The effect of hydrostatic pressure on the interpretation of the supine pressor test.

To assess the effects of hydrostatic pressure on the interpretation of the supine pressor test (SPT), blood pressure (BP) measurements were made with a mercury sphygmomanometer in 35 normotensive primigravid patients between 28 and 32 weeks' gestation. BP was recorded in both left lateral recumbent and supine positions. Blood pressure was also recorded in both arms simultaneously by separate observers. A positive supine pressor test was defined as a diastolic BP rise of 20 mm Hg. There were no positive SPTs when BP was recorded in the lower arm despite the development of preeclampsia in eight patients. When BP was corrected for the hydrostatic effect of position, no correlation was seen between position change and blood pressure (p greater than 0.05). Diastolic BP was significantly higher (p less than 0.01) in patients who later developed preeclampsia than in patients who remained normotensive.