Heart rate variability during sympatho-excitatory challenges: comparison between spontaneous and metronomic breathing

Controlled breathing and autonomic rhythms: Influence of auditory versus visual cues

We compared standard metrics of autonomic control in 20 humans (10 female) during spontaneous and controlled breathing. Subjects controlled breathing at 0.25 Hz following a metronome (auditory) or scrolling waveforms (visual). Respiratory rates and heart rates were lower during spontaneous breathing compared with auditory and visual. One heart rate variability metric was higher during visual compared with spontaneous breathing, but baroreflex sensitivity and muscle sympathetic nerve activity were not affected by breathing cues. A majority of subjects (86%) perceived that breathing to auditory cues was more difficult compared with visual cues, but this elevated perceived stress did not manifest physiologically.

Worry is associated with robust reductions in heart rate variability: a transdiagnostic study of anxiety psychopathology

BACKGROUND

Individuals with anxiety disorders display reduced resting-state heart rate variability (HRV), although findings have been contradictory and the role of specific symptoms has been less clear. It is possible that HRV reductions may transcend diagnostic categories, consistent with dimensional-trait models of psychopathology. Here we investigated whether anxiety disorders or symptoms of anxiety, stress, worry and depression are more strongly associated with resting-state HRV.

METHODS

Resting-state HRV was calculated in participants with clinical anxiety (n = 25) and healthy controls (n = 58). Symptom severity measures of worry, anxiety, stress, and depression were also collected from participants, regardless of diagnosis.

RESULTS

Participants who fulfilled DSM-IV criteria for an anxiety disorder displayed diminished HRV, a difference at trend level significance (p = .1, Hedges' g = -.37, BF10 = .84). High worriers (Total n = 41; n = 22 diagnosed with an anxiety disorder and n = 19 not meeting criteria for any psychopathology) displayed a robust reduction in resting state HRV relative to low worriers (p = .001, Hedges' g = -.75, BF10 = 28.16).

CONCLUSIONS

The specific symptom of worry - not the diagnosis of an anxiety disorder - was associated with the most robust reductions in HRV, indicating that HRV may provide a transdiagnostic biomarker of worry. These results enhance understanding of the relationship between the cardiac autonomic nervous system and anxiety psychopathology, providing support for dimensional-trait models consistent with the Research Domain Criteria framework.

eNOS gene haplotype is indirectly associated with the recovery of cardiovascular autonomic modulation from exercise

Polymorphisms in the endothelial nitric oxide synthase (eNOS) gene decrease expression and activation of eNOS in vitro, which is associated with lower post-exercise increase in vasodilator reactivity in vivo. However, it is unknown whether such polymorphisms are associated with other eNOS-related phenotypes during recovery from exercise. Therefore, we investigated the impact of an eNOS haplotype containing polymorphic alleles at loci -786 and 894 on the recovery of cardiovascular autonomic function from exercise. Sedentary, non-obese, healthy subjects were enrolled [n = 107, age 32 ± 1 years (mean ± SEM)]. Resting autonomic modulation (heart rate variability, systolic blood pressure variability, and spontaneous baroreflex sensitivity) and vascular reactivity (forearm hyperemic response post-ischemia) were assessed at baseline, 10, 60, and 120 min after a maximal cardiopulmonary exercise test. Besides, autonomic function was assessed by heart rate recovery (HRR) immediately after peak exercise. Haplotype analysis showed that vagal modulation (i.e., HF n.u.) was significantly higher, combined sympathetic and vagal modulation (i.e., LF/HF) was significantly lower and total blood pressure variability was significantly lower post-exercise in a haplotype containing polymorphic alleles (H2) compared to a haplotype with wild type alleles (H1). HRR was similar between groups. Corroborating previous evidence, H2 had significantly lower post-exercise increase in vasodilator reactivity than H1. In conclusion, a haplotype containing polymorphic alleles at loci -786 and 894 had enhanced recovery of autonomic modulation from exercise, along with unchanged HRR, and attenuated vasodilator reactivity. Then, these results suggest an autonomic compensatory response of a direct deleterious effect of eNOS polymorphisms on the vascular function.

