Determinants of the onset of continuous air breathing at birth.

We used mechanical ventilation of the fetal lungs in utero on 11 fetal lambs at 140-145 days gestation to alter fetal blood gases and thus separate the influences of PaO2 and PaCO2 on extrauterine breathing after cord clamping. The fetus was delivered either into a 40 degrees C saline bath or onto a cold table. Mechanical ventilation was stopped 2 min after delivery and the time to onset of continuous air breathing was observed. Also two fetuses were ventilated in utero 5 or more days after chronic instrumentation at 127 days gestation; in these animals the time to onset of breathing (diaphragm EMG) was recorded after stopping the ventilator and occluding the cord. We conclude: (a) hypercapnia is a stimulus to breathing even in hyperoxia and at 40 degrees C; (b) hypocapnia delays the start of extrauterine breathing in hyperoxia at 40 degrees C; (c) hypoxia inhibits breathing in the absence of hypercapnia or cold; (d) cold overrides the effects of hypocapnia in normoxia or moderate hypoxia.