Phase relationship between normal human respiration and baroreflex responsiveness

Carotid baroreflex control of arterial blood pressure at rest and during dynamic exercise in aging humans

The arterial baroreflex is fundamental for evoking and maintaining appropriate cardiovascular adjustments to exercise. We sought to investigate how aging influences carotid baroreflex regulation of blood pressure (BP) during dynamic exercise. BP and heart rate (HR) were continuously recorded at rest and during leg cycling performed at 50% HR reserve in 15 young (22 ± 1 yr) and 11 older (61 ± 2 yr) healthy subjects. Five-second pulses of neck pressure and neck suction from +40 to -80 Torr were applied to determine the full carotid baroreflex stimulus response curve and examine baroreflex resetting during exercise. Although the maximal gain of the modeled stimulus response curve was similar in both groups at rest and during exercise, in older subjects the operating point (OP) was located further away from the centering point (CP) and toward the reflex threshold, both at rest (OP minus CP; -10 ± 3 older vs. 0 ± 2 young mmHg, P < 0.05) and during exercise (OP minus CP; -10 ± 2 older vs. 1 ± 3 young mmHg, P < 0.05). In agreement, older subjects demonstrated a reduced BP response to neck pressure (simulated carotid hypotension) and a greater BP response to neck suction (simulated carotid hypertension). In addition, the magnitude of the upward and rightward resetting of the carotid baroreflex-BP stimulus response curve with exercise was ∼40% greater in older individuals. These data indicate that despite a maintained maximal gain, the ability of the carotid baroreflex to defend against a hypotensive challenge is reduced, whereas responses to hypertensive stimuli are greater with advanced age, both at rest and during exercise.

Postnatal development of baroreflex sensitivity in infancy

Baroreflex sensitivity (BRS) using spontaneous sequence analysis in the time domain is not fully applicable in infancy, as the time delay for heart period to change (heart period delay, HPD) after an arterial pressure change is unknown. We estimated and compared HPD and BRS in the frequency (BRS(sp), HPD(sp)) and time domains (BRS(seq), HPD(seq)) from systolic blood pressure (SBP) and heart period fluctuations. Continuous SBP, using photoplethysmography, and heart period measurements were performed on 30 term infants at 2-4 weeks, 2-3 months and 5-6 months postnatal age. Cross-spectral analysis between SBP and heart period fluctuations was used to estimate BRS(sp) and HPD(sp). Spontaneous sequence analysis was used to estimate BRS using a fixed beat delay of 1-12 beats (BRS(seq)) or a variable delay identified by a novel method accounting for epoch-epoch variability in HPD (BRS(seqvar)). HPD(sp) averaged 3.4 s (approximately 7 beats); BRS(sp) averaged 11.4 ms mmHg(1). BRS(seq) and BRS(seqvar) were consistently lower than BRS(sp) (P < 0.05), but the three BRS estimates were strongly correlated using a HPD of approximately 5-6 beats. BRS(seqvar) resulted in the average estimate (8.9 ms mmHg(1)) closest to BRS(sp) and overall had the strongest correlation with BRS(sp) (R(2) = 0.61; P < 0.001). All three BRS estimates increased progressively with postnatal age, with BRS(sp) averaging 6.4, 10.5 and 16.0 ms mmHg(1) at 2-4 weeks, 2-3 months and 5-6 months, respectively (P < 0.05). Accounting for the HPD of infancy provides estimates of BRS in the time domain that closely parallel spectral estimates, and provides a novel analytical tool to assess normal development and dysfunction of the baroreflex in infants.

Influence of breathing frequency on the pattern of respiratory sinus arrhythmia and blood pressure: old questions revisited

Respiratory sinus arrhythmia (RSA) is classically described as a vagally mediated increase and decrease in heart rate concurrent with inspiration and expiration, respectively. However, although breathing frequency is known to alter this temporal relationship, the precise nature of this phase dependency and its relationship to blood pressure remains unclear. In 16 subjects we systematically examined the temporal relationships between respiration, RSA, and blood pressure by graphically portraying cardiac interval (R-R) and systolic blood pressure (SBP) variations as a function of the respiratory cycle (pattern analysis), during incremental stepwise paced breathing. The principal findings were 1) the time interval between R-R maximum and expiration onset remained the same ( approximately 2.5-3.0 s) irrespective of breathing frequency (P = 0.10), whereas R-R minimum progressively shifted from expiratory onset into midinspiration with slower breathing (P < 0.0001); 2) there is a clear qualitative distinction between pre- versus postinspiratory cardiac acceleration during slow (0.10 Hz) but not fast (0.20 Hz) breathing; 3) the time interval from inspiration onset to SBP minimum (P = 0.16) and from expiration onset to SBP maximum (P = 0.26) remained unchanged across breathing frequencies; 4) SBP maximum and R-R maximum maintained an unchanged temporal alignment of approximately 1.1 s irrespective of breathing frequency (P = 0.84), whereas the alignment between SBP minimum and R-R minimum was inconstant (P > 0.0001); and 5) beta(1)-adrenergic blockade did not influence the respiration-RSA relationships or distinct RSA patterns observed during slow breathing, suggesting that temporal dependencies associated with alterations in breathing frequency are unrelated to cardiac sympathetic modulation. Collectively, these results illustrate nonlinear respiration-RSA-blood pressure relationships that may yield new insights to the fundamental mechanism of RSA in humans.

Cardiac autonomic regulation after lung exposure to carbon nanotubes

The ultrafine (UF) component of airborne pollution may impair cardiovascular autonomic control, a high-risk condition for cardiovascular adverse events. Since engineered nanoparticles, such as single-walled carbon nanotubes (SWCNTs) share physicochemical properties with UF, they might have similar adverse effects. Aim of the study was to evaluate arterial baroreflex function (BRF) at baseline, 24 h after the first instillation, immediately before the second one, and 2 weeks later, in adult Wystar-Kyoto conscious rats undergoing two intratracheal instillations of SWCNT (eight rats) or phosphate buffer saline (PBS) (five rats) at 2-week interval. During each session, 30-min continuous recording of arterial pressure and pulse interval was performed by a telemetered catheter implanted in the abdominal aorta of the rats. BRF was studied by the sequence technique. SWCNTs dispersed in PBS (1 mg/ml) were administered immediately after sonication (1 microg/g body weight). A significant decrease in the number of baroreflex sequences (from 498 +/- 27.1 at baseline to 287 +/- 40.2 at the recording performed after 4 weeks; P < 0.05) was observed in SWCNT-instilled rats, whereas no significant change was detected in controls. These data suggest that SWCNTs may alter the BRF, thus affecting the autonomic cardiovascular control regulation.

Influence of ageing on carotid baroreflex peak response latency in humans

The stability of a physiological control system, such as the arterial baroreflex, depends critically upon both the magnitude (i.e. gain or sensitivity) and timing (i.e. latency) of the effector response. Although studies have examined resting arterial baroreflex sensitivity in older subjects, little attention has been given to the influence of ageing on the latency of peak baroreflex responses. First, we compared the temporal pattern of heart rate (HR) and mean arterial blood pressure (BP) responses to selective carotid baroreceptor (CBR) unloading and loading in 14 young (22 +/- 1 years) and older (61 +/- 1 years) subjects, using 5 s pulses of neck pressure (NP, +35 Torr) and neck suction (NS, -80 Torr). Second, CBR latency was assessed following pharmacological blockade of cardiac parasympathetic nerve activity in eight young subjects, to better understand how known age-related reductions in parasympathetic nerve activity influence CBR response latency. In response to NP, the time to the peak increase in HR and mean BP were similar in young and older groups. In contrast, in response to NS the time to peak decrease in HR (2.1 +/- 0.2 vs. 3.8 +/- 0.2 s) and mean BP (6.7 +/- 0.4 vs. 8.3 +/- 0.2 s) were delayed in older individuals (young vs. older, P < 0.05). The time to peak HR and mean BP were delayed in young subjects following cardiac parasympathetic blockade (glycopyrrolate). Collectively, these data suggest that ageing is associated with delayed peak cardiovascular responses to acute carotid baroreceptor loading that may be, in part, due to age-related reductions in cardiac parasympathetic tone.

