Sympathoadrenal and cardiovascular reactivity in pregnancy-induced hypertension. II. Responses to tilting.

Sympathoadrenal and cardiovascular responses to tilting were studied in patients with pregnancy-induced hypertension and healthy control subjects during the last trimester of pregnancy and 8 to 12 weeks post partum. Blood volumes were lower in the patients with pregnancy-induced hypertension during pregnancy (0.065 versus 0.081 L/kg, p less than 0.01) but not post partum. Tilting induced significantly smaller increases in heart rate and arterial plasma norepinephrine concentrations and smaller changes in blood pressure during pregnancy as compared to after pregnancy in both groups. Forearm vascular resistance increased significantly in both groups after pregnancy but only in the patients with pregnancy-induced hypertension during pregnancy. The forearm vasoconstrictor response to tilting was, in fact, totally abolished in the third trimester of normal pregnancy. The hypertensive patients had higher arterial plasma epinephrine levels at rest and greater epinephrine and norepinephrine responses to tilting than the control subjects during pregnancy. Normal pregnancy appears to reduce the circulatory and sympathoadrenal responses to orthostatic stress, presumably because of volume expansion that allows venous return to be better maintained in the upright position. The less pronounced pregnancy-induced increase in blood volume in patients with pregnancy-induced hypertension appears to explain the increased sympathoadrenal and forearm vascular reactivity in this group during pregnancy.