PULMONARY INTERSTITIAL PRESSURE IN ANESTHETIZED PARALYZED NEWBORN RABBITS

In anesthetized paralyzed term newborn rabbits at various postgestatio nal ages (from birth up to 16 days), we measured by micropuncture tech nique the hydraulic pressure of the pulmonary interstitium (Pip), the extrapleural parietal interstitium, and the pleural liquid. Birth data refer to cesarian-delivered nonbreathing rabbits. Pip increased from 0.5 +/- 2 to 6 +/- 0.7 cmH(2)O from birth up to 2 h and then decreased , becoming subatmospheric at 5 h and attaining -6 +/- 1.6 cmH(2)O at 1 6 days. Over the same period of time, pressure in the extrapleural par ietal interstitium and the pleural liquid remained fairly constant at an average value of approximately -1.5 and -2 cmH(2)O, respectively. T he wet-to-dry weight ratio of the lungs decreased from 7.8 +/- 0.4 to 4.9 +/- 0.1 at 16 days. Plasma protein concentration was 4.2 +/- 0.4 g /dl at birth, decreased to 3.2 +/- 0.5 g/dl at 1 h from delivery, and increased back to 4 +/- 0.6 g/dl at 16 days. Pleural liquid protein co ncentration was 3 +/- 0.1 g/dl at birth and decreased to 1.2 +/- 0.2 g /dl at 16 days. In the first hours of postnatal life, the marked incre ase in Pip appears to be a key factor in favoring fluid clearance from pulmonary interstitium into the pulmonary capillaries and the pleural space. This factor vanishes after similar to 6 h because of the marke d decrease in Pip.