Alveolar liquid pressure measured by micropuncture in isolated lungs of mature and immature fetal rabbits.

Increased alveolar surface tension due to surfactant deficiency is thought to result in a negative pressure surrounding pulmonary capillaries and to promote fluid filtration. To test this hypothesis, alveolar liquid pressure (Pliquid) was measured by micropuncture in isolated lungs of mature and immature fetal rabbits (with and without surfactant replacement) at different air inflation pressures (Pairway). Lung maturity was assessed by air pressure-volume (P-V) curves. Pliquid was correlated with surfactant content in the lungs and with alveolar size. Pliquid was lower in immature (2.3 +/- 0.7 cmH2O) than in mature (8.4 +/- 1.0 cmH2O) lungs at comparable Pairway (25 cmH2O) (P less than 0.01). The mean linear intercept, a measure of airspace dimensions was similar in all lungs (42.1 +/- 2.0 micron), but alveolar wash phospholipid/g wet lung was lower in immature than in mature lungs (0.05 +/- 0.01 vs. 0.49 +/- 0.30 mg) (P less than 0.01). Surfactant replacement in immature lungs resulted in P-V curves and Pliquid similar to those of mature lungs. If pericapillary interstitial liquid pressure approximates Pliquid, surfactant deficiency will predispose preterm infants to pulmonary edema.