The effect of body position on maternal cardiovascular function during sleep and wakefulness in late pregnancy

An association between the increased risk of late stillbirth and the maternal supine sleeping position has been recently established. The risk of stillbirth following supine sleep has been suspected to occur as a result of aortocaval compression by the gravid uterus. A number of studies conducted during wakefulness have reported compromised cardiovascular function during supine rest, as demonstrated by reductions in cardiac output, blood pressure and utero-placental blood flow. It remains unclear whether similar effects are also present during sleep, due to the presence of key sleep-specific changes in cardiovascular function. To investigate the changes in maternal cardiovascular function between the supine and left-lateral positions during wakefulness and non-rapid eye movement (NREM) sleep in late pregnancy. Twenty-nine women with a singleton pregnancy between 24.7 and 36.7 weeks’ gestation participated in a single overnight sleep study. Physiological measures (blood pressure, heart rate, heart rate variability – HRV, and pulse arrival time – PAT) were measured and recorded throughout the night using standard polysomnography equipment and the Portapres Model-2 device. As the present study evaluated cardiovascular changes during natural rest and sleep in pregnancy, participants were not given explicit instructions on which position to adopt. Body position was continuously recorded using a position monitor and verified with video recording. No changes in systolic, diastolic or mean arterial blood pressure were observed between the left-lateral and supine positions during wakefulness or sleep. However, heart rate was significantly higher in the supine position compared to the left during wakefulness (<i>p=</i> .03), with a similar trend present during sleep (<i>p</i>= .11). A significantly shorter PAT was measured in the supine position (compared to the left) during wakefulness (<i>p=</i> .01) and sleep (<i>p=</i> .01). No change in HRV measures was observed between the left and supine positions in either state. Blood pressure did not appear to differ significantly between the left-lateral and supine positions during wakefulness and sleep. The lack of blood pressure differences may reflect elevated sympathetic activity during rest and sleep in the supine position (compared to the left), suggesting that some degree of compensation for aortocaval compression may still be possible during sleep.

近期已有研究证实，妊娠晚期孕妇仰卧位睡眠与死产风险升高存在关联。此前推测仰卧位睡眠导致的死产风险升高，可能源于妊娠子宫对主动脉及腔静脉的压迫。多项清醒状态下的研究表明，仰卧位静息时心血管功能会受损，具体表现为心输出量、血压及子宫胎盘血流量降低。鉴于睡眠状态下心血管功能存在特有的关键变化，目前尚不清楚睡眠时是否也会出现类似的心血管受损效应。

本研究旨在探究妊娠晚期孕妇在清醒状态及非快速眼动（non-rapid eye movement, NREM）睡眠时，仰卧位与左侧卧位下的心血管功能变化。本研究共纳入29名孕周为24.7~36.7周的单胎妊娠孕妇，完成1次夜间睡眠监测。研究采用标准多导睡眠监测设备及Portapres Model-2型设备，整夜监测并记录多项生理指标：血压、心率、心率变异性（heart rate variability, HRV）及脉搏到达时间（pulse arrival time, PAT）。

鉴于本研究旨在评估孕妇自然静息及睡眠状态下的心血管变化，研究人员未对受试者的体位选择作出明确要求。研究采用体位监测设备持续记录孕妇体位，并通过视频录像进行验证。无论在清醒状态还是睡眠状态，左侧卧位与仰卧位下的收缩压、舒张压及平均动脉压均无显著差异。但在清醒状态下，仰卧位时的心率显著高于左侧卧位（p=0.03）；睡眠状态下也呈现类似趋势（p=0.11）。无论清醒状态还是睡眠状态，仰卧位下的脉搏到达时间均显著短于左侧卧位（均p=0.01）。两种状态下，左侧卧位与仰卧位的心率变异性指标均无显著变化。无论清醒状态还是睡眠状态，左侧卧位与仰卧位的血压仍未出现显著差异。

血压无显著差异的现象，可能反映了仰卧位静息及睡眠时交感神经活动较左侧卧位时更强，提示睡眠状态下仍可在一定程度上代偿妊娠子宫对主动脉腔静脉的压迫。