Autonomic control of heart rate during exercise studied by heart rate variability spectral analysis

The duration of impairment of autonomic control after anticholinergic drug administration in humans

Impaired parasympathetic control of heart rate is associated with increased incidence of cardiac dysrhythmias and ischemia. Anticholinergic drugs, commonly administered during reversal of neuromuscular blockade, suppress parasympathetic control in the early postoperative period. This could potentially be detrimental in patients at risk of cardiovascular complications. The duration of parasympathetic impairment by two anticholinergic drugs were compared in this double-blind randomized cross-over study. Fourteen healthy volunteers received a single intravenous injection of atropine 20 micrograms/kg or glycopyrrolate 8 micrograms/kg during two different study sessions. The methods of spontaneous baroreflex analysis and spectral analysis of heart rate variability generated indices of beat-by-beat parasympathetic modulation of heart rate. Both drugs resulted in a marked decrease in baroreflex sensitivity and high-frequency heart rate variability. The times to return to baseline values were approximately doubled after atropine compared to glycopyrrolate (177 +/- 22 vs 82 +/- 8 min for baroreflex sensitivity, 212 +/- 16 vs 111 +/- 14 min for high-frequency power, and 171 +/- 18 vs 95 +/- 18 min for high-frequency power normalized to total power; P < 0.01 for all variables). Atropine leads to more prolonged impairment of parasympathetic control than equipotent doses of glycopyrrolate, and its use may thus be less desirable in high-risk patients in the early postoperative period.

Heart rate variability at rest and exercise: influence of age, gender, and physical training

The adaptive responses of the cardiovascular system to regular physical activity appear to include a reduction in sympathetic (SNS) activity and an increase in parasympathetic (PNS) activity during rest and at different absolute intensities of exercise. In a cross-sectional design, trained individuals who exercised at least 5 days/week for 45 min/day or more were compared with age- and gender-matched untrained controls. There was a relative bradycardia in the trained groups at rest and at the same absolute intensity of exercise in both young and middle-aged subjects. There were no differences in indicators of PNS and SNS activity at rest in young subjects. There was a reduced SNS activity and increased PNS activity in middle-aged trained subjects versus their age-matched controls. Spectral analysis was capable of showing changes in autonomic control of heart rate at rest and across intensities of exercise. These data showed significant shifts to increased PNS and reduced SNS indicators at rest in trained versus untrained middle-aged subjects and in females in both age groups versus males.

Bradycardia during baroreflex stimulation and active or passive stressor tasks: cardiorespiratory fitness and hostility

We examined whether the resting bradycardia associated with cardiorespiratory fitness extends to lowered heart rate (HR) and mean arterial pressure during novel passive or active laboratory stressors and to a longer heart period during stimulation of the carotid-cardiac vagal baroreflex, independently of the Type A behavior pattern (TABP) and hostility. Forty-four normotensive Caucasian men (18-35 years of age) completed the Structured Interview for TABP, the Cook-Medley Hostility Scale, and a peak oxygen uptake (Vo2peak) test. Vo2peak was inversely related to HR prior to, during, and after each stressor and was positively related to heart period during baroreflex testing. Hostility was inversely related to heart period during baroreflex testing. TABP had no effects. Our findings indicate that cardiorespiratory fitness and low hostility are independently associated with a bradycardia during stimulation of the carotid-cardiac baroreflex, consistent with a possible enhancement of cardiac vagal tone.

Invincible Athletics program: aerobic exercise and performance without strain

This study compared physiologic pattern during conventional aerobic exercise to those during Invincible Athletics--a program emphasizing balance and comfort during exercise to increase strength, endurance, and mind-body coordination gradually without the negative effects of the stress/recovery cycle. While heart rats were similar during both workouts, during Invincible Athletics: (1) perceived comfort tended to be higher and perceived exertion tended to be lower; (2) breath rates were significantly lower; (3) respiratory sinus arrhythmia and central-parietal alpha relative power were significantly higher; and (4) endurance was significantly longer. Respiratory sinus arrhythmia decreases with physiologic and mental stress. Heightened respiratory sinus arrhythmia and EEG alpha relative power along with subjective reports of greater comfort, less exertion, and observed increased endurance suggest that during Invincible Athletics athletes may be more easily adapting to the demands made upon them.

