Sympathetic baroreflex gain in normotensive pregnant women

Abstract

Muscle sympathetic nerve activity is increased during normotensive pregnancy while mean arterial pressure is maintained or reduced, suggesting baroreflex resetting. We hypothesized spontaneous sympathetic baroreflex gain would be reduced in normotensive pregnant women relative to nonpregnant matched controls. Integrated muscle sympathetic burst incidence and total sympathetic activity (microneurography), blood pressure (Finometer), and R-R interval (ECG) were assessed at rest in 11 pregnant women (33 ± 1 wk gestation, 31 ± 1 yr, prepregnancy BMI: 23.5 ± 0.9 kg/m(2)) and 11 nonpregnant controls (29 ± 1 yr; BMI: 25.2 ± 1.7 kg/m(2)). Pregnant women had elevated baseline sympathetic burst incidence (43 ± 2 vs. 33 ± 2 bursts/100 heart beats, P = 0.01) and total sympathetic activity (1,811 ± 148 vs. 1,140 ± 55 au, P < 0.01) relative to controls. Both mean (88 ± 3 vs. 91 ± 2 mmHg, P = 0.4) and diastolic (DBP) (72 ± 3 vs. 73 ± 2 mmHg, P = 0.7) pressures were similar between pregnant and nonpregnant women, respectively, indicating an upward resetting of the baroreflex set point with pregnancy. Baroreflex gain, calculated as the linear relationship between sympathetic burst incidence and DBP, was reduced in pregnant women relative to controls (-3.7 ± 0.5 vs. -5.4 ± 0.5 bursts·100 heart beats(-1)·mmHg(-1), P = 0.03), as was baroreflex gain calculated with total sympathetic activity (-294 ± 24 vs. -210 ± 24 au·100 heart beats(-1)·mmHg(-1); P = 0.03). Cardiovagal baroreflex gain (sequence method) was not different between nonpregnant controls and pregnant women (49 ± 8 vs. 36 ± 8 ms/mmHg; P = 0.2). However, sympathetic (burst incidence) and cardiovagal gains were negatively correlated in pregnant women (R = -0.7; P = 0.02). Together, these data indicate that the influence of the sympathetic nervous system over arterial blood pressure is reduced in normotensive pregnancy, in terms of both long-term and beat-to-beat regulation of arterial pressure, likely through a baroreceptor-dependent mechanism.

Keywords: baroreflex control; blood pressure; pregnancy; sympathetic nerve activity.

Fig. 1.

Sample microneurography recordings from a nonpregnant (A) and a pregnant woman (B). BMI, body mass index; DBP, diastolic blood pressure; hb, heartbeat.

Fig. 2.

Sympathetic burst incidence baroreflex gain in normotensive nonpregnant and pregnant women in the third trimester of gestation. A: muscle sympathetic nerve activity (MSNA) burst incidence was significantly elevated in normotensive pregnant women relative to nonpregnant controls. DBPs were similar between groups, indicating an upward resetting of the sympathetic baroreflex curve with pregnancy. The linear portion of the sympathetic baroreflex curves were less steep in pregnant women relative to the nonpregnant control participants (−3.7 ± 0.5 vs. −5.4 ± 0.5 bursts·100 heart beats⁻¹·mmHg⁻¹; P = 0.03), indicating a blunting of sympathetic baroreflex gain with pregnancy. B and C: sample sympathetic baroreflex curves in nonpregnant and pregnant women, respectively. BI, burst incidence. Data are means ± SE.

Fig. 3.

Total sympathetic activity baroreflex gain. The set point of the baroreflex was reset upward in normotensive pregnant women relative to nonpregnant controls. The linear portion of the total sympathetic baroreflex curve was less steep in pregnant women relative to nonpregnant women (−294 ± 24 vs. −210 ± 24 au·100 heart beats⁻¹·mmHg; P = 0.03). Data are means ± SE.

Fig. 4.

Cardiovagal baroreflex gain. The set point of the cardiovagal baroreflex was reset downward in pregnant women relative to nonpregnant controls, favoring shorter R-R intervals without significant changes in systolic bloods pressure (SBP) during pregnancy. However, the slopes of the cardiovagal baroreflex curves were not significantly different between pregnant and nonpregnant women (36 ± 8 vs. 49 ± 8 ms/mmHg; P = 0.2). Data are means ± SE.