Phasic Cardiac Reactivity to Psychological Stress as a Function of Aerobic Fitness Level

Cardiovascular adjustments to physical exertion are known to be affected by aerobic fitness level. This raises the possibility that the cardiovascular responses associated with psychological stress may also be related to aerobic fitness. This study was undertaken to determine if phasic cardiac reactivity to stress would be affected by aerobic fitness level, and to make inferences regarding the autonomic mechanisms which might mediate such an effect. Twenty-four males were given a standard physical fitness test and then divided into high and low aerobic fitness level groups. Within two weeks of this assessment, the heart rate and electrocardiographic T-wave amplitude responses of these subjects were monitored during the performance of hard and easy versions of a mental arithmetic task. Analysis of the heart rate data revealed no aerobic fitness level effect. There was, however, greater T-wave amplitude attenuation in the low as compared to the high aerobic fitness level group on hard trials. This pattern of results suggests that psychologically-elicited sympathetic cardiac reactivity is reduced by enhanced aerobic fitness level.     

Aerobic Fitness, Physical Activity, and Psychophysiological Reactions to Mental Tasks

The association between aerobic fitness, exercise, and psychophysiological reactivity was assessed in cross-sectional and prospective analyses. Seventy-five healthy but sedentary adults carried out a sub-maximal exercise test and easy and difficult problem solving tasks. Blood pressure, heart rate, skin conductance level, respiration rate, tidal volume, and oxygen consumption were monitored and additional heart rate was also computed. Differences between relatively fit and unfit individuals were found in respiration rate during tasks and in skin conductance level during post-task recovery periods, with a tendency toward diminished heart rate reactivity in fitter people. Subjects were subsequently allocated to four conditions: high intensity aerobic training, moderate intensity aerobic training, an undemanding strength and flexibility program (designed as an attention-placebo condition), and waiting list control. Training programs were conducted over a 10-week period, and were followed by a second laboratory session. Appropriate changes in aerobic performance over the training period were observed in the 12-min run/walk test. There were no important modifications in psychophysiological stress reactions associated with the different experimental conditions. These results are discussed in relation to the literature concerning the effects of fitness and physical activity on physiological response patterns.     

Association of Aerobic Fitness with Pulse Rate and Subjective Responses to Psychological Stress

Ten high fit and 10 low fit subjects first sat quietly during a baseline period and then participated in a mildly stressful task (recall of digits backwards). Pulse rates and levels of subjective arousal were assessed during the baseline period and during the task performance period. Initial analyses indicated that task performance resulted in general increases in pulse rates, subjective cognitive arousal, and subjective somatic arousal. More importantly, it was found that high fit subjects evinced a smaller pulse rate increase in response to stress than did low fit subjects, but the high and low fit subjects did not differ in their subjective responses to stress. These results are consistent with a growing body of research which indicates that a high level of aerobic fitness is associated with reduced physiological reactivity to psychological stress.     

Failure to alter sympathoadrenal response to psychosocial stress following aerobic training

This experiment assessed the impact of aerobic training on sympathoadrenal, heart rate and subjective responses to psychosocial stress. Subjects were six previously sedentary males who demonstrated marked improvement in fitness level following 10 weeks of training. Plasma samples, heart rate and subjective arousal ratings were obtained prior to, during, and following exposure to stressful mental tasks within a laboratory session. With the exception of training-related bradycardia which was manifest throughout the session, no changes in plasma epinephrine and norepinephrine concentrations, heart rate or subjective reactivity or recovery were seen. These findings are inconsistent with the hypothesized beneficial effects of aerobic training on stress-response. Methodological factors such as self-selection inherent in previous correlation work or the duration of the training program should be considered in this context. The potential contribution of training-related bradycardia to enhanced coping with challenging situations is explored.     

Effects of aerobic conditioning on cardiovascular sympathetic response to and recovery from challenge

Exercise has widely documented cardioprotective effects, but the mechanisms behind these effects are still poorly understood. Here, we test the hypothesis that aerobic training lowers cardiovascular sympathetic responses to and speeds recovery from challenge. We conducted a randomized, controlled trial contrasting aerobic versus strength training on indices of cardiac (pre‐ejection period, PEP) and vascular (low‐frequency blood pressure variability, LF‐BPV) sympathetic responses to and recovery from psychological and orthostatic challenge in 149 young, healthy, sedentary adults. Aerobic and strength training did not alter PEP or LF‐BPV reactivity to or recovery from challenge. These findings, from a large randomized, controlled trial using an intent‐to‐treat design, show that moderate aerobic exercise training has no effect on PEP and LF‐BPV reactivity to or recovery from psychological or orthostatic challenge. In healthy young adults, the cardioprotective effects of exercise training are unlikely to be mediated by changes in sympathetic activity.     