Sympathetic dysfunction in vasovagal syncope and the postural orthostatic tachycardia syndrome

Orthostatic intolerance is the inability to tolerate the upright posture and is relieved by recumbence. It most commonly affects young women and has a major impact on quality of life and psychosocial well-being. Several forms of orthostatic intolerance have been described. The most common one is the recurrent vasovagal syncope (VVS) phenotype which presents as a transient and abrupt loss of consciousness and postural tone that is followed by rapid recovery. Another common type of orthostatic intolerance is the postural orthostatic tachycardia syndrome (POTS) which is characterized by an excessive rise in heart rate upon standing and is associated with symptoms of presyncope such as light-headedness, fatigue, palpitations, and nausea. Maintenance of arterial pressure under condition of reduced central blood volume during the orthostasis is accomplished in large part through sympathetic efferent nerve traffic to the peripheral vasculature. Therefore sympathetic nervous system (SNS) dysfunction is high on the list of possible contributors to the pathophysiology of orthostatic intolerance. Investigations into the role of the SNS in orthostatic intolerance have yielded mixed results. This review outlines the current knowledge of the function of the SNS in both VVS and POTS.

Endothelial nitric oxide synthase polymorphisms and adaptation of parasympathetic modulation to exercise training

PURPOSE

There is a large interindividual variation in the parasympathetic adaptation induced by aerobic exercise training, which may be partially attributed to genetic polymorphisms. Therefore, we investigated the association among three polymorphisms in the endothelial nitric oxide gene (-786T>C, 4b4a, and 894G>T), analyzed individually and as haplotypes, and the parasympathetic adaptation induced by exercise training.

METHODS

Eighty healthy males, age 20-35 yr, were genotyped by polymerase chain reaction-restriction fragment length polymorphism analysis, and haplotypes were inferred using the software PHASE 2.1. Autonomic modulation (i.e., HR variability and spontaneous baroreflex sensitivity) and peak oxygen consumption (VO(2peak)) were measured before and after training (running, moderate to severe intensity, three times per week, 60 min·day(-1), during 18 wk).

RESULTS

Training increased VO(2peak) (P < 0.05) and decreased mean arterial pressure (P < 0.05) in the whole sample. Subjects with the -786C polymorphic allele had a significant reduction in baroreflex sensitivity after training (change: wild type (-786TT) = 2% ± 89% vs polymorphic (-786TC/CC) = -28% ± 60%, median ± quartile range, P = 0.03), and parasympathetic modulation was marginally reduced in subjects with the 894T polymorphic allele (change: wild type (894GG) = 8% ± 67% vs polymorphic (894GT/TT) = -18% ± 59%, median ± quartile range, P = 0.06). Furthermore, parasympathetic modulation percent change was different between the haplotypes containing wild-type alleles (-786T/4b/894G) and polymorphic alleles at positions -786 and 894 (-786C/4b/894T) (-6% ± 56% vs -41% ± 50%, median ± quartile range, P = 0.04).

CONCLUSIONS

The polymorphic allele at position -786 and the haplotype containing polymorphic alleles at positions -786 and 894 in the endothelial nitric oxide gene were associated with decreased parasympathetic modulation after exercise training.

Blunted vagal reactivity predicts stress-precipitated tobacco smoking

RATIONALE

Long-term smoking can lead to changes in autonomic function, including decreased vagal tone and altered stress responses. One index of the inability to adapt to stress may be blunted vagal reactivity. Stress is a primary mechanism involved in relapse to smoking, but mechanisms leading to stress-precipitated relapse are not well understood.

OBJECTIVES

Using an experimental paradigm of stress-precipitated smoking behavior, we examined whether autonomic reactivity mediates the relationship between stress and smoking. High-frequency heart rate variability (HF-HRV), a putative measure of vagal tone, and the ratio of low-to-high frequency HRV (LF/HF), a measure of sympathovagal balance, were assessed.

METHODS

Using a within-subjects design, 32 nicotine-dependent, 15-h abstinent smokers (a subgroup from McKee et al. (J Psychopharmacol 25(4):490-502, 2011)) were exposed to individualized script-driven imagery of stressful and relaxing scenarios and assessed on the ability to resist smoking and subsequent ad-lib smoking. HRV was monitored throughout each laboratory session (maximum 60 min following imagery).

RESULTS

As expected, stress and ad-lib smoking additively decreased HF-HRV and increased LF/HF. Blunted stress-induced HF-HRV responses reflecting decreased vagal reactivity were associated with less time to initiate smoking and increased craving relief and reinforcement from smoking. These relationships were specific to HF-HRV following stress as neither baseline HF-HRV, HF-HRV following relaxing imagery, or LF/HF predicted smoking behavior.

