EFFECTS OF HEAD-DOWN TILT ON ATRIAL-NATRIURETIC-PEPTIDE AND THE RENINSYSTEM IN PREGNANCY

We studied the effects of head-down tilt to 10 degrees for 30 minutes: on plasma atrial natriuretic peptide and the renin-aldosterone system in 8 preeclamptic pregnant women, 8 healthy pregnant women, and 11 non pregnant women of fertile age. Mean arterial blood pressure did not ch ange in the pregnant groups but increased significantly in the nonpreg nant control subjects. Heart rate decreased significantly in preeclamp tic women but remained unchanged in both control groups. Baseline atri al natriuretic peptide concentration was significantly higher in both preeclamptic (66+/-4 pmol/L) and pregnant (54+/-6 pmol/L) control subj ects compared with nonpregnant subjects (40+/-2 pmol/L), but the diffe rence between the pregnant groups was not significant. Head-down tilti ng induced a significant increase in atrial natriuretic peptide only i n healthy pregnant women. Baseline plasma renin activity and aldostero ne concentrations were significantly higher in pregnant control subjec ts compared with both the preeclamptic and nonpregnant groups. The dif ferences between the preeclamptic and nonpregnant control groups were nonsignificant. After head-down tilting, plasma renin activity decreas ed significantly only in nonpregnant control subjects, whereas aldoste rone decreased significantly in preeclamptic and nonpregnant control s ubjects. In preeclampsia, atrial natriuretic peptide release followed blood pressure and not changes in cardiac output. When all 27 women we re studied, a correlation between atrial natriuretic peptide and mean arterial pressure was found in the left lateral supine position. The r esults suggest that pregnant women developing preeclampsia lose their usual hemodynamic control and show reactions resembling the nonpregnan t state when subjected to head-down tilt.