Correlation between baroreflex gain and 24-h indices of heart rate variability

SEDENTARY LIFESTYLE IN ADOLESCENTS IS ASSOCIATED WITH IMPAIRMENT IN AUTONOMIC CARDIOVASCULAR MODULATION

ABSTRACT Introduction Regular physical exercise, or a more active lifestyle, are important to prevent cardiovascular diseases, reducing not only cardiovascular mortality but also promoting a reduction in the risk factors related to these diseases. Objective To assess the association between physical inactivity and heart rate variability (HRV) in adolescents. Methods This was a cross-sectional study of the Diagnostic Type with Level of Evidence II according to the Oxford table. One hundred and twenty-nine adolescents were evaluated, aged 15 and 17 years old, divided into four groups: male teenagers who were insufficiently active (IAM n = 28) or physically active (FAM n = 29), and female teenagers who were insufficiently active (IAF n = 42) or physically active (FAF n = 30). The level of physical activity was assessed by the IPAQ. The HRV was evaluated in the time and frequency domains. Results Reduced pulse interval variance was observed in the insufficiently active male or female groups (5089 ± 378 ms2 and 4335 ± 276 ms2 respectively) compared to the physically active groups (9106 ± 606 ms2 and 6182 ± 366 ms2 respectively). Moreover, the insufficiently active groups presented higher cardiac sympathetic/vagal balance values (0.81 ± 0.05 and 0.80 ± 0.05 respectively) compared to the physically active groups (0.63 ± 0.05 and 0.55 ± 0.05 respectively). Conclusion A physically active lifestyle was associated with better cardiovascular autonomic modulation in adolescents. Level of Evidence II; Diagnostic Studies - Investigating a Diagnostic Test.

Cardiac Baroreflex, HRV, and Statistics: An Interdisciplinary Approach in Hypertension

Interests about the fine underpinnings of cardiovascular beat-by-beat variability have historical roots. Over the last decades, various aspects of the relationships between arterial pressure and heart period were taken as a proxy of the baroreflex in physiology and medicine, stimulating the interest of investigators in several interconnected scientific fields, in particular, bioengineering, neurophysiology, and clinical medicine. Studies of the overall system facilitated the emergence of a simplified negative (vagal) feedback model of the baroreflex and overshadowed the simultaneous interaction with excitatory, sympathetic positive-feedback mechanisms that would, however, better suit the model of a "paired antagonistic (parasympathetic/sympathetic) innervation of the internal organs." From the bioengineering side, the simplicity of obtaining the series of subsequent RR intervals stimulated the analysis of beat-by-beat variations, providing a multitude of heart rate variability (HRV) indices considered as proxies of the underlying sympatho-vagal balance, and participating to the management of several important clinical conditions, such as hypertension. In this context, advanced statistical methods, used in an integrated manner and controlling for age and gender biases, might help shed new light on the relationship between cardiac baroreflex, assessed by the frequency domain index α, and the HRV indices with the varying of systolic arterial pressure (SAP) levels. The focus is also on a novel unitary Autonomic Nervous System Index (ANSI) built as a synthesis of HRV considering its three most informative proxies [RR, RR variance, and the rest-stand difference in the normalized power of low-frequency (LF) variability component]. Data from a relatively large set of healthy subjects (n = 1154) with a broad range of SAP [from normal (n Nt = 778) to elevated (n Ht = 232)] show that, e.g., α and ANSI significantly correlate overall (r = 0.523, p < 0.001), and that this correlation is lower in hypertensives (r = 0.444, p < 0.001) and higher in pre-hypertensives (r = 0.618, p < 0.001) than in normotensives (r = 0.5, p < 0.001). That suggests the existence of curvilinear "umbrella" patterns that might better describe the effects of the SAP states on the relationships between baroreflex and HRV. By a mix of robust, non-parametric and resampling statistical techniques, we give empirical support to this study hypothesis and show that the pre-hypertensive group results at the apex/bottom in most of the studied trends.

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.

