Heart rate variability in athletes and nonathletes at rest and during head-up tilt.

The purpose of the present study was to determine if autonomic heart rate modulation, indicated by heart rate variability (HRV), differs during supine rest and head-up tilt (HUT) when sedentary and endurance-trained cyclists are compared. Eleven sedentary young men (S) and 10 trained cyclists (C) were studied. The volunteers were submitted to a dynamic ECG Holter to calculate HRV at rest and during a 70 masculine HUT. The major aerobic capacity of athletes was expressed by higher values of VO2 at anaerobic threshold and peak conditions (P < 0.05). At rest the athletes had lower heart rates (P < 0.05) and higher values in the time domain of HRV compared with controls (SD of normal RR interval, SDNN, medians): 59.1 ms (S) vs 89.9 ms (C), P < 0.05. During tilt athletes also had higher values in the time domain of HRV compared with controls (SDNN, medians): 55.7 ms (S) vs 69.7 ms (C), P < 0.05. No differences in power spectral components of HRV at rest or during HUT were detected between groups. Based on the analysis of data by the frequency domain method, we conclude that in athletes the resting bradycardia seems to be much more related to changes in intrinsic mechanisms than to modifications in autonomic control. Also, HUT caused comparable changes in sympathetic and parasympathetic modulation of the sinus node in both groups.

Effects of age and physical activity on the autonomic control of heart rate in healthy men.

The effects of the aging process and an active life-style on the autonomic control of heart rate (HR) were investigated in nine young sedentary (YS, 23 +/- 2.4 years), 16 young active (YA, 22 +/- 2.1 years), 8 older sedentary (OS, 63 +/- 2.4 years) and 8 older active (OA, 61 +/- 1.1 years) healthy men. Electrocardiogram was continuously recorded for 15 min at rest and for 4 min in the deep breathing test, with a breath rate of 5 to 6 cycles/min in the supine position. Resting HR and RR intervals were analyzed by time (RMSSD index) and frequency domain methods. The power spectral components are reported in normalized units (nu) at low (LF) and high (HF) frequency, and as the LF/HF ratio. The deep breathing test was analyzed by the respiratory sinus arrhythmia indices: expiration/inspiration ratio (E/I) and inspiration-expiration difference (deltaIE). The active groups had lower HR and higher RMSSD index than the sedentary groups (life-style condition: sedentary vs active, P < 0.05). The older groups showed lower HFnu, higher LFnu and higher LF/HF ratio than the young groups (aging effect: young vs older, P < 0.05). The OS group had a lower E/I ratio (1.16) and deltaIE (9.7 bpm) than the other groups studied (YS: 1.38, 22.4 bpm; YA: 1.40, 21.3 bpm; OA: 1.38, 18.5 bpm). The interaction between aging and life-style effects had a P < 0.05. These results suggest that aging reduces HR variability. However, regular physical activity positively affects vagal activity on the heart and consequently attenuates the effects of aging in the autonomic control of HR.

Functional evaluation of sympathetic and parasympathetic system in Chagas' disease using dynamic exercise.

Ten normal subjects and 14 patients with chronic Chagas' disease (seven with and seven without heart disease) underwent dynamic exercise on a cycle ergometer. Heart rate (HR), pulmonary ventilation (V), oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory quotient (RQ) were measured. Increasing workloads (25, 50, 100, and 150 W) were applied for 4 min and intercalated with resting periods. The main objective of this protocol was to analyse heart rate response in relation to the other cardiorespiratory variables in order to evaluate the functional conditions of the sympathetic and parasympathetic cardiac efferents. Analysis of the results showed that (a) the group of chagasic patients with heart disease had lower heart rates (p less than 0.05) than normal subjects during the initial 10 s (delta HR 0-10 s) of effort (fast component); (b) the difference between the normal subjects and chagasic patients without heart disease was not statistically significant; (c) the abnormalities in heart rate response were due to depression of parasympathetic efferent action on the sinus node; (d) the slow heart rate response (delta HR 1-4 min), which expresses the degree of sympathetic stimulation of the sinus node, was comparable in the three groups studied, thus showing unimpaired adrenergic responses during dynamic exercise in Chagas' disease; and (e) the V, VO2, VCO2, and RQ values were normal at all workloads in each group, suggesting that vagal dysfunction does not affect oxygen transport at these submaximal levels of dynamic exercise.