Aerobic Fitness and Opioidergic Inhibition of Cardiovascular Stress Reactivity

The role of endogenous opioids in aerobic fitness-induced decrements in cardiovascular stress reactivity was examined by comparing the effects of opioid antagonism with naltrexone on responses to stress in young adults with high versus low levels of aerobic fitness. Two hundred forty subjects were given an activity questionnaire and males with the highest (Fit) and lowest (Nonfit) aerobic activity profiles were recruited for maximal oxygen consumption (VO2max) treadmill testing and psychological stress testing (final sample N = 28). Heart rate and blood pressures were measured during performance on a computer-controlled arithmetic task after pretreatment with either naltrexone (Trexan, DuPont) or a placebo. During placebo challenges, Fit subjects, compared with Nonfit, showed lower heart rate reactivity during stress and lower mean arterial blood pressures immediately before and during recovery from stress. Naltrexone eliminated these reactivity differences by increasing heart rate reactivity and raising mean arterial blood pressure in Fit subjects. These data suggest that aerobic fitness is associated with enhanced opioidergic inhibition of circulatory stress reactivity. Opioidergic modulatory effects on stress reactivity may comprise an important mechanism in fitness-associated risk reduction for cardiovascular disease.     

Generalization of cardiovascular response: supportive evidence for the reactivity hypothesis.

Cardiovascular responses were monitored while 36 subjects completed a battery of laboratory stressors comprising mental arithmetic, a reaction time task, a speech task, and the forehead cold pressor. Inter-task consistency was assessed for each of 6 physiological parameters for all task pairings. Considerable inter-task consistency for reactivity scores was seen among the psychological stressors for all variables. The question of such consistency between the cold pressor and the psychological tasks was then addressed. The pattern of consistency was not as clear-cut in this case. For systolic blood pressure and pre-ejection period, reactivity scores to the cold pressor did not correlate with those to any of the psychological tasks. In contrast, cardiac output and total peripheral resistance responses showed considerable consistency. The importance of determining the nature of the relationship between psychological and physical stressors is discussed.     

Stress-reactivity in psychosis: evidence for an affective pathway to psychosis

This paper will review a series of studies using the Experience Sampling Method that suggest that altered sensitivity to stress is an endophenotype for psychosis. The Experience Sampling Method is a structured diary technique allowing the assessment of emotional reactivity to stressors occurring in normal daily life. Elevated emotional reactivity to stress was found in subjects vulnerable to psychosis, suggesting that affective responses to stressors in the flow of daily life are an indicator of genetic and/or environmental liability to psychosis. Indeed, the small stressors in daily life associated with affective responses also predict more intense moment-to-moment variation of subtle positive psychotic experiences. Increased emotional reactivity was found to be independent from cognitive impairments, and argued to constitute evidence of an affective pathway to psychosis that may underlie a more episodic, reactive, good-outcome type of psychosis. Evidence for this hypothesis was found in data suggesting that the experience of stressful life events and early trauma were associated with increased stress-sensitivity, and that women were more likely to display elevated stress-reactivity. These findings are discussed in the light of recent biological and psychological mechanisms.     

Cardiac vagal tone indices of temperamental reactivity and behavioral regulation in young children

The relation between two dimensions of vagal tone (V_na), indexed by a baseline measure of V_na and suppression of V_na, and temperamental reactivity and behavioral regulation was investigated. Forty-one children were observed during a series of episodes designed to elicit temperamental reactivity and behavioral regulation. Heart rate was recorded during these baseline, positive, negative, and delay episodes, from which measures of vagal tone were computed. Across the entire sample, vagal tone decreased from the baseline episode to the three affect tasks. Baseline measures of vagal tone were related to the tendency to show a decrease in vagal tone: Children who consistently suppressed vagal tone (showed a decrease to all the affect tasks) had higher baseline vagal tone. Baseline vagal tone was related to temperamental reactivity for the positive and negative tasks, but not the delay tasks. Vagal suppression (vagal difference score) was related to several of the behavioral regulation strategies used by the children in the affect-eliciting situations. These findings are discussed in terms of the adaptive value of physiological regulation in the development of regulatory behaviors that may be critical to social development. ©1997 John Wiley & Sons, Inc. Dev Psychobiol 31: 125–135, 1997     

Paced Respiratory Sinus Arrhythmia as an Index of Cardiac Parasympathetic Tone During Varying Behavioral Tasks

