Withdrawal Of Vagal Tone Research Articles

Cardiovascular reactivity to stress in long-term yoga practitioners.

Bhramari Pranayama yogic breathing is a very simple exercise of voluntary breathing which mitigates stress reactivity through autonomic modulation of heart rate. To study the effects of long-term practice of the yoga breathing exercise on cardiovascular reactivity to head-up tilt (HUT) test. All the participants were subjected to a 10-item yoga questionnaire. Based on Likert scale scoring, 32 subjects (group I) were selected as long-term yogic breathing practitioners, and autonomic function tests using heart rate variability (HRV) and a provocative stress test and HUT test were conducted. Cardiovascular parameters obtained were compared with controls who had never been engaged in any form of yoga (group II). A highly significant decrease in values of both high (HF) and low frequency (LF) variables was observed in group II as compared to group I. However, a highly significant rise in LF/HF has been observed in group I compared to group I (P = 0.004), further indicating a greater withdrawal of vagal tone during the HUT test and also during recovery (P = 0.001). The results and discussion of HRV analysis during provocative tests concluded that autonomic imbalance was present in the subjects who were not exposed to any form of yoga. Thus, it is advisable to practice yoga in any form regularly to mitigate the early onset of autonomic dysfunction.

Maintained barostatic regulation of heart rate in digesting snakes (Boa constrictor).

When snakes digest large meals, heart rate is accelerated by withdrawal of vagal tone and an increased non-adrenergic-non-cholinergic tone that seems to stem from circulating blood-borne factors exerting positive chronotropic effects. To investigate whether this tonic elevation of heart rate impairs the ability for autonomic regulation of heart during digestion, we characterised heart rate responses to pharmacological manipulation of blood pressure in the snake Boa constrictor through serial injections of sodium nitroprusside and phenylephrine. Both fasting and digesting snakes responded with a robust tachycardia to hypotension induced by sodium nitroprusside, with digesting snakes attaining higher maximal heart rates than fasting snakes. Both fasting and digesting snakes exhibited small reductions of the cardiac chronotropic response to hypertension, induced by injection of phenylephrine. All heart rate changes were abolished by autonomic blockade with the combination of atropine and propranolol. The digesting snakes retained the capacity for compensatory heart rate responses to hypotension, despite their higher resting values, and the upward shift of the barostatic response curve enables snakes to maintain the cardiac limb of barostatic regulation for blood pressure regulation.

Effects of Heat Stress on Heart Rate Variability in Free-Moving Sheep and Goats Assessed With Correction for Physical Activity.

Heart rate variability (HRV) is the heart beat-to-beat variation under control of the cardiovascular function of animals. Under stressed conditions, cardiac activity is generally regulated with an upregulated sympathetic tone and withdrawal of vagal tone; thus, HRV monitoring can be a non-invasive technique to assess stress level in animals especially related to animal welfare. Among several stress-induced factors, heat stress is one of the most serious causes of physiological damage to animals. The aim of this study was to assess the effects of heat stress on HRV in small ruminants under free-moving conditions. In three experimental periods (June, August, and October), inter-beat intervals in sheep and goats (three for each) in two consecutive days were measured. HRV parameters were calculated from the inter-beat interval data by three types of analyses: time domain, frequency domain, and non-linear analyses. The temperature–humidity index (THI) was used as an indicator of heat stress, and vectorial dynamic body acceleration (VeDBA) was calculated to quantify the physical activity of the animals tested. First, we investigated correlations of THI and VeDBA with HRV parameters; subsequently, THI was divided into five categories according to the values obtained (≤ 65, 65–70, 70–75, 75–80, and >80), and the effects of the THI categories on HRV parameters were investigated with and without correcting for the effects of physical activity based on the VeDBA. The results indicated that HRV significantly decreased with increasing THI and VeDBA. For non-linear HRV parameters that were corrected for the effects of physical activity, it was suggested that there would be a threshold of THI around 80 that strongly affected HRV; high heat stress can affect the autonomic balance of animals non-linearly by inducing the sympathetic nervous system. In conclusion, to assess psychophysiological conditions of unrestrained animals by HRV analysis, the confounding effect of physical activity on HRV should be minimized for a more precise interpretation of the results.

An Exploratory Study Testing Autonomic Reactivity to Pain in Women with Sensory Over-Responsiveness.