Parasympathetic contribution to bradycardia induced by endurance training in man.

Seven normal subjects of sedentary habits were submitted to a 10 week period of endurance physical training on a cycloergometer. The training programme produced a mean 15.6 +/- 1.4% (+/- SE) increase in VO2max (from 39.7 +/- 2.0 ml . kg-1 . min-1 to 45.9 +/- 2.4 ml . kg-1 . min-1) and a reduction in resting heart rate (HR) from 69 +/- 1.9 to 58 +/- 1.7 beats . min-1 in the supine position. After pharmacological blockade of the parasympathetic system with atropine sulphate, HR rose on average by 53 +/- 3.9 beats . min-1 before training and 47 +/- 3.6 beats . min-1 after training, the difference being statistically nonsignificant. The magnitude of respiratory sinus arrhythmia (RSA) was similar before and after the period of physical conditioning. The respiratory variation in HR ( Delta HR) at the 1 litre tidal volume was 20 +/- 2.4 beats . min-1 and 20 +/- 2.6 beats . min-1 before and after training, respectively. At the 2 litre tidal volume, these values were 25 +/- 3.2 and 27 +/- 4.5 beats . min-1. Similar results were obtained with the RSA test when a group of 13 sedentary individuals (VO2max = 39.4 +/- 1.3 ml . kg-1 . min-1) was compared with a group of 7 athletes who are medium distance runners (VO2max = 53.8 +/- 1.3 ml . kg-1 . min-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Use of the Wasserman equation in optimization of the duration of the power ramp in a cardiopulmonary exercise test: a study of Brazilian men.

This study aimed to analyze the agreement between measurements of unloaded oxygen uptake and peak oxygen uptake based on equations proposed by Wasserman and on real measurements directly obtained with the ergospirometry system. We performed an incremental cardiopulmonary exercise test (CPET), which was applied to two groups of sedentary male subjects: one apparently healthy group (HG, n=12) and the other had stable coronary artery disease (n=16). The mean age in the HG was 47±4 years and that in the coronary artery disease group (CG) was 57±8 years. Both groups performed CPET on a cycle ergometer with a ramp-type protocol at an intensity that was calculated according to the Wasserman equation. In the HG, there was no significant difference between measurements predicted by the formula and real measurements obtained in CPET in the unloaded condition. However, at peak effort, a significant difference was observed between oxygen uptake (V˙O2)peak(predicted)and V˙O2peak(real)(nonparametric Wilcoxon test). In the CG, there was a significant difference of 116.26 mL/min between the predicted values by the formula and the real values obtained in the unloaded condition. A significant difference in peak effort was found, where V˙O2peak(real)was 40% lower than V˙O2peak(predicted)(nonparametric Wilcoxon test). There was no agreement between the real and predicted measurements as analyzed by Lin's coefficient or the Bland and Altman model. The Wasserman formula does not appear to be appropriate for prediction of functional capacity of volunteers. Therefore, this formula cannot precisely predict the increase in power in incremental CPET on a cycle ergometer.

Abnormal baroreflex control of heart rate in decompensated congestive heart failure and reversal after compensation.