CONCLUSIONS

The current findings are the first to experimentally demonstrate that stress-precipitated decreased vagal reactivity predicts the ability to resist smoking. Findings suggest that strategies that normalize vagal reactivity in early abstinent smokers may lead to improved smoking cessation outcomes.

Cardiac vagal control and respiratory sinus arrhythmia during hypercapnia in humans

Normoxic hypercapnia may increase high-frequency (HF) power in heart rate variability (HRV) and also increase respiratory sinus arrhythmia (RSA). Low-frequency (LF) power may remain unchanged. In this study, 5-min ECG recordings (N = 10) were analyzed in time and frequency domains while human subjects breathed normoxic 5% CO2 (5%CO2) or room air (RA). Tidal volume (VT), inhalatory (TI), and exhalatory (TE) times of breaths in the final minute were measured. ECG time domain measures were unaffected by CO2 inhalation (P > 0.05). Following natural logarithmic transformation (LN), LFLN was unaltered (RA: 7.14 +/- 0.95 vs. 5%CO2: 7.35 +/- 1.12, P > 0.05), and HFLN increased (RA: 7.65 +/- 1.37 vs. 5%CO2: 8.58 +/- 1.11, P < 0.05) with CO2 inhalation. When changes in total power (NU) were corrected, LF(NU) decreased (RA: 34.4 +/- 22.9 vs. 5%CO2: 23.8 +/- 23.1, P < 0.01), and HFNU increased (RA: 56.5 +/- 22.3 vs. 5%CO2: 66.8 +/- 22.9, P < 0.01) with CO2 inhalation. TI (RA: 2.0 +/- 1.0 vs. 5%CO2: 1.9 +/- 0.8 s) and TE (RA: 2.5 +/- 1.1 vs. 5%CO2: 2.4 +/- 0.9 s) remained unchanged, but VT increased with CO2 inhalation (RA: 1.1 +/- 0.3 vs. 5%CO2: 2.0 +/- 0.8 L, P < 0.001). Heart rates during inhalation (RA: 35.2 +/- 4.4, 5%CO2: 34.5 +/- 4.8 beats min(-1)) were different from heart rates during exhalation (RA: 28.8 +/- 4.4, 5%CO2: 29.1 +/- 3.1 beats min(-1)). Hypercapnia did not increase the clustering of heart beats during inhalation, and we suggest that the HF component may not adequately reflect RSA.

Sample entropy of electrocardiographic RR and QT time-series data during rest and exercise

Sample entropy (SampEn) is a measure of the complexity of data. Few studies have compared the SampEn of electrocardiographic cardiac interval (RR) data (SampEn-RR) during differing physiological states, and none have examined SampEn for the corresponding QT interval (SampEn-QT). The aim of this study was to quantify SampEn-RR and SampEn-QT during rest and for a range of exercise workloads. Specific objectives were to assess both the utility of SampEn for discriminating between physiological states and the relationship of SampEn-RR with traditional measures of heart rate variability (HRV). Twelve males of similar age, mass and aerobic fitness participated. A three-lead ECG was recorded continuously during pre-exercise, progressive bicycle exercise and recovery, and beat-to-beat RR and QT intervals were quantified for sinus beats. SampEn and HRV were calculated within consecutive 1 min periods throughout. Consistent estimation of SampEn-RR and SampEn-QT was possible with an appropriate choice of SampEn parameters. SampEn-RR was sensitive to differing physiological conditions, but its discriminating ability was poorer than that of linear HRV indices. SampEn-RR was also negatively correlated with normalized LF and LF/HF parameters. We interpret changes in SampEn for RR and QT data in terms of the altered autonomic nervous system (ANS) control of either the atrial or the ventricular myocardium (or both) during discrete physiological states. We speculate that greater complexity in QT data might be explained by a direct ANS influence on the ventricular myocardium.

