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32: The effect of supine recumbency on fetal aortic and umbilical blood flow and heart rate patterns

      Objective

      In this study we investigated the hemodynamic and cardiovascular response of the human fetus to maternal supine recumbency in low risk pregnancies.

      Study Design

      One hundred sixty one low-risk pregnant women participated in the study. Gestational age ranged from 36 weeks to term. Thirty minutes recordings of FHR, uterine contractions and fetal movements were obtained from each woman both in supine and left lateral decubitus positions, so that each woman served as her own control. All recordings were sampled into a computer and subsequently analyzed by a set of computer programs. In 38 women Doppler flow measurements were also obtained from the descending aorta and the intra-hepatic portion of the umbilical vein in supine and left lateral decubitus positions. Results are expressed as the mean ±SEM.

      Results

      The volume flow rate in the descending aorta was 239.9±38/6 ml/min/kg in the supine position compared to 257.3±44.ml/min/kg in the left lateral position. The respecrive values in the umbilical vein were 112.6±5.7 and 117.8±6.3 ml/min/kg. The mean number of FHR accelerations decreased from 10.3±0.46 bpm in the lateral position to 7.7±0.42 in the supine position (p<0.0001). The bandwidth heart rate variability decreased from 9.1±0.41 bpm in the lateral position to 7.54 ±0.35 bpm in the supine position (p<0.002). There were significantly more non-reactive traces in the supine position compared to the lefta lateral position (p<0.007) with a likelihood ratio of 8 for a non-reactive trace in the supine postion (p<0.005).

      Conclusion

      Supine recumbency in late pregnancy leading to aortic and vena-caval compression, a decrease in cardiac output and in systemic blood pressure, is associated with reduced aortic and umbilical venous flow in the fetus. Non-reeassuring FHR patterns are significantly more prevalent in the supine position as reflected by a decrease in the number of acceleretations, an increase in the frequency of non-reactive traces and in a supression of all measures of fetal heart rate variation.