Congestive heart failure (CHF) causes impairment of baroreflex control of heart rate (HR). To determine if this derangement is reversible, the cardiac chronotropic control was assessed in 10 patients with class IV chronic CHF of various etiologies before and after compensation achieved by bed rest, salt restriction, diuretics and vasodilators. Mean time between the 2 studies was 15 +/- 3 days. The management was modified 3 days before the second autonomic evaluation, so as to reestablish the same diet and pharmacologic conditions of the previous study. Compensation led to significant reduction in symptom-based class, body weight, and pulmonary and systemic congestion. Mean +/- standard error of the mean HR responses (beats/min) before and after compensation were, respectively: (1) to atropine (0.04 mg/kg): 10 +/- 2 and 27 +/- 2 (p less than 0.01); (2) to handgrip (30% maximum capacity, 1 minute): 9 +/- 2 and 19 +/- 3 (p less than 0.005); (3) to headup tilt (5 minutes): 4 +/- 3 and 20 +/- 4 (p less than 0.005). Mean +/- standard error of the mean baroreflex sensitivity (ms/mm Hg) of RR responses to phenylephrine and amyl nitrate-induced changes in systolic pressure was, respectively, in each condition: phenylephrine, 0.9 +/- 0.2 and 8 +/- 2.3 (p less than 0.05); amyl nitrate, 0.3 +/- 0.2 and 4.1 +/- 1.1 (p less than 0.05). A significant correlation between improvement in HR responses to atropine and tilt and changes in body weight was obtained. These findings show a reversible component of impaired baroreflex control of HR in severe CHF, possibly due to its congestive effects.

Myocardial perfusion abnormalities in chronic Chagas' disease as detected by thallium-201 scintigraphy.

Most patients with chronic Chagas' heart disease complain of chest pain. The pathophysiology of this symptom is unknown, although myocardial necrosis and fibrosis are frequent necropsy findings and cardiac autonomic impairment is a prominent feature of the disease. To evaluate the possibility of an ischemic cause for these abnormalities in 23 patients (18 men, aged 32 to 60 years, mean 42) with chronic Chagas' disease complaining of chest pain, thallium-201 myocardial scintigraphy was performed after maximal effort and 4-hour redistribution. Regional wall motion was assessed by radionuclide and contrast angiography. Heart rate responses to sinus respiratory arrhythmia, atropine, phenylephrine and Valsalva maneuver were evaluated in all patients and in 22 normal control subjects. Coronary angiography was performed in 16 patients. Only 1 patient had chest pain and no ischemic electrocardiographic changes occurred in any case during the effort test. Scintigraphic analysis of 7 segments per patient showed perfusion defects in at least 1 segment in all patients. Of 161 myocardial segments 16 showed fixed, 10 reversible, and 22 paradox defects (reverse redistribution). The majority (75%) of the fixed perfusion defects occurred in dyssynergic regions, whereas reverse redistribution predominated in regions with normal wall motion (82%). The reversible defects were present in normal or mildly hypokinetic regions. Markedly impaired parasympathetic cardiac control was present but no significant coronary abnormalities were seen in any of the 16 patients undergoing angiography. It is concluded that whereas fixed defects are likely to correspond to fibrotic or necrotic lesions, reversible and paradox perfusion defects may be caused by regional flow or metabolism derangements, possibly related to abnormal parasympathetic control of the coronary microcirculation.

Multiple pulmonary thromboembolism diagnosed by two-dimensional echocardiography.

A 55-year-old man was admitted to our hospital because of progressive dyspnea, orthopnea and hemoptysis. Two-dimensional echocardiography multiple echo-dense images 1-4 mm in diameter were consistently seen crossing the tricuspid valve. A hypothesis of multiple pulmonary thromboembolism was raised. Clinical and postmortem evidence were later obtained to confirm this diagnosis.

Cholinergic bronchomotor tone and airway caliber in insulin-dependent diabetes mellitus.