Cardiovascular Autonomic Responses in the VCD Rat Model of Menopause: Effects of Short- and Long-Term Ovarian Failure

After menopause, hypertension elevates the risk of cardiac diseases, one of the major causes of women's morbidity. The gradual depletion of ovarian follicles in rats, induced by 4-vinylcyclohexene diepoxide (VCD), is a model for studying the physiology of menopause. 4-Vinylcyclohexene diepoxide treatment leads to early ovarian failure (OF) and a hormonal profile comparable to menopause in humans. We have hypothesized that OF can compromise the balance between sympathetic and parasympathetic tones of the cardiovascular system, shifting toward dominance of the former. We aimed to study the autonomic modulation of heart and blood vessels and the cardiovascular reflexes in rats presenting short-term (80 days) or long-term (180 days) OF induced by VCD. Twenty-eight-day-old Wistar rats were submitted to VCD treatment (160 mg/kg, intraperitoneally) or vehicle (control) for 15 consecutive days and experiments were conducted at 80 or 180 days after the onset of treatment. Long-term OF led to an increase in the sympathetic activity to blood vessels and an impairment in the baroreflex control of the heart, evoked by physiological changes in arterial pressure. Despite that, long-term OF did not cause hypertension during the 180 days of exposure. Short-term OF did not cause any deleterious effect on the cardiovascular parameters analyzed. These data indicate that long-term OF does not disrupt the maintenance of arterial pressure homeostasis in rats but worsens the autonomic cardiovascular control. In turn, this can lead to cardiovascular complications, especially when associated with the aging process seen during human menopause.

Cumulative stress and autonomic dysregulation in a community sample

Whether cumulative stress, including both chronic stress and adverse life events, is associated with decreased heart rate variability (HRV), a non-invasive measure of autonomic status which predicts poor cardiovascular outcomes, is unknown. Healthy community dwelling volunteers (N = 157, mean age 29 years) participated in the Cumulative Stress/Adversity Interview (CAI), a 140-item event interview measuring cumulative adversity including major life events, life trauma, recent life events and chronic stressors, and underwent 24-h ambulatory ECG monitoring. HRV was analyzed in the frequency domain and standard deviation of NN intervals (SDNN) calculated. Initial simple regression analyses revealed that total cumulative stress score, chronic stressors and cumulative adverse life events (CALE) were all inversely associated with ultra low-frequency (ULF), very low-frequency (VLF) and low-frequency (LF) power and SDNN (all p < 0.05). In hierarchical regression analyses, total cumulative stress and chronic stress each was significantly associated with SDNN and ULF even after the highly significant contributions of age and sex, with no other covariates accounting for additional appreciable variance. For VLF and LF, both total cumulative stress and chronic stress significantly contributed to the variance alone but were not longer significant after adjusting for race and health behaviors. In summary, total cumulative stress, and its components of adverse life events and chronic stress were associated with decreased cardiac autonomic function as measured by HRV. Findings suggest one potential mechanism by which stress may exert adverse effects on mortality in healthy individuals. Primary preventive strategies including stress management may prove beneficial.

Baroreflex sensitivity and its association with arrhythmic events in Chagas disease

Background

Sudden death is the leading cause of death in Chagas disease (CD), even in patients with preserved ejection fraction (EF), suggesting that destabilizing factors of the arrhythmogenic substrate (autonomic modulation) contribute to its occurrence.

Objective

To determine baroreflex sensitivity (BRS) in patients with undetermined CD (GI), arrhythmogenic CD with nonsustained ventricular tachycardia (NSVT) (GII) and CD with spontaneous sustained ventricular tachycardia (STV) (GIII), to evaluate its association with the occurrence and complexity of arrhythmias.

Method

Forty-two patients with CD underwent ECG and continuous and noninvasive BP monitoring (TASK force monitor). The following were determined: BRS (phenylephrine method); heart rate variability (HRV) on 24-h Holter; and EF (echocardiogram).

Results

GIII had lower BRS (6.09 ms/mm Hg) as compared to GII (11.84) and GI (15.23). The difference was significant between GI and GIII (p = 0.01). Correlating BRS with the density of ventricular extrasystoles (VE), low VE density (<10/h) was associated with preserved BRS. Only 59% of the patients with high VE density (> 10/h) had preserved BRS (p = 0.003). Patients with depressed BRS had higher VE density (p = 0.01), regardless of the EF. The BRS was the only variable related to the occurrence of SVT (p = 0.028).

