INITIATION AND MAINTENANCE OF CONTINUOUS BREATHING AT BIRTH

Changes in arterial PCO2 (PaCO2) and body temperature normally occurri ng at the moment of birth may play a role in the initiation and mainte nance of continuous breathing. To clarify these mechanisms, five chron ically instrumented fetal lambs were connected to an extracorporeal me mbrane oxygenation (ECMO) system. ECMO was initiated in utero at a flo w rate sufficient to support the fetus totally, the umbilical cord was occluded, and the fetuses were delivered into a warm isotonic saline bath. Breathing activity was present periodically before connection to the ECMO system and on ECMO during fetal normocapnia and normoxia. Ne ar delivery there were no breathing movements, because all ewes were l abor. After delivering the fetuses into the warm saline bath, breathin g movements remained episodic, being absent during high voltage electr ocortical activity, whereas fetal PaCO2 remained constant. However, af ter 36-192 min, breathing activity became present continuously in all animals, at a time when fetal central temperature decreased. Once init iated, continuous breathing could be stopped by reducing the PaCO2. We conclude that maintenance of fetal PaCO2 and a slow decrease in centr al temperature after cord occlusion delays the establishment of contin uous breathing, and that the level of PaCO2 is important in the mainte nance of breathing activity during early postnatal life.