Effects of acetylsalicylic acid on the ductus arteriosus and circulation in fetal lambs in utero.

Intra-arterial and intravenous catheters were inserted in six fetal lambs at 125-130 days of gestation. On the following day, fetal arterial pressures and blood gases were monitored and fetal cardiac output and its distribution were measured by injection of radionuclide-labeled microspheres 15 mum in diameter. Acetylsalicylic acid, 55-90 mg/kg of estimated fetal weight, then was administered into the fetal stomach. Fetal pulmonary arterial pressure rose significantly after an average of 58 minutes, increasing the pressure difference between the pulmonary artery and the aorta from 2 +/- 0.3 (SEM) mm Hg during control to 11.2 +/- 1.6 mm Hg. Resistance across the ductus arteriosus rose from 4.2 +/- 0.5 (SEM) to 27.4 +/- 4.01 units, and flow fell from 495 +/- 44 (SEM) to 409 +/- 20 ml/minute. The proportion of combined ventricular output distributed to the placenta, adrenals, heart, and lungs increased, whereas the proportion of combined ventricular output distributed to the brain, liver, intestine, kidneys, and upper and lower body fell. In two fetuses infusion of prostaglandin E1 reversed the pulmonary hypertension. Inhibition of prostaglandin synthesis in fetal lambs produced constriction of the ductus arteriosus and redistribution of cardiac output. It is probable that prostaglandins, particularly E1, are involved in regulation of blood flow through the ductus arteriosus and various vascular beds in the normal resting fetus.