Maternal treatment with a cardioselective beta-blocking agent--consequences for the ovine fetus during intermittent asphyxia.

To evaluate the effect of chronic beta 1-adrenoceptor blockade on physiological adaptation to asphyxia a study was done on exteriorized sheep fetuses of 127-142 days gestational age. Eleven pregnant ewes were infused with metoprolol for 5 days prior to experiment. Another 10 ewes were infused with saline and served as controls. Asphyxia was induced by intermittent complete obstruction of maternal placental blood flow. Fetal electro-cardiogram, heart rate, cardiac output, myocardial contractility and cerebral blood flow were measured together with blood pH, lactate and hypoxanthine. Neurophysiological responses were evaluated by changes in somatosensory evoked electroencephalogram. The beta 1-blocked fetuses showed less responsiveness in myocardial contractility and heart rate during reoxygenation. This curtailed reaction resulted in accelerated lactic acidosis, increased break-down of intracellular energy rich substances and impaired cerebral function. Nine of the ten controls survived the experiment and 8 of them regained their somatosensory evoked EEG potentials, whereas 7 of the 11 beta-blocked fetuses survived and only 3 regained original somatosensory evoked EEG potentials. It is concluded that beta 1-adrenoceptor blockade impairs the adaptive responses to asphyxia in the ovine fetus and decreases its ability to survive severe asphyxia.