Fetal hemodynamic and fetoplacental vascular response to exogenous arginine vasopressin

Fetal hemodynamics and fetoplacental blood flow were measured in chronically instrumented ovine fetuses during intravenous infusion of arginine vasopressin. Vasopressin was infused at rates ranging from 1 to 300 ng/min/kg estimated fetal mass. This range of infusion rates produces plasma arginine vasopressin levels observed throughout a wide range of fetal stress. No maternal effects were observed at any infusion rates used in this investigation. Fetal heart rate declined linearly with the log of the infusion rate. Mean fetal arterial pressure showed a sigmoidal response to log arginine vasopressin infusion rate, reaching a plateau at 30 ng/min/kg. Umbilical vascular resistance increased throughout the entire range of infusion rates. Fetoplacental blood flow decreased with increasing infusion rate but decreased only 4% to 13% throughout the range of infusion rates that produce plasma arginine vasopressin levels commonly observed during fetal stress. Because umbilical vascular resistance continued to rise after arterial pressure reached a plateau, fetoplacental blood flow decreased 31% at the highest infusion rate. However, the plasma vasopressin level associated with this infusion rate is probably in excess of that associated with severe fetal distress. The fetoplacental vascular bed is, therefore, either relatively insensitive to arginine vasopressin, or is capable of autoregulation in the face of high circulating levels of arginine vasopressin.