Effect of halothane on cardiac output and regional flow in the fetal lamb in utero.

We studied the effect of halothane on the fetal cardiovascular system of six lambs in utero by measuring fetal heart rate and femoral arterial blood pressure and by injecting labeled microspheres during a control period and again after 60 and 90 min of halothane anesthesia administered to six pregnant ewes at an inspired concentration of 1.5%. There were no significant effects on maternal cardiovascular function or acid-base balance, but fetal blood pressure decreased significantly by 27% after 8 min of halothane anesthesia and remained at this level for the duration of the experiment. However, there were no significant changes either in fetal regional blood flow to the vital organs or in fetal cardiac output. Fetal oxygenation and acid-base status remained stable. We conclude that in normal fetal lamb in utero the decrease in mean fetal arterial blood pressure associated with maternal halothane anesthesia is due to a decrease in peripheral vascular resistance because regional blood flow and acid-base status are well maintained.