Conclusion

The BRS is preserved in undetermined CD. The BRS impairment increases as disease progresses, being more severe in patients with more complex ventricular arrhythmias. The degree of autonomic dysfunction did not correlate with EF, but with the density and complexity of ventricular arrhythmias.

Nocturnal snoring decreases daytime baroreceptor sensitivity

BACKGROUND

In patients with obstructive sleep apnea heart rate variability and baroreceptor sensitivity during night and daytime are impaired. Snoring without obstructive sleep apnea may already influence heart rate variability and baroreceptor sensitivity during daytime.

METHODS

Cardiovascular daytime testing was performed in 11 snorers and age, BMI, and gender matched controls. Sleep apnea and snoring were quantified by sleep recordings. Paced breathing was performed during daytime with ECG, non-invasive blood pressure, and respiration recorded. Heart rate variability and blood pressure variability were analyzed in the time and frequency domain. Baroreceptor sensitivity (alpha gain) was calculated.

RESULTS

In snorers a significant increase in high frequency systolic blood pressure variability (SBPV-HF) compared to control group (0.37 mm Hg(2) vs. 0.11 mm Hg(2) for 12 breaths and 0.35 mm Hg(2) vs. 0.10 mm Hg(2) for 15 breaths) was demonstrated. Furthermore a lower baroreceptor sensitivity was found in snorers compared to controls (9.2 ms/mm Hg vs. 16.2 ms/mm Hg for 12 breaths and 8.5 ms/mm Hg vs. 17.4 ms/mm Hg for 15 breaths per minute) using the paced breathing protocol. Mean heart rate was elevated in snorers as well.

CONCLUSIONS

Snorers may have a reduced parasympathetic tone during daytime rather than an increased sympathetic tone.

Metabolic energy correlates of heart rate variability spectral power associated with a 900-calorie challenge

We studied healthy males challenged with a 900 Cal test beverage and correlated EE with the raw (ms²) and normalized units (nu) of total power (TP), low frequency/high frequency (LF/HF) and VLF spectral power of heart rate variability (HRV). The correlations were evaluated during 20 min of normal breathing (NB, control) and 20 min of paced breathing (PB) at 12 breaths·min⁻¹ (0.2 Hz). EE was not significantly correlated with any of the HRV variables before the metabolic challenge. After the challenge, EE was positively correlated with LF/HF and with VLF; VLF was also positively correlated with LF/HF during both NB and PB. These findings suggest that EE may be a correlate of LF/HF and of VLF spectral power of HRV in healthy adolescent/young adult males. The association of lower resting energy expenditure with lower amounts of VLF spectral power may occur in individuals with predilections for obese phenotypes.

Daytime baroreflex sensitivity in patients with primary insomnia

Insomnia has been linked to cardiovascular disease and among these especially hypertension and changes in autonomic function. One marker for cardiovascular risk is baroreflex sensitivity (BRS). We investigate daytime BRS in patients with primary insomnia in order to assess cardiovascular risk. Twenty-one patients (18 females/3 males) with primary insomnia according to DSM-IV were recruited. Careful investigations excluded confounding sleep disorders such as sleep-disordered breathing and periodic limb movements. An age-matched control group with 21 healthy subjects (18 females/3 males) underwent the same investigations. To assess BRS, an experimental protocol with paced breathing during daytime was performed. ECG and continuous non-invasive blood pressure were recorded to obtain spontaneous BRS by calculating the α index (BRS-α) and also by transfer function analysis (TF-BRS). There were no differences at daytime between insomnia patients and controls neither in BRS-α (8.1 ms/mmHg, range 5.8-14.7 vs. 9.6 ms/mmHg, range 6.9-15.8) nor in TF-BRS (5.8 ms/mmHg, range 2.4-16.8 vs. 5.4 ms/mmHg, range 2.3-11.4). Also there were no differences in absolute, low or high frequency bands of heart rate or blood pressure variability between the two groups. We could show that primary insomnia may be not associated with daytime parameters of autonomic imbalance (e.g., baroreflex sensitivity) which are known as non-classical risk markers of cardiovascular disease.

