A longitudinal study of maternal cardiovascular autonomic responses to position change during pregnancy

Abstract

New Zealand and international research has shown that there is an association between going to sleep in the supine position in late pregnancy and an increased risk of stillbirth. Lying supine in late pregnancy results in a significant decrease in maternal cardiac output and blood flow through the abdominal aorta and the inferior vena cava. These changes in blood flow are partially compensated for by an increase in blood flow through the azygos vein, but this does not fully explain the physiological mechanisms behind maintenance of blood flow. This thesis aimed to describe maternal cardiovascular autonomic responses while resting in different positions throughout a normal healthy pregnancy and post-partum. This information would help to determine normal cardiovascular autonomic activity in response to acute physiological stressors. A cohort of pregnant women was studied longitudinally throughout their pregnancy and once postpartum. A control group of nulligravid women was also assessed. Non-invasive cardiovascular autonomic assessments included measurement of heart rate and blood pressure responses in the left lateral, right lateral, and supine positions, as well as in response to a deep breathing and an orthostatic manoeuvre. The standardised methodology used in this research as well as inclusion of numerous recumbent positions and a nulligravid control group has contributed novel data to the existing knowledge base. Maternal cardiovascular autonomic responses, assessed by changes in heart rate and blood pressure, were significantly affected by gestation. Gestational effects began in early pregnancy. Cardiovascular autonomic responses to physiological stressors (e.g. the standing position, and the supine position in late pregnancy) became significantly different with advancing gestation. These changes largely resolved by 6 weeks post-partum. In addition, a sub-group of women were identified who had an unstable heart rate pattern while standing in late pregnancy. Preliminary analysis of these women concluded that there were statistically significant differences in blood pressure and sample entropy between the two groups. Further analysis of women with unstable heart rate patterns while standing in late pregnancy has been identified as an important future direction. It is recommended that nulligravid women are used as controls in future research.