Background: Difficulty modulating sensory input related to multi-sensory integration dysfunction, specifically the sensory over-responsive (SOR) type, is associated with psychological distress and hyperalgesia in children and adults. Scares reports suggest atypical autonomic nervous system (ANS) reactivity to innocuous sensory stimuli in children with SOR. Thus, the ANS may contribute to sensory stimuli responses and psychological distress. This exploratory study aimed to characterize the ANS reactivity to single and dual pain stimulation, and in relation to psychological distress in adults with SOR. Methods: Healthy women with SOR (n = 9) vs. without SOR (n = 9) underwent two runs of single pain stimulation and a third run comprised of dual pain stimulation. Pain was self-rated, while heart rate variability was measured and analyzed in the time and frequency domains. In addition, questionnaires assessing anxiety and somatization were utilized. Results: While controls demonstrated a vagal tone withdrawal (root mean square of successive differences in R-R-intervals; (RMSSD)) p = 0.029 from base-line to the third run, this was absent in the SOR group. However, no group differences were found in pain ratings. Furthermore, groups differed in the correlations between R-R mean and the level of both anxiety (p = 0.006) and somatization (p < 0.001); while in the SOR group, higher levels of anxiety and somatization correlated with shorter R-R intervals, the opposite was found in the control group. Conclusions: This is the first study to demonstrate in women with SOR atypical vagal tone reactivity to challenging pain load. Vagal tone reactivity is related to both pain ratings and psychological distress.

Normal tension glaucoma versus primary open-angle glaucoma - The autonomic perspective

Background: Dissimilarities in the risk factors concerned with primary open angle glaucoma (POAG) and normal tension glaucoma (NTG) suggest these two forms of glaucoma to be separate clinical entities, each having their own pathogenesis, yet the relationship between them is not clear convincing. Aims and Objectives: The aim of the study was to compare the autonomic activity between healthy controls, POAG, and NTG patients by heart rate variability (HRV). Materials and Methods: The study was conducted on 30 age- and sex-matched healthy controls (Group І), 30 patients with NTG (Group ІІ), and 30 patients with POAG (Group ІІІ) of age between 45 and 65 years. The basal HRV was analyzed at rest followed by stress tests, handgrip testing (HGT) and passive head-up tilt (HUT), and post recovery. Data obtained were statistically analyzed using SPSS 20. Results: Significant raised values of low frequency variables (LF [nu] [58.94 ± 6.556] and [73.39 ± 4.04]) in Group ІІ and Group ІІІ, respectively, as compared to Group І (52.13 ± 2.97). LF/HF ratio was significantly rise in Groups ІІ and ІІІ with P < 0.000 during HGT. LF/HF ratio was significantly high (P < 0.006) and (P < 0.004) in Groups ІІ and ІІІ, respectively, as compared to Group І during HUT. Conclusion: The above findings of HRV suggested greater withdrawal of vagal tone and persistently overdrive of sympathetic activity due to sympathovagal imbalance indicated ocular vascular alterations more in NTG than in POAG as compared to normal healthy subjects.

Testing the vagal withdrawal hypothesis during light exercise under autonomic blockade: a heart rate variability study.

We performed the first analysis of heart rate variability (HRV) at rest and during exercise under full autonomic blockade on the same subjects, to test the conjecture that vagal tone withdrawal occurs at exercise onset. We hypothesized that between rest and exercise there would be 1) no differences in total power (PTOT) under parasympathetic blockade, 2) a PTOT fall under β1-sympathetic blockade, and 3) no differences in PTOT under blockade of both autonomic nervous system branches. Seven men [24 (3) yr, mean (SD)] performed 5-min cycling (80 W) supine, preceded by 5-min rest during control and with administration of atropine, metoprolol, and atropine + metoprolol (double blockade). Heart rate and arterial blood pressure were continuously recorded. HRV and blood pressure variability were determined by power spectral analysis, and baroreflex sensitivity was determined by the sequence method. At rest, PTOT and the powers of low- and high-frequency components of HRV (LF and HF, respectively) were dramatically decreased with atropine and double blockade compared with control and metoprolol, with no effects on LF-to-HF ratio and on the normalized LF (LFnu) and HF (HFnu). During exercise, patterns were the same as at rest. Comparing exercise with rest, PTOT varied as hypothesized. For systolic and diastolic blood pressure, resting PTOT was the same in all conditions. During exercise, in all conditions, PTOT was lower than in control. Baroreflex sensitivity decreased under atropine and double blockade at rest and under control and metoprolol during exercise. The results support the hypothesis that vagal suppression determined disappearance of HRV during exercise.NEW & NOTEWORTHY This study provides the first demonstration, by systematic analysis of heart rate variability at rest and during exercise under full autonomic blockade on the same subjects, that suppression of vagal activity is responsible for the disappearance of spontaneous heart rate variability during exercise. This finding supports previous hypotheses on the role of vagal withdrawal in the control of the rapid cardiovascular response at exercise onset.