Respiratory modulation of cardiovagal baroreflex sensitivity

Emerging evidence has suggested that with minimal prerequisite training, slow deep breathing around 0.10 Hz can acutely enhance cardiovagal baroreflex sensitivity (BRS) in humans. Such reports have led to the speculation that behavioral interventions designed to reduce breathing frequency may serve a therapeutic role in ameliorating depressed baroreflex function in conditions such as chronic heart failure, essential hypertension, and obstructive airway disease. This study sought to test the hypothesis that slow controlled breathing acutely enhances cardiovagal baroreflex function in young healthy volunteers. Distinct from earlier studies, however, baroreflex function was examined ( n = 30) using the classical pharmacological modified Oxford method, which enabled the assessment of cardiovagal BRS through experimentally driven baroreceptor stimulation across a wide range of blood pressures. For a comparison against existing evidence, spontaneous cardiovagal BRS was also assessed using the α-index and sequence method. Compared with fast breathing (0.25 Hz), slow breathing (0.10 Hz) was associated with an increase in the α-index (8.1 ± 14 ms/mmHg, P < 0.01) and spontaneous up-sequence BRS (10 ± 11 ms/mmHg, P < 0.01). In contrast, BRS derived from spontaneous down sequences and the modified Oxford method were unaltered by slow breathing. The lack of change in BRS derived from the modified Oxford method challenges the concept that slow breathing acutely augments arterial baroreflex function in otherwise healthy humans. Our results also provide further evidence that spontaneous BRS may not reflect the BRS determined by experimentally driven baroreceptor stimulation.

Lateralization of expression of neural sympathetic activity to the vessels and effects of carotid baroreceptor stimulation

Human studies suggest that cardiovascular neural sympathetic control is predominantly modulated by the right cerebral hemisphere. It is unknown whether post-ganglionic sympathetic activity [muscle sympathetic nerve activity (MSNA)] shows any functional asymmetry. Eight right-handed volunteers (3 women and 5 men, 32 +/- 2 yr of age) underwent ECG, beat-by-beat blood pressure, respiratory activity, and simultaneous right and left MSNA recordings during spontaneous and controlled breathing (CB, 15 breaths/min, 0.25 Hz). Dynamic carotid baroreceptor stimulation was obtained by 0.1-Hz sinusoidal suction, from 0 to -50 mmHg, randomly applied to the right, left, and combined right and left sides of the neck during CB. Laterality was assessed by changes in the MSNA burst rate (in bursts/min, and bursts/100 beats), strength [amplitude (A) and area (AA)], and the oscillatory component at 0.1 Hz during baroreceptor stimulation. Amplitude parameters were normalized by CB burst mean amplitude and area of the same side. At rest, the right and left MSNA burst rate and total MSNA activity were similar. Conversely, the right MSNA normalized burst A(N) (1.36 +/- 0.18) and AA(N) (1.31 +/- 0.16) were larger than the left MSNA A(N) (1.04 +/- 0.09) and AA(N) (1.02 +/- 0.08). Unilateral and bilateral carotid baroreflex stimulation abolished the right prevalence of A(N) and AA(N). In conclusion, the right lateralization of sympathetic activity to the vessels is indicated by normalized burst strength parameters of bilateral MSNA recordings at rest during spontaneous breathing. Carotid baroreceptor stimulation disrupted such expression of MSNA lateralization possibly by disturbing the synchronizing action of right cerebral hemisphere.

Carotid baroreflex testing using the neck collar device

A neck chamber device for stimulation of carotid sinus baroreceptors by changing carotid transmural pressure was first described in 1957 by Ernsting and Parry and, with several modifications, has been extensively used in a number of physiological and clinical studies. This article outlines the evolution of neck chamber devices and describes some of the advantages and limitations of the technique. We also describe the responses in healthy subjects and the changes observed in patients with some disorders affecting the autonomic nervous system.

Spontaneous baroreflex measures are unable to detect age-related impairments in cardiac baroreflex function during dynamic exercise in humans

The dynamic relationship between 'spontaneous' fluctuations in arterial blood pressure (BP) and heart rate (HR) is increasingly being used to provide an estimate of resting cardiac baroreflex sensitivity. Given the ease of use and clinical utility, spontaneous methods are now also being used to examine cardiac baroreflex sensitivity in distinct subject groups during various laboratory stressors and tasks encountered during daily life, such as physical activity. However, the utility of such spontaneous measures to estimate cardiac baroreflex function during exercise remains unclear, particularly when comparing groups. Therefore, we tested the ability of spontaneous indices to detect age-related differences in cardiac baroreflex function during dynamic exercise. Beat-to-beat HR and BP were measured in eighteen healthy young subjects (24 +/- 1 years) and sixteen healthy middle-aged subjects (59 +/- 1 years) at rest and during steady-state leg cycling. Estimates of spontaneous cardiac baroreflex sensitivity using the sequence technique (G(SEQ)) and low-frequency transfer function gain (G(TF)) were compared with the operating point (G(OP)) and maximal gain (G(MAX)) of the full carotid-cardiac baroreflex function curve. At rest G(SEQ), G(TF), G(OP) and G(MAX) were all significantly lower in older subjects. During moderate-intensity steady-state exercise no differences were observed in G(SEQ) and G(TF) (older 0.26 +/- 0.03 beats min(-1) mmHg(-1) versus younger 0.32 +/- 0.04 beats min(-1) mmHg(-1); P > 0.05), whereas G(OP) and G(MAX) (older -0.21 +/- 0.02 beats min(-1) mmHg(-1) versus younger -0.39 +/- 0.03 beats min(-1) mmHg(-1); P < 0.05) remained lower in older subjects. These data indicate that spontaneous measures of cardiac baroreflex sensitivity alone provide limited information when comparing age-groups during exercise, which makes genuine differences in baroreflex function difficult to identify.

Effect of position on valsalva maneuver: supine versus 20 degree position

Blood pressure changes in response to the Valsalva maneuver (VM), which reflect the integrity of the baroreflex that regulates blood pressure. Performing this maneuver in the standard supine position often prevents adequate venous preload reduction, resulting in a rise rather than a fall in blood pressure, the "flat-top" Valsalva response. We determined whether performing the VM at a 20 degree angle of head-up tilt (20 degrees ) improves preload reduction, thereby reducing the frequency of flat-top responses, improving reflex vasoconstriction, and increasing the Valsalva ratio. One hundred thirty patients were evaluated in a prospective study. Each patient performed the VM in both supine and 20 degrees positions.Flat-top responses were present in 18% of subjects when supine. Twenty degree angle of head-up tilt position reduced the flat-top response by 87%. The components of the response that are dependent on preload reduction (Valsalva ratio and phases II_E, II_L, and IV) also showed significant improvement with 20 degrees .A 20 degree angle of tilt is sufficient to reduce venous preload, decreasing flat-top response rate and improving the Valsalva ratio and the morphology of the VM. We recommend this modification for laboratory evaluation of the VM, whenever a flat-top response is seen.

Treating hypertension in type II diabetic patients with device-guided breathing: a randomized controlled trial

The objective of the study was to evaluate the efficacy of device-guided breathing to lower blood pressure (BP) in hypertensive type II diabetic patients. A randomized controlled trial was carried out in four urban family practice clinics in Israel. Non-insulin-dependent diabetic, hypertensive patients with uncontrolled BP, receiving antihypertensive therapy or those non-medicated were enrolled. Baseline characteristics of the 66 patients who completed the study (33 intervention and 33 control) were: 62% men, age 62+/-8 years (mean+/-s.d.); body mass index 29+/-5 kg/m2; systolic BP 148+/-11 mm Hg and diastolic BP 81+/-9 mm Hg. The intervention group used a device (RESPeRATE), which interactively guides the user towards slow and regular breathing by synchronizing respiration voluntarily to musical tones for 15 min daily for an 8-week period. The control group continued with their regular treatment. BP was measured in the clinic at baseline, after 4 weeks and at 8 weeks. Medication was unchanged for 4 weeks prior to and during the study period. The main outcome measure was the office BP change from baseline to the end of the 8-week period. BP was reduced in the treatment group (mean+/-s.e.) systolic -10.0+/-1.8 mm Hg and diastolic -3.6+/-1.3 mm Hg (P<0.0001 and P<0.01), but not in the controls +1.6+/-2.1 and -1.0+/-1.4 mm Hg P>0.4 and P>0.4, respectively. Test for between group difference P<0.0001 and P=0.08. The subjects were highly compliant with the treatment, performing 75% of the requested exercise sessions. Greater BP reduction was observed with increased compliance with device usage (P=0.01 and P=0.001). It is concluded that self-treatment with device-guided breathing at home for 8 weeks by non-insulin-dependent diabetic patients was associated with a substantial reduction in office systolic BP.

