of the baroreflex does not depend on the absolute value and sign of arterial pressure variations

Objective.The percentages of cardiac and sympathetic baroreflex patterns detected via baroreflex sequence (SEQ) technique from spontaneous variability of heart period (HP) and systolic arterial pressure (SAP) and of muscle nerve sympathetic activity (MSNA) burst rate and diastolic arterial pressure (DAP) are utilized to assess the level of the baroreflex engagement. The cardiac baroreflex patterns can be distinguished in those featuring both HP and SAP increases (cSEQ++) and decreases (cSEQ--), while the sympathetic baroreflex patterns in those featuring a MSNA burst rate decrease and a DAP increase (sSEQ+-) and vice versa (sSEQ-+). The present study aims to assess the modifications of the involvement of the cardiac and sympathetic arms of the baroreflex with age and postural stimulus intensity.Approach.We monitored the percentages of cSEQ++ (%cSEQ++) and cSEQ-- (%cSEQ--) in 100 healthy subjects (age: 21-70 years, 54 males, 46 females), divided into five sex-balanced groups consisting of 20 subjects in each decade at rest in supine position and during active standing (STAND). We evaluated %cSEQ++, %cSEQ--, and the percentages of sSEQ+- (%sSEQ+-) and sSEQ-+ (%sSEQ-+) in 12 young healthy subjects (age 23 ± 2 years, 3 females, 9 males) undergoing incremental head-up tilt.Main results.We found that: (i) %cSEQ++ and %cSEQ-- decreased with age and increased with STAND and postural stimulus intensity; (ii) %sSEQ+- and %sSEQ-+ augmented with postural challenge magnitude; (iii) the level of cardiac and sympathetic baroreflex engagement did not depend on either the absolute value of arterial pressure or the direction of its changes.Significance.This study stresses the limited ability of the cardiac and sympathetic arms of the baroreflex in controlling absolute arterial pressure values and the equivalent ability of both positive and negative arterial pressure changes in soliciting them.

Age- and sex-related differences in sympathetic vascular transduction and neurohemodynamic balance in humans

Bursts of muscle sympathetic nerve activity (MSNA) and the ensuing vasoconstriction are pivotal determinants of beat-by-beat blood pressure regulation. Although age and sex impact blood pressure regulation, how these factors affect the central and peripheral arcs of the baroreflex remains unclear. In 27 young [25 (SD 3) yr] males (YM; n = 14) and females (YF; n = 13) and 23 older [71 (SD 5) yr] males (OM; n = 11) and females (OF; n = 12), femoral artery blood flow, blood pressure, and MSNA were recorded for 10 min of supine rest. Sympathetic baroreflex sensitivity (i.e., central arc) was quantified as the relationship between diastolic blood pressure and MSNA burst incidence. Signal averaging was used to determine sympathetic vascular transduction into leg vascular conductance (LVC) for 12 cardiac cycles following MSNA bursts (i.e., peripheral arc). Older adults demonstrated attenuated sympathetic transduction into LVC (both P < 0.001) following MSNA bursts, and smaller increases in sympathetic transduction as a function of MSNA burst size and firing pattern compared with young adults (range, P = 0.004-0.032). YM (r2 = 0.36; P = 0.032) and OM (r2 = 0.51; P = 0.014) exhibited an inverse relationship between the central and peripheral arcs of the baroreflex, whereas females did not (YF, r2 = 0.03, P = 0.621; OF, r2 = 0.06, P = 0.445). MSNA burst incidence was inversely related to sympathetic transduction in YM and OF (range, P = 0.03-0.046) but not in YF or OM (range, P = 0.360-0.603). These data indicate that age is associated with attenuated sympathetic vascular transduction, whereas age- and sex-specific changes are present in the relationship between the central and peripheral arcs of the baroreflex regulation of blood pressure.NEW & NOTEWORTHY Sympathetic vascular transduction is attenuated in older compared with young adults, regardless of biological sex. Males, but not females (regardless of age), demonstrate an inverse relationship between central (sympathetic baroreflex sensitivity) and peripheral (sympathetic vascular transduction) components of the baroreflex arc. Young males and older females exhibit an inverse relationship between resting sympathetic outflow and sympathetic vascular transduction. Our results indicate that age and sex exert independent and interactive effects on sympathetic vascular transduction and sympathetic neurohemodynamic balance in humans.

Testosterone-associated blood pressure dysregulation in women with androgen excess polycystic ovary syndrome

We tested the hypothesis that hyperandrogenemia in androgen excess polycystic ovary syndrome (AE-PCOS) is a primary driver in blood pressure (BP) dysregulation via altered sympathetic nervous system activity (SNSA), reduced integrated baroreflex gain and increased renin-angiotensin system (RAS) activation. We measured resting SNSA (microneurography), integrated baroreflex gain, and RAS with lower body negative pressure in obese insulin-resistant (IR) women with AE-PCOS [n = 8, 23 ± 4 yr; body mass index (BMI) = 36.3 ± 6.4 kg/m2] and obese IR controls (n = 7, control, 29 ± 7 yr; BMI = 34.9 ± 6.8 kg/m2), at baseline (BSL), after 4 days of gonadotropin-releasing hormone antagonist (ANT, 250 μg/day) and 4 days of ANT + testosterone (ANT + T, 5 mg/day) administration. Resting BP was similar between groups for systolic blood pressure (SBP; 137 ± 14 vs. 135 ± 14 mmHg, AE-PCOS, control) and diastolic BP (89 ± 21 vs. 76 ± 10 mmHg, AE-PCOS, control). BSL integrated baroreflex gain was similar between groups [1.4 ± 0.9 vs. 1.0 ± 1.3 forearm vascular resistance (FVR) U/mmHg], but AE-PCOS had lower SNSA (10.3 ± 2.0 vs. 14.4 ± 4.4 burst/100 heartbeats, P = 0.04). In AE-PCOS, T suppression increased integrated baroreflex gain, which was restored to BSL with ANT + T (4.3 ± 6.5 vs. 1.5 ± 0.8 FVR U/mmHg, ANT, and ANT + T, P = 0.04), with no effect in control. ANT increased SNSA in AE-PCOS (11.2 ± 2.4, P = 0.04). Serum aldosterone was greater in AE-PCOS versus control (136.5 ± 60.2 vs. 75.7 ± 41.4 pg/mL, AE-PCOS, control, P = 0.04) at BSL but was unaffected by intervention. Serum angiotensin-converting enzyme was greater in AE-PCOS versus control (101.9 ± 93.4 vs. 38.2 ± 14.7 pg/mL, P = 0.04) and reduced by ANT in AE-PCOS (77.7 ± 76.5 vs. 43.4 ± 27.3 µg/L, ANT, and ANT + T, P = 0.04) with no impact on control. Obese, IR women with AE-PCOS showed decreased integrated baroreflex gain and increased RAS activation compared with control.NEW & NOTEWORTHY Here we present evidence for an important role of testosterone in baroreflex control of blood pressure and renal responses to baroreceptor unloading in women with a common, high-risk androgen excess polycystic ovary syndrome (AE-PCOS) phenotype. These data indicate a direct effect of testosterone on the vascular system of women with AE-PCOS independent of body mass index (BMI) and insulin-resistant (IR). Our study indicates that hyperandrogenemia is a central underlining mechanism of heightened cardiovascular risk in women with PCOS.

Posture-related changes in sympathetic baroreflex sensitivity during normal pregnancy

Normal pregnancy is associated with vast adjustments in cardiovascular autonomic control. Sympathetic baroreflex sensitivity has been reported to be attenuated during pregnancy in animal models, but most studies in humans are cross-sectional and findings from longitudinal case studies are inconclusive. It remains unclear how sympathetic baroreflex sensitivity is altered longitudinally during pregnancy within an individual in different body postures. Therefore, this study examined the impact of posture on sympathetic baroreflex sensitivity in 24 normal-weight normotensive pregnant women. Spontaneous sympathetic baroreflex sensitivity was assessed during early (6-11 weeks) and late (32-36 weeks) pregnancy and 6-10 weeks postpartum in the supine posture and graded head-up tilt (30° and 60°). In addition, data from the postpartum period were compared with (and no different to) 18 age-matched non-pregnant women to confirm that the postpartum period was reflective of a non-pregnant condition (online supplement). When compared with postpartum (-3.8 ± 0.4 bursts/100 heartbeats/mmHg), supine sympathetic baroreflex sensitivity was augmented during early pregnancy (-5.9 ± 0.4 bursts/100 heartbeats/mmHg, P < 0.001). However, sympathetic baroreflex sensitivity at 30° or 60° head-up tilt was not different between any phase of gestation (P > 0.05). When compared to supine, sympathetic baroreflex sensitivity at 60° head-up tilt was significantly blunted during early (Δ2.0 ± 0.7 bursts/100 heartbeats/mmHg, P = 0.024) and late (Δ1.5 ± 0.6 bursts/100 heartbeats/mmHg, P = 0.049) pregnancy but did not change postpartum (Δ0.4 ± 0.6 bursts/100 heartbeats/mmHg, P = 1.0). These data show that time-course changes in sympathetic baroreflex sensitivity are dependent on the posture it is examined in and provides a foundation of normal blood pressure regulation during pregnancy for future studies in women at risk for adverse pregnancy outcomes.

Sympathetic neural reactivity to the Trier social stress test

Sympathetic responsiveness to laboratory mental stress is highly variable, making interpretations of its role in stress reactivity challenging. The present study assessed muscle sympathetic nerve activity (MSNA, microneurography) responsiveness to the Trier social stress test (TSST), which employs an anticipatory stress phase, followed by a public speaking and mental arithmetic task. We hypothesized that sympathetic reactivity to the anticipatory phase would offer a more uniform response between individuals due to elimination of confounds (i.e. respiratory changes, muscle movement, etc.) observed during more common stress tasks. Participants included 26 healthy adults (11 men, 15 women, age: 25 ± 6 years, body mass index: 24 ± 3 kg/m² ). Continuous heart rate (electrocardiogram) and beat-to-beat blood pressure (finger plethysmography) were recorded from all participants, while MSNA recordings were obtained in 20 participants. MSNA burst frequency was significantly reduced during anticipatory stress. During the speech, although burst frequency was unchanged, total MSNA was significantly increased. Changes in diastolic arterial pressure were predictive of changes in MSNA during anticipatory (β = -0.680, P = 0.001), but not the speech (P = 0.318) or mental maths (P = 0.051) phases. Lastly, sympathetic reactivity to anticipatory stress was predictive of subsequent reactivity to both speech (β = 0.740, P = 0.0002) and maths (β = 0.663, P = 0.001). In conclusion, anticipatory social stress may offer a more versatile means of assessing sympathetic reactivity to mental stress in the absence of confounds and appears to predict reactivity to subsequent mental stress paradigms. KEY POINTS: Cardiovascular reactivity to laboratory mental stress is predictive of future health outcomes. However, reactivity of the sympathetic nervous system to mental stress is highly variable. The current study assessed peripheral muscle sympathetic nerve activity in response to the Trier social stress test, a psychosocial stressor that includes anticipatory stress, public speaking and mental arithmetic. Our findings demonstrate that sympathetic neural activity is consistently reduced during anticipatory stress. Conversely, the classically observed inter-individual variability of sympathetic responsiveness was observed during speech and maths tasks. Additionally, sympathetic reactivity to the anticipatory period accurately predicted how an individual would respond to both speech and maths tasks, outlining the utility of anticipatory stress in future research surrounding stress reactivity. Utilization of the Trier social stress test in autonomic physiology may offer an alternative assessment of sympathetic responsiveness to stress with more consistent inter-individual responsiveness and may be a useful tool for further investigation of stress reactivity.

