Determining blood pressure in pregnancy: Positional hydrostatic effects

OBJECTIVE: To evaluate positional hydrostatic effects on blood pressure determination during pregnancy. STUDY DESIGN: We studied 30 normotensive, pregnant women at 34-41 weeks of gestation. Blood pressures were taken in the sitting, left lateral, right lateral and supine positions with a two-minute stabilization period between positions. The bisacromial diameter was measured. Multivariate analysis of variance for repeated measures was used to evaluate the affect of position on blood pressure. RESULTS: Mean systolic pressure in the right arm was 2.6 mm Hg greater than that in the left arm (P < .05). There was no difference between the arms in diastolic blood pressure. Immediate blood pressure in the lower arm was no greater than in the higher arm in lateral positions, and there were no other significant positional effects. Observed blood pressures were significantly different than those theoretically expected on the basis of hydrostatic effects (P < .0001). CONCLUSION: Positional effects on blood pressure in the lateral positions do not appear immediately (within two minutes), indicating that hydrostatic pressure does not account for these changes. The well-documented blood pressure reduction from longer duration in the lateral position does not appear to be an artifact of hydrostatic effect. Repositioning pregnant women in the supine position to have the cuff at the level of the heart is unnecessary and often undesirable when fetal perfusion is an important consideration. We suggest that American Heart Association blood pressure guidelines stating that all measurements be taken with the cuff at the level of the heart to avoid hydrostatic pressure change be revised for pregnancy.