Cardiac autonomic adjustments to normal human pregnancy: insight from spectral analysis of R-R interval and systolic arterial pressure variability

Objective To assess the adaptation in autonomic control mechanisms that acc ompanies the marked haemodynamic changes, such as increases in cardiac size and output, that occur in the course of normal human pregnancy. Design We studied 14 healthy pregnant women (aged 30 +/- 1 years) before th e 6th week (early stage) and within weeks 32-34 (late stage) of pregnancy, while they were at rest or in a state of active orthostatism (standing), wh ich enhances sympathetic activity. Methods We used echocardiography to assess cardiac volumes and mass, and sp ectral analysis of the R-R interval and systolic arterial pressure variabil ity to obtain indices of autonomic regulation of the circulation. This non- invasive methodology, recently validated with direct recordings of muscle s ympathetic nerve activity, furnishes quantitative markers of sympathetic mo dulation of the sino-atrial node (low frequency component, LF in normalized units, nu), vagal modulation (high frequency component, HF in normalized u nits, nu) and the overall arterial pressure-heart rate baroreflex gain (alp ha index). Results Late pregnancy was characterized by an increase in cardiac size and volumes and by a reduction of R-R interval, R-R interval variance and the a index, together with an increase in the LF/HF ratio (from 1.4 +/- 0.4 to 5.6 +/- 1.9). Changes in markers of autonomic modulation of the sino-atrial node normally induced by the standing position were blunted. Conclusions The late stage of normal human pregnancy appears to be characte rized by alterations in the autonomic control of the circulation and by att enuated responsiveness to active standing, possibly as a consequence of the accompanying increase in cardiac size. (C) Lippincott Williams & Wilkins.