Human sinus arrhythmia: inconsistencies of a teleological hypothesis

Respiratory sinus arrhythmia (RSA) may serve an inherent function in optimizing pulmonary gas exchange efficiency via clustering and scattering of heart beats during the inspiratory and expiratory phases of the respiratory cycle. This study sought to determine whether physiological levels of RSA, enhanced by slow paced breathing, caused more heart beats to cluster in inspiration. In 12 human subjects, we analyzed the histogram distribution of heart beats throughout the respiratory cycle during paced breathing at 12, 9, and 6 breaths/min (br/min). The inspiratory period-to-respiratory period ratio was fixed at approximately 0.5. RSA and its relationship with respiration was characterized in the phase domain by average cubic-spline interpolation of electrocardiographic R wave-to-R wave interval fluctuations throughout all respiratory cycles. Although 6 br/min breathing was associated with a significant increase in RSA amplitude (P < 0.01), we observed no significant increase in the proportion of heart beats in inspiration (P = 0.34). Contrary to assumptions in the literature, we observed no significant clustering of heart beats even with high levels of RSA enhanced by slow breathing. The results of this study do not support the hypothesis that RSA optimizes pulmonary gas exchange efficiency via clustering of heart beats in inspiration.

Effect of muscle metaboreflex activation on carotid-cardiac baroreflex function in humans

Whether the activation of metabolically sensitive skeletal muscle afferents (i.e., muscle metaboreflex) influences cardiac baroreflex responsiveness remains incompletely understood. A potential explanation for contrasting findings of previous reports may be related to differences in the magnitude of muscle metaboreflex activation utilized. Therefore, the present study was designed to investigate the influence of graded intensities of muscle metaboreflex activation on cardiac baroreflex function. In eight healthy subjects (24 +/- 1 yr), the graded isolation of the muscle metaboreflex was achieved by post-exercise ischemia (PEI) following moderate- (PEI-M) and high- (PEI-H) intensity isometric handgrip performed at 35% and 45% maximum voluntary contraction, respectively. Beat-to-beat heart rate (HR) and blood pressure were measured continuously. Rapid pulse trains of neck pressure and neck suction (+40 to -80 Torr) were applied to derive carotid baroreflex stimulus-response curves. Mean blood pressure increased significantly from rest during PEI-M (+13 +/- 3 mmHg) and was further augmented during PEI-H (+26 +/- 4 mmHg), indicating graded metaboreflex activation. However, the operating point gain and maximal gain (-0.51 +/- 0.09, -0.48 +/- 0.13, and -0.49 +/- 0.12 beats.min(-1).mmHg(-1) for rest; PEI-M and PEI-H) of the carotid-cardiac baroreflex function curve were unchanged from rest during PEI-M and PEI-H (P > 0.05 vs. rest). Furthermore, the carotid-cardiac baroreflex function curve was progressively reset rightward from rest to PEI-M to PEI-H, with no upward resetting. These findings suggest that the muscle metaboreflex contributes to the resetting of the carotid baroreflex control of HR; however, it would appear not to influence carotid-cardiac baroreflex responsiveness in humans, even with high-intensity activation during PEI.

Exercise intensity influences cardiac baroreflex function at the onset of isometric exercise in humans

We sought to examine the influence of exercise intensity on carotid baroreflex (CBR) control of heart rate (HR) and mean arterial pressure (MAP) at the onset of exercise in humans. To accomplish this, eight subjects performed multiple 1-min bouts of isometric handgrip (HG) exercise at 15, 30, 45 and 60% maximal voluntary contraction (MVC), while breathing to a metronome set at eupneic frequency. Neck suction (NS) of -60 Torr was applied for 5 s at end expiration to stimulate the CBR at rest, at the onset of HG (<1 s), and after approximately 40 s of HG. Beat-to-beat measurements of HR and MAP were recorded throughout. Cardiac responses to NS at onset of 15% (-12 +/- 2 beats/min) and 30% (-10 +/- 2 beats/min) MVC HG were similar to rest (-10 +/- 1 beats/min). However, HR responses to NS were reduced at the onset of 45% and 60% MVC HG (-6 +/- 2 and -4 +/- 1 beats/min, respectively; P < 0.001). In contrast to HR, MAP responses to NS were not different from rest at exercise onset. Furthermore, both HR and MAP responses to NS applied at approximately 40s of HG were similar to rest. In summary, CBR control of HR was transiently blunted at the immediate onset of high-intensity HG, whereas MAP responses were preserved demonstrating differential baroreflex control of HR and blood pressure at exercise onset. Collectively, these results suggest that carotid-cardiac baroreflex control is dynamically modulated throughout isometric exercise in humans, whereas carotid baroreflex regulation of blood pressure is well-maintained.

Influence of age on cardiac baroreflex function during dynamic exercise in humans

We investigated the influence of aging on cardiac baroreflex function during dynamic exercise in seven young (22 +/- 1 yr) and eight older middle-aged (59 +/- 2 yr) healthy subjects. Carotid-cardiac baroreflex function was assessed at rest and during moderate-intensity steady-state cycling performed at 50% heart rate reserve (HRR). Five-second pulses of neck pressure and neck suction from +40 to -80 Torr were applied to determine the operating point gain (G(OP)) and maximal gain (G(MAX)) of the full carotid-cardiac baroreflex function curve and examine baroreflex resetting during exercise. At rest, mean arterial pressure (MAP) and heart rate were similar between the younger and older subjects. In contrast, the resting G(OP) and G(MAX) were significantly lower in the older subjects. The increase in MAP from rest to exercise was greater in the older subjects (Delta +20 +/- 2 older vs. Delta +6 +/- 3 younger mmHg; P < 0.001). However, the G(OP) was similar in both groups during exercise because of a reduction in the younger subjects. In contrast, G(MAX) was unchanged from rest and therefore remained lower in older subjects (-0.19 +/- 0.05 older vs. -0.42 +/- 0.05 younger beats.min(-1).mmHg(-1); 50% HRR; P < 0.001). Furthermore, exercise resulted in an upward and rightward resetting of the cardiac baroreflex function curve in both groups. Collectively, these findings suggest that the cardiac baroreflex function curve appropriately resets during exercise in older subjects but operates at a reduced G(MAX) primarily because of age-related reductions in carotid-cardiac control manifest at rest.

Autonomic responses during motion sickness induced by virtual reality

OBJECTIVE

To examine the development of subjective symptoms and heart rate variability (HRV) during motion sickness induced by virtual reality (VR).

METHODS

Subjects were 10 healthy young volunteers. During VR immersion, subjects were immersed in a visual-vestibular conflict produced by VR. The levels of the subjective symptoms were assessed by Graybiel's and Hamilton's criteria. HRV was determined by measuring microvascular blood flow or electrocardiogram.

RESULTS

Subjective symptoms evaluated by Graybiel's and Hamilton's criteria were gradually worsened during VR. Power spectrum analysis of HRV demonstrated a gradual increase in the low frequency but no change in the high frequency during VR. In this study, individual subjective symptoms were not correlated with the individual result of power spectrum analysis.

CONCLUSION

These findings indicate that there was an increase in sympathetic nervous activity, but no change in parasympathetic nervous activity during motion sickness induced by VR. Given the large inter-individual variability and the reliability of subjective measures, it is not surprising that there is scarcely a relation between the subjective symptoms and the results of power spectrum analysis.

Increases in central blood volume modulate carotid baroreflex resetting during dynamic exercise in humans

We sought to determine if resetting of the carotid-vasomotor baroreflex function curve during exercise is modulated by changes in central blood volume (CBV). CBV was increased during exercise by altering: (1) subject posture (supine versus upright) and (2) pedal frequency (80 versus 60 revolutions min(-1) (r.p.m.)); while oxygen uptake ( ) was kept constant. Eight male subjects performed three exercise trials: upright cycling at 60 r.p.m. (control); supine cycling at 60 r.p.m. (SupEX) and upright cycling at 80 r.p.m. to enhance the muscle pump (80EX). During each condition, carotid baroreflex (CBR) function was determined using the rapid neck pressure (NP) and neck suction (NS) protocol. Although mean arterial pressure (MAP) was significantly elevated from rest (88 +/- 2 mmHg) during all exercise conditions (P < 0.001), the increase in MAP was lower during SupEX (94 +/- 2 mmHg) and 80EX (95 +/- 2 mmHg) compared with control (105 +/- 2 mmHg, P < 0.05). Importantly, the blood pressure responses to NP and NS were maintained around these changed operating points of MAP. However, in comparison to control, the carotid-vasomotor baroreflex function curve was relocated downward and leftward when CBV was increased during SupEX and 80EX. These alterations in CBR resetting occurred without any differences in or heart rate between the exercise conditions. Thus, increasing CBV and loading the cardiopulmonary baroreflex reduces the magnitude of exercise-induced increases in MAP and CBR resetting. These findings suggest that changes in cardiopulmonary baroreceptor load influence carotid baroreflex resetting during dynamic exercise.

