Purpose: The aim of this study was to determine the hemodynamic and blood gas effects of inhalational anesthetics on the maternal-fetal sheep unit. The principal hypothesis, tested in chronically instrumented near-term pregnant ewes, was that sevoflurane anesthesia may be safe and useful for the mother and fetus during pregnancy, compared with isoflurane.
Methods: Six chronically instrumented pregnant and 3 nonpregnant ewes were tested repeatedly to establish the minimum alveolar concentration (MAC) for sevoflurane and isoflurane to be used in the hemodynamic and blood gas studies. Progressively increasing concentrations of sevoflurane or isoflurane in oxygen were administered to 12 pregnant ewes. Uterine blood flow, maternal and fetal heart rates, blood pressure, arterial blood gases, and intra-amniotic pressure were subsequently measured.
Results: The MAC of sevoflurane was 1.52 +/- 0.1 15% and 1.92 +/- 0.17% in pregnant and nonpregnant ewes, respectively; while the MAC of isoflurane in the pregnant and nonpregnant sheep was 1.02 +/- 0.12% and 1.42 +/- 0.19%, respectively. In both the sevoflurane and isoflurane groups, changes in maternal and fetal blood gases were minimal during exposure to low-dose (0.5-1.0 MAC) inhaled concentrations. Although uterine blood flow was maintained and the fetus remained well oxygenated at higher concentrations of both agents (2.0 MAC of either agent), the agents produced decreases in maternal and fetal arterial pressure.
Conclusion: A "low-dose" concentration (0.5-1.0 MAC) of sevoflurane may be safe and useful for both mother and fetus during near-term pregnancy. However, a high concentration (1.5-2.0 MAC) of sevoflurane or isoflurane may induce hemodynamic instability in the mother and fetus when administered.