Aging is associated with enhanced central, but impaired peripheral arms of the sympathetic baroreflex arc

We tested the hypothesis that the baroreflex control of action potential (AP) subpopulations would be blunted in older compared to young adults. Integrated muscle sympathetic nerve activity (MSNA) and the underlying sympathetic APs were obtained using microneurography and a continuous wavelet analysis approach, respectively, during 5 minutes of supine rest in 13 older (45-75 years, 6 females) and 14 young (21-30 years, 7 females) adults. Baroreflex threshold relationships were quantified as the slope of the linear regression between MSNA burst probability (%) and diastolic blood pressure (mmHg), or AP cluster firing probability (%) and diastolic blood pressure (mmHg). Integrated MSNA baroreflex threshold gain was greater in older compared to young adults (older: -7.6±3.6 %/mmHg vs. Young: -3.5±1.5 %/mmHg, P<0.001). Similarly, the baroreflex threshold gain of AP clusters was modified by aging (group-by-cluster effect: P<0.001) such that older adults demonstrated greater baroreflex threshold gains of medium-sized AP clusters (e.g., Cluster 4, older: -8.2±3.2 %/mmHg vs. Young: -3.6±1.9 %/mmHg, P=0.003) but not for the smallest- (Cluster 1, older: -1.6±1.9 %/mmHg vs. Young: -1.0±1.7 %/mmHg, P>0.999) and largest-sized (Cluster 10, older: -0.5±0.5 %/mmHg vs. Young: -0.2±0.1 %/mmHg, P=0.819) AP clusters compared to young adults. In contrast, the peak change in mean arterial pressure (MAP) following a spontaneous MSNA burst (i.e., sympathetic transduction) was impaired with aging (older: -0.7±0.3 mmHg vs. Young: 1.8±1.2 mmHg, P<0.001). We conclude that aging is associated with elevated baroreflex control over high-probability AP content of sympathetic bursts that may compensate for impaired sympathetic neurovascular transduction.

Action potential amplitude and baroreflex resetting of action potential clusters mediate hypoxia-induced sympathetic long-term facilitation

KEY POINTS

Acute isocapnic hypoxia resets the arterial baroreflex and permits long-lasting sympathoexcitation called sympathetic long-term facilitation. Our understanding of sympathetic long-term facilitation following hypoxia in humans is based on multiunit muscle sympathetic nerve activity and does not fully characterize the underlying baroreflex control of sympathetic neuronal subpopulations or their discharge/recruitment strategies. We show that sympathetic long-term facilitation is mediated by baroreflex resetting of sympathetic action potential clusters to higher arterial pressure operating points, a reduction in the percentage of action potentials firing asynchronously, and a shift toward larger amplitude action potential activity. The results advance our fundamental understanding of how the sympathetic nervous system mediates sympathetic long-term facilitation following exposure to acute isocapnic hypoxia in humans.

ABSTRACT

Baroreflex resetting permits sympathetic long-term facilitation (sLTF) following hypoxia; however, baroreflex control of action potential (AP) clusters and AP recruitment patterns facilitating sLTF is unknown. We hypothesized that baroreflex resetting of arterial pressure operating points (OPs) of AP clusters and recruitment of large-amplitude APs would mediate sLTF following hypoxia. Eight men (age: 24 (3) yrs; BMI: 24 (3) kg/m² ) underwent 20-min isocapnic hypoxia (P_ET O₂ : 47 (2) mmHg) and 30-min recovery. Multi-unit microneurography (muscle sympathetic nerve activity; MSNA) and a continuous wavelet transform with matched mother wavelet was used to detect sympathetic APs during baseline, hypoxia, early (first 5-min), and late recovery (last 5-min). AP amplitude (normalized to largest baseline AP amplitude), percent APs occurring outside a MSNA burst (% asynchronous APs), and proportion of APs firing in small (1-3), medium (4-6), and large (7-10) normalized cluster sizes was calculated. Normalized clusters were used to assess baroreflex OPs and sensitivity. Hypoxia increased total MSNA activity, which remained elevated during recovery (P<0.0001). Baroreflex OPs were shifted rightward for all clusters in recovery, with no effect on slope. Compared to baseline, AP amplitude was elevated by 3 (2) % and 4 (2) % while asynchronous APs were reduced by 9 (5) % and 7 (6) % in early and late recovery, respectively. In early recovery, the proportion of APs firing in large clusters was increased compared to baseline. Hypoxia-induced sLTF is mediated by baroreflex resetting of AP clusters to higher OPs, reduced asynchronous AP firing, and increased contribution from large-amplitude APs. Abstract figure legend Eight healthy men underwent 20-min isocapnic hypoxia and 30-min recovery. The study tested the hypothesis that baroreflex resetting of arterial pressure operating points (OPs) of action potential (AP) clusters and recruitment of large-amplitude APs would mediate sympathetic long-term facilitation (sLTF) following acute hypoxic exposure. Hypoxia increased multi-unit muscle sympathetic nerve activity (MSNA; measured via microneurography), which remained elevated throughout recovery. Sympathetic APs were detected in the filtered MSNA neurogram using a continuous wavelet transform with matched mother wavelet. An effect of condition revealed that compared to baseline, AP amplitude was elevated while asynchronous APs were reduced in early and late recovery, respectively. Our findings show that AP amplitude distributions are shifting towards larger AP amplitudes in all subjects following hypoxia. Our findings indicate that hypoxia-induced sLTF is mediated by baroreflex resetting of AP clusters to higher OPs, reduced asynchronous AP firing, and increased contribution from large-amplitude APs. This article is protected by copyright. All rights reserved.

The Feasibility of Baroreflex Sensitivity Measurements in Heart Failure Subjects: The Role of Slow-patterned Breathing

INTRODUCTION

Increased muscle sympathetic nerve activity (MSNA) indicates an adverse outcome in heart failure. Decreased baroreflex modulation of MSNA is well known feature of the disease. The determinability of cardiovagal baroreflex sensitivity (BRS) in heart failure is low, however, the determinability of sympathetic BRS is not known.

METHODS

We have assessed the spontaneous, MSNA burst incidence-based baroreflex index (BRSsymp) in 33 stable heart failure patients and in 10 healthy controls using the traditional r≥0.5 cut-off for acceptable individual diastolic pressure-burst incidence slopes, and also a more stringent r≥0.7 cut-off. We have also assessed the influence of 6/min breathing.

RESULTS

The determinability of BRSsymp in heart failure patients was 64% during spontaneous breathing with r≥0.5 cut-off, and 39% using the r≥0.7 cut-off. The determinability of these indices further decreased during 6/min breathing, dropping to 29% with the r≥0.7 cut-off. In contrast, the determinability of the cardiovagal BRS indices increased significantly with 6/min breathing (from 24% to 66%; p<0.001). Patients who still had determinable BRSsymp at the r≥0.7 cut-off had a significantly lower baseline burst incidence than those with an undeterminable index (70±14 vs.89±10 burst/100 cycles; p<0.002). Neither the 6/min breathing, nor the r≥0.7 cut-off limit influenced the high availability of BRSsymp in healthy subjects.

CONCLUSION

The determinability of BRSsymp in heart failure patients is limited, especially with the 0.7 limit for correlation. Undeterminable BRSsymp in patients is associated with higher sympathetic activity. 6/min breathing improves the determinability of cardiovagal BRS indices, but not that of BRSsymp. This article is protected by copyright. All rights reserved.

Ventricular-Vascular Uncoupling in Heart Failure: Effects of Arterial Baroreflex-Induced Sympathoexcitation at Rest and During Exercise

Autonomic alterations in blood pressure are primarily a result of arterial baroreflex modulation of systemic vascular resistance and cardiac output on a beat-by-beat basis. The combined central and peripheral control by the baroreflex likely acts to maintain efficient energy transfer from the heart to the systemic vasculature; termed ventricular-vascular coupling. This level of control is maintained whether at rest or during exercise in healthy subjects. During heart failure, the ventricular-vascular relationship is uncoupled and baroreflex dysfunction is apparent. We investigated if baroreflex dysfunction in heart failure exacerbated ventricular-vascular uncoupling at rest, and during exercise in response to baroreceptor unloading by performing bilateral carotid occlusions in chronically instrumented conscious canines. We observed in healthy subjects that baroreceptor unloading caused significant increases in effective arterial elastance (Ea) at rest (1.2 ± 0.3 mmHg/ml) and during exercise (1.3 ± 0.2 mmHg/ml) that coincided with significant increases in stroke work (SW) (1.5 ± 0.2 mmHg/ml) and (1.6 ± 0.2 mmHg/ml) suggesting maintained ventricular-vascular coupling. Heart Failure significantly increased the effect of baroreceptor unloading on Ea at rest (3.1 ± 0.7 mmHg/ml) and during exercise (2.3 ± 0.5 mmHg/ml) whereas no significant increases in stroke work occurred, thus signifying further ventricular-vascular uncoupling. We believe that the enhanced ventricular-vascular uncoupling observed during baroreceptor unloading only worsens the already challenged orthostatic and exercise tolerance and thereby contributes to poor exercise performance and quality of life for heart failure patients.