It has been suggested that the autonomic bronchomotor tone may be altered in diabetes. In the present study, we assessed the cholinergic bronchomotor tone in 34 insulin-dependent diabetic patients and in a control group of 32 healthy subjects (group C). As an index of the intensity of cholinergic tone to the airways, we measured the increase in specific airway conductance (Gaw/VL) induced by aerosol administration of atropine sulfate. In all of the patients and normal individuals the autonomic cardiovascular activity was also evaluated by the tilting test and by the magnitude of the respiratory sinus arrhythmia (RSA). In 19 patients without symptoms of autonomic neuropathy (AN) (group D-1), the autonomic cardiovascular activity was comparable to that of group C. The other 15 patients presented with at least one symptom of AN and a depressed heart rate (HR) control when submitted to the tests of autonomic activity (group D-2). Before atropine administration, Gaw/VL was significantly higher (p less than 0.05) in group D-2 (2.48 +/- 0.12 s-1.kPa-1 [mean +/- SE]) than in group D-1 (2.11 +/- 0.10 s-1.kPa-1). Aerosol atropine caused a significant increase (p less than 0.001) in airway caliber in all three groups; however, the increase in Gaw/VL was significantly lower in group D-2 (0.26 +/- 0.05 s-1.kPa-1) when compared with group D-1 (0.63 +/- 0.09 s-1.kPa-1; p less than 0.01) and group C (0.67 +/- 0.06 s-1.kPa-1; p less than 0.001). A weak but significant (p less than 0.02) correlation was observed between the increases in Gaw/VL provoked by atropine and the magnitude of RSA. Our findings suggest that the reduction in parasympathetic bronchomotor tone may cause an increase in basal airway caliber in diabetic patients with AN, compared to patients without AN.

Effects of blood viscosity on proximal flow convergence calculations of regurgitant flow rate and jet dimensions as evaluated by color Doppler flow mapping: an in vitro study.

There are limited data on the potential influence of blood viscosity on the quantification of valvular regurgitation by color Doppler in the clinical setting. This study was designed to evaluate the effects of blood viscosity on jet dimensions and the proximal flow convergence (proximal isovelocity surface area, PISA) method of estimating valvular insufficiency severity. We used an in vitro flow model filled with human blood at varying hematocrits (15%, 35%, and 55%) and blood viscosity (blood/water viscosity: 2.6, 4.8, 9.1) in which jets were driven through a known orifice (16 mm(2)) into a 110-mL compliant receiving chamber (compliance: 2.2 mL/mm Hg) by a power injection pump. Blood injections (2 and 4 mL) at flow rates of 4, 6, 8, 10, and 12 mL/s were performed. Proximal flow convergence and spatial distribution of jets were imaged by a 3.5-MHz transducer. Pressure and volume in the flow model were kept constant before each injection. Ultrasound settings were the same for all experiments. Jet area decreased significantly with increasing blood viscosity, but the difference in jet dimensions was much larger for lower than for higher flow rates and for highest blood viscosity. Estimation of flow rate by the PISA method was not significantly influenced by blood viscosity. Blood viscosity has a major influence in jet area, especially for lower flow rates, but did not change significantly the grading of regurgitation by the PISA method. Thus this factor should be considered for determining the method of choice when quantification of valvular regurgitation is performed in patients with anemia or polycythemia.

Cardiovascular effects of berberine in patients with severe congestive heart failure.

Berberine, an alkaloid of the protoberberine family, has been shown to have strong positive inotropic and peripheral resistance-lowering effects in dogs with and without heart failure. To determine the acute cardiovascular effects of berberine in humans, 12 patients with refractory congestive heart failure were studied before and during berberine intravenous infusion at rates of 0.02 and 0.2 mg/kg per min for 30 minutes. The lower infusion dose produced no significant circulatory changes, apart from a reduction in heart rate (14%). The 0.2 mg/kg per min dose elicited several significant changes: (a) Decreases in systemic (48%, p less than 0.01) and pulmonary vascular resistance (41%, p less than 0.01), and in right atrium (28%, p less than 0.05) and left ventricular end-diastolic pressures (32%, p less than 0.01). (b) Increases in cardiac index (45%, p less than 0.01), stroke index (45%, p less than 0.01), and LV ejection fraction measured by contrast angiography (56%, p less than 0.01). (c) Increases in hemodynamic and echocardiographic indices of LV performance: peak measured velocity of shortening (45%, p less than 0.01), peak shortening velocity at zero load (41%, p less than 0.01), rate of development of pressure at developed isovolumic pressure of 40 mmHg (20%, p less than 0.01), percent fractional shortening (50%, p less than 0.01), and the mean velocity of circumferential fiber shortening (54%, p less than 0.01). (d) Decrease of arteriovenous oxygen difference (28%, p less than 0.05) with no changes in total body oxygen uptake, arterial oxygen tension, or hemoglobin dissociation properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of electromyographic activity and heart rate responses to isometric exercise. The role played by muscular mass and type.

