The effect of position and delay on stroke and minute distance in late pregnancy

Stroke and minute distance were measured in 12 women in late pregnancy using Doppler ultrasound. In each patient two sets of measurements were made every 2 min for 20 min in the supine and left lateral positions, the sequence of positions being reversed on the second occasion. Within each of the four 20 min measurement periods there were no significant changes with time in stroke distance, heart rate, or minute distance, or in their coefficients of variation. From this evidence there is no justification for delaying the measurement of cardiac output until a 'steady state' is reached. The measurements were influenced both by the sequence and the position in which they were taken, the effects being additive. Overall, stroke distance was 5.1% greater in the left lateral than in the supine position, heart rate was 3.7% less and minute distance not significantly different. Independent of posture, stroke distance was 6.5% greater in the second period of measurement than in the first, heart rate was 3% less and minute distance 2.8% more. In late pregnancy the relative magnitude of cardiac output in the left lateral and supine positions has to be interpreted in the light of a stepwise increase of aortic blood flow with change of posture.