Toward understanding respiratory sinus arrhythmia: relations to cardiac vagal tone, evolution and biobehavioral functions

Respiratory sinus arrhythmia (RSA, or high-frequency heart-rate variability) is frequently employed as an index of cardiac vagal tone or even believed to be a direct measure of vagal tone. However, there are many significant caveats regarding vagal tone interpretation: 1. Respiratory parameters can confound relations between RSA and cardiac vagal tone.2. Although intraindividual relations between RSA and cardiac vagal control are often strong, interindividual associations may be modest.3. RSA measurement is profoundly influenced by concurrent levels of momentary physical activity, which can bias estimation of individual differences in vagal tone.4. RSA magnitude is affected by beta-adrenergic tone.5. RSA and cardiac vagal tone can dissociate under certain circumstances.6. The polyvagal theory contains evolution-based speculations that relate RSA, vagal tone and behavioral phenomena. We present evidence that the polyvagal theory does not accurately depict evolution of vagal control of heart-rate variability, and that it ignores the phenomenon of cardiac aliasing and disregards the evolution of a functional role for vagal control of the heart, from cardiorespiratory synchrony in fish to RSA in mammals. Unawareness of these issues can lead to misinterpretation of cardiovascular autonomic mechanisms. On the other hand, RSA has been shown to often provide a reasonable reflection of cardiac vagal tone when the above-mentioned complexities are considered. Finally, a recent hypothesis is expanded upon, in which RSA plays a primary role in regulation of energy exchange by means of synchronizing respiratory and cardiovascular processes during metabolic and behavioral change.

Heart rate variability, blood pressure variability, and baroreflex sensitivity in overtrained athletes

OBJECTIVE

To assess the effects of abruptly intensified physical training on cardiovascular control.

DESIGN

Retrospective longitudinal study.

SETTING

Research laboratory.

PARTICIPANTS

Ten healthy athletes (5 men and 5 women) from track and field as well as triathlon.

INTERVENTIONS

A 2-week training camp, including daily stepwise increasing cycling tests, running of 40 minutes, and additional cycling of 60 minutes.

MAIN OUTCOME MEASUREMENTS

Time and frequency domain parameters of resting heart rate and blood pressure variability (HRV and BPV) and baroreflex sensitivity (BRS), before, during, and after the training camp.

RESULTS

We found significantly reduced HRV during the training camp (mean beat-to-beat interval: 1042 [937 to 1194] ms vs. 933 [832 to 1103] ms vs. 1055 [947 to 1183] ms, P < 0.01; root-mean-square of beat-to-beat interval differences: 68 [52 to 95] ms vs. 52 [38 to 71] ms vs. 61 [48 to 78] ms, P < 0.05). Further, BRS was significantly reduced: 25.2 (20.4 to 40.4) ms/mmHg vs. 17.0 (12.9 to 25.7) ms/mmHg vs. 25.7 (18.8 to 29.1) ms/mmHg, P < 0.05. These effects disappeared at a large degree after 3 to 4 days of recovery.

CONCLUSION

Abruptly intensified physical training results in an altered autonomic cardiovascular activity towards parasympathetic inhibition and sympathetic activation that can be monitored by means of HRV and BRS analyses and might provide useful markers to avoid the overtraining syndrome.

Is slow wave sleep an appropriate recording condition for heart rate variability analysis?

Heart rate variability (HRV) analysis holds increasing interest but electrocardiographic (ECG) recordings are strongly disturbed by body movements, changes in environment and respiration. Here we give arguments for the use of slow wave sleep (SWS) as an appropriate recording condition. Sixteen healthy subjects aged 21-31 years (10 males, 6 females) underwent polygraphic sleep, ECG, and respiratory recordings during one experimental night. HRV was analyzed in 5-min SWS segments and compared to data collected during quiet wake in the morning with controlled breathing, using for each individual the same respiratory frequency as that recorded during SWS. SWS has two major advantages. First, it is a quiet sleep period, free of any external confounding events and is characterized by fewer body movements or arousals that cause abrupt heart rate (HR) increases which disrupt the ECG signal. Second, SWS avoids the deleterious effect of controlled breathing on HRV. Respiratory cycles were spontaneously more regular during SWS than during generally used wake (Standard deviation (SD) of the respiratory cycles was 0.27+/-0.02 s during SWS vs 0.42+/-0.07 s during wake under controlled breathing; p<0.01). Compared to quiet wake, the SD of normal R-R intervals reflecting global variability was significantly lower during SWS (54.3+/-4.7 vs 78.8+/-6.1 ms; p<0.001) and the normalized high frequency power was increased (0.57+/-0.04 vs 0.51+/-0.03; p<0.05), suggesting a higher parasympathetic control of the heart. Thus, SWS offers a "self-controlled" and undisturbed moment of observation for assessing time and frequency domain HRV indexes. Its relevance as an optimal ECG recording condition has to be confirmed in various experimental conditions.