Interrelationships among measures of autonomic activity and cardiovascular risk factors during orthostasis and the oral glucose tolerance test

Overstimulation of sympathetic nervous system activity is related to atherosclerotic cardiovascular disease risk, but the role of parasympathetic activity in this association is not clear. This study evaluated sympathetic and parasympathetic function by spectral analysis of heart rate variability and plasma levels of norepinephrine (NE) epinephrine (EPI), dihydroxyphenylglycol (DHPG), dihydroxyphenylalanine (DOPA) and dihydroxyphenylacetic acid (DOPAC). It also examined the interrelationships among these parameters and established atherosclerotic cardiovascular disease risk factors in 53 men (mean age 59.5 years). During supine rest, low-frequency power correlated positively with high-frequency power (r = 0.58, p < 0.001), plasma NE correlated with plasma DHPG (r = 0.41, p < 0.001) and plasma DOPA with DOPAC (r = 0.47, p < 0.001) but neither low- nor high-frequency power was correlated with plasma levels of any catechol. Among risk factors, plasma NE correlated with fasting insulin and mean arterial blood pressure, and urine NE correlated with body mass index. Both low- and high-frequency power correlated positively with insulin levels. Orthostasis decreased high-frequency power and increased low-frequency power and plasma NE levels. During the oral glucose tolerance test, both high- and low-frequency power increased, plasma NE levels were unchanged, and plasma EPI levels decreased [88.5 +/- 18 (SEM) versus 52.5 +/- 12 pM, p = 0.001]. The results suggest that orthostasis decreases and the oral glucose tolerance test increases parasympathetic outflows, whereas both stimuli increase sympathetic outflows. Among all atherosclerotic cardiovascular disease risk factors, hyperinsulinaemia showed the strongest association with autonomic nervous system activity, especially parasympathetic activity. Estimates of sympathetic responses obtained from power spectral analysis of heart rate variability agree poorly with those from plasma levels of catechols, possibly because of a parasympathetic contribution to low-frequency power and independence of sympathoneural outflows to the arm and heart.

Insulin resistance and autonomic function in traumatic lower limb amputees

This study examined plasma insulin response to oral glucose load and autonomic nervous system activity in male lower limb amputees (n = 52) aged 50-65 years, compared to matched controls (n = 53). The groups had similar body mass index, blood pressure and plasma lipid levels. The amputees had higher mean fasting plasma insulin levels (18.4 +/- 9.7 (SD) versus 13.7 +/- 5.1 mU/l, p = 0.005) and during an oral glucose tolerance test (OGTT) (1 h levels 88.1 +/- 45.3 versus 62.1 +/- 42.7, p = 0.016) with similar plasma glucose levels, indicating insulin resistance. At baseline with the subjects supine, there were no group differences in low- or high-frequency power of heart rate variability or in plasma levels of norepinephrine (NE) or epinephrine (EPI). In response to orthostasis, the groups had similarly increased plasma NE levels. During the OGTT, amputees had significantly larger increments in low-frequency power than did controls (2.2 +/- 1.3 versus 1.6 +/- 0.9 (beats/min)2 respectively, p < 0.01) and plasma NE levels increased significantly in amputees (1595 +/- 849 versus 1941 +/- 986 pM, p = 0.0008) but not in controls. At 1 h after glucose administration, plasma EPI levels were decreased significantly from baseline in both groups; at both 1 and 2 h after glucose administration, plasma EPI levels were higher in the amputees than controls. Amputees appear to have a combination of enhanced sympathoneural responsiveness and attenuated suppression of adrenomedullary secretion during glucose challenge. As catecholamines antagonize insulin effects, one possible explanation for insulin resistance in amputees is hyperglycaemia-induced sympathoneural activation and a failure of hyperglycaemia to decrease adrenomedullary secretion.