This study addresses a number of unresolved issues regarding the employment of respiratory sinus arrhythmia as an index of tonic parasympathetic cardiac control in psychophysiological investigations. These questions include the following: (1) Does respiratory sinus arrhythmia reflect cardiac vagal tone under conditions in which alterations in parasympathetic control are expected to be mild to moderate? (2) Are variations in human respiratory sinus arrhythmia that occur in response to varying behavioral demands independent of beta-adrenergic effects on the heart? (3) To what extent do typical experimental tasks apparently affect tonic cardiac vagal control? Twelve healthy male subjects were administered a joint alpha- and β-adrenoreceptor pharmacological blocker on one day and a placebo on another (balanced across subjects). On both days, respiratory sinus arrhythmia and heart period were monitored during a number of different experimental tasks while subjects continuously paced their respiration. Results indicated that respiratory sinus arrhythmia, under controlled respiratory conditions, is uninfluenced by variations in sympathetic activity, and provides a reasonably sensitive index of cardiac vagal tone, even when alterations in parasympathetic tone are not large. Furthermore, our findings suggest that cardiac vagal tone is responsive to varying behavioral demands and may interact in different ways with β-adrenergic mechanisms.     

Responses to laboratory psychosocial stress in postpartum women

OBJECTIVE: Lactation has been associated with attenuated hormonal responses to exercise stress in humans. This study was designed to determine the effect of lactation on hypothalamic-pituitary-adrenal axis, autonomic nervous system, and anxiety responses to psychological stress. METHOD: The Trier Social Stress Test was administered to 24 lactating women, 13 postpartum nonlactating women, and 14 healthy control women in the early follicular phase of the menstrual cycle. Lactating women were stressed at least 40 minutes after last feeding their infant. RESULTS: ACTH, cortisol, heart rate, diastolic blood pressure, systolic blood pressure, and subjective anxiety ratings were all significantly increased in response to the psychological stress (all p <.0001). There were no differences among the three groups in any of these responses to the stress. However, postpartum nonlactating women did have a persistently higher systolic blood pressure and lower cardiac vagal tone than the lactating women and control subjects. In addition, the typical negative correlation between cardiac vagal tone and heart rate was consistently higher in lactating women than nonlactating postpartum women and controls, which suggests stronger vagal control of heart rate in lactating women. In addition, there was no change in oxytocin or allopregnanolone in response to the stress, and baseline oxytocin and allopregnanolone levels did not differ among the three groups. CONCLUSIONS: These results indicate that physiological and subjective responses to social stress are not attenuated in lactating women tested at least one hour after feeding their infant. However, enhanced vagal control of cardiac reactivity was observed in lactating women. In addition, postpartum women who did not lactate showed evidence of increased sympathetic and decreased parasympathetic nervous system tone.     

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.     

Hemodynamics of Blood Pressure Responses During Active and Passive Coping

This laboratory study was designed to address a number of interrelated issues regarding cardiovascular reactivity to psychological stress. One objective was to extend the previous research comparing cardiovascular responses during active versus passive coping, by comparing responses to two task conditions designed to be similar in all ways except the opportunity to make a response influencing the task's outcome. A second objective was to compare responses to two different passive film tasks, which differed in outcome uncertainty and the degree of vicarious active coping achieved through identification with the role portrayed by the actors. A third objective was to evaluate whether individuals are predisposed to exhibit a particular hemodynamic pattern underlying their blood pressure adjustments, independently of the task demands imposed. Ninety healthy young adult male subjects were tested in pairs on a series of tasks that included a competitive reaction-time task, an active as well as a passive phase of a team reaction-time task, and passive viewing of two film segments. The tasks demanding active coping responses tended to raise blood pressure due primarily to an increase in cardiac output, while vascular resistance fell. During passive coping demands cardiac output increased to a lesser extent, but vascular resistance also tended to increase, thereby raising blood pressure by their synergistic effects. However, these patterns were not typical of all participating subjects. On the basis of their cardiac output and vascular resistance responses to the competitive reaction-time task, one third of the subjects were categorized as being high myocardial reactors (n = 30) and another third high vascular reactors (n = 31). Post-hoc analyses of responses to the other tasks, based on these categorizations, indicated that the hemodynamic basis of reactivity is an individual characteristic only partially modified by coping demands. The active/passive coping dimension is discussed both conceptually and in relation to the role of stress in the etiology of hypertension.     