Association of body fat percentage and heart rate variability measures of sympathovagal balance

AIMS

We tested the hypothesis that body fat percentage determines cardiac sympathovagal balance in healthy subjects.

MAIN METHODS

Heart rate variability (HRV) measurements were made of the standard deviation of the normal-normal RR intervals (SDNN) and the low frequency/high frequency (LF/HF) ratio, from time domain and fast Fourier transform spectral analysis of electrocardiogram RR intervals during trials of uncontrolled and controlled (paced) breathing at 0.2Hz. Body fat percentage was measured by dual energy x-ray absorptiometric (DEXA) scanning. Significance of differences between uncontrolled and controlled (paced) breathing was determined by analysis of variance and correlations between body fat percentage and HRV measurements by Pearson's coefficient at P<0.05.

KEY FINDINGS

Percent body fat was negatively correlated with LF/HF during the uncontrolled breathing (r=-0.56, two-tailed P<0.05, one-tailed P<0.01) but not during the paced breathing trial (r=-0.34, (P>0.1).

SIGNIFICANCE

We conclude that sympathetic activity produced by paced breathing at 0.2Hz can obscure the relationship between body fat percentage and sympathovagal balance and that high body fat percentage may be associated with low sympathetic modulation of the heart rate in healthy adolescent/young adult males.

Blood pressure regulation in neurally intact human vs. acutely injured paraplegic and tetraplegic patients during passive tilt

We investigated autonomic control of cardiovascular function in able-bodied (AB), paraplegic (PARA), and tetraplegic (TETRA) subjects in response to head-up tilt following spinal cord injury. We evaluated spectral power of blood pressure (BP), baroreflex sensitivity (BRS), baroreflex effectiveness index (BEI), occurrence of systolic blood pressure (SBP) ramps, baroreflex sequences, and cross-correlation of SBP with heart rate (HR) in low (0.04-0.15 Hz)- and high (0.15-0.4 Hz)-frequency regions. During tilt, AB and PARA effectively regulated BP and HR, but TETRA did not. The numbers of SBP ramps and percentages of heartbeats involved in SBP ramps and baroreflex sequences increased in AB, were unchanged in PARA, and declined in TETRA. BRS was lowest in PARA and declined with tilt in all groups. BEI was greatest in AB and declined with tilt in all groups. Low-frequency power of BP and the peak of the SBP/HR cross-correlation magnitude were greatest in AB, increased during tilt in AB, remained unchanged in PARA, and declined in TETRA. The peak cross-correlation magnitude in HF decreased with tilt in all groups. Our data indicate that spinal cord injury results in decreased stimulation of arterial baroreceptors and less engagement of feedback control as demonstrated by lower 1) spectral power of BP, 2) number (and percentages) of SBP ramps and barosequences, 3) cross-correlation magnitude of SBP/HR, 4) BEI, and 5) changes in delay between SBP/HR. Diminished vasomotion and impaired baroreflex regulation may be major contributors to decreased orthostatic tolerance following injury.

CARDIOVASCULAR AUTONOMIC REGULATION IN SUBJECTS WITH NORMAL BLOOD PRESSURE, HIGH-NORMAL BLOOD PRESSURE and RECENT-ONSET HYPERTENSION

