Kinetics of heart rate and catecholamines during exercise in humans

Summary To elucidate the role of factors other than the nervous system in heart rate (f _c) control during exercise, the kinetics off _c and plasma catecholamine concentrations were studied in ten heart transplant recipients during and after 10-min cycle ergometer exercise at 50 W. Thef _c did not increase at the beginning of the exercise for about 60 s. Then in the eight subjects who completed the exercise it increased following an exponential kinetic with a mean time constant of 210 (SEM 22) s. The two other subjects were exhausted after 5 and 8 min of exercise during whichf _c increased linearly. At the cessation of the exercise,f _c remained unchanged for about 50 s and then decreased exponentially with a time constant which was unchanged from that at the beginning of exercise. In the group of eight subjects plasma noradrenaline concentration ([NA]) increased after 30 s to a mean value above resting of 547 (SEM 124) pg · ml^–1, showing a tendency to a plateau, while adrenaline concentration ([A]) did not increase significantly. In the two subjects who became exhausted an almost linear increase in [NA] occurred up to about 1,300 pg · ml^–1 coupled with a significant increase in [A]. During recovery an immediate decrease in [NA] was observed towards resting values. The values of thef _c increase above resting levels determined at the time of blood collection were linearly related with [NA] increments both at the beginning and end of exercise with a similar slope, i.e. about 2.5 beats · min^–1 per 100 pg · ml^–1 of [NA] change. These findings would seem to suggest that in the absence of heart innervation the increase inf _c depends on plasma [NA].     

Changes in human heart rate during sleep following daily physical exercise

Summary Four fit, healthy young men (aged 20) volunteered for the experiment. After a 5 day control period, they marched for 6 consecutive days (from 09:00 h to 17:00 h) for 34 km/day at a speed of 6 km/h, with an energy expenditure of 35% of individual max. A recovery period of 5 consecutive days began immediately after the exercise period. Sleep records and electrocardiograms were taken every night during the three periods from 22:00 h to 06:00 h.During the exercise period the night time heart rates increased by about 10%, compared to the previous control condition, and returned to normal during the recovery period.The relation between heart rate changes and sleep stages remained identical throughout the three experimental periods. Three subjects showed an increase in heart rate during paradoxical sleep, compared to the preceding slow wave sleep, while one subject experienced the reverse.The tonic increases in heart rate are discussed in relation to changes in body temperature, sleep patterns, blood composition and hormonal status induced by the physical exercise performed.Supported by grant N 77/1198 from the Direction des Recherches et Etudes Techniques (Délégation Ministérielle pour l'Armement)     

Changes in the leucocyte count during and after brief intense exercise

Summary Twelve healthy male volunteers exercised at 200 W on a cycle ergometer for 8 min or until exhausted, if sooner. Retrospectively, subjects fell into two groups. During the last minute of exercise at 200 W, those in group 1 (n = 5) had a mean respiratory exchange ratio (R) of 1.06 (SD 0.01) and were working at a mean of 79% (SD 4%) of their maximum oxygen consumption ( ) as measured in a separate incremental load test. For subjects in group 2 (n =7),R was 1.31 (SD 0.08) and their was maximal (mean 101%. SD 3% . Plasma lactate, and adrenaline concentrations rose to higher levels during exercise in subjects in group 2 than in those in group 1. At the finish of exercise, the leucocyte count and the plasma lactate concentration immediately began to fall in subjects in group 1 whereas in group 2 subjects both rose for several minutes before falling. Plasma catecholamine concentrations fell rapidly in both groups during recovery.A preliminary account of this work was presented to a meeting of the Physiological Society (March 1991, London, UK).     