PP.38.09] EFFECTS OF MILD DYNAMIC EXERCISE ON CIRCULATORY AND AUTONOMIC FUNCTION IN HEALTHY VOLUNTEERS

Objective: Valuable insights into BP regulation have arisen from the study of neurocirculatory responses to exercise. However most studies have focused exclusively on evaluation of responses to maximal/submaximal exercise with concomitant profound increase in muscle metabolic demand. We studied the effects of mild exercise in healthy volunteers to obviate the confounding effects of large metabolic changes. Design and method: 8 healthy volunteers undertook very mild dynamic exercise (< 3 METs) of 1 minute duration. Non-invasive arterial BP was monitored in all 8 subjects. In 5 subjects the arterial BP recording was used to derive haemodynamic variables (PulseCO®, LiDCO plc, London), including cardiac output (CO), mean arterial pressure (MAP), systemic vascular resistance (SVR) and heart rate. Autonomic nervous system investigation was undertaken in 7 subjects with high resolution monitoring of arterial pressure and electrocardiography to derive indices of sympathetic and parasympathetic tone (NeuroScope®, Medifit Instruments Ltd, London). Results: CO increased from an average resting value of 4.8 l min-1 to 6.2 l min-1 during exercise (a 30.4% increase, p = 0.003) and settled to 4.8 l min-1 afterwards. The reciprocal of SVR, conductance (100/SVR) was used to determine the role of the periphery on CO in positive terms (the other driver for the increase in CO being MAP change). The average increase in MAP and conductance was 4.4% (p = ns) and 23.8% (p = 0.01) respectively.Cardiac sensitivity to baroreceptor function (CSB) fell from an average of 5.4 ± 1.2 SEM to 2.9 ± 1.2 SEM (p < 0.05) and cardiac vagal tone (CVT) fell from an average of 6.8 ± 1.3 SEM on a linear vagal scale to 4.1 ± 1.3 SEM, (p < 0.05). Following exercise rebound occurred with both to values above baseline. Conclusions: The main change driving the increase in CO was an increase in conductance rather than change in MAP. This is in contrast to dynamic cardiovascular exercise at higher workloads where MAP increases immediately and progressively with workload. Withdrawal of vagal tone at such mild exercise intensity is a new finding and is consistent with gradual heart rate increase as exercise intensity increases from the lowest levels.

Autonomic regulation of heart rate during specific dynamic action associated with digestion in the bullfrog Lithobates catesbeianus

Feeding causes a substantial rise in the aerobic metabolism of all vertebrates, which is met by a rise in heart rate to secure adequate oxygen delivery, as well as intestinal nutrient absorption and transport between organs. To study the autonomic regulation of this postprandial tachycardia in an amphibian, we placed arterial catheters in individuals of the bullfrog, Lithobates catesbeianus (Shaw, 1802), to measure their blood pressure and heart rate. Heart rate in experimental subjects increased from a fasting value of 42.5 ± 2.5 to 52.5 ± 1.6 and 57.5 ± 2.1 min-1 at 24 and 33 hours, respectively, after meals that corresponded to 5% of their body weight. Using an infusion of atropine and propranolol, we demonstrated that part of the postprandial tachycardia can be ascribed to a progressive decrease in parasympathetic tone (26 ± 5% in fasting frogs to 6 ± 5% by 33 hours into digestion), whereas the sympathetic tone remained unaltered at approximately 20%. In addition to the withdrawal of vagal tone, digestion was also associated with an increase in the heart rate upon double-blockade (i.e., the combination of atropine and propranolol) from a fasting value of 44.5 ± 0.9 to 50.8 ± 1.9 min-1 at 33 hours. This indicates that the postprandial tachycardia in frogs, as previously demonstrated for snakes, is partially governed by excitation of non adrenergic, non cholinergic (NANC) factors.

