Norepinephrine elevation in the fetal lamb: oxygen consumption and cardiac output

Abstract
To determine if placental diffusion reserves exceed fetal O2 requirements, fetal O2 consumption (.ovrhdot.VO2) was increased by infusing 1.7-11.5 .mu.g of norepinephrine (NE) .cntdot. min-1. After 50 min of infusion .ovrhdot.VO2 rose 25% to 10.2 from 8.2 ml .cntdot. min-1 .cntdot. kg fetal wt-1. Placental CO diffusing capacity remained essentially unchanged from control, 0.49 .+-. 0.05 (SE) ml .cntdot. min-1 .cntdot. torr-1. .cntdot. kg-1. During the first 5 min of NE infusion fetal arterial blood pressure increased 29%, while heart rate decreased 15%. Coronary, pulmonary and umbilical blood flow, expressed per mg fetal wt, as determined by use of labeled microspheres, increased 50, 162 and 25%, respectively (P < 0.05), although fetal cardiac output remained constant at 538 .+-. 23 (SE) ml .cntdot. min-1 .cntdot. kg-1. NE-blood pressure dose-response relations were determined for the fetus. Blood pressure increase with dose up to 1 .mu.g .cntdot. min-1 .cntdot. kg-1, but failed to rise further with higher doses. Fetal .ovrhdot.VO2 increases with NE infusion. Placental reserve for O2 diffusion exceeds normal requirements. NE infusion is associated with increased blood pressure, bradycardia and a redistribution of blood flows to the heart, lungs and placenta despite a constant cardiac output.