Comparison of supine and upright positions on autonomic nervous activity in late pregnancy: the role of aortocaval compression

In order to understand the effect of aortocaval compression on autonomic ne rvous activity we compared the effects of supine and upright postures on me asures of heart rate variability in late pregnancy. Thirty-two women in lat e pregnancy and 23 nonpregnant age-matched women were studied. Both time an d frequency domain heart rate variability measures were compared between th e supine and upright positions, and between pregnant women and controls. Th e pregnant women had smaller mean RR interval, standard deviation of RR int ervals and normalised high-frequency power, and a larger low- to high-frequ ency power ratio than nonpregnant women in the supine position. When the po sition was changed from upright to supine in nonpregnant women, the percent age change in the mean RR interval and normalised high-frequency power were increased, whereas the percentage change in the coefficient of variation o f RR interval and low- to high-frequency power ratio were decreased. In con trast, the percentage change in normalised high-frequency power was decreas ed and the percentage change in low- to high-frequency power ratio was incr eased in pregnant women when the position was changed from upright to supin e. These results indicate that the vagal enhancing effect seen in normal su bjects when changing position from upright to supine was reversed in women in late pregnancy Aortocaval compression in the supine position in pregnant women might be responsible for this reversal in the autonomic response to lying down.