Analysis of rapid heart rate variability in the assessment of anticholinergic drug effects in humans

Anticholinergic agents have widespread therapeutic indications in clinical medicine. In addition, certain other drug groups-such as neuroleptics, antidepressants and antihistamines-possess distinct anticholinergic properties that reduce tolerance and compliance. Especially in patients with heart disease, attention should be paid to cardiac anticholinergic drug effects. The analysis of short-term heart rate variability (HRV) provides a noninvasive tool to estimate vagal cholinergic outflow. In this review article, we present the basic principles of the most relevant techniques to study rapid HRV: the time domain analysis methods RMSSD and pNN50, and the high-frequency (HF) spectral component of HRV. We provide examples of previously reported effects of anticholinergic agents on these measures and also describe how adrenergic drugs may influence them. We have the following recommendations for a clinical pharmacologist investigating anticholinergic agents. (1) If the breathing rate of the study subject can be controlled during the assessment and the electrocardiogram recordings contain good-quality, stationary segments that are at least a few minutes long, then the HF power of HRV should be the method of choice. (2) During uncontrolled conditions, RMSSD should be included in the analyses, because it is less affected by changes in the respiratory pattern and it can be measured from shorter segments of electrocardiogram data. (3) Reduced short-term HRV suggests an anticholinergic, but not necessarily an antimuscarinic drug effect, since the inhibition of cholinergic vagal efferent activity may also originate from central or peripheral adrenergic influences.

Two-rescuer CPR results in hyperventilation in the ventilating rescuer

The "Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care--International Consensus on Science" recommend a tidal ventilation volume of 10 ml/kg body-weight without the use of supplemental oxygen during two-rescuer adult cardiopulmonary resuscitation (CPR). This relates to a ventilation volume of about 6.4 l/min. Additionally, the first aid provider ventilating the victim will breathe for him/herself during the external chest compression period adding another 3.2 l/min of ventilation. Finally, a deep breath is recommended before each ventilation to increase the end-expiratory oxygen concentration of the air exhaled. To investigate the effects of these recommendations, 20 healthy volunteers were asked to perform two-rescuer CPR in a lung model connected to a BLS-manikin. End-tidal carbon dioxide, oxygen saturation, and heart rate were recorded continuously. Capillary blood gas samples were collected and non-invasive blood pressure was recorded prior to the start of external chest compressions and immediately after the end of each measurement period. Furthermore, hyperventilation related symptoms reported by the volunteers were also recorded. The data reveal a significant decrease in capillary and end-tidal carbon dioxide pressure in the volunteers (P < 0.001). Additionally, in 75% of test persons multiple hyperventilation associated symptoms occurred. Ventilation during two-rescuer CPR performed according to the Guidelines 2000 may cause injury to the health of first aid providers. To minimize hyperventilation, both rescuers should exchange their positions at intervals of 3-5 min. These data challenge the recommendation to take a deep breath prior to each ventilation.

Relation between physical exertion and heart rate variability characteristics in professional cyclists during the Tour of Spain

BACKGROUND

Continued exposure to prolonged periods of intense exercise may unfavourably alter neuroendocrine, neuromuscular, and cardiovascular function.

OBJECTIVE

To examine the relation between quantifiable levels of exertion (TRIMPS) and resting heart rate (HR) and resting supine heart rate variability (HRV) in professional cyclists during a three week stage race.

METHOD

Eight professional male cyclists (mean (SEM) age 27 (1) years, body mass 65.5 (2.3) kg, and maximum rate of oxygen consumption (VO(2)max) 75.6 (2.2) ml/kg/min) riding in the 2001 Vuelta a España were examined for resting HR and HRV on the mornings of day 0 (baseline), day 10 (first rest day), and day 17 (second rest day). The rest days followed stages 1-9 and 10-15 respectively. HR was recorded during each race stage, and total HR time was categorised into a modified, three phase TRIMPS schema. These phases were based on standardised physiological laboratory values obtained during previous VO(2)max testing, where HR time in each phase (phase I = light intensity and less than ventilatory threshold (VT; approximately 70% VO(2)max); phase II = moderate intensity between VT and respiratory compensation point (RCP; approximately 90% VO(2)max); phase III = high intensity (>RCP)) was multiplied by exertional factors of 1, 2, and 3 respectively.

