Relationship between the ovine fetal umbilical artery blood flow waveform and umbilical vascular resistance

Abstract

Ultrasonic umbilical artery blood flow velocity waveform indexes decrease throughout gestation as umbilical blood flow increases. However, it is not known whether the decrease in indexes such as systolic/diastolic ratio is a direct result of a change in umbilical vascular resistance. Umbilical artery blood flow was measured as vasoactive agents angiotensin II, arginine vasopressin, norepinephrine, serotonin, and the thromboxane A2 sympathomimetic agent U-46619 were infused into the inferior vena cava of chronically instrumented ovine fetuses. Angiotensin II, arginine vasopressin, norepinephrine, serotonin, and U-46619 all significantly increased fetal arterial pressure levels. Angiotensin II, arginine vasopressin, and U-46619 significantly reduced umbilical blood flow and fetal heart rate. All five pressor agents increased umbilical vascular resistance. Systolic/diastolic ratios were obtained from pulsatile umbilical blood flow tracings. Systolic/diastolic ratios before infusions were significantly correlated with baseline values of umbilical blood flow (r = -0.55) and umbilical vascular resistance (r = 0.52). Systolic/diastolic ratios decreased with increasing infusion rates of angiotensin II, arginine vasopressin, norepinephrine, and serotonin, and increased with infusion of U-46619. There was no significant correlation between systolic/diastolic ratio and either umbilical blood flow or umbilical vascular resistance during infusion of angiotensin II, arginine vasopressin, or serotonin. Systolic/diastolic ratio decreased with increasing umbilical vascular resistance during norepinephrine infusion (r = -0.44). Systolic/diastolic ratio changed in the predicted direction only when umbilical vascular resistance was increased by infusion of U-46619 (r = 0.80). We conclude that umbilical artery waveform analysis is not an adequate predictor of acute changes in umbilical blood flow or vascular resistance.