Efeito da terapia estrogênica sobre o controle autonômico da freqüência cardíaca e a capacidade aeróbia de mulheres saudáveis.

Abstract

The effects of female sex hormones on the cardiovascular system have been the topic of much discussion and controversy in the literature. Nevertheless, many scientists believe that estrogens play an important cardioprotective role in premenopausal women, with their effects being observed directly on blood vessels or indirectly by the promotion of an antiatherogenic lipid profile. In recent years, studies have reported that estrogen hormone levels may also influence autonomic control of heart rate (HR) and exercise tolerance. However, other researches have found no modification of these parameters in function of hormone therapy. Within this context, three studies were conducted to verify whether estrogen therapy (ET) could attenuate the age-related decline in autonomic control of HR under resting and exercise conditions and aerobic capacity of healthy women. Thirteen young women (mean age: 24 years), 10 postmenopausal women undergoing ET (PMET, mean age: 53 years) and 15 postmenopausal women not undergoing ET (PMnET, mean age: 56 years) were studied. Hormonal treatment consisted of 0.625 mg/day of conjugated equine estrogens. In the first study, the effect of age and ET on HR variability (HRV) under resting conditions in the supine and sitting positions was evaluated. HRV was analyzed by time (TD) and frequency domain (FD) methods. In this study, higher values of the temporal indices of HRV were observed for the young group. In the analysis of FD, the PMnET group presented lower values in the indices reflecting vagal activity and higher values in the indices reflecting sympathetic activity compared to the young group (supine position) and to the PMET group (sitting position). These results suggest that HRV decreases during aging and that ET may attenuate this process by promoting a reduction of sympathetic activity on the heart and contributing to the cardioprotective effect of estrogen hormones. In the second study, the effect of age and ET on the autonomic control of HR during dynamic exercise and anaerobic threshold (AT) was evaluated. Dynamic exercise was performed on a cycle ergometer starting at 15 W and followed by 5 W increments, until the loss of HR response stabilization was identified by a semiparametric model, characterizing AT. The autonomic control of HR during exercise was analyzed by vagal withdrawal at the beginning of exercise and by calculating the rMSSD index of the stable interval of each workload level. The vagal withdrawal and the rMSSD index were higher for the young group at the workloads studied. The young group also presented higher workload and HR values at AT compared to the postmenopausal groups. These results suggest that autonomic modulation of HR during exercise and aerobic capacity are strongly influenced by age. Hypoestrogenism and ET had no effect on the variables studied. In the third study, the effect of age and ET on cardiorespiratory responses during a cardiopulmonary exercise test was evaluated. This test was performed on a cycle ergometer with 10 to 20 W/min increments until physical exhaustion. The AT was determined by graphic visual analysis of the curves for carbon dioxide output and oxygen uptake ( O2). Higher workload and HR values both at AT and at the peak of exercise were observed for the young group. HR was similar between groups at AT and significantly higher at peak exercise for the young group. The percentages of AT in relation to peak exercise for O2 and HR values were higher for the postmenopausal groups. These results suggest that ET had no effect on cardiorespiratory responses during the incremental exercise test. In conclusion, the results obtained in the three studies suggest that the vagal-protective effect of estrogen hormones detected at rest is not maintained during exercise. In addition, exercise tolerance does not seem to depend on the physiological levels of estrogens. On this basis, the present findings support the importance of the prescription of physical exercise in the clinical orientation for climacteric women