Vasomotor sympathetic neural responses during upright tilt in early human pregnancy

Abstract

Pregnancy‐induced hypertension (PIH) and preeclampsia (PE) is proposed to be associated with a hyperadrenergic state. However, the state of sympathetic neural control in normal pregnancy, especially in early pregnancy, is unknown. We tested the hypothesis that sympathetic neural responses during orthostasis are enhanced in early pregnancy in humans. Six healthy young early pregnant (4–7 wk of gestation) women and 12 age, race and BMI‐matched non‐pregnant women (controls) were studied. Muscle sympathetic nerve activity (MSNA), blood pressure (BP), heart rate (HR), and respiration (nasal cannula) were recorded continuously supine and during a graded upright tilt (30° and 60° for 8 min each). We found that MSNA was much greater in early‐pregnant women than in controls supine (33±7 vs 14±9 bursts/min, P<0.001) and during the graded tilt (44±6 and 56±5 vs 25±13 and 35±10 bursts/min, P<0.001 for both group and protocol). However, changes of MSNA (ΔMSNA) from supine to 30° (11±8 vs 12±5 bursts/min, P=0.746) and 60° tilt (23±10 vs 22±6 bursts/min, P=0.640) were similar between early pregnant women and controls (P=0.690 for interaction). BP responses did not differ between the groups, while increases in HR at 60° tilt tended to be greater in early‐pregnant women than in controls (ΔHR, 23±13 vs 16±8 beats/min, P=0.087). These results suggest that vasomotor sympathetic activity increases markedly, but sympathetic neural responses during acute orthostasis remain unchanged in early human pregnancy. It seems likely that sympathetic hyperactivity is a universal characteristic in normal pregnancy and marked sympathetic activation alone may not be the cause for PIH or PE.