Luiz Eduardo Barreto Martins

Autonomic nervous control of the heart rate during isometric exercise in normal man.

The relative contribution of the efferent components of the autonomic nervous system to the regulation of tachycardia induced by isometric exercise was assessed in 23 normal males. The isometric exercise (handgrip) was performed at the maximum intensity tolerated by the individual over a period of 10 s (maximal voluntary contraction — MVC) and at levels equivalent to 75, 50 and 25% of MVC for 20, 40 and 10 s, respectively. The study was performed both under control conditions and after pharmacological blockade with atropine (12 individuals) or propranolol (11 individuals). Under control conditions, the heart rate (HR) responses to isometric effort were dependent on the intensity and duration of the exercise, showing a tendency towards progressive elevation with the maintenance of muscular contraction at the levels studied. The tachycardia evoked by this effort was of considerable magnitude and of rapid onset, especially at the more intense levels of activity. Parasympathetic blockade markedly decreased tachycardia, which manifested itself during the first 10 s of exercise at all levels of intensity, whereas sympathetic blockade markedly modified the HR response after 10 s of effort at the 75 and 50% MVC levels. A slight depression of the tachycardiac response could be observed already after 10 s of maximum effort after propranolol. The present results suggest that the autonomic regulation of these responses is based on a biphasic mechanism, with the initial phase depending on the rapid withdrawal of the parasympathetic influence, followed by a marked sympathetic contribution to the induction of tachycardia after 10 s of isometric contraction or even a little before at maximum exertion.

The use of isometric exercise as a means of evaluating the parasympathetic contribution to the tachycardia induced by dynamic exercise in normal man.

Fourteen normal subjects were submitted to isometric exercise (IE), dynamic exercise (DE) and a combination of the two (IE+DE). The main purpose of the present study was to use IE as a means of evaluating the mechanism of the heart rate (HR) increase induced by DE. To this end, the magnitude of the IE (handgrip) was standardized so as to cause an elevation of HR almost exclusively by vagal withdrawal: IE was performed using a dynamometer straingauge system with a linear response at 75% of maximum voluntary contraction (MVC) for 10s, repeated at 1 min intervals. The change in HR evoked by IE under control conditions was compared with that evoked during DE, and during the corresponding recovery period. DE was performed by the legs, with the subject in the seated position for 4 min, at workloads of 55 and 105 watts, separated by a rest period. In the combined protocol, IE was performed at the beginning of DE, as well as at 1, 2 and 3 min during DE, and at 0, 1, 2, 3 and 5 min during recovery period. The following results were obtained: (1) IE associated with DE always induced smaller increase in heart rate than IE alone, and this effect was more marked at 105 than at 55W; this finding suggested a workload-dependent vagal withdrawal at the very beginning of DE that was sustained until the end of effort. (2) During DE, each IE induced a similar increase in HR, even at workload of 105 W, when the HR increased gradually in response to DE itself; these results suggest that, this gradual tachycardia (from the 1st to the 4th min of DE) was not vagus dependent. (3) The standardized IE used here permitted functional evaluation of the dynamics of vagal withdrawal without resorting to pharmacological blockade of the autonomic nervous system.

Measurement of anaerobic threshold during dynamic exercise in healthy subjects: comparison among visual analysis and mathematical models

The anaerobic threshold (AT) during dynamic physical exercise is a good parameter to quantify oxygen uptake and transport which reflects the functional cardiorespiratory reserve in men. The increasing use of computerized ergospirometric systems has allowed recording, processing and plotting ventilatory variables, cycle by cycle, on a real time basis. So, response changes in carbon dioxide production (VCO/sub 2/) may be used as a non-invasive measurement of AT. In the present study a custom software was developed to apply bi-segmental models (linear-linear and linear-quadratic) using the least square method for fitting VCO/sub 2/ data. The lowest value of the residual sum of squares found in VCO/sub 2/ graphic plots corresponded to AT, expressed as oxygen uptake. Data analysis showed that, despite the good correlation documented between the linear-linear model and the classical visual method, the mathematical method underestimates the AT values when compared to the visual one.

Evaluation of the Autonomic Nervous System of the Heart in Male Patients with Mitral Valve Prolapse Syndrome Using Respiratory Sinus Arrhythmia and Dynamic Exercise

The autonomic nervous system of the heart was evaluated in two male groups composed of 11 patients with mitral valve prolapse and of 10 normal subjects, using the heart rate response in two types of tests: respiratory sinus arrhythmia at rest and dynamic exercise. Sinus arrhythmia was of higher magnitude in patients with mitral valve prolapse when compared to the control group; however, the differences reached statistical significance only at a respiratory frequency of 7 cycles/min. With respect to dynamic exercise (25, 50, 100, 150 W during 4 min), the heart rate response, either in terms of the early, vagus-dependent fast tachycardia (first 10 s), or the late, sympathetic-dependent tachycardia (1-4 min) was normal in both groups studied, the same occurring with aerobic exercise capacity evaluated by measurement of the anaerobic threshold. Thus, our results show that in the group of male patients with mitral valve prolapse studied here, the parasympathetic abnormalities, if present, are of questionable physiological significance and do not affect the sympathetic and parasympathetic control of heart rate during dynamic exercise.