Fetal and maternal influences on arterial oxygen levels in the sheep fetus.

Continuous recordings of arterial oxygen tension (PaO2) and percent oxygen saturation (SaO2) were made in 8 instrumented fetal sheep during late gestation. Fetal behavioural states were monitored from recordings of rapid eye movements, the electrocorticogram, and the nuchal electromyogram (EMG). In addition, fetal breathing and swallowing movements and uterine motility were measured using EMG techniques. Variations in PaO2 and SaO2 were related to fetal muscular activity, non-labour uterine contractions and maternal postural changes. Significant reductions in the variability of PaO2 and SaO2 were brought about in 7 of the 8 fetuses by skeletal muscle paralysis. Variation was further reduced in each fetus by paralysis of the myometrium by a beta-adrenergic agonist. Remaining variations were largely attributable to postural changes by the ewe. It is concluded that fetal skeletal muscle activity contributes to reduced amounts of oxygen in fetal arterial blood, although the mechanism by which this occurs has not yet been identified.