The purpose of the present study was to examine the relationship between the electromyographic (EMG) activity and heart rate (HR) responses induced by isometric exercise performed by knee extension (KE) and flexion (KF) in men. Fifteen healthy male subjects, 21 +/- 1.3 years (mean +/- SD), were submitted to KE and KF isometric exercise tests at 100% of maximal voluntary contraction (MVC). The exercises were performed with one leg (right or left) and with two legs simultaneously, for 10 s in the sitting position with the hip and knee flexed at 90 degrees. EMG activity (root mean square values) and HR (beats/min) were recorded simultaneously both at rest and throughout the sustained contraction. The HR responses to isometric exercise in KE and KF were similar when performed with one and two legs. However, the HR increase was always significantly higher in KE than KF (P < 0.05), whereas the EMG activity was higher in KE than in KF (P < 0.05), regardless of the muscle mass (one or two legs) involved in the effort. The correlation coefficients between HR response and the EMG activity during KE (r = 0.33, P > 0.05) and KF (r = 0.15, P > 0.05) contractions were not significant. These results suggest that the predominant mechanism responsible for the larger increase in HR response to KE as compared to KF in our study could be dependent on qualitative and quantitative differences in the fiber type composition found in each muscle group. This mechanism seems to demand a higher activation of motor units with a corresponding increase in central command to the cardiovascular centers that modulate HR control.

Leg endurance training has no effect on the autonomic control of heart rate during isometric exercise.

1. Heart rate (HR) response to isometric exercise (handgrip) was investigated in 7 normal males of sedentary habits before and after endurance training involving the muscles of the lower limbs. Thirteen additional sedentary individuals and 7 middle-distance runners were also studied. Isometric exercise was performed at 100, 75 and 50% of maximum voluntary contraction (MVC) during 10, 20 and 40 s, respectively. Training produced a 15.6 +/- 1.4% (mean +/- SEM) increase in VO2max and a reduction in resting HR from 69 +/- 1.9 to 58 +/- 1.7 bpm. HR was monitored throughout each period of isometric exercise. 2. The pattern of HR response to static effort performed by untrained muscles was comparable before and after training as well as in athletes and sedentary individuals during the first 10 s of contraction, a period during which tachycardia is mainly mediated by vagal release. After the first 10 s, when the sympathetic influence on tachycardia becomes evident, athletes and trained individuals showed a slight but nonsignificant tendency toward lower HR increases. 3. These results do not demonstrate any appreciable alteration in the efferent activity of autonomic components induced by aerobic training of the leg muscles when isometric exercise is performed with untrained muscles.

Sympathetic and parasympathetic changes in heart rate control during dynamic exercise induced by endurance training in man.

1. Seven healthy young men of sedentary habits were submitted to a 10-week period of endurance physical training on a cycle ergometer. The training program caused a 15% increase in maximal oxygen consumption (VO2max) and a 16% reduction in resting heart rate (HR). Before and after training, these volunteers performed dynamic exercise (DE) on a cycle ergometer at loads of 25, 50, 75, 100 and 150 w for 4 min at each level. The same exercise protocol was applied to 13 sedentary individuals and to 7 athletes (medium distance runners) who showed a VO2max of 39.4 and 53.8 ml/kg, respectively. HR was continuously monitored throughout the period of effort at each workload. 2. During the first 10 s of DE, a period when tachycardia is mediated almost exclusively by vagal withdrawal, the athletes presented a more rapid increase in HR than sedentary subjects. The same tendency was observed in the sedentary individuals after the training period, although of a lesser magnitude. 3. During the DE phase in which sympathetic mediation plays an important role (between 30 s and 4 min), the athletes presented a lower HR increase than the sedentary individuals, and the same response pattern was observed in the group submitted to physical training. Total HR increase (from 0 to 4 min) induced by DE was lower in athletes than in sedentary subjects and was not changed by training of the sedentary subjects. 4. These results suggest that aerobic training decreases the slow sympathetic and increases the fast parasympathetic contribution to HR during dynamic exercise at the same absolute workloads. 5. These functional changes in the autonomic control of HR may or may not be associated with modifications of absolute HR values which increase from rest to the end of exercise. In contrast to what happens in athletes, the autonomic adaptations observed after short-term aerobic training may occur during DE without changes in the total HR response.

