The effect of continuous or pulsatile administration of oxytocin to ewes at 126 to 136 days' gestation on myometrial activity and fetal oxygenation

Oxytocin was infused either continuously or in pulses to pregnant ewes to compare the responses of myometrial activity (contractures) and changes in fetal arterial Po₂. Group 1 (n = 5) was infused with saline solution throughout the experiment. In group 2 (n = 5) oxytocin was infused continuously (160 μU · kg^-1 · min^-1) for 7 days whereas group 3 (n = 5) ewes received 960 μU · kg^-1 · min^-1 for 5 minutes every 30 minutes for 7 days. Contracture frequency increased in both oxytocin-infused groups. With continuous infusion contracture frequency returned to preinfusion levels during the last 4 days of infusion, whereas with pulsatile infusion contracture frequency remained increased throughout the infusion period. Fetal arterial Po₂ was decreased throughout the 7 days of infusion in both oxytocin-infused groups. Pulsatile group fetal arterial Po₂ levels remained decreased after the oxytocin infusion was stopped. These findings show; (1) Myometrial response to oxytocin in late pregnant ewes is influenced by the mode of administration; (2) administration of oxytocin to pregnant ewes resulted in a decrease in fetal Po₂, and thus fetal hypoxemia cannot be attributed exclusively to increased contracture frequency.