Cardiac vagal tone withdrawal in conscious treated hypertensive patients and normal healthy controls during baroreceptor unloading and baroreceptors loading

Cardiac vagal tone withdrawal in conscious treated hypertensive patients and normal healthy controls during baroreceptor unloading and baroreceptors loading

Variant cardiovascular regulation in children with postural tachycardia syndrome

Postural tachycardia syndrome (POTS) manifests as marked tachycardia while standing. We noticed two forms of circulatory response to orthostatic stress in POTS. We investigated cardiovascular and autonomic nervous response to orthostatic stress in the two forms. We studied 79 patients with POTS and 38 healthy control subjects (Ct). Beat-to-beat blood pressure (BP) and heart rate (HR) were non-invasively and continuously measured in the supine and standing positions. Autonomic nervous function was evaluated on power spectral analysis of HR variability and diastolic BP variability. We divided the subjects into two groups: standing-induced tachycardia (SI group; increase in HR ≥35 beats/min) and supine tachycardia (Su group; standing HR ≥115 beats/min with standing-induced HR increase <35 beats/min). The Su group had higher supine BP and HR compared with the other groups, indicating dominant sympathetic control of the heart in the supine position. While rising, the SI group had a higher increase in HR than the Ct group, indicating excessive withdrawal of vagal tone. The Su group had a smaller increase in HR and a greater decrease of systolic BP and cardiac index by standing compared with the SI group. These results suggest that compensatory mechanisms of sympathetic function during standing failed in the Su group, probably because of exhaustion by the nearly maximum effort to generate sympathetic drive even in the supine position with low central blood volume. There is a difference between the two types of POTS, in the balance of resting autonomic function and hemodynamic response to standing.

Hemodynamic actions and mechanisms of systemically administered α-MSH analogs in mice

α-Melanocyte-stimulating hormone (α-MSH) regulates important physiological functions including energy homeostasis and inflammation. Potent analogs of α-MSH, [Nle4, d-Phe7]-α-MSH (NDP-α-MSH) and melanotan-II (MT-II), are widely used in pharmacological studies, but the hemodynamic effects associated with their systemic administration have not been thoroughly examined. Therefore, we investigated the hemodynamic actions of these compounds in anesthetized and conscious C57Bl/6N mice using peripheral routes of administration. NDP-α-MSH and MT-II induced mild changes in blood pressure and heart rate in anesthetized mice compared to the effects observed in conscious mice, suggesting that anesthesia distorts the hemodynamic actions of α-MSH analogs. In conscious mice, NDP-α-MSH and MT-II increased blood pressure and heart rate in a dose-dependent manner, but the tachycardic effect was more prominent than the pressor effect. Pretreatment with the melanocortin (MC) 3/4 receptor antagonist SHU9119 abolished these hemodynamic effects. Furthermore, the blockade of β1-adrenoceptors with metoprolol prevented the pressor effect and partly the tachycardic action of α-MSH analogs, while the ganglionic blocker hexamethonium abrogated completely the difference in heart rate between vehicle and α-MSH treatments. These findings suggest that the pressor effect is primarily caused by augmentation of cardiac sympathetic activity, but the tachycardic effect seems to involve withdrawal of vagal tone in addition to sympathetic activation. In conclusion, the present results indicate that systemic administration of α-MSH analogs elevates blood pressure and heart rate via activation of MC3/4 receptor pathways. These effects and the consequent increase in cardiac workload should be taken into account when using α-MSH analogs via peripheral routes of administration.

Alternating Ventricular Preexcitation

A 45-year-old woman diagnosed with Wolff-Parkinson-White syndrome preferred treatment with propafenone rather than catheter ablation. A 12-lead electrocardiogram (Fig. 1) after the drug administration showed sinus arrhythmia; alternating ventricular preexcitation; and alternating T wave inversion in the inferior leads during normal AV conduction. Fig. 1 Ventricular preexcitation may disappear because of decreased conduction delay over the AV node, relative to the accessory pathway; this was similar to that observed on vagal tone withdrawal. However, loss of ventricular preexcitation associated with decreased sinus cycle length was observed, which excluded the first possibility as a mechanism for intermittent preexcitation. The other mechanisms include prolongation of the refractory period (RP) of the accessory pathway (AP) due to increased vagal tone1) and linking. Linking is defined as repetitive conduction over 1 of 2 potential pathways, with persistent conduction block in the other pathway maintained by concealed retrograde conduction.2),3) However, the alternating preexcitation cannot be attributed to linking and was most probably caused by prolongation of the antegrade RP of the AP by propafenone greater than sinus cycle length (Fig. 2). Fig. 2 Intermittent ventricular preexcitation is associated with a long antegrade RP of the AP and a prolonged preexcited RR interval during atrial fibrillation.4) Twave inversion in the inferior leads occurs because of the memory effect.5) Alternating ventricular preexcitation is a rare electrocardiographic phenomenon associated with RP of the AP greater than the length of the sinus cycle.