Arterial Baroreflex Inhibits Muscle Metaboreflex Induced Increases in Effective Arterial Elastance: Implications for Ventricular-Vascular Coupling

The ventricular-vascular relationship assesses the efficacy of energy transferred from the left ventricle to the systemic circulation and is quantified as the ratio of effective arterial elastance to maximal left ventricular elastance. This relationship is maintained during exercise via reflex increases in cardiovascular performance raising both arterial and ventricular elastance in parallel. These changes are, in part, due to reflexes engendered by activation of metabosensitive skeletal muscle afferents-termed the muscle metaboreflex. However, in heart failure, ventricular-vascular uncoupling is apparent and muscle metaboreflex activation worsens this relationship through enhanced systemic vasoconstriction markedly increasing effective arterial elastance which is unaccompanied by substantial increases in ventricular function. This enhanced arterial vasoconstriction is, in part, due to significant reductions in cardiac performance induced by heart failure causing over-stimulation of the metaboreflex due to under perfusion of active skeletal muscle, but also as a result of reduced baroreflex buffering of the muscle metaboreflex-induced peripheral sympatho-activation. To what extent the arterial baroreflex modifies the metaboreflex-induced changes in effective arterial elastance is unknown. We investigated in chronically instrumented conscious canines if removal of baroreflex input via sino-aortic baroreceptor denervation (SAD) would significantly enhance effective arterial elastance in normal animals and whether this would be amplified after induction of heart failure. We observed that effective arterial elastance (E_a), was significantly increased during muscle metaboreflex activation after SAD (0.4 ± 0.1 mmHg/mL to 1.4 ± 0.3 mmHg/mL). In heart failure, metaboreflex activation caused exaggerated increases in E_a and in this setting, SAD significantly increased the rise in E_a elicited by muscle metaboreflex activation (1.3 ± 0.3 mmHg/mL to 2.3 ± 0.3 mmHg/mL). Thus, we conclude that the arterial baroreflex does buffer muscle metaboreflex induced increases in E_a and this buffering likely has effects on the ventricular-vascular coupling.

Test-retest reliability of the Valsalva maneuver in spinal cord injury

Objective: To determine the test-retest reliability of quantitative and qualitative baroreflex sensitivity (BRS) parameters derived from the Valsalva maneuver (VM) in individuals with traumatic cervical SCI.Design: Test-retest reliability.Setting: Tertiary rehabilitation center.Participants: Fourteen participants with cervical SCI (ranging from C3-C8 neurological level).Outcome Measurements: Beat-to-beat systolic blood pressure (SBP) traces (finger photoplethysmography) were obtained during a 15-second forced expiration at two time points (7.6 ± 2.9 days between sessions) to assess VM reliability. Test-retest reliability of BRS metrics from derived from the VM (Valsalva ratio; VR, pressure recovery time; PRT, vagal baroreflex sensitivity; BRSv, adrenergic baroreflex sensitivity; BRSa₁, and total recovery; TR) were assessed by intra-class correlation coefficient (ICC, with 95% confidence interval; CI) and by qualitative reproducibility (V, N, or M pattern).Results: ICCs for quantitative parameters were (CI): VR = 0.894 (0.703-0.965), TR = 0.927 (0.789-0.976), BRSa₁ = 0.561 (0.149-0.911), PRT = 0.728 (0.343-0.904), BRSv = 0.243 (-0.309-0.673). Qualitatively, 12 subjects (85.7%) demonstrated reproducible VM patterns at both time points (3 "M" pattern, 8 "V" pattern and one "N" pattern).Conclusion: VR (a measure of cardiovagal function) and TR (a measure of sympathetic adrenergic function) are reliable quantitative parameters that can be derived from SBP response to VM in participants with SCI. Qualitative waveform analysis was reproducible in 12/14 participants. This provides the foundational evidence required to pursue further validity testing to establish a role for VM in the assessment of autonomic functions in SCI.

Determinants of orthostatic cerebral oxygenation assessed using near-infrared spectroscopy

BACKGROUND

To understand the relationship between blood pressure changes during standing up and clinical outcome, cerebral oxygenation needs to be measured, which may be performed using near-infrared spectroscopy (NIRS). However, the role of potential determinants of NIRS-derived orthostatic cerebral oxygenation, i.e., age, sex, type of postural change (i.e., standing up from sitting versus supine position), blood pressure (BP) and baroreflex sensitivity (BRS) is still unknown and needed to better interpret findings from studies using orthostatic NIRS measurements.

METHODS

34 younger (median age 25 years, inter quartile range (IQR) 22-45) and 31 older adults (median age 77 years, IQR 72-81) underwent BP, BRS and NIRS measurements during standing up from sitting and supine position. Linear regression models were used to assess the potential determinant role of age, sex, type of postural change, BP and BRS in orthostatic cerebral oxygenation drop and recovery. Orthostatic cerebral oxygenation test-retest reliability was assessed using intra class correlations.

RESULTS

Younger age, male sex and standing up from supine compared to sitting position were positively associated with cerebral oxygenation drop; older age and standing up from sitting compared to supine position were associated with higher cerebral oxygenation recovery. Test-retest reliability was highest (ICC > 0.83) during standing up from supine position.

CONCLUSION

Based on the findings of this study, age, sex and type of postural change are significant determinants of NIRS-derived orthostatic cerebral oxygenation and should be taken into account in the interpretation of NIRS measurements. In the design of new studies, standing up from supine position is preferable (higher reliability) over standing up from sitting position.

Sympathetic Neural Control in Humans with Anxiety-Related Disorders

Numerous conceptual models are used to describe the dynamic responsiveness of physiological systems to environmental pressures, originating with Claude Bernard's milieu intérieur and extending to more recent models such as allostasis. The impact of stress and anxiety upon these regulatory processes has both basic science and clinical relevance, extending from the pioneering work of Hans Selye who advanced the concept that stress can significantly impact physiological health and function. Of particular interest within the current article, anxiety is independently associated with cardiovascular risk, yet mechanisms underlying these associations remain equivocal. This link between anxiety and cardiovascular risk is relevant given the high prevalence of anxiety in the general population, as well as its early age of onset. Chronically anxious populations, such as those with anxiety disorders (i.e., generalized anxiety disorder, panic disorder, specific phobias, etc.) offer a human model that interrogates the deleterious effects that chronic stress and allostatic load can have on the nervous system and cardiovascular function. Further, while many of these disorders do not appear to exhibit baseline alterations in sympathetic neural activity, reactivity to mental stress offers insights into applicable, real-world scenarios in which heightened sympathetic reactivity may predispose those individuals to elevated cardiovascular risk. This article also assesses behavioral and lifestyle modifications that have been shown to concurrently improve anxiety symptoms, as well as sympathetic control. Lastly, future directions of research will be discussed, with a focus on better integration of psychological factors within physiological studies examining anxiety and neural cardiovascular health. © 2022 American Physiological Society. Compr Physiol 12:1-33, 2022.

Endovascular baroreflex amplification and the effect on sympathetic nerve activity in patients with resistant hypertension: A proof-of-principle study

BACKGROUND

First in human studies suggest that endovascular baroreflex amplification (EVBA) lowers blood pressure (BP). To explore potential mechanisms for BP reduction, this study examines the effects of EVBA on muscle sympathetic nerve activity (MSNA) and baroreceptor sensitivity (BRS).

METHODS

In a single-center sub-study of the CALM-DIEM study (Controlling And Lowering blood pressure with the MobiusHD-Defining Efficacy Markers), 14 patients with resistant hypertension were treated with EVBA. Microneurography and non-invasive continuous BP measurements were performed at baseline and three months after MobiusHD implantation. The primary outcome was change in MSNA. Secondary outcomes were change in baroreflex sensitivity (BRS), cardiovascular responses to a sympathetic stimulus, BP, heart rate (HR) and heart rate variability (HRV).

RESULTS

The primary endpoint was obtained in 10 of 14 patients enrolled in the sub-study. MSNA burst frequency and burst incidence decreased in 6 of 10 patients: mean change -4.1 bursts/min (95% confidence interval -12.2 to 4.0) and -3.8 bursts/100 heartbeats (-15.2 to 7.7). MSNA spike frequency and spike count decreased in 8 of 10 patients: mean change -2.8 spikes/sec (-7.3 to 1.8) and -3.0 spikes/heartbeat (-6.1 to 0.1). Change in MSNA and BP were not correlated. Office BP decreased by -14/-6 mmHg (-27 to -2/-15 to 3). We observed a trend towards decreased HR (-5 bpm, -10 to 1) and increased total power HRV (623 msec2, 78 to 1168). In contrast, BRS and cardiovascular responses remained unchanged after EVBA.

CONCLUSIONS

In this proof-of-principle study, EVBA did not significantly decrease MSNA in patients with resistant hypertension. EVBA did not impair baroreflex function.

TRIAL REGISTRATION

Clinical trial registration at NCT02827032.

Prenatal Exercise and Cardiovascular Health (PEACH) Study: Impact on Muscle Sympathetic Nerve (Re)Activity

PURPOSE

Women who develop gestational hypertension have evidence of elevated muscle sympathetic nerve activity (MSNA) in early pregnancy, which continues to rise after diagnosis. Exercise has been shown to play a preventative role in the development of gestational hypertension and has been shown to reduce resting and reflex MSNA in nonpregnant populations. We sought to investigate whether aerobic exercise affected the sympathetic regulation of blood pressure between the second and third trimesters of pregnancy.

METHODS

We conducted a randomized controlled trial of structured aerobic exercise (n = 31) compared with no intervention (control, n = 28) beginning at 16-20 wk and continuing until 34-36 wk of gestation (NCT02948439). Women in the exercise group were prescribed aerobic activity at 50%-70% of their heart rate reserve, on 3-4 d·wk-1 for 25-40 min with a 5-min warm-up and 5-min cool-down (i.e., up to 160 min total activity per week). At preintervention and postintervention assessments, data from ~10 min of quiet rest and a 3-min cold pressor test were analyzed to determine sympathetic nervous system activity and reactivity.

RESULTS

MSNA was obtained in 51% of assessments. Resting MSNA burst frequency and burst incidence increased across gestation (main effect of gestational age, P = 0.002). Neurovascular transduction was blunted in the control group (P = 0.024) but not in exercisers (P = 0.873) at the postintervention time point. Lastly, MSNA reactivity during the cold pressor test was not affected by gestational age or exercise (P = 0.790, interaction).

CONCLUSIONS

These data show that exercise attenuates both the rise in MSNA and the blunting of neurovascular transduction. This may partially explain the lower risk of developing gestational hypertension in women who are active during their pregnancies.