RESULTS

Multivariate analysis of variance showed that total TRIMPS for race stages 1-9 (2466 (90)) were greater than for stages 10-15 (2055 (65)) (p<0.0002). However, TRIMPS/day were less for stages 1-9 (274 (10)) than for stages 10-15 (343 (11)) (p<0.01). Despite a trend to decline, no difference in supine resting HR was found between day 0 (53.2 (1.8) beats/min), day 10 (49.0 (2.8) beats/min), and day 17 (48.0 (2.6) beats/min) (p = 0.21). Whereas no significant group mean changes in HR or HRV indices were noted during the course of the race, significant inverse Pearson product-moment correlations were observed between all HRV indices relative to total TRIMPS and TRIMPS/day accumulated in race stages 10-15. Total TRIMPS correlated with square root of mean squared differences of successive RR intervals (r = -0.93; p<0.001), standard deviation of the RR intervals (r = -0.94; p<0.001), log normalised total power (r = -0.97; p<0.001), log normalised low frequency power (r = -0.79; p<0.02), and log normalised high frequency power (r = -0.94; p<0.001).

CONCLUSION

HRV may be strongly affected by chronic exposure to heavy exertion. Training volume and intensity are necessary to delineate the degree of these alterations.

Orthostatic modification of ventilatory dynamic response to carbon dioxide perturbations

In order to determine whether changes in ventilatory control contribute to the observed decrease in arterial partial pressure of carbon dioxide (PaCO(2)) during head up tilt, we assessed ventilatory dynamic sensitivity to changes in PaCO(2) during supine and 70 degrees passive head up tilt. In 24 adult normals, we stimulated the ventilatory control system by switching inspired CO(2) between room air and room air+5% CO(2) in a pseudo random binary sequence. A Box-Jenkins model was used to compute ventilatory response to CO(2). Airflow, CO(2), non-invasive beat by beat blood pressure, ECG and cerebral blood flow velocity (Doppler) were recorded. During tilt, sensitivity of the ventilatory controller to CO(2) disturbance increased (from 0.45 to 0.72 L/min/mm Hg, p<0.005); minute ventilation increased (7.63 to 8.47 L/min, p<0.01), end tidal CO(2) (ETCO(2)), cerebral blood flow velocity (CBF) and baroreflex sensitivity decreased (46.9 to 42.9 mm Hg, p<0.001; 84.9 to 72.9 cm/s, p<0.001; and 17.6 to 5.5 ms/mm Hg, p<0.001). The primary observation from our study was that the sensitivity of ventilatory control system to perturbations in ETCO(2) increased during tilt. Taken together with decrease in mean levels of ETCO(2) and an increase in minute ventilation, these results suggest that during tilt, a change in the regulated level or 'set point' of PaCO(2) may occur.

Dynamic cardiorespiratory interaction during head-up tilt-mediated presyncope

In 28 healthy adults, we compared the dynamic interaction between respiration and cerebral autoregulation in 2 groups of subjects: those who did and did not develop presyncopal symptoms during 70 degrees passive head-up tilt (HUT), i.e., nonpresyncopal (23 subjects) and presyncopal (5 subjects). Airflow, CO2, cerebral blood flow velocity (CBF), ECG, and blood pressure (BP) were recorded. To determine whether influences of mean BP (MBP) and systolic SP (SBP) on CBF were altered in presyncopal subjects, coherencies and transfer functions between these variables and mean and peak CBF (CBFm and CBFp) were estimated. To determine the influence of end-tidal CO2 (ETco2) on CBF, the relative CO2 reactivity (%change in CBFm per mmHg change in ETco2) was calculated. We found that in presyncopal subjects before symptoms during HUT, coherence between SBP and CBFp was higher (P=0.02) and gains of transfer functions between BP (MBP and SBP) and CBFm were larger (MBP, P=0.01; SBP, P=0.01) in the respiratory frequency region. In the last 3 min before presyncope, presyncopals had a reduced relative CO2 reactivity (P=0.005), likely a consequence of the larger decrease in ETco2. We hypothesize that the CO2-mediated increase in resistance attenuates autoregulation such that the relationship between systemic and cerebral hemodynamics is enhanced. Our results suggest that an altered cardiorespiratory interaction involving cerebral hemodynamics may contribute in the cascade of events during tilt that culminate in unexplained syncope.

