Alteration of arterial gas composition by positive pressure ventilation in the unanesthetized fetal lamb in utero

To study the effectiveness of in utero ventilation in altering fetal arterial gas composition, we ventilated 15 near-term fetal lambs with a range of inspired gas mixtures. To accomplish this, double lumen nasogastric tubes were surgically placed in the tracheas of 15 near-term (35 days' gestation) fetal lambs. After 4 ± 1 SD postoperative days the fetuses were respired by positive pressure ventilation. 13 of these fetuses were also ventilated with their umbilical cords completely occluded. Ventilation was maintained for an average of 4.5 h (range 2.5-7.5 h). All 15 fetuses were effectively oxygenated on room air when exposed to a small net positive end expiratory pressure. In fetuses with intact umbilical cords, PaO₂ could be maintained at levels between 11 and 280 mm Hg and PaCO₂ from 36 to 139 mm Hg by altering the inspired O₂ and CO₂ with a gas mixing device. During umbilical cord occlusion, PaO₂ was regulated from 2.2 to 103 mm Hg and PaCO₂ from 37 to 187 mm Hg.