The uptake of isoflurane by the foetal lamb in utero: effect on regional blood flow.

Because isoflurane has recently been approved for clinical use in anaesthesia, we have studied the effect of this agent in the foetus using the pregnant ewe as an animal model. Eight pregnant ewes of 120-125 days gestation were surgically prepared with indwelling catheters and tracheostomy. Prior to anaesthesia, labelled microspheres were injected into the foetal circulation to determine cardiac output and regional blood flows to all organs. The ewes were anaesthetized with a constant inspired concentration of 2.0 per cent isoflurane in oxygen. Blood samples were drawn to construct an uptake curve for both mother and foetus. At 60 and 96 minutes of anaesthesia, microspheres were injected into the foetal circulation to measure changes in organ blood flow from the control period. Isoflurane crossed the placenta and appeared in the foetal circulation within two minutes. By 96 minutes, maternal and foetal arterial levels were 116.3 +/- 9.9 and 99.3 +/- 9.1 mEq/L (0.98 vol% and 0.75 vol%). There were no significant changes in foetal blood pressure or pulse rate but foetal pH decreased significantly from 7.39 +/- 0.02 to 7.26 +/- 0.2 (mean +/- SEM) and base excess decreased from -1.1 +/- 1.5 to -6.2 +/- 0.7. Foetal cardiac index decreased from 390.8 +/- 26.9 ml X kg-1 X min-1 to 292.0 +/- 13.8 after 96 minutes of anaesthesia. There were no significant changes in any of the maternal cardiovascular or acid-base parameters.