Test-Retest Reliability of Heart Rate Variability and Respiration Rate at Rest and during Light Physical Activity in Normal Subjects

BACKGROUND

A variable that remains stable over repeated measurements (in stable conditions) is ideal for tracking modifications of the clinical state. The aim of the present study is to examine test-retest reliability of time-domain heart rate variability and respiration rate measurements using a portable device on normal subjects during rest and light physical activity.

METHODS

Twenty-six normal subjects [18 females and 8 males aged 28 +/- 6 years and 34 +/- 12 years (mean +/- SD), respectively] underwent two measurements for time-domain heart rate variability (SDNN and RMSSD) and respiration rate, with 7 days in between. Measurements took place under three conditions: lying down in a laboratory, cycling in a laboratory and sleeping in an ambulatory surrounding. Reliability was assessed statistically by calculating intra-class correlation coefficients (ICC).

RESULTS

Reliability was found to be good to excellent for both time-domain heart rate variability (SDNN: ICC values between 0.74 and 0.85, RMSSD: ICC values between 0.75 and 0.98) and for respiration rate (ICC values between 0.77 and 0.96).

CONCLUSIONS

Both time-domain heart rate variability and respiration rate can be reliably assessed. However, we advise reliability research in a clinical setting before using the device for tracking modifications in a clinical state.

Psychophysiological training of operators in adaptive biofeedback cardiorhythm control

A new individual computerized technique for psychophysiological training of operators before performing psychomotor activity on a computer model (psychomotor concentration and spatial orientation test) was developed. Qualitative criteria for the prediction of safe operator activity were formulated. Preliminary testing of operators' activity quality showed great dispersion of individual results: The amount of errors ranged from 0 to 56 and the rate of information processing varied from 1.01 up 3.56 bit/s. Subjects with initially identified respiratory sinus arrhythmia or synchronization caused by respiratory movements committed minimal recognition errors in initial stages at a high rate of information processing. The number of errors remained unchanged after the biofeedback cardio-training cycle, with the rate of information processing increasing noticeably. Subjects without inherent harmonics developed harmonics after sessions of cardiorhythm biofeedback control, and their operator activity quality improved significantly, making fewer mistakes and increasing the rate of information processing. Biofeedback control led not only to the restoration of respiratory sinus arrhythmia, a favorable diagnostic sign, as revealed by cardiorhythmograms, but also resulted in improvement of the quality of operator activity.

Short-term effects of a brief respiratory training on baroreceptor cardiac reflex function in normotensive and mild hypertensive subjects

Baroreceptor cardiac reflex sensitivity is reduced in hypertension and is considered a powerful prognostic factor in cardiovascular health. This study analyzes the acute effects of a brief respiratory training on baroreceptor sensitivity and on two new proposed baroreflex parameters: baroreceptor power (i.e., the percentage of cardiac beats regulated by the baroreflex) and effectiveness (i.e., the frequency in which the baroreflex responds to transient alterations in blood pressure). Twenty-two participants, 10 primary mild hypertensives and 12 normotensives, learned and practiced a respiratory pattern characterized by breathing at 6 bpm, with time of expiration being twice time of inspiration, predominantly abdominal, and with pursed lips. Baroreceptor parameters are differentiated in terms of increases ("up" sequences) or decreases ("down" sequences) in blood pressure. Irrespective of the groups, the breathing manipulation increased baroreceptor sensitivity (only in the "up" sequences), power, and effectiveness (only in the "down" sequences). These results suggest that this type of respiratory training could be used as a promising intervention to increase baroreceptor cardiac function in primary hypertension.

Can the blood pressure effects of COX-2 selective inhibitors be explained by changes in plasma aldosterone levels?

OBJECTIVE

The increased cardiovascular risk associated with rofecoxib may relate to differential effects on blood pressure (BP) in comparison with other cyclooxygenase-2 (COX-2) selective inhibitors. Rofecoxib is uniquely metabolized by cytosol reductase and may compete with aldosterone for metabolism. We hypothesized that the effect of rofecoxib on BP may be due to an increase in aldosterone levels.

DESIGN

Prospective, randomized, cross-over study.

SETTING

Tertiary institution.

PATIENTS

Eleven patients (all female, age 65 +/- 11 years) with osteoarthritis and hypertension.

INTERVENTIONS

Patients received rofecoxib 50 mg once/day or celecoxib 400 mg once/day for 8 weeks, followed by cross-over after a 2-week wash-out period.

OUTCOME MEASURES

Office BP, heart rate (HR), plasma aldosterone and aldosterone related-markers, markers of sympathetic and parasympathetic activity, catecholamines and heart rate variability (HRV) were measured.

RESULTS

Rofecoxib caused an increase in BP compared to celecoxib; a change in recumbent systolic BP +/- SD (6.1 +/- 11.0 versus -7.0 +/- 12.5 mmHg, P < 0.05) and recumbent diastolic BP +/- SD (3.6 +/- 7.6 versus -1.7 +/- 5.7 mmHg). There was no difference in median change in plasma aldosterone levels between rofecoxib and celecoxib (-31 +/- 171.5 versus 43 +/- 147 pg/ml, P = 0.173). The other parameters measured were not different between the two agents.

CONCLUSIONS

Rofecoxib therapy results in a significant increase in systolic BP compared to celecoxib. As plasma aldosterone levels were not different between the two drugs, the differential effect of rofecoxib on BP compared to celecoxib appears to be unlikely due to competition by rofecoxib for aldosterone metabolism.

Carotid baroreceptor stimulation alters cutaneous vascular conductance during whole-body heating in humans

Prior studies investigating carotid baroreflex control of the cutaneous vasculature have yielded mixed findings. However, previously used methodological and analytical techniques may limit the ability to detect carotid baroreflex-mediated changes in cutaneous vascular conductance (CVC). The aim of this study was to test the hypothesis that dynamic carotid baroreceptor stimulation (i.e. 5 s trials) using neck pressure (NP, simulated carotid hypotension) and neck suction (NS, simulated carotid hypertension) will decrease and increase CVC, respectively, during normothermic and whole-body heating conditions in resting humans. Data were obtained from nine subjects (age, 31 +/- 2 year). The ratio of forearm skin blood flux (laser-Doppler flowmetry) and arterial blood pressure (Finapres) was used as an index of CVC. Multiple 5 s trials of NP (+40(Torr)) and NS (-60(Torr)), as well as breath-hold/airflow control trials, were applied during end-expiratory breath-holds while subjects were normotheric and heat stressed (change in core temperature approximately 0.75 degrees C). CVC responses to each NP and NS trial were averaged into 1 s intervals during the following periods: 3 s prestimulus, 5 s during stimulus, and 5 s poststimulus. Peak CVC responses (3 s average) to NP and NS were compared to prestimulus values using paired t test. During normothermia, NP decreased CVC by 0.032 +/- 0.007 arbitrary units (a.u.) mmHg(-1); (P < 0.05); however, breath-hold/airflow control trials resulted in similar decreases in CVC. NS did not change CVC (Delta = 0.002 +/- 0.005 a.u. mmHg(-1); P = 0.63). During whole-body heating, NP decreased CVC (by 0.16 +/- 0.04 a.u. mmHg(-1); (P < 0.05), whereas NS increased CVC by 0.07 +/- 0.03 a.u. mmHg(-1); (P < 0.05). Furthermore, these changes were greater than, or directionally different from, the breath-hold/airflow control trials. These findings indicate that carotid baroreceptor stimulation elicits dynamic changes in CVC and that these changes are more apparent during whole-body heating.

