Effects of isoxsuprine on maternal and fetal acid-base balance and circulation.

Prematurity is a large contributing factor to neonatal morbidity. An obstetric alternative to premature labor is administration of isoxsuprine, an effective inhibitor of uterine contractions. Six crossbred unanesthetized pregnant ewes were used to determine the effects on fetal and maternal cardiovascular and acid-base status of 10 maternal intravenous infusions of 30 mg isoxsuprine over 30 minutes (1 mg/min). Isoxsuprine caused slight maternal hypotension and a transient acidosis, both of which returned to baseline values within 60 minutes. Maternal tachycardia persisted throughout the experiment. A slight decrease in the oxygen percent saturation of the hemoglobin and the calculated O2 content was observed after the infusion. There was no significant change in uterine blood flow. Maternal lactate concentration transiently increased at 60 minutes only. In the fetus, isoxsuprine caused transient hypotension and tachycardia. Fetal arterial PCO2, calculated bicarbonate concentration, and O2 content all decreased slightly at 45 minutes only. Fetal hyperglycemia was observed after the infusion throughout the experiment. All other fetal values returned to baseline levels within 60 minutes. It is suggested that 30 mg isoxsuprine can cause tolerable maternal and fetal metabolic and physiologic effects when administered intravenously to the unanesthetized pregnant ewe.