EFFECT OF DRUGS ON CHEMORECEPTOR RESPONSIVENESS IN FETAL SHEEP

This study was designed to examine the effects of the drugs ketamine, morphine, pentobarbital, and propranolol on fetal chemoreceptor respon siveness. Eleven fetal lambs (gestational age 125-133 d) were chronica lly instrumented with a catheter in a hindlimb artery and vein and a f orelimb artery, a carotid arterial oximeter catheter was placed in six of these fetuses. An inflatable cuff occluder was placed around the m aternal hypogastric artery. Acute fetal hypoxemia was induced repeated ly by reducing uterine blood flow. Fetal heart rate, arterial pressure , and carotid arterial oxygen saturation were monitored continuously b efore and after administering ketamine, morphine, pentobarbital, or pr opranolol to the fetus. The ratio Delta heart rate/Delta O-2 saturatio n has been shown previously to be a reproducible index of chemoreflex response. The differences in baseline values and changes with drugs we re compared by multiple regression analysis coded by effects. Chemoref lex response was markedly attenuated by ketamine and morphine but not by pentobarbital or propranolol. Because the cardiovascular response t o hypoxemia is blunted by some drugs, caution should be exercised in i nterpreting heart rate responses to hypoxemia in the fetus when these drugs have been administered to the mother.