Influence of different respiratory maneuvers on exercise-induced cardiac vagal inhibition

Physical exercise inhibits cardiac vagal activity. To study the relationship between heart rate (HR) and respiratory pattern, we applied the 4-s exercise test (4sET) and measured cardiac vagal index (CVI) in 30 healthy subjects who served as their own controls, using the standard plus three additional variations, essentially respiratory, of the original protocol: (a) a maximum inspiratory apnea of 16 s, of which 8 s were in the pre-exercise phase (4sET(insp)); (b) free respiratory pattern (4sET(unc)); and (c) maximum expiratory apnea of 12 s (4sET(exp)). The respective results were expressed by the following CVIs: CVI(insp), CVI(unc) and CVI(exp). CVI was determined in a continuous digital ECG recording through a specific ratio of two RR interval durations. The results [(mean +/- SEM)] for the four different maneuvers were as follows: CVI (1.56 +/- 0.05), CVI(insp) (1.55 +/- 0.05), CVI(unc) (1.63 +/- 0.05) and CVI(exp) (1.37 +/- 0.02). ANOVA-Bonferroni significant differences were only found between CVI(exp) and CVI(insp) (P = 0.009), CVI(unc) (P < 0.001) and CVI (P = 0.003). Dividing our sample in terciles according to CVI values, those with lower CVI, showed an attenuation of biphasic HR response after a 15 s maximum inspiratory apnea. We conclude that cardiac vagal reflex seems to be influencing the biphasic HR response modulation after a 12 s inspiratory apnea as described in the original protocol of 4sET, and this appears to be the option that best discriminates the cardiac vagal reflex, with less variability in the maneuvers when subjects are divided in terciles.

Autonomic nervous system influence on arterial baroreflex control of heart rate during exercise in humans

A combination of sympathoexcitation and vagal withdrawal increases heart rate (HR) during exercise, however, their specific contribution to arterial baroreflex sensitivity remains unclear. Eight subjects performed 25 min bouts of exercise at a HR of 90, 120, and 150 beats min-1, respectively, with and without metoprolol (0.16 +/- 0.01 mg kg(-1); mean +/- S.E.M.) or glycopyrrolate (12.6 +/- 1.6 microg kg-1). Carotid baroreflex (CBR) function was determined using 5 s pulses of neck pressure (NP) and neck suction (NS) from +40 to -80 Torr, while transfer function gain (GTF) was calculated to assess the linear dynamic relationship between mean arterial pressure and HR. Spontaneous baroreflex sensitivity (SBR) was evaluated as the slope of sequences of three consecutive beats in which systolic blood pressure and the R-R interval of the ECG either increased or decreased, in a linear fashion. The beta-1 adrenergic blockade decreased and vagal cardiac blockade increased HR both at rest and during exercise (P < 0.05). The gain at the operating point of the modelled reflex function curve (GOP) obtained using NP and NS decreased with workload independent of beta-1 adrenergic blockade. In contrast, vagal blockade decreased GOP from -0.40 +/- 0.04 to -0.06 +/- 0.01 beats min-1 mmHg-1 at rest (P < 0.05). Furthermore, as workload increased both GOP and SBR, and GOP and GTF were correlated (P < 0.001), suggesting that the two dynamic methods applied to evaluate arterial baroreflex (ABR) function provide the same information as the modelled GOP. These findings suggest that during exercise the reduction of arterial baroreceptor reflex sensitivity at the operating point was a result of vagal withdrawal rather than an increase in sympathetic activity.

Intramuscular pressure-induced inhibition of cardiac contraction: implications for cardiac-locomotor synchronization

The synchronization of cardiac and locomotor rhythms has been suggested to enhance the efficiency of arterial delivery to active muscles during rhythmic exercise, but direct evidence showing such a functional role has not been provided. In this study, we tested the hypothesis that the heartbeat is coupled with intramuscular pressure (IMP) changes so as to time the delivery of blood through peripheral tissues when the IMP is lower. To this end, we developed a computer-controlled, dynamic, thigh cuff occlusion device that enables bilateral thigh cuffs to repeatedly inflate and deflate, one side after the other, to simulate rhythmic IMP changes during bipedal locomotion. Nine healthy subjects were examined, and three different occlusion pressures (50, 80, and 120 mmHg) were applied separately to the thigh cuffs of normal subjects while they were sitting. Alternate occlusions of the bilateral thigh cuffs administered at the frequency of the mean heart rate produced significant phase synchronization between the cardiac and cuff-occlusion rhythms when 120 mmHg pressure was applied. However, synchronization was not observed when the occlusion pressure was 50 or 80 mmHg. During synchronization, heartbeats were most likely to occur in phases that did not include overlap between the peak arterial flow velocity in the thigh and elevated cuff pressure. We believe that phase synchronization occurs so that the cardiac cycle is timed to deliver blood through the lower legs when IMP is not maximal. If this can be extrapolated to natural locomotion, synchronization between cardiac and locomotor activities may be associated with the improved perfusion of exercising muscles.

Baroreflex sensitivity in nonapneic snorers and control subjects before and after nasal continuous positive airway pressure

HYPOTHESIS

We hypothesized that baroreflex sensitivity is decreased during wakefulness and non-rapid eye movement sleep in normotensive, nonapneic snorers who are otherwise healthy. Moreover, we hypothesized that nocturnal alterations in baroreflex sensitivity are abolished during the application of nasal continuous positive airway pressure (nCPAP).

DESIGN

The sequencing technique was used to measure baroreflex sensitivity in 16 normotensive nonapneic snorers and 16 control subjects matched for age, height, weight, gender, and race. Subsequently, baroreflex sensitivity was measured in 12 of 16 snorers and 14 of 16 control subjects during the application of nCPAP.

RESULTS

Mean (+/- SE) baroreflex sensitivity was reduced during sleep in the nonapneic snoring group (wakefulness, 20.99 +/- 1.46 ms/mm Hg; sleep, 15.85 +/- 1.49 ms/mm Hg), but not in the control group (wakefulness, 21.82 +/- 2.48 ms/mm Hg; sleep, 23.54 +/- 2.18 ms/mm Hg). This reduction was abolished by the application of nCPAP in the snoring group (before nCPAP therapy, 16.30 +/- 2.17 ms/mm Hg; during nCPAP therapy, 20.63 +/- 2.40 ms/mm Hg). The application of nCPAP did not alter baroreflex sensitivity in the control group (before nCPAP therapy, 23.54 +/- 2.18 ms/mm Hg; during nCPAP therapy, 22.56 +/- 1.73 ms/mm Hg). BP was not significantly different between the snoring and control groups either before or during nCPAP application.

CONCLUSIONS

Our findings suggest that nocturnal alterations in baroreflex sensitivity may exist in nonapneic snoring subjects prior to alterations in other cardiovascular variables.

Carotid baroreflex control of leg vasculature in exercising and non-exercising skeletal muscle in humans

Carotid baroreflex (CBR) function was examined in five men and three women (25 +/- 1 years) using the variable-pressure neck collar technique at rest and during dynamic, one-legged knee extension exercise at 7 W and 25 W. The CBR exhibited control of leg vascular conductance (LVC) at rest and during exercise in both an exercising leg (EL) and a non-exercising leg (NEL) across a wide range of pressures from +40 Torr neck pressure (NP) to -80 Torr neck suction (NS). Specifically, increases in LVC (% change) in response to NS were no different across -20 to -80 Torr in either EL or NEL compared to rest, P > 0.05. However, CBR-mediated decreases in percentage LVC in response to NP were attenuated in EL at both 7 W (16 +/- 1%) and 25 W (12 +/- 1%) compared to rest (40 +/- 3%; P < 0.05) as well as compared to responses in the NEL (36 +/- 6% at 7 W and 36 +/- 7% at 25 W; P < 0.05). This decrease in vascular responsiveness in EL was associated with a reduction in the gain of the percentage muscle sympathetic nerve activity (%MSNA)-%LVC relationship compared to rest (P < 0.05). Collectively, these data indicate that, despite a clear attenuation of the vascular response to MSNA in the exercising leg, CBR-mediated changes in mean arterial pressure were no different between rest and exercise.

Arterial blood pressure and carotid baroreflex function during arm and combined arm and leg exercise in humans

AIM

During arm cranking (A) blood pressure is higher than during combined arm and leg exercise (A + L), while the carotid baroreflex (CBR) is suggested to reset to control a higher blood pressure in direct relation to work intensity and the engaged muscle mass.

METHOD

This study evaluated the function of the CBR by using neck pressure and neck suction during upright A, L and A + L in 12 subjects and, in order to evaluate a potential influence of the central blood volume on the CBR, also during supine A in five subjects. Exercise intensities for A and L were planned to elicit a heart rate response of c. 100 and 120 beats min(-1), respectively, in the upright position and both workloads were maintained during A + L and supine A.

RESULTS

The CBR operating point, corresponding to the pre-stimulus blood pressure, was 88 +/- 6 mmHg (mean +/- SE) at rest. During upright A, L and A + L and supine A it increased to 109 +/- 9, 95 +/- 7, 103 +/- 7 and 104 +/- 4 mmHg, respectively, and it was thus higher during upright A than during A + L and supine A (P < 0.05). In addition, the CBR threshold and saturation pressures, corresponding to the minimum and maximum carotid sinus pressure, respectively, were higher during upright A than during supine A, A + L, L and at rest (P < 0.05) with no significant change in the maximal reflex gain.