Continuous positive airway pressure increases heart rate variability in congestive heart failure

OBJECTIVES

Our objective was to determine whether continuous positive airway pressure augments the low heart rate variability of congestive heart failure, a marker of poor prognosis.

BACKGROUND

Nasal continuous positive airway pressure improves ventricular function in selected patients with heart failure.

METHODS

In 21 sessions in 16 men (mean [+/- SE] age 56 +/- 2 years) with New York Heart Association functional class II to IV heart failure, we assessed the effects of 45 min with (n = 14) and without (as a time control, n = 7) nasal continuous positive airway pressure (10 cm of water) on heart rate variability and end-expiratory lung volume. Coarse-graining spectral analysis was used to derive total spectral power (PT), its nonharmonic component (fractal power [PF]) and the low (0.0 to 0.15 Hz [PL]) and high (0.15 to 0.50 Hz [PH]) frequency components of harmonic power. Standard deviation of the RR interval, high frequency power and the PH/PT ratio were used to estimate parasympathetic activity in the time and frequency domains, and the PL/PH ratio was used to estimate cardiac sympathetic activity in the frequency domain.

RESULTS

Use of continuous positive airway pressure increased end-expiratory lung volume by 445 +/- 82 ml (p < 0.01) and both time (p < 0.006) and frequency domain indexes of heart rate variability: Total spectral power (p < 0.01), nonharmonic power (p < 0.023) and low (p < 0.04) and high (p < 0.05) frequency components of harmonic power all increased. Time alone had no effect on these variables. By comparison, the PH/PT ratio increased during nasal continuous positive airway pressure (p < 0.004), whereas the PL/PH ratio was unchanged. Breathing rate remained constant in both groups.

CONCLUSIONS

Short-term application of nasal continuous positive airway pressure increases heart rate variability and time and frequency domain indexes of parasympathetic activity without influencing cardiac sympathetic activity. This increase may occur reflexively, through stimulation of pulmonary mechanoreceptor afferents.

Effects of passive tilt and submaximal exercise on spectral heart rate variability in ventricular fibrillation patients without significant structural heart disease

It has been shown that tilt and exercise elicit significant changes in autonomic activity in normal subjects and that submaximal exercise causes a greater reduction in heart rate variability (HRV) in animals susceptible to ventricular fibrillation (VF). Whether there is an abnormal HRV response to tilt and exercise in patients at risk of sudden cardiac death (SCD) remains unknown. Short-term HRV before and during passive tilt and exercise was studied in 12 survivors of out-of-hospital cardiac arrest with documented VF and compared with 12 age- and sex-matched normal controls. No patient had significant structural heart disease or left ventricular dysfunction. HRV was computed as total-frequency (TF, 0.01 to 1.00 Hz), low-frequency (LF, 0.04 to 0.15 Hz) and high-frequency (HF, 0.15 to 0.40 Hz) components. There was no significant difference between normal controls and SCD survivors in HRV before or during tilt or submaximal exercise testing. The HF component was significantly decreased during tilt compared with that in the supine position in both normal controls (5.85 +/- 0.61 vs 5.08 +/- 0.95 In(msec2), p = 0.005) and patients (5.58 +/- 1.49 versus 4.74 +/- 1.18 In(msec2), p = 0.003). There was again no significant change in the TF or LF components during tilt in either patients or controls. All frequency components were significantly decreased during submaximal exercise testing in both patients and controls. However, there was no significant difference in any of these tilt- and exercise-induced changes in HRV between normal controls and SCD survivors.(ABSTRACT TRUNCATED AT 250 WORDS)

Pitfalls in the interpretation of spectral analysis of the heart rate variability during exercise in humans