Heart rate dynamics during a treadmill cardiopulmonary exercise test in optimized beta-blocked heart failure patients

BACKGROUND: Calculating the maximum heart rate for age is one method to characterize the maximum effort of an individual. Although this method is commonly used, little is known about heart rate dynamics in optimized beta-blocked heart failure patients. AIM: The aim of this study was to evaluate heart rate dynamics (basal, peak and % heart rate increase) in optimized beta-blocked heart failure patients compared to sedentary, normal individuals (controls) during a treadmill cardiopulmonary exercise test. METHODS: Twenty-five heart failure patients (49+/-11 years, 76% male), with an average LVEF of 30+/-7%, and fourteen controls were included in the study. Patients with atrial fibrillation, a pacemaker or noncardiovascular functional limitations or whose drug therapy was not optimized were excluded. Optimization was considered to be 50 mg/day or more of carvedilol, with a basal heart rate between 50 to 60 bpm that was maintained for 3 months. RESULTS: Basal heart rate was lower in heart failure patients (57+/-3 bpm) compared to controls (89+/-14 bpm; p<0.0001). Similarly, the peak heart rate (% maximum predicted for age) was lower in HF patients (65.4+/-11.1%) compared to controls (98.6+/-2.2; p<0.0001). Maximum respiratory exchange ratio did not differ between the groups (1.2+/-0.5 for controls and 1.15+/-1 for heart failure patients; p=0.42). All controls reached the maximum heart rate for their age, while no patients in the heart failure group reached the maximum. Moreover, the % increase of heart rate from rest to peak exercise between heart failure (48+/-9%) and control (53+/-8%) was not different (p=0.157). CONCLUSION: No patient in the heart failure group reached the maximum heart rate for their age during a treadmill cardiopulmonary exercise test, despite the fact that the percentage increase of heart rate was similar to sedentary normal subjects. A heart rate increase in optimized beta-blocked heart failure patients during cardiopulmonary exercise test over 65% of the maximum age-adjusted value should be considered an effort near the maximum. This information may be useful in rehabilitation programs and ischemic tests, although further studies are required.     

Aerobic exercise intensity and time of stressor administration influence cardiovascular responses to psychological stress

This study examined cardiovascular responses as a function of time following exercise in which participants were exposed to a laboratory stressor. Ninety (42 women) young (18-35 years old) nonsmoking normotensive participants engaged in 30 min of high and low intensity (75-80% and 50-55% VO(2) max) aerobic exercise and a sedentary control condition. Participants were randomly assigned to a laboratory stressor 5, 30, or 60 min following the exercise bout. Results indicate that low and high intensity exercise significantly reduce heart rate (HR) and systolic and diastolic blood pressure reactivity and HR recovery values. An inverse relationship between intensity of exercise and subsequent cardiovascular reactivity was found. These findings suggest attenuated stress responses following acute exercise depend both on exercise intensity and the time of exposure to psychological stress following exercise.     

8-OH-DPAT prevents cardiac arrhythmias and attenuates tachycardia during social stress in rats

The aim of this study was to apply a behavioural stress paradigm for studying the neural mechanisms underlying stress-induced arrhythmias, and to test whether such arrhythmias could be suppressed by systemic administration of 8-OH-DPAT, a 5-HT1A agonist possessing central sympatholytic properties. The study was conducted on adult male rats instrumented for telemetric recordings of ECG, body temperature and locomotor activity. In the first experiment, rats were subjected to social defeat after either 8-OH-DPAT (100 µg/kg s.c.) or vehicle injection. In the second experiment, prior to vehicle/8-OH-DPAT administration, animals were pre-treated with zatebradine, a blocker of the pacemaker current. 8-OH-DPAT caused prolongation of basal RR interval, increase in locomotion and hypothermia. Subjecting vehicle-treated animals to social defeat caused shortening in RR interval, increase in locomotor activity and hyperthermia, and provoked the occurrence of premature ventricular and supraventricular beats; all these effects were substantially attenuated by 8-OH-DPAT. Zatebradine caused prolongation of RR interval. In zatebradine/vehicle-treated rats, the incidence of ventricular and supraventricular premature beats during defeat increased 2.5-fold and 3.5-fold, respectively. 8-OH-DPAT administered after zatebradine significantly reduced these stress-induced arrhythmias. We conclude that: i) pharmacologically induced prolongation of RR interval may contribute to an increased susceptibility to stress-induced cardiac arrhythmias, possibly due to the prolongation of the ventricular diastolic period with restored excitability; and ii) systemic administration of 8-OH-DPAT abolishes these arrhythmic events, likely by suppressing stress-induced cardiac sympathetic outflow.     