CONCLUSION

These findings demonstrate that during combined arm and leg and exercise in the upright position the CBR resets to a lower blood pressure than during arm cranking likely because the central blood volume is enhanced by the muscle pump of the legs.

Spontaneous 'baroreflex sequences' occur as deterministic functions of breathing phase

Parallel increases or decreases of systolic pressures and R-R intervals occur spontaneously in healthy resting humans, and are thought to be expressions of vagal baroreflex physiology. We studied ten healthy supine young adults, and tested the null hypothesis that spontaneous baroreflex sequences are distributed uniformly throughout the breathing cycle. We recorded the electrocardiogram, photoplethysmographic arterial pressure, respiration (pneumobelt), and peroneal nerve muscle sympathetic activity in supine subjects who breathed spontaneously, or held their breaths in inspiration after 2 min of hyperventilation with 100% oxygen. We analysed pairs of three or more increasing or decreasing systolic pressures and R-R intervals with linear regression, and related the gain and timing of the onset of such sequences to the phase of respiration, and to preceding muscle sympathetic nerve activity. We found that baroreflex sequences occur erratically, at a frequency about one-third that of breathing. However, when baroreflex sequences do occur, the timing of their onset is dictated by the phase of respiration. Parallel increases of systolic pressures and R-R intervals ('up' sequences) begin just before and after the beginning of expiration, and parallel decreases of systolic pressures and R-R intervals ('down' sequences) begin during late expiration and inspiration. Average gains of up and down baroreflex sequences triggered by muscle sympathetic bursts are comparable during breathing and apnoea. However, the latencies between sympathetic bursts and baroreflex sequences are less during breathing than during apnoea. We propose that parallel systolic pressure--R-R interval sequences are expressions of arterial baroreflex physiology, and that the nearly fixed timing of such sequences within breaths reflects simply respiratory gating of muscle sympathetic bursts.

Effects of simulated obstructive sleep apnoea on the human carotid baroreceptor-vascular resistance reflex

Obstructive sleep apnoea (OSA), which is characterized by periodic inspiratory obstruction, is associated with hypertension and possibly with changes in the baroreceptor reflex. In this investigation we induced changes in inspiratory resistance and in inspiratory oxygen and carbon dioxide content, which simulate some of the changes in OSA, to determine whether this caused changes in the gain or setting of the carotid baroreflex. In eight healthy subjects (aged 21-62 years) we changed the stimulus to carotid baroreceptors, using neck chambers and graded pressures of -40 to +60 mmHg, and assessed vascular resistance responses in the brachial artery from changes in blood pressure (Finapres) divided by brachial artery blood flow velocity (Doppler ultrasound). Stimulus-response curves were defined during (a) sham (no additional stimulus), (b) addition of an inspiratory resistance (inspiratory pressure -10 mmHg), (c) breathing asphyxic gas (12% O(2), 5% CO(2)), and (d) combined resistance and asphyxia. Sigmoid or polynomial functions were applied to the curves and maximum differentials (equivalent to peak gain) and the corresponding carotid pressures (equivalent to 'set point') were determined. The sham test had no effect on either gain or 'set point'. Inspiratory resistance alone had no effect on blood pressure and did not displace the curve. However, it reduced gain from -3.0 +/- 0.6 to -2.1 +/- 0.4 units (P < 0.05). Asphyxia alone did increase blood pressure (+7.0 +/- 1.1 mmHg, P < 0.0005) and displaced the curve to higher pressures by +16.8 +/- 2.1 mmHg (P < 0.0005). However, it did not affect gain. The combination of resistance and asphyxia both reduced gain and displaced the curve to higher pressures. These results suggest that inspiratory resistance and asphyxia cause changes in the baroreceptor reflex which could lead to an increase in blood pressure. These changes, if sustained, could provide a mechanism linking hypertension to obstructive sleep apnoea.

Contribution of the respiratory rhythm to sinus arrhythmia in normal unanesthetized subjects during positive-pressure mechanical hyperventilation

The precise contribution of the CO2-dependent respiratory rhythm to sinus arrhythmia in eupnea is unclear. The respiratory rhythm and sinus arrhythmia were measured in 12 normal, unanesthetized subjects in normocapnia and hypocapnia during mechanical hyperventilation with positive pressure. In normocapnia (41 +/- 1 mmHg), the respiratory rhythm was always detectable from airway pressure and inspiratory electromyogram activity. The amplitude of sinus arrhythmia (138 +/- 21 ms) during mechanical hyperventilation with positive pressure was not significantly different from that in eupnea. During the same mechanical hyperventilation pattern but in hypocapnia (24 +/- 1 mmHg), the respiratory rhythm was undetectable and the amplitude of sinus arrhythmia was significantly reduced (to 40 +/- 5 ms). These results show a greater contribution to sinus arrhythmia from the respiratory rhythm during hypocapnia caused by mechanical hyperventilation than previously indicated in normal subjects during hypocapnia caused by voluntary hyperventilation. We discuss whether the respiratory rhythm provides the principal contribution to sinus arrhythmia in eupnea.

Carotid baroreflex function during and following voluntary apnea in humans

Muscle sympathetic nerve activity (MSNA) and arterial pressure increase concomitantly during apnea, suggesting a possible overriding of arterial baroreflex inhibitory input to sympathoregulatory centers by apnea-induced excitatory mechanisms. Apnea termination is accompanied by strong sympathoinhibition while arterial pressure remains elevated. Therefore, we hypothesized that the sensitivity of carotid baroreflex control of MSNA would decrease during apnea and return upon apnea termination. MSNA and heart rate responses to -60-Torr neck suction (NS) were evaluated during baseline and throughout apnea. Responses to +30-Torr neck pressure (NP) were evaluated during baseline and throughout 1 min postapnea. Apnea did not affect the sympathoinhibitory or bradycardic response to NS (P > 0.05); however, whereas the cardiac response to NP was maintained postapnea, the sympathoexcitatory response was reduced for 50 s (P < 0.05). These data demonstrate that the sensitivity of carotid baroreflex control of MSNA is not attenuated during apnea. We propose a transient rightward and upward resetting of the carotid baroreflex-MSNA function curve during apnea and that return of the function curve to, or more likely beyond, baseline (i.e., a downward and leftward shift) upon apnea termination may importantly contribute to the reduced sympathoexcitatory response to NP.

Effects of unilateral and bilateral carotid baroreflex stimulation on cardiac and neural sympathetic discharge oscillatory patterns

BACKGROUND

Left and right carotid baroreflex afferents participate in generating the spontaneous variability of heart rate (HR), arterial pressure (AP), and muscle sympathetic nerve activity (MSNA), but the relative contribution of each side is unclear. Pathophysiological conditions unilaterally affecting carotid baroreceptor function might result in abnormal changes of HR, AP, and MSNA variability, thus markedly affecting prognosis. We tested the hypothesis that unilateral carotid baroreceptor perturbation might differentially affect HR, AP, and MSNA variability compared with stimulation of the opposite side.

METHODS AND RESULTS

In 12 healthy volunteers, 4 sinusoidal neck suction procedures (0.1 Hz, from 0 to -50 mm Hg) were applied at the right, left, and combined right and left sides of the neck, in concordance or with phase opposition. Respiration was controlled at 0.25 Hz. Power spectrum analysis assessed the changes in the 0.1-Hz oscillatory component of the R-R interval, systolic AP (SAP), and MSNA variability induced by rhythmic baroreceptor stimulation. Mean R-R interval, SAP, and MSNA were unchanged during all procedures. The increase of the 0.1-Hz component of R-R and SAP variability during right and combined right and left carotid baroreceptor stimulation was greater than the changes induced by left-sided stimulation. The increase in the 0.1-Hz oscillatory component of MSNA variability was similar during all neck suction procedures.

CONCLUSIONS

Right carotid baroreflex loading was as efficient as bilateral stimulation and more effective than left carotid suction in modulating R-R and SAP variability. There was no asymmetry in neural sympathetic discharge responses after single-sided carotid baroreceptor stimulation.