Increased finger arterial blood pressure after exercise detraining in women with parental hypertension: autonomic tasks

The effects of exercise detraining on resting finger arterial blood pressure (BP), the carotid-cardiac vagal baroreflex, and BP and heart rate (HR) responses to mental arithmetic and forehead cold exposure were studied in young (19 ± 1.1 years) normotensive women with parental history of hypertension. Following 8 weeks of aerobic exercise for 25 min, 3 days week^?1 at an intensity of 60% V˙O _{2 peak}, subjects ceased training for 6–8 weeks. After detraining, V˙O _{2 peak} (mL kg^?1 min^?1) was reduced by 11.5% (41.1 ± 6.9 to 36.4 ± 4.8) coincident with an ≈ 10 %increase in submaximal exercise heart rate. Responses to the laboratory tasks were then compared. Detraining was accompanied by increases (P <0.05) in resting systolic (SBP) (113.6 ± 8.9 to 121.2 ± 9.0), diastolic (DBP) (63.0 ± 8.4 to 68.3 ± 6.8), and mean arterial (MAP) (78.7 ±8.4 to 84.2 ± 7.3) BP (mmHg). None of the above changes occurred in sedentary matched-control subjects. Systolic blood pressure was elevated during forehead cold exposure and MAP was elevated during mental arithmetic after detraining, but the rates of response and recovery for SBP, DBP and MAP were not altered by detraining. Despite higher submaximal exercise HR after detraining, HR responses to autonomic challenges, including the carotid-cardiac vagal baroreflex, were unchanged between training and detraining. Our results indicate that exercise detraining increases resting finger arterial BP in young normotensive women at risk for hypertension with no effects on the rate of response or recovery of heart rate and BP during autonomic tasks known to elicit sympathetic and carotid-cardiac vagal activities in this population. The use of auscultatory brachial artery pressures in a similar study of women diagnosed with hypertension will clarify the clinical meaning of our findings.     

Respiratory Sinus Arrhythmia and Cardiovascular Responses to Stress

The parasympathetic nervous system provides mechanisms that could attenuate sympathetically mediated heart rate stress responses and might have even more general antagonistic actions on stress reactivity. Individuals characterized by higher levels of parasympathetic tone might, through such mechanisms, be less reactive when stimuli elicit sympathetically mediated responses. Respiratory sinus arrhythmia (RSA) is considered to be a noninvasive index of cardiac parasympathetic (vagal) tone. The present study investigated whether individual differences in RSA level at rest could predict variations among individuals in the magnitude of cardiovascular responses to psychological stress. None of the measures of resting respiratory sinus arrhythmia, derived from spectral analysis of beat-to-beat changes in resting heart rate, predicted the observed variations in cardiovascular task reactivity. However, scores reflecting respiratory sinus arrhythmia as the percentage of total heart rate variability (RSAnorm) were negatively correlated with blood pressure levels, both at rest and during the task. Furthermore, subjects with higher scores for RSAnorm demonstrated a faster adaptation of heart rate responses during stress, which suggests the development of parasympathetic antagonism to ongoing sympathetic arousal. Although a simple relationship between respiratory sinus arrhythmia and reactivity was not observed, these results encourage further investigation of RSA measures as psychophysiological indices of individual differences in parasympathetic (vagal) cardiac tone, or perhaps of general parasympathetic/sympathetic balance, which could modulate the expression of potentially pathogenic stress responses.     

Aerobic Fitness Affects Cardiovascular and Catecholamine Responses to Stressors

Subjects of varying degrees of aerobic fitness were subjected to four laboratory stressors in a test of the hypothesis that aerobic fitness is associated with decreased responsiveness to stressors other than exercise. Blood pressure, heart rate, norepinephrine, epinephrine, and psychological responses to a film of industrial accidents (passive psychological stressor), the Stroop word color task (active psychological stressor), the cold pressor test (passive physical stressor), and running to exhaustion on a treadmill (active physical stressor) were measured. Baseline systolic blood pressure and relative diastolic responses to the film, Stroop task, and exercise were smaller in fit subjects over 40 than in less fit subjects of the same age group. Heart rates were lower in fit subjects at most times, except during and after maximal exercise. Norepinephrine was lower after 9 min of exercise in fit subjects, but was much higher at exhaustion, after these subjects had accomplished more work. Norepinephrine levels fell rapidly and were not different among groups 3 and 10 min after exercise. There was no preferential generalization of the “fitness effect” to the active psychological task.     