Cardiovascular activity during mental stress following vigorous exercise in sportsmen and inactive men

Thirty-six competitive sportsmen and 36 inactive men participated in a two-session experiment. Session 1 involved exercise to exhaustion so as to assess maximal oxygen consumption (V?o_{2 max}. In Session 2, both groups were randomized into three experimental conditions: 20 min of exercise at high intensity (70%V?o_{2 max}) or moderate intensity (50% V˙o_{2 max}) or a light exercise control. Following 30 min of recovery, all subjects performed mental arithmetic and public speech tasks in a counterbalanced order. Cardiovascular, electrodermal, respiratory, and subjective variables were recorded. Sportsmen had higher V˙o_{2 max}, lower body fat, and lower resting heart rate (HR) than inactive men. A postexercise hypotensive response was observed among subjects in the 70% and 50% V˙o_{2 max} conditions, accompanied by baroreceptor reflex inhibition in the 70% condition. Systolic pressure was lower during mental arithmetic and during recovery from the speech task in the high-intensity than in the control group. Diastolic pressure was lower following mental arithmetic in the high-intensity group. No differences in HR reactivity, electrodermal, or respiratory parameters were observed, but baroreceptor reflex sensitivity was inhibited during mental arithmetic. The results are discussed in relation to previous reports of suppressed cardiovascular reactivity to mental stress tests following vigorous exercise and the role of stress-related processes in the antihypertensive response to physical training.     

Sex-related differences in free plasma catecholamines in individuals of similar performance ability during graded ergometric exercise

Summary Sex-related differences of catecholamine responses were evaluated in nine healthy women and six age-matched men at rest and during incremental treadmill erxercise. Heart rate, oxygen uptake ( ), glucose and lactate blood levels as well as the free plasma catecholamines, noradrenaline and adrenaline, were determined. No significant differences were observed for these parameters between the two groups at rest. The females had relative and maximal running velocities similar to the males, which points to a comparable dynamic performance ability. However, at identical work loads, noradrenaline, adrenaline and glucose levels were significantly higher in women than in men. Lactate, heart rate and relative showed a similar tendency at submaximal exercise levels, indicating higher strain at identical stress levels in women. The reason for the higher sympathetic activity in women at identical work loads may be their relatively smaller skeletal muscle mass in relation to the loads during this test.Supported by Bundesinstitut für Sportwissenschaften, Cologne-Lövenich, FRG     

Glucose and free fatty acid utilization during prolonged exercise in prepubertal boys in relation to catecholamine responses

Summary Ten prepubertal boys performed 60-min cycle exercise at about 60% of their maximal oxygen uptake as previously measured. To measure packed cell volume, plasma glucose, free fatty acids (FFA), glycerol and catecholamines, blood samples were drawn at rest using a heparinized cathether and at the 15th, 30th and 60th min of the exercise and after 30 min of recovery. At rest, the blood glucose concentrations were at the lowest values for normal. Exercise induced a small decrease of blood glucose which was combined with an abrupt increase of the noradrenaline concentration during the first 15 min. The FFA and glycerol concentrations increased throughout the exercise linearly with that of adrenaline. Compared to adults, the FFA uptake expressed per minute and per litre of oxygen uptake was greater in children. These results suggested that it is difficult for children to maintain a constant blood glucose concentration and that prolonged exercise provided a real stimulus to hypoglycaemia. An immediate and large increase in noradrenaline concentration during exercise and a greater utilization of FFA was probably used by children to prevent hypoglycaemia.     