Gender differences in the variables of exercise treadmill test in type 2 diabetes mellitus

Exercise capacity, like some other variables of exercise stress test, is a strong predictor of cardiovascular and overall mortality. Many confounding factors, including gender, have been found to affect exercise capacity. This study evaluated whether exercise capacity differs in age-matched type 2 diabetic Nigerian men and their women counterparts and the hemodynamic variables of exercise treadmill test that correlate with exercise capacity in them. A total of 61 type 2 diabetics (male = 34; female = 27) aged 30 to 60 years who were recruited through the Medical Out-patient Department of OAUTHC, Ile Ife, Nigeria, underwent symptom-limited maximal treadmill exercise using Bruce protocol. Patients had comparable clinical and demographic patterns. There was no gender difference in the chronotropic response to exercise. Males had faster heart rate recovery (HRR) than females. Though both sexes had similar resting systolic blood pressure (SBP), males had significantly higher peak SBP than females (216.2 ± 23.7 mmHg vs 203.3 ± 21.7 mmHg; P = 0.03). Exercise capacity was significantly higher in males (7.5 ± 2.0 METs) than females (6.4 ± 1.5 METs); P = 0.01. Significant correlates of exercise capacity in both sexes were fasting plasma glucose, resting diastolic blood pressure, Duke Treadmill Score, and HRR. Majority of the patients were in moderate DUKE risk subgroup and there was no statistically significant difference between males and females in this regard. Gender difference occurs in the exercise capacity of diabetic patients and the factors associated with this disparity may be related to gender differences in resting heart rate and HRR, both reflecting a withdrawal of vagal tone.

The Bainbridge and the “Reverse” Bainbridge Reflexes

Francis A. Bainbridge demonstrated in 1915 that an infusion of saline or blood into the jugular vein of the anesthetized dog produced tachycardia. His findings after transection of the cardiac autonomic nerve supply and injection of the cholinergic blocking drug atropine demonstrated that the tachycardia was reflex in origin, with the vagus nerves constituting the afferent limb and a withdrawal of vagal tone the primary efferent limb. Subsequent investigators demonstrated that the increase in venous return was detected by stretch receptors in the right and left atria. In the 1980s, it was shown convincingly that the Bainbridge reflex was present in primates, including humans, but that the reflex was much less prominent than in the dog. This difference may be due to a more dominant arterial baroreceptor reflex in humans. A "reverse" Bainbridge reflex has been proposed to explain the decreases in heart rate observed under conditions in which venous return is reduced, such as during spinal and epidural anesthesia, controlled hypotension, and severe hemorrhage. The Bainbridge reflex is invoked throughout the anesthesia literature to describe the effect of changes in venous return on heart rate in patients in the surgical and critical care settings, but a critical analysis of the experimental and clinical evidence is lacking. Our main objectives in this review are to summarize the history of the Bainbridge reflex, to describe its anatomy and physiology, and to discuss the evidence for and against it having an influence on heart rate changes observed clinically. The interaction of the Bainbridge reflex with the arterial baroreceptor and Bezold-Jarisch reflexes is discussed.

Autonomic control of cardiorespiratory interactions in fish, amphibians and reptiles

Control of the heart rate and cardiorespiratory interactions (CRI) is predominantly parasympathetic in all jawed vertebrates, with the sympathetic nervous system having some influence in tetrapods. Respiratory sinus arrhythmia (RSA) has been described as a solely mammalian phenomenon but respiration-related beat-to-beat control of the heart has been described in fish and reptiles. Though they are both important, the relative roles of feed-forward central control and peripheral reflexes in generating CRI vary between groups of fishes and probably between other vertebrates. CRI may relate to two locations for the vagal preganglionic neurons (VPN) and in particular cardiac VPN in the brainstem. This has been described in representatives from all vertebrate groups, though the proportion in each location is variable. Air-breathing fishes, amphibians and reptiles breathe discontinuously and the onset of a bout of breathing is characteristically accompanied by an immediate increase in heart rate plus, in the latter two groups, a left-right shunting of blood through the pulmonary circuit. Both the increase in heart rate and opening of a sphincter on the pulmonary artery are due to withdrawal of vagal tone. An increase in heart rate following a meal in snakes is related to withdrawal of vagal tone plus a non-adrenergic-non-cholinergic effect that may be due to humoral factors released by the gut. Histamine is one candidate for this role.

