EFFECTS OF HUMAN PREGNANCY ON CARDIAC AUTONOMIC ACTIVITY ABOVE AND BELOW THE VENTILATORY ANAEROBIC THRESHOLD 23

Abstract

This study was conducted to test the hypothesis that cardiac autonomic balance is altered by human pregnancy. Subjects were 5 physically active pregnant volunteers studied between 27-37 weeks gestation. A reference group of 5 nonpregnant subjects matched according to age, height, pre-pregnant body mass and parity was also studied. Subjects exercised on a constant work rate cycle ergometer for 4 minutes at 20 watts followed by a ramp increase in work rate of 20 watts/min until fatigue. Ventilatory threshold (Tvent) was determined using the V-slope method (J. Appl. Physiol. 60:2020, 1986). Subjects then performed 2 additional submaximal exercise tests on a separate day. The testing protocol involved 4 minutes of pedalling at a work rate of 20 watts followed by a ramp increase to a level which corresponds to 60 or 110% of the work rate at Tvent. R-R interval data were collected (≥ 512 cycles) and stored using a computerized system. Fast Fourier Transform Analysis was performed to plot the R-R interval spectrum. Low frequency(0-0.15 Hz) and high frequency (0.15-0.5 Hz) power were calculated from data obtained at rest and at each level of exercise. The ratio of high frequency power:total power was used as an index of cardiac parasympathetic activity and the ratio of low to high frequency power was used as an index of sympathetic activity (J. Appl. Physiol. 71: 1136, 1991). Blood pressure in the finger (Ohmeda 2300 Finapres) was measured at rest and during exercise on a beat-to-beat basis for evaluation of spontaneous baroreflex function(Hypertension 12:214, 1988). In the resting state, cardiac parasympathetic modulation (as reflected by HRV high frequency power/total power and spontaneous baroreflex slope) were reduced and cardiac sympathetic modulation (as reflected by low frequency power/high frequency power) was increased in late gestation (p<0.05). At work rates above Tvent, cardiac sympathetic modulation was reduced in late gestation vs. the nonpregnant state. These findings support our original experimental hypothesis.