Heart rate variability during dynamic exercise in elderly males and females

It has been proposed that cardiac control is altered in the elderly. Power spectral analysis of heart rate variability (HRV) was performed on 12 male and 11 female elderly subjects (mean age 74 years) while at rest in supine and sitting positions, and at steady states during 5 min of exercise (35–95% peak oxygen consumption, V˙O_2peak). There were no differences in power, measured as a percentage of the total of the high frequency peak (HF, centred at about 0.25 Hz; 13% in males vs 12% in females), low frequency peak (LF, centred at 0.09 Hz; 25% in males and 22% in females), and very low frequency component (VLF, at 0.03 Hz; 66% in males and 69% in females) between body positions at rest. There was no difference in spectral power between male and female subjects. Total power decreased as a function of oxygen consumption during exercise, LF% did not change up to about 14 ml · kg⁻¹ · min⁻¹ (40% and 80% V˙O_2peak in males and females, respectively), then decreased towards minimal values in both genders. HF% power and central frequency increased linearly with metabolic demand, reaching higher values in male subjects than in female subjects at V˙O_2peak, while VLF% remained unchanged. Thus, the power spectra components of HRV did not reflect the changes in autonomic activity that occur at increasing exercise intensities, confirming previous findings in young subjects, and indicated similar responses in both genders. Accepted: 30 November 1999     

Lung-to-lung circulation times during exercise in heart failure

Circulation time (the transit time for a bolus of blood through the circulatory system) is a potential index of cardiac dysfunction in chronic heart failure (HF). In healthy subjects, circulation time falls as cardiac output (Q) rises during exercise, however little is known about this index in HF. In this study we examined the relationship between lung-to-lung circulation time (LLCT) during exercise in ten HF (53 ± 14 year, resting ejection fraction = 23 ± 8%) and control subjects (51 ± 18 year). We hypothesized that HF patients would have slower LLCT times during exercise when compared to control subjects. Each subject completed two identical incremental exercise tests during which LLCT was measured in one test and Q measured in the other. Q was measured using the open circuit C₂H₂ washin technique and circulation time measured using an inert gas technique. In HF patients and control subjects, LLCT decreased and Q increased from rest (HF:LLCT = 53.6 ± 8.2 s, Q = 4.3 ± 1.1 l min⁻¹; control: LLCT = 55.3 ± 10.9 s, Q = 4.5 ± 0.5 l min⁻¹) to peak exercise (HF:LLCT = 20.6 ± 3.9* s, Q = 8.8 ± 2.5* l min⁻¹; control:LLCT = 14.9 ± 2.4 s, Q = 16.5 ± 1.2 l min⁻¹; *P < 0.05 vs control). LLCT was significantly (P < 0.05) slower for the HF group when compared to the control group during submaximal exercise and at peak exercise. However, at a fixed Q the HF subjects had a faster LLCT. We hypothesize that the faster LLCT at a fixed Q for HF patients, may be the result of a more intensive peripheral vasoconstriction of non-active beds and a better redistribution of blood flow.     

Plasma catecholamines and heart rate at the beginning of muscular exercise in man

Summary The relationship between the time course of heart rate and venous blood norepinephrine (NE) and epinephrine (E) concentrations was studied in 7 sedentary young men before and during 3 bicycle exercises of 5 min each (respectively 23±2.8%, 45±2.6% and 65±2.4% , mean ±SE). During the low level exercise the change in heart rate is monoexponential ( =5.7±1.2s) and no increment above the resting level of NE (NE) or of E (E) occurs. At the medium and highest intensity of exercise: a) the change in heart rate is biexponential, for the fast and the slow component averaging about 3 and 80 s respectively; b) NE (but not E) increases continuously with time of exercise; c) at the 5th min of exercise heart rate increments are related to NE; d) between 20s and 5 min, at corresponding sampling times, the heart rate of the slow component is linearly related to NE. At exercise levels higher than 33% the increase in heart rate described by the slow component of the biexponential kinetic could be due to an augmented sympathetic activity revealed by increased NE blood levels.     

Effect of graded changes in extracellular muscle volume on cardiovascular drives during static exercise