Baroreflex-mediated changes in cardiac output and vascular conductance in response to alterations in carotid sinus pressure during exercise in humans

We sought to quantify the contribution of cardiac output (Q) and total vascular conductance (TVC) to carotid baroreflex (CBR)-mediated changes in mean arterial pressure (MAP) during mild to heavy exercise. CBR function was determined in eight subjects (25 +/- 1 years) at rest and during three cycle exercise trials at heart rates (HRs) of 90, 120 and 150 beats min-1 performed in random order. Acute changes in carotid sinus transmural pressure were evoked using 5 s pulses of neck pressure (NP) and neck suction (NS) from +40 to -80 Torr (+5.33 to -10.67 kPa). Beat-to-beat changes in HR and MAP were recorded throughout. In addition, stroke volume (SV) was estimated using the Modelflow method, which incorporates a non-linear, three-element model of the aortic input impedance to compute an aortic flow waveform from the arterial pressure wave. The application of NP and NS did not cause any significant changes in SV either at rest or during exercise. Thus, CBR-mediated alterations in Q were solely due to reflex changes in HR. In fact, a decrease in the carotid-HR response range from 26 +/- 7 beats min-1 at rest to 7 +/- 1 beats min-1 during heavy exercise (P = 0.001) reduced the contribution of Q to the CBR-mediated change in MAP. More importantly, at the time of the peak MAP response, the contribution of TVC to the CBR-mediated change in MAP was increased from 74 +/- 14 % at rest to 118 +/- 6 % (P = 0.017) during heavy exercise. Collectively, these findings indicate that alterations in vasomotion are the primary means by which the CBR regulates blood pressure during mild to heavy exercise.

Heart rate variability and sympathovagal balance: pharmacological validation

RATIONALE

We validated heart rate (HR) and six time and six frequency domain measures of heart rate variability (HRV) as estimators of autonomic outflow in 44 young healthy male subjects. Gold standards for autonomic outflow were the Rosenblueth-Simeone factors m (sympathetic tone) and n (vagal tone), and the sympathovagal balance m·n, determined by two-stage complete autonomic blockade.

METHODS

Rank correlations were computed between HR and the HRV measures obtained before autonomic blockade, and m, n and m·n. Also, the maximal mean performances (averaged sensitivity and specificity) for HR and HRV as discriminators between low and high values of m, n or m·n were computed.

RESULTS

The spectral HRV measures showed less good correlations and performances than the time domain HRV measures. Correlations with sympathetic tone were all below 0.31. Respiratory sinus arrhythmia during 15 cycles/min metronome breathing was superior in estimating vagal tone and sympathovagal balance (correlations -0.71/-0.73; both performances 0.82), heart rate scored similarly for assessing the sympathovagal balance (correlation 0.71; performance 0.82).

CONCLUSIONS

It does not appear justified to evaluate HR or HRV in terms of sympathetic tone, vagal tone, or sympathovagal balance. HR and HRV are specifically weak in assessing sympathetic tone. Respiratory sinus arrhythmia during 15 cycles/min metronome breathing is superior in assessing vagal tone. Current HRV analysis techniques offer no advantages compared with HR in assessing the sympathovagal balance.

The human respiratory gate

Respiratory activity phasically alters membrane potentials of preganglionic vagal and sympathetic motoneurones and continuously modulates their responsiveness to stimulatory inputs. The most obvious manifestation of this 'respiratory gating' is respiratory sinus arrhythmia, the rhythmic fluctuations of electrocardiographic R-R intervals observed in healthy resting humans. Phasic autonomic motoneurone firing, reflecting the throughput of the system, depends importantly on the intensity of stimulatory inputs, such that when levels of stimulation are low (as with high arterial pressure and sympathetic activity, or low arterial pressure and vagal activity), respiratory fluctuations of sympathetic or vagal firing are also low. The respiratory gate has a finite capacity, and high levels of stimulation override the ability of respiration to gate autonomic responsiveness. Autonomic throughput also depends importantly on other factors, including especially, the frequency of breathing, the rate at which the gate opens and closes. Respiratory sinus arrhythmia is small at rapid, and large at slow breathing rates. The strong correlation between systolic pressure and R-R intervals at respiratory frequencies reflects the influence of respiration on these two measures, rather than arterial baroreflex physiology. A wide range of evidence suggests that respiratory activity gates the timing of autonomic motoneurone firing, but does not influence its tonic level. I propose that the most enduring significance of respiratory gating is its use as a precisely controlled experimental tool to tease out and better understand otherwise inaccessible human autonomic neurophysiological mechanisms.

The human respiratory gate

Respiratory activity phasically alters membrane potentials of preganglionic vagal and sympathetic motoneurones and continuously modulates their responsiveness to stimulatory inputs. The most obvious manifestation of this 'respiratory gating' is respiratory sinus arrhythmia, the rhythmic fluctuations of electrocardiographic R-R intervals observed in healthy resting humans. Phasic autonomic motoneurone firing, reflecting the throughput of the system, depends importantly on the intensity of stimulatory inputs, such that when levels of stimulation are low (as with high arterial pressure and sympathetic activity, or low arterial pressure and vagal activity), respiratory fluctuations of sympathetic or vagal firing are also low. The respiratory gate has a finite capacity, and high levels of stimulation override the ability of respiration to gate autonomic responsiveness. Autonomic throughput also depends importantly on other factors, including especially, the frequency of breathing, the rate at which the gate opens and closes. Respiratory sinus arrhythmia is small at rapid, and large at slow breathing rates. The strong correlation between systolic pressure and R-R intervals at respiratory frequencies reflects the influence of respiration on these two measures, rather than arterial baroreflex physiology. A wide range of evidence suggests that respiratory activity gates the timing of autonomic motoneurone firing, but does not influence its tonic level. I propose that the most enduring significance of respiratory gating is its use as a precisely controlled experimental tool to tease out and better understand otherwise inaccessible human autonomic neurophysiological mechanisms.

Carotid baroreflex control of leg vascular conductance at rest and during exercise

We sought to test the hypothesis that the carotid baroreflex (CBR) alters mean leg blood flow (LBF) and leg vascular conductance (LVC) at rest and during exercise. In seven men and one woman, 25 +/- 2 (SE) yr of age, CBR control of LBF and LVC was determined at rest and during steady-state one-legged knee extension exercise at approximately 65% peak O(2) uptake. The application of 5-s pulses of +40 Torr neck pressure and -60 Torr neck suction significantly altered mean arterial pressure (MAP) and LVC both at rest and during exercise. CBR-mediated changes in MAP were similar between rest and exercise (P > 0.05). However, CBR-mediated decreases in LVC (%change) to neck pressure were attenuated in the exercising leg (16.4 +/- 1.6%) compared with rest (33 +/- 2.1%) and the nonexercising leg (23.7 +/- 1.9%) (P < 0.01). These data suggest CBR control of blood pressure is partially mediated by changes in leg vascular tone both at rest and during exercise. Furthermore, despite alterations in CBR-induced changes in LVC during exercise, CBR control of blood pressure was well maintained.

Activity patterns of cardiac vagal motoneurons in rat nucleus ambiguus

Extracellular recordings were made in the right nucleus ambiguus of urethane-anesthetized rats from 33 neurons that were activated at constant latency from the craniovagal cardiac branch. Their calculated conduction velocities were in the B-fiber range (1.6-13.8 m/s, median 4.2), and most (22/33) were silent. Active units were confirmed as cardiac vagal motoneurons (CVM) by the collision test for antidromic activation and by the presence of cardiac rhythmicity in their resting discharge (9/9). Brief arterial pressure rises of 20-50 mmHg increased the activity in five of five CVM by 0.1 +/- 0.02 spikes. s(-1). mmHg(-1) from a resting 3.8 +/- 1.2 spikes/s; they also recruited activity in two of four previously silent cardiac branch-projecting neurons. CVM firing was modulated by the central respiratory cycle, showing peak activity during inspiration (8/8). Rat CVM thus show firing properties similar to those in other species, but their respiratory pattern is distinct. These findings are discussed in relation to mechanisms of respiratory sinus arrhythmia.

Altered sympathetic and parasympathetic activity in lung transplantation patients at rest and following autonomic perturbation

STUDY OBJECTIVES

To investigate the nature and extent of the alteration in autonomic function following heterotopic lung transplantation.

DESIGN

Measures of cardiac parasympathetic nervous system activity (PNSA) and systemic sympathetic nervous system activity (SNSA) were compared in lung transplant patients and age-matched healthy subjects, both at rest and following autonomic perturbation.

SETTING

Lung transplantation service of a university teaching hospital.

PATIENTS AND PARTICIPANTS

Twenty-two lung transplant patients (mean [+/- SEM] age, 50.5 +/- 2.4 years) and 13 healthy subjects (mean age, 48.2 +/- 3.7 years).

