ObjectiveTo examine whether an increase in sympathetic nervous tone contributes to the augmented response to cardiovascular reflex testing in preeclamptic women.MethodsMaternal electrocardiograms were recorded from 11 nonpregnant women and 25 normotensive and 15 preeclamptic nulliparous women at term, during periods of quiet respiration in the left-lateral position and after shifting to the supine position. Power spectral analysis was applied to epochs of 512 consecutive beat-to-beat intervals to determine the contribution of sympathetic tone, parasympathetic tone, and respiratory sinus arrhythmia to heart rate variability.ResultsBoth normotensive and preeclamptic pregnant women showed a significant decrease in respiratory sinus arrhythmia and an increase in sympathetic tone compared with nonpregnant women. In nonpregnant and in normotensive pregnant women, shifting from the left-lateral to the supine position did not cause any change in autonomic characteristics. In contrast, preeclamptic women demonstrated a marked increase in power within the very lowfrequency range representing sympathetic tone, from 288 ± 214 to 556 ± 322 second2/Hz, in response to the same challenge (P< .05).ConclusionThird-trimester pregnancy is characterized by sympathetic overactivity. When complicated by preeclampsia, sympathetic overreactivity to cardiovascular reflex testing is observed. Our data support the notion that the pathophysiologic phenomena that characterize preeclampsia are mediated not only by circulating or locally acting vasoactive substances, but also, at least in part, by an increase in sympathetic nervous tone.