Anaerobic threshold estimation by statistical modelling.

Anaerobic threshold (AT) is usually estimated as a change point problem by visual analysis of the cardiorespiratory response to incremental dynamic exercise. In this study, two phase linear (TPL) models of the linear-linear and linear-quadratic type were used for the estimation of AT. The correlation coefficient between the classical and statistical approaches was 0.88, and 0.89 after outlier exclusion. The TPL models provide a simple method for estimating AT that can be easily implemented using a digital computer for the automatic pattern recognition of AT.

Use of isometric exercise to demonstrate cardiac parasympathetic impairment in the digestive form of Chagas' disease.

Standardized handgrip at 100% of maximal voluntary capacity was used to study heart rate responses to isometric exercise in patients with the digestive form of Chagas' disease. The chronotropic responses of the group were significantly lower (P less than 0.001) than those exhibited by control subjects, and comparable to those observed in a group of cardiac chagasic patients. These results show that severe impairment of autonomic control of the heart may occur in chagasic patients apparently having only digestive tract involvement.

Role of bradykinin in postprandial hypotension in humans.

A transient significant decrease in mean arterial blood pressure (MAP) from 107 +/- 3 to 98 +/- 3 mmHg (P < 0.05) was observed in elderly (59-69 years of age), healthy volunteers 25-30 min following ingestion of a test meal. In young volunteers (22-34 years of age), a postprandial decrease of MAP from 88 +/- 3 to 83 +/- 4 mmHg was also noted but it was not statistically significant. A 40% decrease in bradykinin (BK) content of circulatory high molecular weight kininogen had previously been observed in human subjects given the same test meal. We presently demonstrate by specific ELISA that the stable pentapeptide metabolite (1-5 BK) of BK increases from 2.5 +/- 1.0 to 11.0 +/- 2.5 pg/ml plasma (P < 0.05) in elderly volunteers and from 2.0 +/- 1.0 to 10.3 +/- 3.2 pg/ml plasma (P < 0.05) in young volunteers 3 h following food intake. This result suggests that ingestion of food stimulates BK release from kininogen in normal man. Postprandial splanchnic vasodilatation, demonstrated by a decrease of plasma half-life of intravenously administered indocyanine green (ICG), a marker of mesenteric blood flow to the liver, from 4.4 +/- 0.4 to 3.0 +/- 0.1 min (P < 0.05) in young volunteers and from 5.2 +/- 1.0 to 4.0 +/- 0.5 min (P < 0.05) in elderly volunteers, accompanied BK release. The participation of BK in this response was investigated in subjects given the BK-potentiating drug captopril prior to food intake. Postprandial decreases of ICG half-lives were not changed by this treatment in either young or elderly subjects, a result which may indicate that BK released following food intake plays no role in postprandial splanchnic vasodilatation in normal man.

Subtle cardiac autonomic impairment in Chagas' disease detected by baroreflex sensitivity testing.