CORP: Standardizing methodology for assessing spontaneous baroreflex control of muscle sympathetic nerve activity in humans

The use of spontaneous bursts of muscle sympathetic nerve activity (MSNA) to assess arterial baroreflex control of sympathetic nerve activity has seen increased utility in studies of both health and disease. However, methods used for analyzing spontaneous MSNA baroreflex sensitivity are highly variable across published studies. Therefore, we sought to comprehensively examine methods of producing linear regression slopes to quantify spontaneous MSNA baroreflex sensitivity in a large cohort of subjects (n = 150) to support a standardized procedure for analysis that would allow for consistent and comparable results across laboratories. The primary results demonstrated that 1) consistency of linear regression slopes was considerably improved when the correlation coefficient was above -0.70, which is more stringent compared with commonly reported criterion of -0.50, 2) longer recording durations increased the percentage of linear regressions producing correlation coefficients above -0.70 (1 min = 15%, 2 min = 28%, 5 min = 53%, 10 min = 67%, P < 0.001) and reaching statistical significance (1 min = 40%, 2 min = 69%, 5 min = 78%, 10 min = 89%, P < 0.001), 3) correlation coefficients were improved with 3-mmHg versus 1-mmHg and 2-mmHg diastolic blood pressure (BP) bin size, and 4) linear regression slopes were reduced when the acquired BP signal was not properly aligned with the cardiac cycle triggering the burst of MSNA. In summary, these results support the use of baseline recording durations of 10 min, a correlation coefficient above -0.70 for reliable linear regressions, 3-mmHg bin size, and importance of properly time-aligning MSNA and diastolic BP. Together, these findings provide best practices for determining spontaneous MSNA baroreflex sensitivity under resting conditions for improved rigor and reproducibility of results.

Preeclampsia is not associated with elevated muscle sympathetic reactivity

To determine whether increased chemoreflex tonic activity is associated with augmented muscle sympathetic nervous system activity (MSNA) in women diagnosed with preeclampsia. Women with preeclampsia (n = 19; 32 ± 5 yr old, 31 ± 3 wk of gestation) were matched by age and gestational age with pregnant women (controls, n = 38, 32 ± 4 yr old, 31 ± 4 wk gestation; 2:1 ratio). MSNA (n = 9 preeclampsia) was assessed during baseline, peripheral chemoreflex deactivation (hyperoxia), and a cold pressor test (CPT). Baroreflex gain and diastolic blood pressure at which there is a 50% likelihood of MSNA occurring (T50) and plasma noradrenaline concentrations were measured. Baseline mean arterial pressure (MAP: 106 ± 11 vs. 87 ± 10 mmHg, P < 0.0001), noradrenaline concentrations (498 ± 152 pg/mL vs. 326 ± 147, P = 0.001), and T50 (79 ± 7 vs. 71 ± 9 mmHg, P = 0.02) were greater in women with preeclampsia than in controls. However, baseline MSNA (burst incidence [BI]: 41 ± 16 vs. 45 ± 13 bursts/100 hb, P = 0.4) was not different between groups. Responses to hyperoxia (ΔBI -5 ± 7 vs. -1 ± 8 bursts/100 hb, P = 0.1; ΔMAP -1 ± 3 vs. -2 ± 3 mmHg, P = 0.7) and CPT (ΔBI 15 ± 7 vs. 12 ± 11 bursts/100 hb, P = 0.6; ΔMAP 10 ± 4 vs. 12 ± 11 mmHg, P = 0.6) were not different between groups. Our findings question the assumption that increased MSNA contributes to hypertension in women with preeclampsia. The chemoreflex does not appear to contribute to an increase in MSNA in women with preeclampsia.NEW & NOTEWORTHY We wanted to determine whether increased chemoreflex tonic activity is associated with augmented muscle sympathetic nervous system activity (MSNA) in women diagnosed with preeclampsia. The chemoreflex does not contribute to increased MSNA in women with preeclampsia. Our data also challenge the belief that preeclampsia is associated with sympathetic neural hyperactivity. Thus, targeting sympathetic neural hyperactivity as therapeutic strategy is unlikely to be the most efficacious approach to treatment and management.

Sympathetic arterial baroreflex hysteresis in humans: different patterns during low- and high-pressure levels

Fluctuations in diastolic pressure modulate muscle sympathetic nerve activity (MSNA) through the arterial baroreflex. A higher sympathetic baroreflex sensitivity (sBRS) to pressure falls compared with rises has been reported; however, the underlying mechanisms are unclear. We assessed whether beat-to-beat falling and rising diastolic pressures operate on two distinct baroreflex response curves. Twenty-two men (32 ± 8 yr) underwent sequential bolus injections of nitroprusside and phenylephrine (modified Oxford test) with continuous recording of heart rate, blood pressure, and MSNA. The weighted negative linear regression slope between falling or rising diastolic pressure and MSNA burst incidence quantified sBRS_fall and sBRS_rise, respectively. The diastolic pressure evoking a MSNA burst incidence of 50 (T₅₀) was calculated. sBRS_fall was greater than sBRS_rise (-6.24 ± 2.80 vs. -4.34 ± 2.16 bursts·100 heartbeats^-1·mmHg^-1, P = 0.01) and had a narrower operating range (14 ± 8 vs. 20 ± 10 mmHg, P = 0.01) that was shifted rightward (T_50, 75 ± 9 and 70 ± 11 mmHg, P < 0.001). At diastolic pressures below baseline, sBRS_fall was less than sBRS_rise (-1.81 ± 1.31 vs. -3.59 ± 1.70 bursts·100 heartbeats^-1·mmHg^-1, P = 0.003) as low absolute pressures operated closer to the saturation plateau on the falling, compared with the rising pressure curve. At pressures above baseline, sBRS_fall was greater than sBRS_rise (-5.23 ± 1.94 and -3.79 ± 1.67 bursts·100 heartbeats^-1·mmHg^-1, P = 0.03). These findings demonstrate that the sympathetic arterial baroreflex possesses two response curves for processing beat-to-beat diastolic pressure falls and rises. The falling pressure curve is rightward shifted, which reduces sensitivity to falling pressure at low absolute pressures. This demonstrates that the direction of the hysteresis is influenced by the prevailing pressure level relative to each baroreflex response curve.NEW & NOTEWORTHY The findings show that the arterial baroreflex processes diastolic pressure dependent on the direction of pressure change from the previous beat, yielding two distinct baroreflex response curves to falling and rising pressure. Overall, the falling pressure curve is rightward shifted and more sensitive. The rightward shift caused a hysteresis reversal at hypotensive pressures as the falling pressure saturation plateau of the sigmoid response curve occurred at higher pressures than the rising pressure curve.

Augmented resting beat-to-beat blood pressure variability in young, healthy, non-Hispanic black men

NEW FINDINGS

What is the central question of this study? The prevalence of hypertension in black individuals exceeds that in other racial groups. Despite this well-known heightened risk, the underlying contributory factors remain incompletely understood. We hypothesized that young black men would exhibit augmented beat-to-beat blood pressure variability compared with white men and that black men would exhibit augmented total peripheral resistance variability. What is the main finding and its importance? We demonstrate that young, healthy black men exhibit greater resting beat-to-beat blood pressure variability compared with their white counterparts, which is accompanied by greater variability in total peripheral resistance. These swings in blood pressure over time might contribute to the enhanced cardiovascular risk profile in black individuals.

ABSTRACT

The prevalence of hypertension in black (BL) individuals exceeds that in other racial groups. Recently, resting beat-to-beat blood pressure (BP) variability has been shown to predict cardiovascular risk and detect target organ damage better than ambulatory BP monitoring. Given the heightened risk in BL individuals, we hypothesized young BL men would exhibit augmented beat-to-beat BP variability compared with white (WH) men. Furthermore, given studies reporting reduced vasodilatation and augmented vasoconstriction in BL individuals, we hypothesized that BL men would exhibit augmented variability in total peripheral resistance (TPR). In 45 normotensive men (24 BL), beat-to-beat BP (Finometer) was measured during 10-20 min of quiet rest. Cardiac output and TPR were estimated (Modelflow method). Despite similar resting BP, BL men exhibited greater BP standard deviation (e.g. systolic BP SD; BL, 7.1 ± 2.2 mmHg; WH, 5.4 ± 1.5 mmHg; P = 0.006) compared with WH men, which was accompanied by a greater TPR SD (P = 0.003), but not cardiac output SD (P = 0.390). Other traditional measures of variability provided similar results. Histogram analysis indicated that BL men exhibited a greater percentage of cardiac cycles with BPs higher (> +10 mmHg higher) and lower (< -8 mmHg lower) than mean systolic BP compared with WH men (interaction, P < 0.001), which was accompanied by a greater percentage of cardiac cycles with high/low TPR (P < 0.001). In a subset of subjects (n = 30), reduced sympathetic baroreflex sensitivity was associated with augmented BP variability (r = -0.638, P < 0.001), whereas cardiac baroreflex sensitivity had no relationship (P = 0.447). Herein, we document an augmented beat-to-beat BP variability in young BL men, which coincided with fluctuations in vascular resistance and reduced sympathetic BRS.

Countdown before voluntary exercise induces muscle vasodilation with baroreflex-mediated decrease in muscle sympathetic nerve activity in humans

We examined whether a countdown (CD) before voluntary cycling exercise induced prospective vascular adjustment for the exercise and, if so, whether and how muscle sympathetic nerve activity (MSNA) was involved in the responses. Young men performed voluntary cycling in a semirecumbent position (n = 14) while middle cerebral artery blood flow velocity (V_MCA; Doppler ultrasonography), heart rate (HR), arterial pressure (AP; finger photoplethysmography), oxygen consumption rate (V̇o₂), oxygen saturation in the thigh muscle (StO2; near-infrared spectrometry), cardiac output (CO; Modelflow method), and total peripheral resistance (TPR) were measured (experiment 1). Another group underwent the same exercise protocol but used only the right leg (n = 10) while MSNA (microneurography) was measured in the peroneal nerve of the left leg (experiment 2). All subjects performed eight trials with a ≥5-min rest between trials. In four trials randomly selected from the eight trials, exercise onset was signaled by a 30-s CD, whereas in the remaining four trials, exercise was started without CD. We found that CD first increased V_MCA, HR, CO, and mean AP, and then decreased TPR and increased StO2 and V̇o₂ (experiment 1; all P < 0.021). Furthermore, the CD-induced increase in mean AP decreased total MSNA and burst frequency (experiment 2; both P < 0.048) through the baroreflex, with decreased TPR and increased StO2 (experiment 2; both P < 0.001). The vasodilation and increased V̇o₂ continued after the start of exercise. Thus CD before starting exercise induced the muscle vasodilatory response with a concomitant reduction in MSNA through the baroreflex to accelerate aerobic energy production after the start of exercise.NEW & NOTEWORTHY Prospective cardiovascular adjustment occurs before starting voluntary exercise, increasing heart rate and arterial pressure followed by muscle vasodilation; however, the precise mechanisms and significance for this vasodilation remain unknown. We found that during the countdown before starting exercise cerebral blood flow velocity increased, followed by increases in heart rate and arterial pressure, which suppressed MSNA through baroreflex, resulting in thigh muscle vasodilation to increase oxygen consumption rate, which might make it easier to start exercise.

