Changes in the pulmonary alveolar subphase at birth in term and premature lambs.

To study the changes in alveolar fluid at birth, we measured alveolar Cl- as soon as possible after onset of ventilation in 11 anesthetized lambs. The lambs ranged from 129 to 144 days gestation and were delivered by cesarean section. Subpleural alveoli were punctured with Cl- selective microelectrodes as soon as 5 min and as late as 135 min after the onset of mechanical ventilation. The [Cl-] was 151 +/- 7 mEq/liter (mean +/- SD, n = 11) in fetal lung fluid collected before ventilation. After about 40 min of ventilation, alveolar Cl- was not different from that in term lambs 24 to 72 h old (94 +/- 6 mEq/liter, n = 21). Assuming first order kinetics the mean t 1/2 was 10.4 min. There was no difference as a function of gestation. Thus, alveolar chloride decreases rapidly and alveolar fluid assumes a mature character very soon after the start of breathing in term and premature lambs. The onset of ventilation appeared to stimulate these rapid changes.