The effect of position on autonomic nervous activity in late pregnancy

Abstract

SummaryWe investigated the effect of three different recumbent positions on autonomic nervous activity in late pregnancy. Thirty pregnant and 24 nonpregnant aged-matched women were studied, and measures of heart rate variability in both time and frequency domains were compared using supine, right and left lateral decubitus positions. In the nonpregnant women, the normalised highfrequency power was greatest in the right lateral decubitus position. In the pregnant women, the normalised high-frequency power was lowest and the low/high-frequency power ratio was greatest in the supine position. Both the percentage decrease of normalised high-frequency power and the percentage increase of low/high-frequency power ratio in the supine and right lateral positions were greater than those in the left lateral position. For women in late pregnancy, the left lateral decubitus position may be beneficial because cardiac vagal activity is least suppressed and cardiac sympathetic activity is least enhanced. Aortocaval compression might be the mechanism underlying the change in cardiac autonomic nervous activity when supine and right lateral decubitus positions are assumed in late pregnancy.Keywords Sympathetic nervous system. Heart; pulse rate, variability. We previously demonstrated that the right lateral decubitus position is associated with the highest vagal tone when three recumbent positions were compared in young normal subjects and recommended this position for routine recumbency in this group [1]. In contrast, Mills and Chaffe observed the sleeping positions adopted by pregnant women of more than 30 weeks gestation and found that the majority adopted a sleeping position that reduced the likelihood of aortocaval compression, with 77% adopting a left tilt, 21% a right tilt and only 2% choosing the supine position [2]. Avoidance of positions which may produce aortocaval compression is desirable because of the higher susceptibility to hypotension from factors that impair normal circulatory control such as regional anaesthesia, labour, fetal blood sampling or general anaesthesia [3,4]. The left lateral decubitus position has been recommended during blood pressure measurement in pregnant women, especially in late pregnancy, by the British Hypertension Society, to minimise the effects of aortocaval compression [5].The aim of this study was to investigate the effect of different recumbent positions on autonomic nervous activity in nonpregnant women and those in late pregnancy by using spectral heart rate variability (HRV) analysis.

Risk of inferior vena cava compression syndrome during fetal MRI in the supine position – a retrospective analysis

“…In the last few decades, several studies have suggested that pregnant women should be sited in the lateral position during the whole fetal MRI examination to avoid the development of VCCS, particularly to avoid severe forms of VCCS that can lead to cardiovascular decompensation [8,13,15,17]. These recommendations are based on studies dealing with the topic of VCCS in pregnant women using different invasive and noninvasive methods.…”

“…These symptoms predominantly occur in the supine position [7]. With regard to the prevention of VCCS, it is recommended that supine positioning of pregnant women during any kind of lengthy diagnostic or therapeutic intervention should be avoided [6,8,15]. This recommendation applies particularly to the evolving technique of fetal magnetic resonance imaging (MRI), where examination times of up to 45 min are often necessary to obtain a complete and diagnostic set of images of the third-trimester fetus [18].…”

Comparison of supine and upright positions on autonomic nervous activity in late pregnancy: the role of aortocaval compression

“…The HRV analysis was performed in the same way as in our previous study [9,10]. In brief, the mean, standard deviation (SD) and coefficient of variation of 512 RR intervals were calculated using standard formulae.…”

“…Lateral tilt was suggested as a means of relieving compression of the inferior vena cava and improving cardiac output [7]. By using HRV analysis, the supine position was found to have the least cardiac vagal activity and the greatest sympathetic activity in three recumbent positions [9]. To evaluate further the effect of aortocaval compression on autonomic nervous activity, we compared the autonomic nervous activity in the supine and upright positions in nonpregnant women with those in women in late pregnancy.…”

Gravity and gravidity: will microgravity assist pregnancy?

“…Ventilation may also produce some discomfort to the mother due to elevation of the diaphragm and other known ventilatory changes, such as the increase of CO 2 and hypoxia sensitivity, of respiratory drive, and of respiratory impedance [4,5]. Finally, supine position may exert inappropriate stimulatory effects on the autonomic nervous system, due to the aorto-caval compression [6][7][8]. Therefore, in theory, the reduction of fetal weight by a microgravity environment may relieve some of the above-mentioned discomforts of pregnancy.…”