Arterial baroreflex regulation of muscle sympathetic single-unit activity in men: influence of resting blood pressure

The arterial baroreflex has dominant control over multiunit muscle sympathetic nerve activity (MSNA) burst occurrence, but whether this extends to all single units or is influenced by resting blood pressure status is unclear. In 22 men (32 ± 8 yr), we assessed 68 MSNA single units during sequential bolus injections of nitroprusside and phenylephrine (modified Oxford). Sympathetic baroreflex sensitivity (sBRS) was quantified as the weighted negative linear regression slope between diastolic blood pressure (DBP) and single-unit spike firing probability and multiple spike firing. Strong negative linear relationships ( r ≥ −0.50) between DBP and spike firing probability were observed in 63/68 (93%) single units (−2.27 ± 1.27%·cardiac cycle−1·mmHg−1 [operating range, 18 ± 8 mmHg]). In contrast, only 45/68 (66%) single units had strong DBP-multiple spike firing relationships (−0.13 ± 0.18 spikes·cardiac cycle−1·mmHg−1 [operating range, 14 ± 7 mmHg]). Participants with higher resting DBP (65 ± 3 vs. 77 ± 3 mmHg, P < 0.001) had similar spike firing probability sBRS (low vs. high, −2.08 ± 1.08 vs. −2.46 ± 1.42%·cardiac cycle−1·mmHg−1, P = 0.33), but a smaller sBRS operating range (20 ± 6 vs. 16 ± 9 mmHg, P = 0.01; 86 ± 24 vs. 52 ± 25% of total range, P < 0.001) and a higher proportion of single units without arterial baroreflex control outside this range [6/31 (19%) vs. 21/32 (66%), P < 0.001]. Participants with higher resting DBP also had fewer single units with arterial baroreflex control of multiple spike firing (79 vs. 53%, P = 0.04). The majority of MSNA single units demonstrate strong arterial baroreflex control over spike firing probability during pharmacological manipulation of blood pressure. Changes in single-unit sBRS operating range and control of multiple spike firing may represent altered sympathetic recruitment patterns associated with the early development of hypertension.

NEW & NOTEWORTHY Muscle sympathetic single units can be differentially controlled during stress. In contrast, we demonstrate that 93% of single units maintain strong arterial baroreflex control during pharmacological manipulation of blood pressure. Interestingly, the operating range and proportion of single units that lose arterial baroreflex control outside of this range are influenced by resting blood pressure levels. Altered single unit, but not multiunit, arterial baroreflex control may represent changes in sympathetic recruitment patterns in early stage development of hypertension.

Heterogeneous baroreflex control of sympathetic action potential subpopulations in humans

KEY POINTS

Emission patterns in muscle sympathetic nerve activity stem from differently sized action potential (AP) subpopulations that express varying discharge probabilities. The mechanisms governing these firing behaviours are unclear. This study investigated the hypothesis that the arterial baroreflex exerts varying control over the different AP subpopulations. During baseline, medium APs expressed the greatest baroreflex slopes, while small and large APs exhibited weaker slopes. On going from baseline to lower body negative pressure (LBNP; simulated orthostatic stress), baroreflex slopes for some clusters of medium APs expressed the greatest increase, while slopes for large APs also increased but to a lesser degree. A subpopulation of previously silent larger APs was recruited with LBNP but these APs expressed weak baroreflex slopes. The arterial baroreflex heterogeneously regulates sympathetic AP subpopulations, exerting its strongest effect over medium APs. Weak baroreflex mechanisms govern the recruitment of latent larger AP subpopulations during orthostatic stress.

ABSTRACT

Muscle sympathetic nerve activity (MSNA) occurs primarily in bursts of action potentials (AP) with subpopulations that differ in size and discharge probabilities. The mechanisms determining these discharge patterns remain unclear. This study investigated the hypothesis that variations in AP discharge are due to subpopulation-specific baroreflex control. We employed multi-unit microneurography and a continuous wavelet analysis approach to extract sympathetic APs in 12 healthy individuals during baseline (BSL) and lower body negative pressure (LBNP; -40, -60, -80 mmHg). For each AP cluster, the baroreflex threshold slope was measured from the linear regression between AP probability (%) and diastolic blood pressure (mmHg). During BSL, the baroreflex exerted non-uniform regulation over AP subpopulations: medium-sized AP clusters expressed the greatest slopes while clusters of small and large APs expressed weaker slopes. On going from BSL to LBNP, the baroreflex slopes for each AP subpopulation were modified differently. Baroreflex slopes (%/mmHg) for some medium APs (cluster 5: -4.4 ± 4 to -9.1 ± 5) expressed the greatest increase with LBNP, while slopes for large APs (cluster 9: -1.3 ± 1 to -2.6 ± 2) also increased, but to a lesser degree. Slopes for small APs present at BSL exhibited reductions with LBNP (cluster 2: -3.9 ± 3 to -2.2 ± 3). Larger previously silent AP clusters recruited with LBNP expressed weak baroreflex regulation (cluster 14: -0.9 ± 1%/mmHg). The baroreflex exerts the strongest control over medium-sized APs. Augmenting baroreflex gain and upward resetting of discrete AP subpopulations active at BSL, as well as recruiting larger previously silent APs with weak baroreflex control, facilitates elevated MSNA during orthostatic stress.

A randomized controlled trial to reduce sedentary time in rheumatoid arthritis: protocol and rationale of the Take a STAND for Health study

Background

Patients with rheumatoid arthritis spend most of their daily hours in sedentary behavior (sitting), a predisposing factor to poor health-related outcomes and all-cause mortality. Interventions focused on reducing sedentary time could be of novel therapeutic relevance. However, studies addressing this topic remain scarce. We aim to investigate the feasibility and efficacy of a newly developed intervention focused on reducing sedentary time, and potential clinical, physiological, metabolic and molecular effects in rheumatoid arthritis.

Methods

The Take a STAND for Health study is a 4-month, parallel-group, randomized controlled trial, in which postmenopausal patients with rheumatoid arthritis will set individually tailored, progressive goals to replace their sedentary time with standing and light-intensity activities. Patients will be recruited from the Clinical Hospital (School of Medicine, University of Sao Paulo) and will be assessed at baseline and after a 4-month follow up. Outcomes will include objectively measured sedentary behavior (primary outcome) and physical activity levels, clinical parameters, anthropometric parameters and body composition; aerobic fitness, muscle function, blood pressure, cardiovascular autonomic function, vascular function and structure, health-related quality of life, and food intake. Blood and muscle samples will be collected for assessing potential mechanisms, through targeted and non-targeted approaches.

Discussion

Findings will be of scientific and clinical relevance with the potential to inform new prescriptions focused on reducing sedentary behavior, a modifiable risk factor that thus far has been overlooked in patients with rheumatoid arthritis.

Trial registration

ClinicalTrials.gov, NCT03186924. Registered on 14 June 2017.

Short-term water deprivation does not increase blood pressure variability or impair neurovascular function in healthy young adults

High dietary salt increases arterial blood pressure variability (BPV) in salt-resistant, normotensive rodents and is thought to result from elevated plasma [Na⁺] sensitizing central sympathetic networks. Our purpose was to test the hypothesis that water deprivation (WD)-induced elevations in serum [Na⁺] augment BPV via changes in baroreflex function and sympathetic vascular transduction in humans. In a randomized crossover fashion, 35 adults [17 female/18 male, age: 25 ± 4 yr, systolic/diastolic blood pressure (BP): 107 ± 11/60 ± 7 mmHg, body mass index: 23 ± 3 kg/m²] completed two hydration protocols: a euhydration control condition (CON) and a stepwise reduction in water intake over 3 days, concluding with 16 h of WD. We assessed blood and urine electrolyte concentrations and osmolality, resting muscle sympathetic nerve activity (MSNA; peroneal microneurography; 18 paired recordings), beat-to-beat BP (photoplethysmography), common femoral artery blood flow (Doppler ultrasound), and heart rate (single-lead ECG). A subset of participants (n = 25) underwent ambulatory BP monitoring during day 3 of each protocol. We calculated average real variability as an index of BPV. WD increased serum [Na⁺] (141.0 ± 2.3 vs. 142.1 ± 1.7 mmol/L, P < 0.01) and plasma osmolality (288 ± 4 vs. 292 ± 5 mosmol/kg H₂O, P < 0.01). However, WD did not increase beat-to-beat (1.9 ± 0.4 vs. 1.8 ± 0.4 mmHg, P = 0.24) or ambulatory daytime (9.6 ± 2.1 vs. 9.4 ± 3.3 mmHg, P = 0.76) systolic BPV. Additionally, sympathetic baroreflex sensitivity (P = 0.20) and sympathetic vascular transduction were not different after WD (P = 0.17 for peak Δmean BP following spontaneous MSNA bursts). These findings suggest that, despite modestly increasing serum [Na⁺], WD does not affect BPV, arterial baroreflex function, or sympathetic vascular transduction in healthy young adults.

Device-guided slow breathing reduces blood pressure and sympathetic activity in young normotensive individuals of both sexes

Slow breathing (SLOWB) is recommended for use as an adjuvant treatment for hypertension. However, the extent to which blood pressure (BP) responses to SLOWB differ between men and women are not well-established. Therefore, we tested the hypothesis that an acute bout of SLOWB would induce larger decreases in BP in males than in females, given that males typically have higher resting BP. We also examined autonomic contributors to reduced BP during SLOWB; that is, muscle sympathetic nerve activity and spontaneous cardiovagal (sequence method) and vascular sympathetic baroreflex sensitivity. We tested normotensive females ( n = 10, age: 22 ± 2 y, body mass index: 22 ± 2 kg/m2) and males ( n = 12, age: 23 ± 3 y, body mass index: 26 ± 4 kg/m2). Subjects were tested at baseline and during the last 5 min of a 15-min RESPeRATE-guided SLOWB session. Overall, SLOWB reduced systolic BP by 3.2 ± 0.8 mmHg (main effect, P < 0.01). Females had lower systolic BP (main effect, P = 0.02); we observed no interaction between sex and SLOWB. SLOWB also reduced muscle sympathetic nerve activity burst incidence by −5.0 ± 1.4 bursts/100 heartbeats (main effect, P < 0.01). Although females tended to have lower burst incidence (main effect, P = 0.1), there was no interaction between sex and SLOWB. Cardiovagal baroreflex sensitivity improved during SLOWB (21.0 vs. 36.0 ms/mmHg, P = 0.03) with no effect of sex. Despite lower overall BP in females, our data support a lack of basement effect on SLOWB-induced reductions in BP, as SLOWB was equally effective in reducing BP in males and females. Our findings support the efficacy of the RESPeRATE device for reducing BP in both sexes, even in young, normotensive individuals.

NEW & NOTEWORTHY We provide support for the effectiveness of device-guided slow breathing for blood pressure reduction in young normotensive women and men. Despite having lower baseline blood pressure and sympathetic nerve activity, women experienced equivalent reductions in both measures in response to RESPeRATE-guided slow breathing as men. Thus, slow breathing appears to be effective in young healthy normotensive individuals of both sexes and may be an ideal preventative therapy against future hypertension.