Summary The effects of graded changes in peripheral extracellular volume on heart rate and blood pressure during isometric exercise were studied in 12 healthy male subjects. Each subject performed four calf ergometer tests with each calf. In all tests, static plantar flexion of one foot was performed in a supine body position with the knee joint flexed to 90°. After a pre-exercise period of 18 min, during which the calf volume was manipulated, the subjects had to counteract a spring force of 120 N for 8 min. In the pre-exercise period the peripheral extracellular volume of the calf muscle to be tested was manipulated in four ways. Test 1: 15 min of rest in the exercise position. During the last 3.5 min, the calf volume was increased by venous congestion [80 mmHg (10.67 kPa) applied to the distal part of the thigh by pneumatic cuff]. Test 2: the same protocol as in test 1 but with 7.5-min venous congestion. Test 3: 15 min of venous congestion. Test 4: the calf volume was decreased by a negative hydrostatic pressure for 15 min (calf raised about 40 cm above heart level). To clamp the changed calf volume, the thigh cuff was rapidely inflated to 300 mmHg (40.0 kPa) at the end of the volume manipulation and the subjects remained resting for a further 3 min. In test 4, the leg of the subject was passively brought into the exercise position. The occlusion was maintained until 2 min after exercise. The calf volume manipulation led to changes ranging from +105 ml (test 3) to –134 ml (test 4) as measured by water displacement plethysmography. The blood pressure response to exercise was inversely related to the calf volume changes while the heart rate response during exercise showed no clearcut relationship to the pretreatments.     

Thermoregulatory stress during rest and exercise in heat in patients with a spinal cord injury

Summary Twelve subjects with spinal cord injuries and four controls (all male) were exposed to heat while sitting at rest or working at each of three environmental temperatures, 30, 35 and 40°C, with a relative humidity of 50%. Exercise was accomplished at a load of 50 W on a friction-braked cycle ergometer which was armcranked or pedalled. Functional electrical stimulation of the legs was provided to the subjects with quadriplegia and paraplegia to allow them to pedal a cycle ergometer. The data showed that individuals with quadriplegia had the poorest tolerance for heat. As an example, in this group, accomplishing armcrank ergometry while working at an environmental temperature of 40°C resulted in an increase in aural temperature of 2°C in 30 min. The aural temperature of individuals with paraplegia working for the same length of time under the same conditions rose approximately 1°C. There was virtually no change in the aural temperature in the control subjects.     

Genetic influences on heart rate variability at rest and during stress

We tested whether the heritability of heart rate variability (HRV) under stress is different from rest and its dependency on ethnicity or gender. HRV indexed by root mean square of successive differences (RMSSD) and high-frequency (HF) power was measured at rest and during 3 stressors in 427 European and 308 African American twins. No ethnic or gender differences were found for any measures. There was a nonsignificant increase in heritability of RMSSD (from 0.48 to 0.58) and HF (from 0.50 to 0.58) under stress. Up to 81% and 60% of the heritabilities of RMSSD and HF under stress could be attributed to genes influencing rest levels. The heritabilities due to genes expressed under stress were 0.11 for RMSSD and 0.23 for HF. The findings suggest that, independent of ethnicity and gender, HRV regulation at rest and under stress is largely influenced by the same genes with a small but significant contribution of stress-specific genetic effects.     

Myocardial Degeneration and Cardiac Arrest in Squirrel Monkey: Physiological and Psychological Correlates

Behavioral factors and cardiovascular changes associated with myocardial degeneration and cardiac arrest induced by shock avoidance stress were studied. Pairs of avoidance and yoked squirrel monkeys were exposed to a 24-hr session. Myocardial degeneration and cardiac arrest were more readily induced in avoidance than yoked monkeys. The cardiac lesions were not related to body weight, aggressive behavior, or number of shocks received during stress, but an increased heart rate without hypertension during the first stress hour was more evident in avoidance than yoked monkeys. These cardiac changes were attributed to an autonomic disturbance associated with the response contingencies of the avoidance situation. An autonomic effect also appeared to be involved with stress-induced death. These deaths were characterized by a sudden, severe bradycardia without initial hypotension and cardiac arrest which was attributed to either parasympathetic activation or sympathetic inhibition. While heart rate decreased in all monkeys, stress-induced death followed only in monkeys which gave up and stopped contending with the stress. Thus, stress-induced myocardial changes and death were related to autonomic disturbances precipitated by psychological stress.     