MEASUREMENTS AND RESULTS

Lung transplant patients had decreased baseline time and frequency domain measures of heart rate variability compared to healthy subjects (root mean square of successive differences in R-R intervals, 11.2 +/- 1.1 vs 30.3 +/- 4.5 ms, respectively [p < 0.005]; LnHP, 2.4 +/- 0.2 vs 4.8 +/- 0.4 ms(2), respectively [p < 0.005]). In addition, lung transplant patients demonstrated an attenuated reduction in LnHP/LnTP following head-up tilt in comparison to healthy subjects (p < 0.05). The baseline recumbent plasma norepinephrine level was increased in lung transplant patients compared to healthy subjects (3.25 +/- 0.43 vs 2.00 +/- 0.27 nmol/L, respectively; p < 0.05), and levels increased in both groups with upright head-up tilt. There were no differences between the two groups in heart rate or mean systolic BP responses to both the Valsalva maneuver and cold pressor testing.

CONCLUSIONS

Lung transplant patients have both reduced PNSA and increased SNSA at rest. Furthermore, these patients appear to have a preserved capacity to respond to autonomic perturbation by increasing SNSA. The mechanisms underlying these observations and their prognostic implications remain to be determined.

Respiratory sinus arrhythmia and the heart rate response to carotid baroreceptor activation after hard dynamic exercise in humans

The aim of this study was to examine the influence of 20 min of hard exercise (HR>160 beats min(-1)) on the efficacy of the cardiac parasympathetic nervous control of heart rate in humans (20-31 years; of either sex). This intensity of exercise was chosen to produce strong activation of the cardiac sympathetic nerves. Using well-controlled stimulus parameters, the efficacy of cardiac parasympathetic control of heart rate was assessed by recording the heart rate response to carotid baroreceptor activation (CBR) and the amplitude of respiratory sinus arrhythmia (RSA). Measurements were made while the subject performed light exercise (100-135 beats min (-1)) before (Control 1) and after very brief (Control 2) and prolonged (20 min; post) periods of hard exercise. There was no difference in the CBR in the three different measurement periods; 0.33 +/- 0.17, 0.38 +/- 0.18 and 0.39 +/- 0.18 beats min(-1) mm Hg(-1) (mean +/- S.D., N=6) for Control 1, Control 2 and post, respectively. At a heart rate of 120 beats min (-1), amplitude of the RSA was 6.1 +/- 2.4, 5.6 +/- 2.4 and 3.3 +/- 2.1 beats min(-1) for Control 1, Control 2 and post, respectively (P<0.001 post vs. Control 1 and Control 2, N=8). The decrease in RSA amplitude following hard exercise may be attributable to an exercise-induced reduction in airway resistance and work of breathing. Overall, these results do not support the hypothesis that sustained hard exercise that produces strong activation of cardiac sympathetic nerves reduces cardiac parasympathetic efficacy.

Effect of hypoxia on arterial baroreflex control of heart rate and muscle sympathetic nerve activity in humans

We tested the hypothesis that acute hypoxia would alter the sensitivity of arterial baroreflex control of both heart rate and sympathetic vasoconstrictor outflow. In 16 healthy, nonsmoking, normotensive subjects (8 women, 8 men, age 20-33 yr), we assessed baroreflex control of heart rate and muscle sympathetic nerve activity by using the modified Oxford technique during both normoxia and hypoxia (12% O(2)). Compared with normoxia, hypoxia reduced arterial O(2) saturation levels from 96.8 +/- 0.3 to 80.7 +/- 1.4% (P < 0.001), increased heart rate from 59.8 +/- 2.4 to 79.4 +/- 2.9 beats/min (P < 0.001), increased mean arterial pressure from 96.7 +/- 2.5 to 105.0 +/- 3.3 mmHg (P = 0.002), and increased sympathetic activity 126 +/- 58% (P < 0.05). The sensitivity for baroreflex control of both heart rate and sympathetic activity was not altered by hypoxia (heart rate: -1.02 +/- 0.09 vs. -1.02 +/- 0.11 beats. min(-1). mmHg(-1); nerve activity: -5.6 +/- 0.9 vs. -6.2 +/- 0.9 integrated activity. beat(-1). mmHg(-1); both P > 0.05). Acute exposure to hypoxia reset baroreflex control of both heart rate and sympathetic activity to higher pressures without changes in baroreflex sensitivity.

Mechanism of blood pressure and R-R variability: insights from ganglion blockade in humans

Spontaneous blood pressure (BP) and R-R variability are used frequently as 'windows' into cardiovascular control mechanisms. However, the origin of these rhythmic fluctuations is not completely understood. In this study, with ganglion blockade, we evaluated the role of autonomic neural activity versus other 'non-neural' factors in the origin of BP and R-R variability in humans. Beat-to-beat BP, R-R interval and respiratory excursions were recorded in ten healthy subjects (aged 30 +/- 6 years) before and after ganglion blockade with trimethaphan. The spectral power of these variables was calculated in the very low (0.0078-0.05 Hz), low (0.05-0.15 Hz) and high (0.15-0.35 Hz) frequency ranges. The relationship between systolic BP and R-R variability was examined by cross-spectral analysis. After blockade, R-R variability was virtually abolished at all frequencies; however, respiration and high frequency BP variability remained unchanged. Very low and low frequency BP variability was reduced substantially by 84 and 69 %, respectively, but still persisted. Transfer function gain between systolic BP and R-R interval variability decreased by 92 and 88 % at low and high frequencies, respectively, while the phase changed from negative to positive values at the high frequencies. These data suggest that under supine resting conditions with spontaneous breathing: (1) R-R variability at all measured frequencies is predominantly controlled by autonomic neural activity; (2) BP variability at high frequencies (> 0.15 Hz) is mediated largely, if not exclusively, by mechanical effects of respiration on intrathoracic pressure and/or cardiac filling; (3) BP variability at very low and low frequencies (< 0.15 Hz) is probably mediated by both sympathetic nerve activity and intrinsic vasomotor rhythmicity; and (4) the dynamic relationship between BP and R-R variability as quantified by transfer function analysis is determined predominantly by autonomic neural activity rather than other, non-neural factors.

Respiratory influences on sympathetic vasomotor outflow in humans

We have attempted to synthesize findings dealing with four types of respiratory system influences on sympathetic outflow in the human. First, a powerful lung volume-dependent modulation of muscle sympathetic nerve activity (MSNA) occurs within each respiratory cycle showing late-inspiratory inhibition and late-expiratory excitation. Secondly, in the intact human, neither reductions in spontaneous respiratory motor output nor voluntary near-maximum increases in central respiratory motor output and inspiratory effort, per sec, influence MSNA modulation within a breath, MSNA total activity or limb vascular conductance. Thirdly, carotid chemoreceptor stimuli markedly increase total MSNA; but most of the MSNA response to chemoreceptor activation appears to be mediated independently of increased central respiratory motor output. Fourthly, repeated fatiguing contractions of the diaphragm or expiratory muscles in the human show a metaboreflex mediated time-dependent increase in MSNA and reduced vascular conductance and blood flow in the resting limb. Recent evidence suggests that these respiratory influences contribute significantly to sympathetic vasomotor outflow and to the distribution of systemic vascular conductances and blood flow in the exercising human.

Respiratory modulation of human autonomic rhythms

We studied the influence of three types of breathing [spontaneous, frequency controlled (0.25 Hz), and hyperventilation with 100% oxygen] and apnea on R-R interval, photoplethysmographic arterial pressure, and muscle sympathetic rhythms in nine healthy young adults. We integrated fast Fourier transform power spectra over low (0.05-0.15 Hz) and respiratory (0.15-0.3 Hz) frequencies; estimated vagal baroreceptor-cardiac reflex gain at low frequencies with cross-spectral techniques; and used partial coherence analysis to remove the influence of breathing from the R-R interval, systolic pressure, and muscle sympathetic nerve spectra. Coherence among signals varied as functions of both frequency and time. Partialization abolished the coherence among these signals at respiratory but not at low frequencies. The mode of breathing did not influence low-frequency oscillations, and they persisted during apnea. Our study documents the independence of low-frequency rhythms from respiratory activity and suggests that the close correlations that may exist among arterial pressures, R-R intervals, and muscle sympathetic nerve activity at respiratory frequencies result from the influence of respiration on these measures rather than from arterial baroreflex physiology. Most importantly, our results indicate that correlations among autonomic and hemodynamic rhythms vary over time and frequency, and, thus, are facultative rather than fixed.