The recent use of spectral analysis of the R-R interval variability to assess the autonomic drive during exercise has produced inconsistent results. The purpose of this study was to assess whether the spectral components of the R-R interval variability reflect different mechanisms at rest and during exercise. Autoregressive spectral analysis of the electrocardiographic and breathing signals was performed in 11 healthy young men at rest and during incremental cycle ergometry. The amplitude of respiratory sinus arrhythmia, i.e. the absolute power of the high frequency spectral component, fell at the onset of exercise, consistent with a reduction in cardiac vagal activity. Conversely, the normalized power of the high frequency component, (i.e. the percentage of R-R interval variance due to the respiratory sinus arrhythmia) increased with increasing work rates. The low frequency spectral component of the R-R interval variability, which when expressed in normalized units is believed to reflect cardiac sympathetic activity, was no longer detectable in severe exercise when the adrenergic drive is known to be elevated. In conclusion, autoregressive spectral analysis of the R-R interval variability does not adequately reflect the autonomic changes that occur during incremental exercise. In particular, the evidence indicates that as the cardiac vagal tone falls with increasing levels of exercise, a greater percentage of the residual power of the high frequency component may be due to non-neural mechanisms.

Autonomic responses of women with parental hypertension. Effects of physical activity and fitness

We studied the moderating effects of cardiorespiratory fitness and physical activity on heart rate and blood pressure responses to psychophysiological stressors and the carotid-cardiac baroreflex in young normotensive women with a parental history of hypertension (n = 31). Testing occurred during the follicular menstrual phase. Subjects were divided into high versus moderate (46.6 +/- 6.5 versus 35.9 +/- 1.9 mL.kg-1.min-1) VO2peak and high versus moderate (1217.7 +/- 98.4 versus 1015.5 +/- 49.4 J.kg-1.wk-1) physical activity groups. The groups did not differ in heart rate or blood pressure responses to mental arithmetic or the cold-face test. However, the highly fit women had longer maximal R-R intervals compared with the moderately fit women when the carotid-cardiac baroreflex was stimulated by negative pressures applied to the neck during resting conditions (P < .01). The carotid-cardiac baroreflex was attenuated during mental arithmetic compared with rest in both the moderately fit and moderately active women but not in the highly fit and highly active groups. We find no evidence that aerobic fitness reduces sympathetic responses to laboratory stressors in young women with parental hypertension. Our findings are consistent with greater parasympathetic tone during sympathetic challenge for the highly fit and highly active subjects. Clarification of autonomic balance during carotid baroreflex stimulation at rest and during sympathetic challenge after exercise training would provide important information regarding mechanisms that regulate cardiovascular responses to autonomic challenge in women at risk for hypertension.

Heart rate variability to monitor autonomic nervous system activity during orthostatic stress

Exposure to microgravity with space flight, or to earth-based analogs such as head-down tilt (HDT) bedrest, results in cardiovascular deconditioning. With reapplication of gravity, deconditioning is most often observed as an elevated heart rate, a narrowed pulse pressure, and possibly even a failure to maintain blood pressure with symptoms of presyncope or syncope. Noninvasive measurements of heart rate variability (HRV) have been used to study cardiovascular control mechanisms during orthostatic stress (head-up tilt and lower body negative pressure [LBNP]). Recently, the authors developed a new approach to study heart rate and blood pressure control mechanisms. Coarse graining spectral analysis (CGSA) allows simultaneous extraction of not only parasympathetic (PNS) and sympathetic (SNS) indicators from the HRV signal, but also determines the fractal dimension (DF), calculated from the slope (beta) of the log spectral power-log frequency relationship. The fractal dimension is an index of the complexity of the cardiovascular control system. Our investigations have indicated patterns of HRV consistent with reduced PNS and increased SNS activity and reduced DF with the HRV signal during orthostatic stress. These findings also indicate that the cardiovascular system may be less stable when operating at a reduced level of complexity.

