The influence of posture upon cardiovascular dynamics throughout pregnancy

Systemic blood pressure (measured with a zero-randomized sphyngmomanometer), stroke volume and heart-rate (measured with a Minnesota Impedance Cardiograph), hematocrit and their derivatives--cardiac output, pulse pressure, mean arterial blood pressure and peripheral vascular resistance, were assessed in 19 nonpregnant women of child-bearing age and in 27 patients who were examined at regular intervals on 12 to 15 occasions from 8 to 11 wk of pregnancy until 6-wk postpartum. Readings were made with the subjects in each of six positions: supine, reclining, left and right lateral, left and right tilt. The purpose of this paper is to compare the results of these different postures. Systolic and diastolic blood pressure readings were lowest in the left lateral and second lowest in the left tilt. We suggest that this reflects a hydrostatic pressure difference. Stroke volume and cardiac output were lowest in the right lateral and second lowest in the right tilt. Thus, the derived values of peripheral resistance were lowest when subjects were turned towards the left and highest when turned towards the right. The rank orders of values were maintained with impressive consistency between subjects (pregnant and nonpregnant) and throughout pregnancy, reaching levels of statistical significance in almost all instances. No evidence was found that aortocaval compression led to a change in the measured attributes of central cardiovascular dynamics. For the 'normal unembarrassed parturient' it is suggested that aortocaval compression is rapidly succeeded by a restablization of these attributes to their previous levels, reflecting the prompt establishment of a suitably diminished circulating blood volume within a reduced vascular capacity. It is nevertheless likely that aortocaval compression will interfere markedly with uteroplacental vascular dynamics and hence pose a hazard to the fetus.