Effect of hypoxemia and acidemia on the fetal cardiac response to acetylcholine: experimental observations in fetal baboons.

Abstract The effect of hypoxemia and acidemia on the fetal cardiac response to acetylcholine (ACh) was studied in 24 baboon fetuses in utero. A bolus injection of ACh was giv8n intravenously to the fetus at a dose that ranged from 5 to 160 µg/kg of estimated fetal weight. The responses were divided into two groups accordiog to fetal oxygenation and acid-base state. Fetuses in group I Were well oxygenated (PaO2, 31 ± 1.4 mm Hg; pHa, 7.32 ± 0.154); only five atrioventricutar (A-V) heart blocks were induced with 50 ± 3.3 µg/kg of ACh. Fetuses in Group II were hypoxemic (PaO2, 18 ± 1.2 mm Hg) and acidemic (pHa, 7.05 ± 0.048); ten A-v heart blacks-were induced with a comparable dose of ACh (45 ± 6.8 µg/kg). Dose response relationship is noted in individual fetuses with multiple dosage injections. These findings suggeetthat the fetal cardiac response to ACh is both dose related and more profound when the fetus is hypoxemic and acidemic, and indicate that the hypoxic and acidotic fetus is at greater risk from strong stimulation of the parasympathetic nervous system. Atropine prevented myocardial conduction defect induced by intravenous injection of ACh even in the severely hypoxemic baboon fetus. This observation suggests that judicious use of atropine could be a life-saving measure when a prolonged severe of cord occlusion is suspected. (Am. J. Obstet. Gynecol. 141:682, 1981.)