The effect of atropine on heart rate and oxygen consumption of the hypoxic fetus.

To determine if vagal blockade during hypoxic bradycardia improves fetal oxygenation, 10 studies were carried out on five chronically instrumented sheep after 0.8 week of gestation. At a mean maternal arterial oxygen tension of 33 +/- 4 mm Hg, the fetal heart rate decreased 20% to 137 +/- 19 bpm, and fetal oxygen consumption decreased 42% to 4.6 +/- 1.5 ml/min/kg of fetus. After 200 micrograms atropine was infused into a fetal vein, fetal heart rate increased immediately to 249 +/- 26 bpm. Fetal oxygen consumption was unchanged. Fetal heart rate increased by 18 bpm after atropine in the normoxic fetus. These studies show that there is a substantial bradycardia due to an increase in vagal activity during hypoxia in the fetus and that blockade of this activity causes a marked tachycardia but no increase in fetal oxygen uptake.