Moderate vagal withdrawal in 3.5-year-old children is associated with optimal performance on executive function tasks.

Vagal tone (measured via respiratory sinus arrhythmia, RSA) and vagal withdrawal (measured by decreases in RSA) have been identified as physiological measures of self-regulation, but little is known how they may relate to the regulation of cognitive activity as measured through executive function (EF) tasks. We expected that baseline measures of vagal tone, thought to be an indicator of attention, would correlate with EF performance. We also predicted that vagal withdrawal would allow for the reorientation of attention that is needed to succeed on EF tasks, but too much withdrawal would be detrimental. RSA measured at baseline was indeed related to EF performance in 220 3.5-year-old children, and those who exhibited a moderate decrease in RSA during the EF tasks outperformed children whose RSA decreased by too little or too much. These findings implicate vagal tone withdrawal as a psychophysiological measure of higher cognitive processes, most likely substantiated through increases in the levels of focused attention.

Cholinergic and adrenergic influences on the heart of the African lungfish Protopterus annectens

Cardiac cholinergic and adrenergic tones were determined in minimally instrumented African lungfish Protopterus annectens. Mean ±S.E. routine heart rate (fH) was 31·6 ± 1·4 beats min−1, cholinergic tone was 34·6 ± 5·2% and adrenergic tone was 9·4 ± 2·3%, while the intrinsic fH after blockade of both adrenergic and cholinergic control systems was 39·1 ± 1·3 beats min−1. It is demonstrated that routine cholinergic tone has probably been underestimated in previous studies on lungfishes, suggesting that withdrawal of vagal tone may provide an important mechanism to increase fH in this group of fishes during, for example, air breathing.

Counterpoint: Cardiac denervation does not play a major role in exercise limitation after heart transplantation

Despite a successful replacement of the failing heart, heart transplant recipients (Htx) generally demonstrate persistent exercise capacity limitations compared with matched sedentary subjects ([4][1], [16][2], [22][3]). Both central (cardiac and pulmonary dysfunctions) and peripheral (endothelial

Heart Rate Dynamics during Shambhavi Mahamudra- A Practice of Isha Yoga

This study was designed to quantify and compare the instantaneous changes of heart rate variability (HRV) during Shambhavi Mahamudra, a practice of Isha yoga. It includes major stages viz. Sukha pranayam, 'AUM' chanting, rapid breathing, and relaxed sitting. The standard lead II electrocardiogram (ECG) signals were acquired from eight healthy, yoga practitioners while performing various stages of Shambhavi Mahamudra. The short-term HRV were extracted from ECG, and analyzed using time, frequency-domain and Poincaré plot methods. The results showed a combination of increased sympathetic tone and withdrawal of vagal tone during Sukha pranayam, an increased sympathetic tone during `AUM' chanting, and withdrawal of vagal tone during rapid breathing. The HRV descriptors were not significantly different during relaxed sitting.

Sex differences in cardiac sympathovagal balance and vagal tone during nocturnal sleep

Objective To analyze sex differences in nocturnal vagal tone and cardiac sympathovagal balance during sleep in healthy men and women. Methods In two groups of healthy, non-obese adults (15 men and 14 women), sleep was polygraphically recorded and heart rate variability was assessed during an awake period prior to sleep and during sleep. Vagal tone was estimated by the high-frequency (HF) power component of heart rate variability, and sympathovagal balance was indexed by the ratio of low-frequency (LF) power to HF power. Results As compared to women, men showed decreases in vagal tone and increases in sympathovagal balance. During rapid eye movement (REM) sleep, a withdrawal of vagal tone occurred with an increase in sympathetic dominance. Men showed a greater increase of sympathovagal balance during REM sleep than women. Secondary analyses covarying for differences in reproductive hormone levels, physical activity, and sleep measures did not alter the results. Conclusions The marked increase in cardiac sympathetic drive during REM sleep in men has implications for understanding sex differences in the risk of cardiovascular events. Additionally, these data offer a pathway to explain the peak in cardiac arrhythmias and sudden cardiac death seen more often in the morning hours.