Saliva electrolytes as a useful tool for anaerobic threshold determination

The purpose of the present study was to determine the anaerobic threshold by analysis of changes in saliva composition during an incremental exercise test on a cycle ergometer. Thirteen healthy males underwent a submaximal test with an initial load of 50 W and load increases of 50 W per 3 min, until capillary blood lactate exceeded 4 mmol.l-1. A maximal test for maximum O2 uptake (VO2max) determination (initial load of 100 W and load increases of 50 W per 2 min) was also performed. Saliva and blood samples were obtained only in the submaximal test. Saliva threshold (Thsa) was defined as the point at which the first increase in either Cl- or Na+ occurred. Catecholamine threshold (Thca) was defined as the point at which a nonlinear increase occurred in either adrenaline or noradrenaline. The lactate (Thla) and ventilatory (Thve) thresholds were determined according to published criteria. No significant differences were found between Thsa values and the other methods of threshold determination. A high correlation was found between Thsa and Thla (r = 0.82, P < 0.01), and Thsa and Thca (r = 0.75, P < 0.05). These results support the validity of Thsa as a new method for noninvasive determination of the anaerobic threshold.

Spontaneous baroreflex by sequence and power spectral methods in humans

Beat-by-beat variations in blood pressure and RR-interval are interrelated by the actions of baroreflex and non-baroreflex responses. This study had two purposes: (1) to examine the spontaneous relationships between RR-interval and systolic blood pressure to determine the relative occurrence of baroreflex and non-baroreflex responses in humans, and (2) to compare the beat-sequence method with a cross spectral estimate of the baroreflex response slope. Eight healthy men were studied during 10 h of quiet, seated rest, and six men and three women were studied during rest, rest plus fixed pace breathing, and a cold pressor test. RR-interval and continuous, non-invasive arterial blood pressure were measured with a computerized system. A baroreflex sequence was defined by a series of at least three consecutive heart beats in which systolic pressure and the following RR-interval either both increased or both decreased. A non-baroreflex relationship was defined by sequences of at least three beats by opposite directional changes of RR-interval and systolic pressure of that beat. The results showed that there were approximately 30% as many non-baroreflex compared to baroreflex slopes. Individual subject mean baroreflex and non-baroreflex slopes were highly correlated (r = 0.72, P < 0.001). Absolute slope values were not different, and they were unaffected by time, fixed pace breathing, or cold pressor test. The data showed the relatively simple beat-by-beat sequence method to yield spontaneous baroreflex response slopes that were quantitatively similar to, and highly correlated with (r = 0.85-0.94), baroreflex response slopes calculated by spectral analysis methods.

Methods in heart rate variability analysis: which tachogram should we choose?

We investigated the practical impact of the representation of the cardiac rhythm--the cardiotachogram--on two elementary spectral indexes in heart rate variability analysis: the low-frequency (0.07-0.14 Hz) and high-frequency (0.14-0.40 Hz) powers. Five commonly used tachograms (inter-beat interval function/series, counts, instantaneous heart rate function/series) were compared. Measurements were done on seven volunteers in the supine and standing positions. Ratios, and their deviations from 100%, of alternative power values were calculated. Mean low-frequency and high-frequency ratio deviations ranged from 0 to 5% and from 6 to 37%. The spectrum of counts yielded on average more (15-37%) high-frequency power. Spectra were incomparable without normalization of the tachogram with respect to heart rate. In conclusion, (i) the choice of a particular spectrum may lead to differing conclusions on the vagal contribution to heart rate variability and (ii) inconclusive results from studies using different tachogram variants can partly be due to the omission of normalization.

Heart rate variability and circulating catecholamine concentrations during steady state exercise in healthy volunteers

OBJECTIVES

To assess whether exercise induced suppression of heart rate variability in the low frequency domain (0.06-0.15 Hz) is related to the increase in circulating catecholamine concentrations.

DESIGN

Randomised crossover trial of three exercise tests characterised by different workloads. Pharmacological simulation of exercise-induced changes in vagal and sympathetic activity.

PARTICIPANTS

Six healthy men with a mean age of 31.2 (SD 3.0) years.

INTERVENTIONS

Three different workloads of steady state cycling ergometry: control state without cycling, cycling at a target heart rate of 100 beats/min, and cycling at a target heart rate of 150 beats/min. Intravenous infusion of atropine (target heart rate 100 beats/min) followed by the additional infusion of adrenaline and noradrenaline.

MAIN OUTCOME MEASURES

Fast Fourier analysis of heart rate variability; blood pressure; and venous plasma concentrations of lactate, adrenaline, and noradrenaline.