Plasma catecholamines during endurance exercise of different intensities as related to the individual anaerobic threshold

The study investigated the concentrations of free plasma catecholamines (CAT), adrenaline and noradrenaline, in comparison to heart rate and lactic acid concentrations during endurance exercises (EE) of different intensities related to the individual anaerobic threshold (IAT). A group of 14 endurance trained male athletes took part in the tests on a treadmill. After an exhausting incremental graded test (increasing 0.5 m · s^–1 every 3 min) to determine t he IAT, the subjects performed EE of 45 min in randomized order with intensities of 85%, 95%, 100% and 105% (E85–E105) of the IAT. The heart rate and CAT increased continuously during all EE. The CAT reacted sensitively to EE above IAT (E105) and showed an overproportional increase in comparison to EE performed with an intensity at or below IAT. At the same time, at exercise intensities up to IAT (E85–E100) a lactate steady state was observed whereas mean lactate concentrations increased during E105. The changes of lactate concentration allowed a better differentiation between E85–E100 as CAT measurements. In E95, E100 and E105 there was a partial overlap of heart rate, which in contrast to lactate concentration only differed by about 5%, so that small variations in heart rate could have coincided with considerable differences of exercise intensity when working at intensities near or above IAT. It was concluded that the range of IAT seemed to represent a real physiological breakpoint which corresponded to the aerobic-anaerobic transition.     

Carotid baroreflex sensitivity at rest and during exercise is not influenced by opioid receptor antagonism

Summary Physical effort involves, along with an increase in the plasma concentration ofβ-endorphin, profound cardiovascular adaptations. The aim of the present study was to investigate with the use of the variable neck chamber technique, the influence of the endogenous opioids on the carotid baroreflex control of blood pressure and heart rate at rest as well as during exercise. Ten normal volunteers exercised in the supine position up to 33% and 66% of their maximal exercise capacity and received, in a randomized doubleblind cross-over protocol, either saline or naloxone (10 mg intravenously, followed by a continuous infusion of 10 mg·h⁻¹). During exercise a progressive attenuation of the carotid baroreceptor reflex control of blood pressure and heart rate was noted. However, neither at rest nor during exercise, did opioid antagonism influence the carotid baroreceptor control of blood pressure and heart rate. Intra-arterial pressure and heart rate also remained unaffected. In contrast, both at rest and during exercise, naloxone administration produced a significant increase in the plasma concentration of cortisol. The latter suggests that in vivo the opioid receptors were effectively antagonized. In conclusion the present study confirms that opioids play only a minor role in cardiovascular homeostasis at rest. In addition, this study demonstrates that they are not involved in the cardiovascular adaptation to exercise, nor in the exercise-related attenuation of the carotid baroreceptor control of pressure and heart rate.     

Blunted cardiac reactions to acute psychological stress predict symptoms of depression five years later: Evidence from a large community study

We recently reported a cross‐sectional negative relationship between cardiovascular reactivity and depressive symptoms. The present analyses examined the prospective association between reactivity and symptoms of depression 5 years later. At the earlier time point, depressive symptoms, measured using the Hospital Anxiety and Depression Scale (HADS), and cardiovascular reactions to a standard mental stress were measured in 1,608 adults comprising three distinct age cohorts: 24‐, 44‐, and 63‐year‐olds. Depression was reassessed using the HADS 5 years later. Heart rate reactions to acute psychological stress were negatively associated with subsequent depressive symptoms; the lower the reactivity the higher the depression scores. This association withstood adjustment for symptom scores at the earlier time point and for sociodemographic factors and medication status. The mechanisms underlying this prospective relationship remain to be determined.     

Women with posttraumatic stress disorder have larger decreases in heart rate variability during stress tasks

The relationship between posttraumatic stress disorder (PTSD) and high frequency heart rate variability (HF-HRV) was investigated during a resting baseline period and two 4-minute laboratory speech tasks. Participants were 20 women with PTSD and 20 age- and gender-matched controls. Parasympathetic nervous system (PNS) cardiac control was measured as HF-HRV (0.12–0.40 Hz) using power spectrum analysis. Participants with PTSD had significantly greater reductions in HF-HRV during two speech tasks (trauma recall and mental arithmetic) than control. These results suggest that PTSD is related to the magnitude of decrease in parasympathetic cardiac control during stress in women. Health implications of altered PNS activity associated with PTSD deserve further study.