1. In the present study, we tested the hypothesis that heart rate variability (HRV) is reduced in recent-onset hypertension and that pressor responses to standard autonomic reflex tests are not any different in hypertensives compared with normotensives. We also hypothesized that subjects with high-normal blood pressure (BP) would be distinguishable from normotensives on the basis of short-term HRV indices. 2. Three groups of subjects, each consisting of 15 men and 10 women, were examined. The first group consisted of subjects with recent-onset hypertension who were not taking antihypertensive medication (mean (+/-SD) age 50 +/- 12 years; BP >/= 140/90 mmHg), the second group consisted of subjects with high-normal BP (mean age 46 +/- 13 years; BP 130-139/85-89 mmHg) and the third group consisted of subjects with normal BP (mean age 48 +/- 12 years; BP < 120/80 mmHg). The aim was to characterize the autonomic state in each group. 3. Blood pressure, heart rate (HR), indices of short-term HRV during supine rest and quiet standing, HR variation during timed deep breathing (HRVdb) and pressor responses to the cold pressor test and sustained isometric handgrip were compared between the groups. 4. Although the three groups were comparable (P > 0.1) in terms of mean HR and low-frequency (LF) power expressed in normalized units at rest and during quiet standing, the standard deviation of normal-to-normal RR intervals (SDNN) during supine rest, LF and high-frequency spectral powers during supine rest and HRVdb were lowest in hypertensives (P </= 0.05 for each), indicating diminished baroreflex modulation of RR intervals in hypertensives. 5. In contrast, LF power was highest in subjects with high-normal BP (P </= 0.05) during supine rest and this is possibly because of higher BP variability. 6. The results suggest that HRVdb provides a simple measure of cardiac vagal effects in hypertensives, the rate-pressure product provides a simple measure of overall HRV in hypertensives and, in clinical hypertension, the arterial baroreflex mechanism is reset to maintain a higher BP through diminished vagal modulation of HR and possibly heightened sympathetic outflow to the heart and resistance vessels.

Relations between insulin sensitivity, fitness and autonomic cardiac regulation in healthy, young men

OBJECTIVES

We hypothesized that insulin sensitivity and vagal cardiac control are independently related in young men after adjustment for fitness and other confounding variables.

DESIGN

Male volunteers aged 21-24 years with high (borderline hypertensive; n = 20) and low-normal (normotensive; n = 21) screening blood pressure (BP) were studied cross-sectionally.

METHODS

Mean R-R interval (RR) and heart rate variability (HRV) were computed from 30-min ECGs, and baroreflex sensitivity (BRS) and latency (phase shift) from 15-min beat-to-beat finger blood pressure (BP) and heart rate recordings. Insulin-adjusted glucose disposal rate (GDR/I) was measured with a 90-min hyperinsulinaemic glucose clamp and fitness by peak oxygen uptake (VO2peak) during a treadmill test.

RESULTS

HRV, baroreflex function, GDR/I, and VO2peak did not differ between the groups. GDR/I correlated positively with time and frequency domain HRV, including high-frequency power (HF) (r = 0.40, P = 0.01) and root-mean squared successive differences (RMSSD) (r = 0.43, P = 0.005), but not BRS or phase shift. GDR/I correlated with VO2peak (r = 0.70, P < 0.0001) and was explained (R = 0.56) by VO2peak (beta = 0.57, P < 0.0001) and RR (beta = 0.29, P = 0.03), independently of HRV and measures of obesity. Conversely, RR (beta = 0.55, P = 0.0004) and HRV, including HF (beta = 0.44, P = 0.006) and RMSSD (beta = 0.46, P = 0.004) were explained by GDR/I, independently of VO2peak.

CONCLUSIONS

Insulin sensitivity and autonomic cardiac control are related independently of physical fitness in young men.

Short-term fluoxetine treatment enhances baroreflex control of sympathetic nervous system activity after hindlimb unloading

Data in humans indicate that individuals with orthostatic hypotension that are refractory to other traditional forms of therapy are responsive to selective serotonin reuptake inhibitor (SSRI) treatment. We tested the hypothesis that SSRI administration would help correct the attenuated baroreflex control of sympathetic nervous system activity in the hindlimb-unloaded (HU) rat model of cardiovascular deconditioning. An initial study was conducted to determine the time course of effects of fluoxetine (Flu) administration on baroreflex control of lumbar sympathetic nerve activity (LSNA) in conscious, chronically instrumented rats. Animals received either vehicle (Veh, sterile water) or 10 mg/kg Flu for 1, 4, or 16 days of treatment. Data indicate that while 1-day and 16-day Flu administration did not affect baroreflex function, baroreflex control of LSNA was enhanced after 4-day (short term) Flu administration. HU rats were then treated with Flu for 4 days and compared with HU rats receiving Veh and to casted control rats maintained in the normal posture that received either Veh or short-term Flu treatment. Similar to pilot data, short-term Flu treatment enhanced baroreflex control of LSNA in both HU rats and control rats. These data taken together indicate that baroreflex control of sympathetic nervous system activity is a possible mechanism responsible for the successful treatment of orthostatic intolerance with Flu.