Cephalic circulation and oxygen consumption before and after birth

Abstract

Cephalic hemodynamics and O2 consumption were studied in fetal and neonatal lambs before and after umbilical cord clamping. Maternal blood pO2 and PCO2 were altered by ventilating the ewe''s lungs with gas mixtures of known composition. Neonatal blood gas tensions were similarly altered through lung ventilation with the same gas mixtures. The findings are as follows: In both the fetal and neonatal states, cephalic blood flow and vascular resistance are regulated mainly by blood pCO2 with the pO2 playing a definite but lesser role; the changes are independent of left ventricular output and blood pressure. Cephalic oxygen consumption remains fairly constant with changing maternal or neonatal blood gas tensions. Interruption of umbilical circulation results in a transitory fall in cephalic blood flow and O2 consumption related to increased vascular resistance probably caused by the sudden changes in blood gas tensions that follow expansion of the lungs and initiation of respiration.