Cardiac and Vascular Sympathetic Baroreflex Control during Orthostatic Pre-Syncope

We hypothesized that sympathetic baroreflex mediated uncoupling between neural sympathetic discharge pattern and arterial pressure (AP) fluctuations at 0.1 Hz during baroreceptor unloading might promote orthostatic pre-syncope. Ten volunteers (32 ± 6 years) underwent electrocardiogram, beat-to-beat AP, respiratory activity and muscle sympathetic nerve activity (MSNA) recordings while supine (REST) and during 80° head-up tilt (HUT) followed by -10 mmHg stepwise increase of lower body negative pressure until pre-syncope. Cardiac and sympathetic baroreflex sensitivity were quantified. Spectrum analysis of systolic and diastolic AP (SAP and DAP) and calibrated MSNA (cMSNA) variability assessed the low frequency fluctuations (LF, ~0.1 Hz) of SAP, DAP and cMSNA variability. The squared coherence function (K²) quantified the coupling between cMSNA and DAP in the LF band. Analyses were performed while supine, during asymptomatic HUT (T₁) and at pre-syncope onset (T₂). During T₂ we found that: (1) sympathetic baroreceptor modulation was virtually abolished compared to T₁; (2) a progressive decrease in AP was accompanied by a persistent but chaotic sympathetic firing; (3) coupling between cMSNA and AP series at 0.1 Hz was reduced compared to T₁. A negligible sympathetic baroreceptor modulation during pre-syncope might disrupt sympathetic discharge pattern impairing the capability of vessels to constrict and promote pre-syncope.

Effects of Prolonged Head-Down Bed Rest on Cardiac and Vascular Baroreceptor Modulation and Orthostatic Tolerance in Healthy Individuals

Orthostatic intolerance commonly occurs after prolonged bed rest, thus increasing the risk of syncope and falls. Baroreflex-mediated adjustments of heart rate and sympathetic vasomotor activity (muscle sympathetic nerve activity – MSNA) are crucial for orthostatic tolerance. We hypothesized that prolonged bed rest deconditioning alters overall baroreceptor functioning, thereby reducing orthostatic tolerance in healthy volunteers. As part of the European Space Agency Medium-term Bed Rest protocol, 10 volunteers were studied before and after 21 days of −6° head down bed rest (HDBR). In both conditions, subjects underwent ECG, beat-by-beat blood pressure, respiratory activity, and MSNA recordings while supine (REST) and during a 15-min 80° head-up tilt (TILT) followed by a 3-min −10 mmHg stepwise increase of lower body negative pressure to pre-syncope. Cardiac baroreflex sensitivity (cBRS) was obtained in the time (sequence method) and frequency domain (spectrum and cross-spectrum analyses of RR interval and systolic arterial pressure – SAP, variability). Baroreceptor modulation of sympathetic discharge activity to the vessels (sBRS) was estimated by the slope of the regression line between the percentage of MSNA burst occurrence and diastolic arterial pressure. Orthostatic tolerance significantly decreased after HDBR (12 ± 0.6 min) compared to before (21 ± 0.6 min). While supine, heart rate, SAP, and cBRS were unchanged before and after HDBR, sBRS gain was slightly depressed after than before HDBR (sBRS: −6.0 ± 1.1 versus −2.9 ± 1.5 burst% × mmHg⁻¹, respectively). During TILT, HR was higher after than before HDBR (116 ± 4 b/min versus 100 ± 4 b/min, respectively), SAP was unmodified in both conditions, and cBRS indexes were lower after HDBR (α index: 3.4 ± 0.7 ms/mmHg; BRS_SEQ 4.0 ± 1.0) than before (α index: 6.4 ± 1.0 ms/mmHg; BRS_SEQ 6.8 ± 1.2). sBRS gain was significantly more depressed after HDBR than before (sBRS: −2.3 ± 0.7 versus −4.4 ± 0.4 burst% × mmHg⁻¹, respectively). Our findings suggest that baroreflex-mediated adjustments in heart rate and MSNA are impaired after prolonged bed rest. The mechanism likely contributes to the decrease in orthostatic tolerance.

The intensity of oscillations of the photoplethysmographic waveform variability at frequencies 0.04-0.4 Hz is effective marker of hypertension and coronary artery disease in males

Background: It is believed that the intensity of oscillations in the photoplethysmographic waveform variability reflects the activity of vascular regulatory mechanisms. However, the relationship of such fluctuations with the state of health is poorly understood.Purpose: The aim of our study was to assess the possibility of using spectral indices that reflect the intensity of oscillations of the photoplethysmographic waveform variability at frequencies 0.04-0.4 Hz as markers of hypertension and coronary artery disease. We did not study women to exclude the influence of menopause and sex hormones on the results.Materials and Methods: We compared synchronous 10-minute records of finger photoplethysmogram and respiration at rest in 30 healthy males (48.8 ± 4.5 years; data presented as Mean ± SD) versus 30 patients with hypertension (aged 49.0 ± 4.3 years) versus 30 patients with stable coronary artery disease (49.2 ± 4.8 years). Percentages of high-frequency and low-frequency ranges in the total power of photoplethysmographic waveform variability spectrum (HF% and LF%), and LF/HF ratio were assessed.Results: HF% are subject to by 2- to 5-fold increase in hypertensive patients (p < .001) and up to an 8-fold increase in patients with coronary artery disease (p < .001) when compared with healthy persons. On the contrary, LF% is reduced by 1.5-5 times in all patients when compared with healthy people (p < .001). We identified cut-off points for each photoplethysmographic index to distinguish patients with coronary artery disease or hypertension from healthy subjects. Multiple logistic regression models based on photoplethysmographic waveform variability indices had sufficient sensitivity and specificity for patients with hypertension or coronary artery disease.Conclusion: Frequency-domain indices of photoplethysmographic waveform variability (in particular, HF%, LF%, and LF/HF) are sufficiently sensitive and specific markers of hypertension and coronary artery disease in adult males.

Characterization of the Asymmetry of the Cardiac and Sympathetic Arms of the Baroreflex From Spontaneous Variability During Incremental Head-Up Tilt

Hysteresis of the baroreflex (BR) is the result of the different BR sensitivity (BRS) when arterial pressure (AP) rises or falls. This phenomenon has been poorly studied and almost exclusively examined by applying pharmacological challenges and static approaches disregarding causal relations. This study inspects the asymmetry of the cardiac BR (cBR) and vascular sympathetic BR (sBR) in physiological closed loop conditions from spontaneous fluctuations of physiological variables, namely heart period (HP) and systolic AP (SAP) leading to the estimation of cardiac BRS (cBRS) and muscle sympathetic nerve activity (MSNA) and diastolic AP (DAP) leading to the estimation of vascular sympathetic BRS (sBRS). The assessment was carried out in 12 young healthy subjects undergoing incremental head-up tilt with table inclination gradually increased from 0 to 60°. Two analytical methods were exploited and compared, namely the sequence (SEQ) and phase-rectified signal averaging (PRSA) methods. SEQ analysis is based on the detection of joint causal schemes representing the HP and MSNA burst rate delayed responses to spontaneous SAP and DAP modifications, respectively. PRSA analysis averages HP and MSNA burst rate patterns after aligning them according to the direction of SAP and DAP changes, respectively. Since cBRSs were similar when SAP went up or down, hysteresis of cBR was not detected. Conversely, hysteresis of sBR was evident with sBRS more negative when DAP was falling than rising. sBR hysteresis was no longer visible during sympathetic activation induced by the orthostatic challenge. These results were obtained via the SEQ method, while the PRSA technique appeared to be less powerful in describing the BR asymmetry due to the strong association between BRS estimates computed over positive and negative AP variations. This study suggests that cBR and sBR provide different information about the BR control, sBR exhibits more relevant non-linear features that are evident even during physiological changes of AP, and the SEQ method can be fruitfully exploited to characterize the BR hysteresis with promising applications to BR branches different from cBR and sBR.

Altered baroreflex sensitivity in young women with a family history of hypertension

A positive family history of hypertension (+FH) is a risk factor for the future development of hypertension. Hypertension is associated with reductions in baroreflex sensitivity (BRS). Therefore, we hypothesized that young women with a +FH [ n = 12, 22 ± 1 yr, body mass index (BMI) 21 ± 1 kg/m², mean arterial pressure (MAP) 79 ± 1 mmHg] would have lower BRS compared with young women without a family history of hypertension (-FH) ( n = 13, 22 ± 1 yr, BMI 21 ± 1 kg/m², MAP 77 ± 2 mmHg, all P > 0.05 between groups). Continuous measurements of muscle sympathetic nerve activity, blood pressure, and electrocardiogram derived R-R interval were recorded at rest and during a Valsalva maneuver. Both cardiovagal BRS and vascular sympathetic BRS were assessed. Resting cardiovagal BRS was reduced in the +FH women (all sequences: -FH 32.3 ± 3.7 vs. +FH 20.2 ± 2.9 ms/mmHg, P = 0.02). Cardiovagal BRS during phase IV (-FH 16.5 ± 2.7 vs. +FH 7.6 ± 1.3 ms/mmHg, P < 0.01) but not phase II (-FH 5.5 ± 0.9 vs. +FH 5.0 ± 0.8 ms/mmHg, P = 0.67) of the Valsalva maneuver was also lower in the +FH women. Vascular sympathetic BRS at rest (-FH -2.38 ± 0.7 vs. +FH -2.33 ± 0.3 bursts· min^-1·mmHg^-1, P = 0.58) and during the Valsalva (-FH -0.74 ± 0.23 vs. +FH -0.66 ± 0.18 bursts·15 s^-1·mmHg^-1, P = 0.79) were not different between groups. These data suggest that healthy young women with a positive family history of hypertension have reduced cardiovagal BRS. This may be one mechanism contributing to the increased incidence of hypertension in this population later in life. NEW & NOTEWORTHY Having a family history of hypertension increases the risk of developing future hypertension. Reductions in baroreflex function have been demonstrated in hypertension and are an important marker for future cardiovascular disease. We show that young women with a family history of hypertension have lower cardiovagal baroreflex sensitivity. This alteration in autonomic function may be one mechanism contributing to the future incidence of hypertension in this patient population.

