Thromboxane receptor blockade causes acidemia in the ovine fetus

The placenta produces the vasoactive eicosanoids thromboxane and prostacyclin. We hypothesized that fetal administration of SQ 29,548, a thromboxane receptor blocker, would lack direct cardiorespiratory effects in the ovine fetus. Continuous monitoring of maternal and fetal heart rates, blood pressures, and common umbilical artery blood flow was performed in six chronically catheterized pregnant ewes. Serial maternal and fetal arterial blood gases and serum lactates were obtained. After 120 minutes of infusion, a significant decrease in fetal mean arterial pressure and a significant increase in fetal heart rate was observed. A significant decrease in fetal arterial pH (7.41 ± 0.02 to 7.33 ± 0.02), PO₂ (26.8 ± 3.2 to 19.8 ± 3.4), HCO₃^- (27.2 ± 1.5 to 24.9 ± 2.0), and base excess (2.9 ± 1.6 to 0.2 ± 2.3) with a significant elevation in PCO₂ (45.4 ± 2.6 to 50.1 ± 3.3) occurred after 120 minutes of SQ 29,548 infusion. SQ 29,548 infusion caused a significant decrease in common umbilical artery flow, from 249.4 ± 19.4 ml ± min^-1 ·; kg^-1 fetal weight to 178.3 ± 17.9 ml · min^-1 · kg^-1 fetal weight at 120 minutes. Fetal blood lactate levels were significantly elevated from 22.0 ± 3.1 to 54.1 ± 3.8 mg/dL after 120 minutes of SQ 29,548 infusion. Prolonged infusion of SQ 29,548 results in umbilical-placental hypoperfusion and significant alterations in acid-base balance in the ovine fetus.