Individual differences in respiratory sinus arrhythmia

To investigate the interindividual differences in respiratory sinus arrhythmia (RSA), recordings of ventilation and electrocardiogram were obtained from 12 healthy subjects for five imposed breathing periods ( TTOT) surrounding each individual's spontaneous breathing period. In addition to the spectral analysis of the R-R interval signal at each breathing period, RSA characteristics were quantified by using a breath-by-breath analysis where a sinusoid was fitted to the changes in instantaneous heart rate in each breath. The amplitude and phase (or delay = phase × TTOT) of this sinusoid were taken as the RSA characteristics for each breath. It was found that for each subject the RSA amplitude- TTOT relationship was linear, whereas the delay- TTOT relationship was parabolic. However, the parameters of these relationships differed between individuals. Linear correlation between the slopes of RSA amplitude versus TTOT regression lines and 1) mean breathing period and 2) mean R-R interval during spontaneous breathing were calculated. Only the correlation coefficient with breathing period was significantly different from zero, indicating that the longer the spontaneous breathing period the lesser the increase in RSA amplitude with increasing breathing period. Similarly, only the correlation coefficient between the curvature of the RSA delay- TTOT parabola and mean breathing period was significantly different from zero; the longer the spontaneous breathing period the larger the curvature of RSA delay. These results suggest that the changes in RSA characteristics induced by changing the breathing period may be explained partly by the spontaneous breathing period of each individual. Furthermore, a transfer function analysis performed on these data suggested interindividual differences in the autonomic modulation of the heart rate.

Influence of Intensive Cycling Training on Heart Rate Variability During Rest and Exercise

The current study examined whether changes in heart rate variability (HRV) following intensive cycling training contribute to the mechanism of training-induced bradycardia. Thirteen healthy untrained subjects, ages 18-27 years, underwent recordings of heart rate (HR) and [Formula: see text] before and after 8 weeks of cycling, 25-60 min/day, 5 days/week at > 80% maximum HR (HRmax). Heart rate recordings were obtained during supine rest and submaximal exercise and were analysed for the following components of HRV: low frequency (LF, 0.041-0.15 Hz); high frequency (HF, 0.15-0.40 Hz); LF/HF ratio and total power (TP, 0-0.40 Hz). At posttraining, VO2max was significantly increased while HR was significantly reduced at rest and all absolute exercise work rates. Training-induced lower HR was accompanied by significantly greater HF and TP during rest as well as LF, HF, and TP during all absolute exercise work rates. Posttraining HR and the majority of HRV measures were similar to pretraining values at the same relative exercise intensity (% HRmax). These results indicated that 8 weeks of intensive cycling training increased HRV and cardiac vagal modulation during rest and absolute exercise work rates but had little effect during relative exercise work rates. Increased vagal modulation resulting from intensive exercise training may contribute to the mechanism of training-induced bradycardia. Key words: exercise training, autonomic nervous system, bradycardia, vagal modulation

Influence of age and moderate-intensity exercise training on heart rate variability in young and mature adults

The purpose of this study was to examine the influence of age and moderate-intensity exercise training on heart rate variability (HRV), and to elucidate further the mechanism of training-induced bradycardia and cardioprotection. Electrocardiograms were recorded from 12 young (18-24 yrs) and 12 mature (29-43 yrs) individuals during supine rest and submaximal moderate exercise. Recordings were obtained prior to, midway, and following 16 weeks of aerobic exercise training designed to improve cardiorespiratory fitness and health. Training resulted in augmented estimated VO2max and bradycardia during rest and submaximal exercise. Total and low frequency components of HRV during exercise were significantly increased for the mature subjects following training whereas other measures of HRV were not significantly changed for either group. It was concluded that training of moderate intensity was insufficient to induce changes in the autonomic control of heart rate for young to mature subjects. The lack of significant HRV changes may suggest the existence of a vagal critical point, below which training-induced increases in vagal modulation may be forthcoming, and above which changes in vagal modulation may be negligible. Training-induced bradycardia and the cardioprotective effect of regular aerobic exercise may result from factors other than an increased vagal modulation.