Spleen reactivity during incremental ascent to altitude

The spleen contains a reservoir of red blood cells that are mobilized into circulation when under physiological stress. Despite the spleen having an established role in compensation to acute hypoxia, no previous work has assessed the role of the spleen during ascent to high altitude. Twelve participants completed 2 min of handgrip exercise at 30% of maximal voluntary contraction at 1,045, 3,440, and 4,240 m. In a subset of eight participants, an infusion of phenylephrine hydrochloride was administered at a dosage of 30 µg/l of predicted blood volume at each altitude. The spleen was imaged by ultrasound via a 2- to 5.5-MHz curvilinear probe. Spleen volume was calculated by the prolate ellipsoid formula. Finger capillary blood samples were taken to measure hematocrit. Spleen images and hematocrit were taken both before and at the end of both handgrip and phenylephrine infusion. No changes in resting spleen volume were observed between altitudes. At low altitude, the spleen contracted in response to handgrip [272.8 ml (SD 102.3) vs. 249.6 ml (SD 105.7), P = 0.009], leading to an increase in hematocrit (42.6% (SD 3.3) vs. 44.3% (SD 3.3), P = 0.023] but did not contract or increase hematocrit at the high-altitude locations. Infusion of phenylephrine led to spleen contraction at all altitudes, but only lead to an increase in hematocrit at low altitude. These data reveal that the human spleen may not contribute to acclimatization to chronic hypoxia, contrary to its response to acute sympathoexcitation. These results are explained by alterations in spleen reactivity to increased sympathetic activation at altitude. NEW & NOTEWORTHY The present study demonstrated that, despite the known role of the human spleen in increasing oxygen delivery to tissues during acute hypoxia scenarios, the spleen does not mobilize red blood cells during ascent to high altitude. Furthermore, the spleen's response to acute stressors at altitude depends on the nature of the stressor; the spleen's sensitivity to neurotransmitter is maintained, while its reflex response to stress is dampened.

Maternal cardioautonomic responses during and following exercise throughout pregnancy

Blood pressure regulation during pregnancy is poorly understood. Cardiovagal baroreflex gain (BRG) is an important contributor to blood pressure regulation through its influence on heart rate. Heart rate fluctuations occur in response to various physiological stimuli and can be measured using heart rate variability (HRV). It is unclear how these mechanisms operate during pregnancy, particularly with regard to exercise. We examined BRG and HRV prior to, during, and following prenatal exercise. Forty-three pregnant (n = 10 first trimester (TM1), n = 17 second trimester (TM2), n = 16 third trimester (TM3)) and 20 nonpregnant (NP) women underwent an incremental peak exercise test. Beat-by-beat blood pressure (photoplethysmography) and heart rate (lead II electrocardiogram) were measured throughout. BRG (the slope of the relationship between fluctuations in systolic blood pressure and the R-R interval) and HRV (root mean square of the successive differences; RMSSD) were assessed at rest, during steady-state exercise (EX), and during active recovery. BRG decreased with gestation and was lower in the TM3 group than in the NP group (17.9 ± 6.9 ms/mm Hg vs 24.8 ± 7.4 ms/mm Hg, p = 0.017). BRG was reduced during EX in all groups. Resting HRV (RMSSD) also decreased with gestation and was lower in the TM3 group than in the NP group (29 ± 17 ms vs 48 ± 20 ms, p < 0.001). RMSSD was blunted during EX in all groups compared with rest. During active recovery, RMSSD was further blunted compared with EX in the NP group but not during pregnancy (TM1, TM2, and TM3). Compared with the nonpregnant controls, the pregnant women had lower BRG and HRV at rest, but comparable cardioautonomic control during both exercise and active recovery following peak exercise.

The Stability and Repeatability of Spontaneous Sympathetic Baroreflex Sensitivity in Healthy Young Individuals

Spontaneous sympathetic baroreflex sensitivity (BRS) is a valuable tool for assessing how well the baroreflex buffers beat-to-beat changes in blood pressure. However, there has yet to be a study involving appropriate statistical tests to examine the stability of sympathetic BRS within an experimental session and the repeatability between separate sessions. The aim of this study was to use intra-class correlations, ordinary least products regression, and Bland–Altman analyses to examine the stability and repeatability of spontaneous sympathetic BRS assessment. In addition, the influence of recording duration on values of BRS was assessed. In eighty-four healthy young individuals (49 males, 35 females), continuous measurements of blood pressure, heart rate and muscle sympathetic nerve activity (MSNA) were recorded for 10 min. In a subgroup of 13 participants (11 male, 2 female) the measurements were repeated on a separate day. Sympathetic BRS was quantified using MSNA burst incidence (BRS_inc) and total MSNA (BRS_total) for the first 5-min period, the second 5-min period, and a 2-min segment taken from the second 5-min period. Intra-class correlation coefficients indicated moderate stability in sympathetic BRS_inc and BRS_total between the first and second 5-min periods in males (BRS_incr = 0.63, BRS_totalr = 0.78) and females (BRS_incr = 0.61, BRS_totalr = 0.47) with no proportional bias, but with fixed bias for BRS_inc in females. When comparing the first 5-min with the 2-min period (n = 76), the intra-class correlation coefficient indicated poor to moderate repeatability in sympathetic BRS_inc and BRS_total for males (BRS_incr = -0.01, BRS_totalr = 0.70) and females (BRS_incr = 0.46, BRS_totalr = 0.39). However, Bland–Altman analysis revealed a fixed bias for BRS_total in males and proportional bias for BRS_total in females, with lower BRS values for 5-min recordings. In the subgroup, intra-class correlations indicated moderate repeatability for measures of BRS_inc (9 male, 2 female, r = 0.63) and BRS_total (6 male, 2 female, r = 0.68) assessed using 5-min periods recorded on separate days. However, Bland–Altman analysis indicated proportional bias for BRS_inc and fixed bias for BRS_total. In conclusion, measures of spontaneous sympathetic BRS are moderately stable and repeatable within and between testing sessions in healthy young adults, provided that the same length of recording is used when making comparisons.

Long-duration bed rest modifies sympathetic neural recruitment strategies in male and female participants

To understand the impact of physical deconditioning with head-down tilt bed rest (HDBR) on the malleability of sympathetic discharge patterns, we studied 1) baseline integrated muscle sympathetic nerve activity (MSNA; microneurography) from 13 female participants in the WISE-2005 60-day HDBR study (retrospective analysis), 2) integrated MSNA and multiunit action potential (AP) analysis in 13 male participants performed on data collected at baseline and during physiological stress imposed by end-inspiratory apnea in a new 60-day HDBR study, and 3) a repeatability study (control; n = 6, retrospective analysis, 4 wk between tests). Neither baseline integrated burst frequency nor incidence were altered with HDBR (both P > 0.35). However, baseline integrated burst latency increased in both HDBR studies (male: 1.35 ± 0.02 to 1.39 ± 0.02 s, P < 0.01; female: 1.23 ± 0.02 to 1.29 ± 0.02 s, P < 0.01), whereas controls exhibited no change across two visits (1.25 ± 0.02 to 1.25 ± 0.02 s, group-by-time interaction, P = 0.02). With the exception of increased AP latency ( P = 0.03), male baseline AP data did not change with HDBR (all P > 0.19). The change in AP frequency on going from baseline to apnea (∆94 ± 25 to ∆317 ± 55 AP/min, P < 0.01) and the number of active sympathetic clusters per burst (∆0 ± 0.2 to ∆1 ± 0.2 clusters/burst, P = 0.02) were greater post- compared with pre-HDBR. The change in total clusters with apnea was ∆0 ± 0.5 clusters pre- and ∆2 ± 0.7 clusters post-HDBR ( P = 0.07). These data indicate that 60-day HDBR modified discharge characteristics in baseline burst latency and sympathetic neural recruitment during apneic stress. NEW & NOTEWORTHY Long-duration bed rest did not modify baseline sympathetic burst frequency in male and female participants, but examination of additional features of the multiunit signal provided novel evidence to suggest augmented synaptic delays or processing times at baseline for all sympathetic action potentials. Furthermore, long-duration bed rest increased reflex-sympathetic arousal to apneic stress in male participants primarily by mechanisms involving an augmented firing rate of action potential clusters active at baseline.

Blood pressure responses to dietary sodium: Association with autonomic cardiovascular function in normotensive adults

Blood pressure responses to dietary sodium vary widely person-to-person. Salt sensitive rodent models display altered autonomic function, a trait thought to contribute to poor cardiovascular health. Thus, we hypothesized that increased salt sensitivity (SS) in normotensive humans would be associated with increased muscle sympathetic nerve activity (MSNA), decreased high frequency heart rate variability (HF-HRV), and decreased baroreflex sensitivity. Healthy normotensive men and women completed 1week of high (300mmol·day-1) and 1week of low (20mmol·day-1) dietary sodium (random order) with 24h mean arterial pressure (MAP) assessed on the last day of each diet to assess SS. Participants returned to the lab under habitual sodium conditions for testing. Forty-two participants are presented in this analysis, 19 of which successful MSNA recordings were obtained (n=42: age 39±2yrs., BMI 24.3±0.5kg·(m2)-1, MAP 83±1mmHg, habitual urine sodium 93±7mmol·24h-1; n=19: MSNA burst frequency 20±2 bursts·min-1). The variables of interest were linearly regressed over the magnitude of SS. Higher SS was associated with increased MSNA (burst frequency: r=0.469, p=0.041), decreased HF-HRV (r=-0.349, p=0.046), and increased LF/HF-HRV (r=0.363, p=0.034). SS was not associated with sympathetic or cardiac baroreflex sensitivity (p>0.05). Multiple regression analysis accounting for age found that age, not SS, independently predicted HF-HRV (age adjusted no longer significant; p=0.369) and LF/HF-HRV (age adjusted p=0.273). These data suggest that age-related salt sensitivity of blood pressure in response to dietary sodium is associated with altered resting autonomic cardiovascular function.

Sympathetic function during whole body cooling is altered in hypertensive adults

During moderate cold exposure, cardiovascular-related morbidity and mortality increase disproportionately in hypertensive adults (HTN); however, the mechanisms underlying this association are not well defined. We hypothesized that whole body cold stress would evoke exaggerated increases in blood pressure (BP) and muscle sympathetic nerve activity (MSNA) in HTN compared with normotensive adults (NTN) and that sympathetic baroreflex function would be altered during cooling in HTN. MSNA (peroneal microneurography) and beat-to-beat BP (Finometer) were measured continuously in 10 NTN (6 men/4 women; age 53 ± 3 yr; resting BP 125 ± 3/79 ± 1 mmHg) and 13 HTN (7 men/6 women; age 58 ± 2 yr; resting BP 146 ± 5/88 ± 2 mmHg) during whole body cooling-induced reductions in mean skin temperature (T_sk; water-perfused suit) from 34.0 to 30.5°C. During cooling, the increase in mean arterial pressure was greater in HTN (NTN: Δ6 ± 2 vs. HTN: Δ11 ± 1 mmHg; P = 0.02) and accompanied by exaggerated increases in MSNA (NTN: Δ8 ± 3 vs. HTN: Δ20 ± 3 bursts/100 heart beats; P < 0.01). The slope of the relation between MSNA and diastolic BP did not change during cooling in NTN (T_sk 34.0°C: -4.4 ± 0.8 vs. T_sk 30.5°C: -5.0 ± 0.3 bursts·100 heart beats^-1·mmHg^-1; P = 0.47) but increased in HTN (T_sk 34.0°C: -3.6 ± 0.4 vs. T_sk 30.5°C: -5.4 ± 0.4 bursts·100 heart beats)^-1·mmHg^-1; P = 0.02). These findings demonstrate that the cooling-induced increases in BP and MSNA are exaggerated in HTN. Furthermore, during cooling, sympathetic baroreflex sensitivity increases in HTN, but not NTN, presumably to allow for baroreflex-mediated buffering of excessive cooling-induced increases in BP. Collectively, these findings suggest that sympathetic function is altered during whole body cooling in hypertension. NEW & NOTEWORTHY These novel findings demonstrate that whole body cooling-induced reductions in mean skin temperature elicited greater increases in blood pressure and muscle sympathetic nerve activity in hypertensive adults. In addition, during moderate cold exposure, sympathetic baroreflex sensitivity increased in hypertensive, but not normotensive, adults.