The sensitivity of baroreflex bradycardia and tachycardia was determined in fourteen patients with Chagas' disease who seemed to be normal with respect to cardiac autonomic control evaluated in terms of heart rate responses to the conventional atropinization and Valsalva maneuver tests. Eleven normal subjects were studied for comparison. Baroreflex sensitivity was determined by relating the beat-to-beat pulse intervals to systolic pressure values during transient phenylephrine- and amyl nitrite-induced changes in arterial pressure. Chagasic patients showed mean bradycardia sensitivity (10.1 +/- 1.3 ms/mmHg) which was significantly lower than that obtained for the control group (16.7 +/- 2.1 ms/mmHg). When only the subgroup of ten patients with overt disease (cardiac and/or digestive form) was considered, the value fell to 8.6 +/- 1.4 ms/mmHg. The lowest individual values were exhibited by the majority of patients with exclusive cardiac or associated cardiac and digestive disease. Patients with only digestive disease or without overt disease (indeterminate form) had values within the normal range. Tachycardia sensitivity (6.3 +/- 0.8 ms/mmHg) was similar to that obtained for the control group (6.6 +/- 1.0 ms/mmHg). Only one patient with associated disease presented a reduced value. These data show that the estimation of baroreflex sensitivity can be used to identify impaired cardiac autonomic control in chronic Chagas' disease not detectable by conventional tests. The reduced baroreflex sensitivity appears to be due to the subtle impairment of the parasympathetic influence on the heart. Furthermore, there is a relationship between the degree of baroreflex sensitivity and the clinical form of organic involvement in Chagas' disease.

Heart rate variability under resting conditions in postmenopausal and young women.

The aim of the present study was to compare the modulation of heart rate in a group of postmenopausal women to that of a group of young women under resting conditions on the basis of R-R interval variability. Ten healthy postmenopausal women (mean +/- SD, 58.3 +/- 6.8 years) and 10 healthy young women (mean +/- SD, 21.6 +/- 0.82 years) were submitted to a control resting electrocardiogram (ECG) in the supine and sitting positions over a period of 6 min. The ECG was obtained from a one-channel heart monitor at the CM5 lead and processed and stored using an analog to digital converter connected to a microcomputer. R-R intervals were calculated on a beat-to-beat basis from the ECG recording in real time using a signal-processing software. Heart rate variability (HRV) was expressed as standard deviation (RMSM) and mean square root (RMSSD). In the supine position, the postmenopausal group showed significantly lower (P<0.05) median values of RMSM (34.9) and RMSSD (22.32) than the young group (RMSM: 62.11 and RMSSD: 49.1). The same occurred in the sitting position (RMSM: 33.0 and RMSSD: 18.9 compared to RMSM: 57.6 and RMSSD: 42.8 for the young group). These results indicate a decrease in parasympathetic modulation in postmenopausal women compared to young women which was possibly due both to the influence of age and hormonal factors. Thus, time domain HRV proved to be a noninvasive and sensitive method for the identification of changes in autonomic modulation of the sinus node in postmenopausal women.

Cardiorespiratory adaptations induced by aerobic training in middle-aged men: the importance of a decrease in sympathetic stimulation for the contribution of dynamic exercise tachycardia.

We investigated the effects of aerobic training on the efferent autonomic control of heart rate (HR) during dynamic exercise in middle-aged men, eight of whom underwent exercise training (T) while the other seven continued their sedentary (S) life style. The training was conducted over 10 months (three 1-h/sessions/week on a field track at 70-85% of the peak HR). The contribution of sympathetic and para-sympathetic exercise tachycardia was determined in terms of differences in the time constant effects on the HR response obtained using a discontinuous protocol (4-min tests at 25, 50, 100 and 125 watts on a cycle ergometer), and a continuous protocol (25 watts/min until exhaustion) allowed the quantification of the parameters (anaerobic threshold, VO2 AT; peak O2 uptake, VO2 peak; power peak) that reflect oxygen transport. The results obtained for the S and the T groups were: 1) a smaller resting HR in T (66 beats/min) when compared to S (84 beats/min); 2) during exercise, a small increase in the fast tachycardia (delta 0-10 s) related to vagal withdrawal (P < 0.05, only at 25 watts) was observed in T at all powers; at middle and higher powers a significant decrease (P < 0.05 at 50, 100 and 125 watts) in the slow tachycardia (delta 1-4 min) related to a sympathetic-dependent mechanism was observed in T; 3) the VO2 AT (S = 1.06 and T = 1.33 l/min) and VO2 peak (S = 1.97 and T = 2.47 l/min) were higher in T (P < 0.05). These results demonstrate that aerobic training can induce significant physiological adaptations in middle-aged men, mainly expressed as a decrease in the sympathetic effects on heart rate associated with an increase in oxygen transport during dynamic exercise.