Umbilical artery flow velocity waveforms and placental vascular resistance during maternal placental outflow obstruction in sheep

Abstract

This study was designed to test the hypothesis that the pulsatility index (PI) of the umbilical artery flow velocity waveform varies as a function of placental vascular resistance. Placental vascular resistance was raised by a one-minute occlusion of the maternal inferior vena cava. Occlusion of the maternal inferior vena cava resulted in a decrease in fetal heart rate from 183 +/- 7.8 beats/min to 142 +/- 8.6 beats/min at the end of occlusion (P less than 0.05). Placental vascular resistance increased from 0.113 +/- 0.021 mmHg.ml-1.min during control to 0.151 +/- 0.033 mmHg.ml-1.min (P less than 0.05) during occlusion. The pulsatility index increased from 1.05 +/- 0.05 to 1.85 +/- 0.4 (P less than 0.05) during occlusion. After parasympathetic blockade with atropine fetal heart rate did not change during occlusion. Placental vascular resistance increased from 0.091 +/- 0.014 before to 0.121 +/- 0.021 mmHg.ml-1.min during occlusion (P less than 0.05). The pulsatility index increased from 0.98 +/- 0.1 before to 1.12 +/- 0.12 during occlusion (P less than 0.05). These results support the hypothesis that, in the fetal sheep, placental vascular resistance is one of the determinants of the pulsatility index of the umbilical artery.