Maternal Cardiovascular Responses to Position Change in Pregnancy

Abstract

The maternal cardiovascular-circulatory system undergoes profound changes almost from the conception of a pregnancy until the postpartum period to support the maternal adaptions required for pregnancy and lactation. Maintenance of cardiovascular homeostasis requires changes in the cardiovascular autonomic responses. Here, we present a longitudinal study of the maternal cardiovascular autonomic responses to pregnancy and maternal position. Over a normal gestation, in the left lateral position there are significant changes in both time and frequency domain parameters reflecting heart rate variability. We show that cardiovascular autonomic responses to physiological stressors (standing and supine positions in late pregnancy) became significantly different with advancing gestation. In the third trimester, 60% of the subjects had an unstable heart rate response on standing, and these subjects had a significantly reduced sample entropy evident in their heart rate variability data. By 6 weeks, postpartum function returned to near the non-pregnant state, but there were consistent differences in high-frequency power when compared to nulligravid cases. Finally, we review complementary evidence, in particular from magnetic resonance imaging, that provides insights into the maternal and fetal impacts of positioning in pregnancy. This demonstrates a clear relationship between supine position and maternal hemodynamic parameters, which relates to compression of the inferior vena cava (p = 0.05). Together, these studies demonstrate new understanding of the physiology of physiological stressors related to position.

Keywords: autonomic control; blood flow; cardiovascular adaptions; magnetic resonance imaging; pregnancy.

Figure 1

Data collection protocol used in each study visit, with right and supine positions randomized to reduce any order effect.

Figure 2

Examples of stable patterns (left) and unstable patterns (middle and right) after an orthostatic manoeuvre. These unstable patterns arose in the third trimester, with 60% of participants exhibiting unstable patterns at this stage in pregnancy.

Figure 3

A summary of the changes in blood flow and oxygen transfer parameters in healthy pregnancies in late gestation derived from MRI imaging studies reviewed here. MRI images are shown in axial planes, using radiological convention for axes. The IVC and aorta are highlighted in blue and red, respectively. NS = non-significant changes.