RESULTS

During the control exercise period there were no changes in the assessed variables compared with the preceding resting period. During exercise at a heart rate of 100 beats/min systolic blood pressure increased and heart rate variability decreased. During exercise at a heart rate of 150 beats/min systolic blood pressure and lactate, adrenaline, and noradrenaline concentrations increased. In addition, low frequency (LF) was lower than during exercise at 100 beats/min, high frequency (HF 0.15-0.80 Hz) resembled that during exercise at 100 beats/min, and diastolic blood pressure was reduced. Infusion of atropine caused no changes in blood pressure or plasma concentrations of lactate, adrenaline, and noradrenaline and decreased heart rate variability. The additional infusion of adrenaline and noradrenaline completely suppressed heart rate variability and increased blood pressure.

CONCLUSIONS

The reduction in LF and HF during exercise at a heart rate of 100 beats/min, which is not characterised by increased plasma catecholamine concentrations, and during atropine infusion suggests that heart rate variability in the supine state is largely influenced by vagal activity. The additional reduction in LF during exercise at 150 beats/min and during catecholamine infusion may reflect a negative feedback of circulating catecholamines on the sympathetic control of heart rate.

Parasympathetic activity is a major modulator of the circadian variability of heart rate in healthy subjects and in patients with coronary artery disease or diabetes mellitus

Autonomic heart rate control was assessed by power spectral analysis of heart rate variability in 24-hour ambulatory electrocardiographic recordings from 23 healthy subjects, 14 patients with coronary artery disease without cardiac dysfunction, and 14 patients with diabetes mellitus. The log value of the ratio of the low-frequency component (LF = 0.04 to 0.15 Hz) to the high-frequency component (HF = 0.15 to 0.5 Hz) and logHF were employed as indexes of sympathetic and parasympathetic activity, respectively. Diurnal and nocturnal logLF, logHF, and log(LF/HF) values were calculated for heart rates of 60, 70, and 80 beats/min. Intergroup differences among these three variables were not significant at any heart rate. Although a heart rate-related decrease in logHF was generally observed, the relationship between log(LF/HF) and heart rate was not consistent. The correlation between diurnal and nocturnal logHF values was significant at all three heart rates (r = 0.63, 0.87, and 0.59), whereas the diurnal log(LF/HF) was correlated with the nocturnal value only at 70 beats/min (r = 0.77). These results suggest that the heart rate during normal daily activities is a reliable indicator of parasympathetic tone, if not sympathetic tone, in healthy subjects and patients with coronary artery disease or diabetes mellitus.

Body position affects the power spectrum of heart rate variability during dynamic exercise

The power spectrum analysis of R-R interval variability (RRV) has been estimated by means of an autoregressive method in six men in supine (S) and sitting (C) postures at rest and during steady-state cycle exercise at about 14%, 28%, 45%, 67% of the maximal oxygen consumption (% VO2max). The total power of RRV decreased exponentially as a function of exercise intensity in a similar way in both postures. Three components were recognized in the power spectra: firstly, a high frequency peak (HF), an expression of respiratory arrhythmia, the central frequency (fcentral) of which increased in both S and C from a resting value of about 0.26 Hz to 0.42 Hz at 67% VO2max; secondly, a low frequency peak (LF) related to arterial pressure control, the fcentral of which remained constant at 0.1 Hz in C, whereas in S above 28% VO2max decreased to 0.07 Hz; and thirdly, a very low frequency component (VLF; less than 0.05 Hz, no fcentral). The power of the three components (as a percentage of the total power) depended on the body posture and the metabolic demand. HF% at rest was 30.3 (SEM 6.6) % in S and 5.0 (SEM 0.8) % in C. During exercise HF% decreased by about 30% in S and increased to 19.7 (SEM 5.5) % at 28% VO2max in C. LF% was lower in S than in C at rest [31.6 (SEM 5.7) % vs 44.9 (SEM 6.4) %; P < 0.05], remaining constant up to 28% VO2max.(ABSTRACT TRUNCATED AT 250 WORDS)

