Effect of continuous infusion of norepinephrine on maternal pelvic and fetal umbilical blood flow in pregnant sheep

The effect of continuous maternal intravenous infusion with norepinephrine on the blood flow in the maternal internal iliac and the median uterine artery was studied in ten chronically instrumented pregnant sheep between 104 and 146 days gestation. Furthermore the effects on umbilical venous blood flow, fetal heart rate and acid-base balance were analyzed. Maternal and fetal blood flows were measured with electromagnetic flow transducers. Norepinephrine was administered to the ewe via a continuous intravenous infusion in increasing sequential doses of 15 minutes duration from 4 to 40 micrograms per minute. Variations of often considerable magnitude associated with e. g. micturition, defecation and fear of the ewe occurred in the maternal pelvic blood flow during the steady state period. The blood flow in the maternal vessels substantially decreased immediately following the onset of the norepinephrine infusion but gradually returned towards the preinfusion level despite the continued drug infusion and except one no significant changes in blood flow were found at the end of each sequential infusion period. Umbilical venous blood flow did not change. No significant changes in fetal arterial blood pressure, heart rate and acid base balance were found. It is concluded that the decrease in maternal pelvic blood flow associated with continuous norepinephrine administration gradually abates with time, possibly by the involvement of local factors such as prostaglandin formation and/or by the phenomenon of down-regulation.