Optimal Maternal Ventilation During Laparotomy with General Anesthesia in Pregnancy in the Ovine Model

General anesthesia during pregnancy is not uncommon, for example, for trauma surgery, cerclage, or cesarean delivery. Current recommendations are to maintain maternal partial pressure of carbon dioxide in arterial blood (paCO2) at 30 mm Hg, which is based solely on the average maternal paCO2 in awake pregnant women. However, there is no evidence that this target, compared to other targets, would enable optimal conditions for the fetus during general anesthesia. Maternal paCO2 can affect uterine blood flow, affinity of hemoglobin for oxygen, and fetal CO2 elimination. In this study, a range of potential targets of maternal paCO2 was investigated in the ovine model, aiming to determine which target is most conducive to physiological fetal blood gas values during laparotomy with general anesthesia.

Ten time-mated pregnant Swifter ewes with a gestational age of 93 to 104 days were used. During the first phase of the experiment, anesthesia was induced, all ewes were ventilated to target a physiological maternal paCO2 of 30 mm Hg, a maternal laparotomy was performed, and a fetal microcatheter was inserted surgically to enable blood sampling from the fetal aorta. Thereafter, in the second phase of the experiment, the 10 pregnant ewes were randomized to 10 different targets of maternal paCO2 between 27 and 50 mm Hg (1 target for each ewe), and maternal ventilation was adjusted accordingly. Forty-five minutes later, maternal and fetal arterial blood gas samples were analyzed. Linear regression models were used to estimate maternal paCO2 enabling physiologic fetal parameters, including fetal paCO2 (primary outcome).

A maternal paCO2 of 27.4 mm Hg (95% confidence interval, 23.1-30.3) enabled physiological fetal paCO2. Each increase in maternal paCO2 by 1 mm Hg, on average, increased fetal paCO2 by 0.94 mm Hg (0.69-1.19). This relationship had a strong correlation (r² = 0.906). No fetuses died during the experiment.

This study provides experimental support for the clinical recommendation to maintain maternal paCO2 close to the physiologic value of 30 mm Hg during general anesthesia for maternal laparotomy in pregnancy as it is conducive to physiological fetal blood gas values. Given the lower bound of the 95% confidence interval, the possibility that a lower maternal paCO2 would improve fetal gas exchange cannot be excluded.