Acute beetroot juice supplementation on sympathetic nerve activity: a randomized, double-blind, placebo-controlled proof-of-concept study

Acute dietary nitrate ([Formula: see text]) supplementation reduces resting blood pressure in healthy normotensives. This response has been attributed to increased nitric oxide bioavailability and peripheral vasodilation, although nitric oxide also tonically inhibits central sympathetic outflow. We hypothesized that acute dietary [Formula: see text] supplementation using beetroot (BR) juice would reduce blood pressure and muscle sympathetic nerve activity (MSNA) at rest and during exercise. Fourteen participants (7 men and 7 women, age: 25 ± 10 yr) underwent blood pressure and MSNA measurements before and after (165–180 min) ingestion of 70ml high-[Formula: see text] (~6.4 mmol [Formula: see text]) BR or [Formula: see text]-depleted BR placebo (PL; ~0.0055 mmol [Formula: see text]) in a double-blind, randomized, crossover design. Blood pressure and MSNA were also collected during 2 min of static handgrip (30% maximal voluntary contraction). The changes in resting MSNA burst frequency (−3 ± 5 vs. 3 ± 4 bursts/min, P = 0.001) and burst incidence (−4 ± 7 vs. 4 ± 5 bursts/100 heart beats, P = 0.002) were lower after BR versus PL, whereas systolic blood pressure (−1 ± 5 vs. 2 ± 5 mmHg, P = 0.30) and diastolic blood pressure (4 ± 5 vs. 5 ± 7 mmHg, P = 0.68) as well as spontaneous arterial sympathetic baroreflex sensitivity ( P = 0.95) were not different. During static handgrip, the change in MSNA burst incidence (1 ± 8 vs. 8 ± 9 bursts/100 heart beats, P = 0.04) was lower after BR versus PL, whereas MSNA burst frequency (6 ± 6 vs. 11 ± 10 bursts/min, P = 0.11) as well as systolic blood pressure (11 ± 7 vs. 12 ± 8 mmHg, P = 0.94) and diastolic blood pressure (11 ± 4 vs. 11 ± 4 mmHg, P = 0.60) were not different. Collectively, these data provide proof of principle that acute BR supplementation can decrease central sympathetic outflow at rest and during exercise. Dietary [Formula: see text] supplementation may represent a novel intervention to target exaggerated sympathetic outflow in clinical populations.

NEW & NOTEWORTHY The hemodynamic benefits of dietary nitrate supplementation have been attributed to nitric oxide-mediated peripheral vasodilation. Here, we provide proof of concept that acute dietary nitrate supplementation using beetroot juice can decrease muscle sympathetic outflow at rest and during exercise in a normotensive population. These results have applications for targeting central sympathetic overactivation in disease.

Neural control of blood pressure in women: differences according to age

PURPOSE

The blood pressure "error signal" represents the difference between an individual's mean diastolic blood pressure and the diastolic blood pressure at which 50% of cardiac cycles are associated with a muscle sympathetic nerve activity burst (the "T50"). In this study we evaluated whether T50 and the error signal related to the extent of change in blood pressure during autonomic blockade in young and older women, to study potential differences in sympathetic neural mechanisms regulating blood pressure before and after menopause.

METHODS

We measured muscle sympathetic nerve activity and blood pressure in 12 premenopausal (25 ± 1 years) and 12 postmenopausal women (61 ± 2 years) before and during complete autonomic blockade with trimethaphan camsylate.

RESULTS

At baseline, young women had a negative error signal (-8 ± 1 versus 2 ± 1 mmHg, p < 0.001; respectively) and lower muscle sympathetic nerve activity (15 ± 1 versus 33 ± 3 bursts/min, p < 0.001; respectively) than older women. The change in diastolic blood pressure after autonomic blockade was associated with baseline T50 in older women (r = -0.725, p = 0.008) but not in young women (r = -0.337, p = 0.29). Women with the most negative error signal had the lowest muscle sympathetic nerve activity in both groups (young: r = 0.886, p < 0.001; older: r = 0.870, p < 0.001).

CONCLUSIONS

Our results suggest that there are differences in baroreflex control of muscle sympathetic nerve activity between young and older women, using the T50 and error signal analysis. This approach provides further information on autonomic control of blood pressure in women.

Non-invasive approach for the assessment of sympathetic baroreflex function: A feasibility study

BACKGROUND

Evaluation of sympathetic baroreflex (sBR) function in humans requires intra-neural recording of muscle sympathetic nerve activity (MSNA) by microneurography.

AIMS

We proposed noninvasive approach for the evaluation of sBR function by applying the threshold-analysis (traditionally, based on MSNA) to systemic vascular resistance (SVR) measurement by photoplethysmography.

METHODS & RESULTS

In nine healthy subjects (5M; age: 25±5y), the threshold-analysis was calculated twice: using MSNA and SVR. Both methods yield comparable results in men (T₅₀(burst-vs.-svr): CV=8.8%, r>0.9; Slope_(burst-svr): CV=30.1%; r>0.9), but not in women.

CONCLUSIONS

SVR-based threshold-analysis is feasible in healthy young subjects and provides a promising alternative to the traditional MSNA-based approach.

Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in rheumatoid arthritis

KEY POINTS

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with an increased risk of cardiovascular mortality. Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity heighten cardiovascular risk, althogh whether such autonomic dysfunction is present in RA is not known. In the present study, we observed an increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in patients with RA compared to matched controls. Pain was positively correlated with sympathetic nerve activity and negatively correlated with cardiac baroreflex sensitivity. The pattern of autonomic dysfunction that we describe may help to explain the increased cardiovascular risk in RA, and raises the possibility that optimizing pain management may resolve autonomic dysfunction in RA.

ABSTRACT

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with increased cardiovascular morbidity/mortality and an incompletely understood pathophysiology. In animal studies, central and blood borne inflammatory cytokines that can be elevated in RA evoke pathogenic increases in sympathetic activity and reductions in baroreflex sensitivity (BRS). We hypothesized that muscle sympathetic nerve activity (MSNA) was increased and BRS decreased in RA. MSNA, blood pressure and heart rate (HR) were recorded in age- and sex-matched RA-normotensive (n = 13), RA-hypertensive patients (RA-HTN; n = 17), normotensive (NC; n = 17) and hypertensive controls (HTN; n = 16). BRS was determined using the modified Oxford technique. Inflammation and pain were determined using serum high sensitivity C-reactive protein (hs-CRP) and a visual analogue scale (VAS), respectively. MSNA was elevated similarly in RA, RA-HTN and HTN patients (32 ± 9, 35 ± 14, 37 ± 8 bursts min^-1 ) compared to NC (22 ± 9 bursts min^-1 ; P = 0.004). Sympathetic BRS was similar between groups (P = 0.927), whereas cardiac BRS (cBRS) was reduced in RA, RA-HTN and HTN patients [5(3-8), 4 (2-7), 6 (4-9) ms mmHg^-1 ] compared to NC [11 (8-15) ms mmHg^-1 ; P = 0.002]. HR was independently associated with hs-CRP. Increased MSNA and reduced cBRS were associated with hs-CRP although confounded in multivariable analysis. VAS was independently associated with MSNA burst frequency, cBRS and HR. We provide the first evidence for heightened sympathetic outflow and reduced cBRS in RA that can be independent of hypertension. In RA patients, reported pain was positively correlated with MSNA and negatively correlated with cBRS. Future studies should assess whether therapies to ameliorate pain and inflammation in RA restores autonomic balance and reduces cardiovascular events.

Simultaneous Characterization of Sympathetic and Cardiac Arms of the Baroreflex through Sequence Techniques during Incremental Head-Up Tilt

We propose a sympathetic baroreflex (sBR) sequence method for characterizing sBR from spontaneous beat-to-beat fluctuations of muscle sympathetic nerve activity (MSNA) and diastolic arterial pressure (DAP). The method exploits a previously defined MSNA variability quantifying the fluctuations of MSNA burst rate. The method is based on the detection of MSNA and DAP sequences characterized by the contemporaneous DAP increase and MSNA decrease or vice versa. The percentage of sBR sequences (SEQ%_sBR) was taken as an indication of the degree of sBR solicitation and the average slope of the regression lines in the (DAP, MSNA) plane was taken as sBR sensitivity (sBRS_SEQ) and expressed in bursts^.s^−1.mmHg⁻¹. sBRS_SEQ was compared to a more traditional estimate based on the baroreflex threshold analysis (sBRS_BTA). An incremental head-up tilt protocol, carried out in 12 young healthy subjects (age: 20–36 yr, median = 22.5 yr, 9 females) sequentially tilted at 0, 20, 30, 40, 60° table inclinations, was utilized to set the sBR sequence method parameters. Traditional sequence analysis was exploited to estimate cardiac baroreflex (cBR) sensitivity (cBRS_SEQ) and percentage of cBR sequences (SEQ%_cBR). The head-up tilt induced the progressive increase of SEQ%_sBR and SEQ%_cBR and gradual decrease of both sBRS_SEQ and cBRS_SEQ, thus suggesting the gradual rise of the sBR and cBR solicitations and the progressive reduction of their effectiveness with the stimulus. sBRS_SEQ was significantly associated with sBRS_BTA. sBRS_SEQ and cBRS_SEQ were significantly correlated as well as SEQ%_sBR and SEQ%_cBR, even though the correlation was not strong, thus suggesting a certain degree of independence between the baroreflex arms. The proposed sBR sequence approach provides a dynamical characterization of the sBR alternative to more traditional static pharmacological and nonpharmacological methods and fully homogenous with the cBR sequence technique.