Changes in baroreceptor sensitivity for heart rate during normotensive pregnancy and the puerperium

Normal pregnancy is associated with marked changes in cardiovascular haemod ynamics, which in part may be due to changes in autonomic control mechanism s. Baroreflex sensitivity for heart rate (BRS) was calculated in the supine and standing positions using power spectral analysis of pulse interval (PI ) and systolic blood pressure (SBP) in 16 normotensive pregnant women and 1 0 normotensive non-pregnant controls. The pregnant women were studied on th ree occasions during their pregnancy (early, mid- and late gestation) and o nce during the puerperium. Supine total SEP variability increased between e arly and late pregnancy by 79% [95% confidence intervals (CI) 30%, 145%; P < 0.001], and supine high-frequency PI variability decreased by 75% (CI 51% , 88%; P < 0.001). Supine BRS fell by 50% (P < 0.001), with values returnin g to early-pregnancy levels in the puerperium, which were similar to those recorded in the control group. Standing SEP variability and BRS values were unchanged during pregnancy and post partum. The low/high frequency ratio o f PI variability, taken as a surrogate measure of sympathovagal balance. in creased by 137% (CI 42%, 296%; P < 0.01) in the supine but not the standing position from early to late pregnancy. This was due to a decrease in high- frequency variability rather than to an increase in low-frequency variabili ty, suggesting that these changes may have been due to vagal withdrawal rat her than increased sympathetic activity. Normotensive pregnancy is associat ed with a marked decrease in supine BRS, although the exact mechanisms for these changes remain unclear. Further studies are required to define whethe r changes in BRS and sympathovagal tone in early pregnancy can be used to p redict the onset of pregnancy-induced hypertension.