Autonomic nervous control of heart rate during blood-flow restricted exercise in man

Power spectra of instantaneous heart rate (fc) allows the estimation of the contribution of sympathetic and parasympathetic control of fc during steady-state conditions. The present study was designed to examine autonomic control of fc as influenced by normal dynamic leg exercise and by ischemic leg exercise. Eight subjects performed supine cycle ergometry at 30% of their control peak work rate, with and without blood-flow restriction. Blood-flow restriction was induced by exposing the exercising legs to a supra-atmospheric pressure of 6.7 kPa (leg positive pressure; LPP). The exercise responses of arterial pressure and fc increased (P < 0.05) by LPP exposure. The exaggerated pressor response may be attributed to a chemoreflex drive originating in the ischemic muscles. Exposure to LPP during exercise also produced a significant decrease in parasympathetically mediated high frequency (HF; 0.15-1.00 Hz) fluctuation of fc, as indicated by a decrease (P < 0.05) in percent HF power compared to the control exercise level. During LPP exercise, the sympathetically mediated very low frequency (VLF; 0-0.05 Hz) fluctuation of fc increased, as indicated by an increase (P < 0.05) in percent VLF power above control exercise levels. Both LPP and control exercise conditions decreased (P < 0.05) power in all frequency ranges of interest compared to their respective resting conditions. The results suggest that the increase in fc associated with normal dynamic exercise was mediated predominantly by parasympathetic withdrawal, whereas the exaggerated fc response during ischemic exercise resulted from a combination of cardiac sympathetic drive and parasympathetic withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of neural mechanisms accompanying different intensities of dynamic exercise

The neural mechanisms accompanying dynamic exercise of different intensities were analyzed in dogs and human subjects by means of autoregressive spectral analysis of heart period and arterial pressure variabilities. In the animal experiments, 8 conscious dogs were examined after implanting a solid state pressure gauge in the left ventricle. Animals were examined at rest and during a treadmill run, at 4 km/h, and 0 degrees incline. The experiments were repeated after chronic alpha 1-adrenoreceptor blockade. During the treadmill run, heart rate and systolic left ventricular pressure increased significantly. Simultaneously, the low frequency (LF, 0.1 Hz) component of pulse interval and of systolic pressure variabilities, ie, markers, respectively, of sympathetic modulation of the SA node and of vasomotor activity, increased significantly (evaluated respectively, in normalized and absolute units). After chronic alpha 1-adrenoreceptor blockade, the increase in LF component of systolic pressure variability was prevented, while that observed in R-R interval variability was maintained. Human studies were carried out with either invasive or noninvasive techniques. In the former approach already described, performed in young hypertensive subjects, arterial pressure was recorded with a high fidelity technique. In the second approach applied to young champion swimmers, only the variability of the R-R interval was examined. In both studies, moderate levels of exercise were accompanied by an increase in the LF component of the spectrum: in the case of arterial pressure variability, this increase was detectable both in absolute and normalized units; vice versa, in the case of R-R variability, since physical exercise is accompanied by a marked abatement of the variance, normalized units had to be used in order to evaluate the shift of the sympathovagal balance in favor of sympathetic overactivity.

Autonomic nervous system responses to exercise in relation to ventilatory threshold

We introduce our recent approach to study autonomic nervous system control of heart rate during exercise by means of heart rate variability (HRV) spectral analysis with special reference to its relationship to ventilatory threshold (Tvent). The rationale for the study was that HRV has been shown to reflect (cardiac) parasympathetic and sympathetic nervous system (PNS and SNS, respectively) activity, together with the underlying complexity of cerebral autonomic system in terms of fractal dimension (DF) of HRV time series. The experimental results showed that PNS was markedly reduced below Tvent, that the rate of change in sympathoadrenal activity indicators (plasma norepinephrine and epinephrine concentrations and SNS indicator) was enhanced above Tvent, and that these changes in PNS and SNS indicators were associated with the appearance of the low-dimensional (low DF) dynamics that might reflect less complex autonomic activity. These findings have been considered with respect to implication for clinical cardiology.