Respiratory Sinus Arrhythmia as an Index of Parasympathetic Cardiac Control During Active Coping

Psychophysiological studies concerning tonic cardiac responses to stressor tasks have tended to emphasize beta-adrenergic effects and to neglect parasympathetic influences and sympathetic-parasympathetic interactions. Much recent evidence indicates that both within- and between-individual variations in cardiac parasympathetic heart rate (HR) control can be easily and reliably assessed by measuring the magnitude of respiratory sinus arrhythmia (RSA). Therefore, we used RSA to index cardiac vagal responses to two active coping tasks. Both tasks consisted of the same video game, in one condition with threat of shock for inferior performance, and in the other without such threat. Twenty subjects underwent both tasks (counterbalanced over subjects), plus a preceding resting baseline period. HR and RSA were continuously measured. Results suggested that cardiac parasympathetic activity was diminished from rest to task, contributing to heart rate responses. Exaggerated HR responses were also associated with extreme parasympathetic withdrawal. Furthermore, task condition X sequence of presentation interaction effects showed that threat of shock was particularly effective in elevating HR and reducing RSA when the threat was presented during the first task condition. A repeated-measures analysis of covariance of HR, attempting to remove the effects of parasympathetic influences upon HR, suggested that sympathetic influences upon HR exceeded any reciprocal vagal effects during the threat condition for those subjects exposed to the threat task first. The findings indicate the importance of considering stress-related parasympathetic effects upon HR as well as sympathetic-vagal interactions.     

Domestic violence and vagal reactivity to peer provocation

This paper examined whether individual differences in children's vagal reactivity to peer provocation were related to domestic violence within the family. It also examined the question of whether conduct-problem children who show vagal augmentation to peer provocation come from families with high levels of domestic violence. During the peer provocation, children were expecting to interact with a difficult peer while vagal reactivity was assessed. Groups were divided into children who showed vagal augmentation and vagal suppression to the stressful peer interaction. Findings indicated that conduct-problem children who showed vagal augmentation to interpersonal challenge came from families with the highest levels of domestic violence. Vagal augmentation was also associated with a greater number of conduct-related problems for those children exposed to high levels of domestic violence. Discussion highlights the role of individual differences in physiological reactivity in understanding children's behavior problems in relation to domestic violence.     

P300, N400, aerobic fitness, and maximal aerobic exercise

Electrophysiological effects of aerobic fitness and maximal aerobic exercise were investigated by comparing P300 and N400 before and after a maximal cycling test. Event-related potentials (ERPs) were obtained from 20 students divided into two matched groups defined by their aerobic fitness level (cyclists vs. sedentary subjects). The session of postexercise ERPs was performed immediately after body temperature and heart rate returned to preexercise values. At rest, no significant differences were observed in ERP parameters between cyclists and sedentary subjects. This finding argued against the hypothesis that ERP modifications may be directly assumed by aerobic fitness level. The postexercise session of ERPs showed a significant P300 amplitude increase and a significant P300 latency decrease in all subjects. Similarly, N400 effect increased significantly after the maximal exercise in all subjects. ERP changes were of the same magnitude in the two groups. The present study argues for a general arousing effect of maximal aerobic exercise, independently of the aerobic fitness level.     

Psychophysiological Reactivity in Cardiac Transplant Recipients

To assess the contribution of the heart's autonomic innervation to reactivity to psychological stressors, hemodynamic responsiveness of the denervated human heart was examined in two studies. In Study 1, cardiac output measured by thermodilution. heart rate, and systolic and diastolic blood pressure responses to a 4-min mental arithmetic task were studied in 7 cardiac transplant patients during routine post-transplant cardiac catheterization. In Study II, 6 cardiac transplant patients, 5 normal controls, and 5 renal transplant patients participated in a 78-min psychophysiological stress protocol during which heart rate, systolic and diastolic pressure, and cardiac output (measured noninvasively by impedance cardiography) as well as serum epinephrine and norepinephrine were measured at baseline and while subjects performed mental arithmetic and reaction time tasks. In Study I, transplant patients showed significant increases, relative to baseline, in heart rate, systolic blood pressure, and cardiac output in response to mental arithmetic. The diastolic blood pressure response was marginally significant. In Study II, mental arithmetic produced significant reactivity in systolic blood pressure and marginally significant increases in heart rate and diastolic blood pressure in cardiac transplant patients. Reaction time produced only marginally significant diastolic blood pressure reactivity. Hemodynamic reactivity of the cardiac transplant group generally was lower than that of the two innervated groups, which generally were similar to each other. Although the small number of subjects makes conclusions tentative, these data suggest that: 1) Cardiac transplant patients are capable of significant reactivity to psychological stressors despite the absence of innervation of the heart, and 2) reactivity to these stressors is diminished relative to innervated control subjects. In the absence of cardiac innervation, reactivity is due to the vascular system and cardiac effects mediated by humoral factors.