Pre-ejection period of cardiac cycles in fetal lamb

The pre-ejection period (PEP) of the fetal cardiac cycle was studied on 22 chronically instrumented pregnant ewes. The PEP was measured from the onset of the Q wave on fetal electrocardiogram to the onset of the upstroke on the fetal arterial blood pressure curve. Lengthening of the PEP was observed in association with an increase in gestational age, fetal body weight, fetal brow-rump length, and fetal heart weight. Consequently, the PEP values were calculated to those at fetal heart weight of 10 grams. The PEPc (calculated) was prolonged by acidosis but no significant relationship was found between the PEPc and arterial blood pO2. An increase in coronary blood flow was associated with a shortening of the PEPc in the fetuses under normal physiologic conditions. Fetal hypoxemia and respiratory acidosis created by administration of 10 per cent O2 and 20 per cent CO2 with 20 per cent O2 to the mother increased fetal coronary flow and was in general associated with a prolonged pre-ejection period.