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 nervous activity, we compared the effects of supine and upright postures on measures of heart rate variability in late pregnancy. Thirty-two women in late pregnancy and 23 nonpregnant age-matched women were studied. Both time and frequency domain heart rate variability measures were compared between the supine and upright positions, and between pregnant women and controls. The pregnant women had smaller mean RR interval, standard deviation of RR intervals and normalised high-frequency power, and a larger low- to high-frequency power ratio than nonpregnant women in the supine position. When the position was changed from upright to supine in nonpregnant women, the percentage change in the mean RR interval and normalised high-frequency power were increased, whereas the percentage change in the coefficient of variation of RR interval and low- to high-frequency power ratio were decreased. In contrast, the percentage change in normalised high-frequency power was decreased and the percentage change in low- to high-frequency power ratio was increased in pregnant women when the position was changed from upright to supine. These results indicate that the vagal enhancing effect seen in normal subjects 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.