Initiation of pulmonary gas exchange by fetal sheep in utero.

The role of umbilical cord occlusion in the initiation of breathing at birth was investigated using unanesthetized fetal sheep that were provided with access to a tracheal supply of hyperoxic air. Near-term fetuses were studied in utero to eliminate extraneous sensory stimuli. Gasping movements began 1.4 +/- 0.1 min after cord occlusion. Breathing was irregular for several minutes before continuous breathing (greater than or equal to 40 min-1) began 6 +/- 1 min after cord occlusion (n = 10). Arterial PO2 rose significantly from 18 +/- 2 mmHg before occlusion and was 115 +/- 15 mmHg immediately before cord release at 15 or 30 min. Breathing continued even during high-voltage electrocortical activity. Cord release caused the breathing rate to decrease from 77 +/- 13 min-1 during the last 5 min of cord occlusion to 5 +/- 3 min-1 10 min after cord release (P less than 0.002; n = 7). Results indicate the change from placental to lung gas exchange can occur in the absence of sensory and thermal changes normally present at birth and that the transition is reversible.