Pulmonary interstitial pressure in premature rabbits

By micropuncture technique we measured pulmonary interstitial pressuure (P_ip) from birth up to 6 h postnatal age in anesthetized and paralyzed cesarian delivered term (31 days gestation) and premature (27 to 30 days gestation) rabbits. In term cesarian delivered rabbits P_ip followed the time course of vaginally delivered rabbits, namely, it increased from about zero at birth up to about 5 cmH₂O at 2 h, as a result of alveolar fluid reabsorption, subsequently it decreased becoming subatmospheric due to progressive interstitial fluid drainage. In ventilated lung regions of premature rabbits, P_ip also peaked to about 5 cmH₂O at 2 h but its subsequent decrease was markedly slowed down while in atelectatic regions of premature rabbits P_ip remained slightly subatmospheric. Up to 6 h, the wet/dry weight ratio of the lung was higher in premature relative to vaginally and cesarian delivered term rabbits (at birth 8.4 ± 0.9 vs. 7.5 ± 0.8). In 29-31 days rabbits, plasma protein concentration at birth was 3.6 ± 0.5 g/dl (within 95% confidence limits for vaginally delivered rabbits, considered as control) while in 27-28 days rabbit it was 3.1 ± 0.4 g/dl (at the lower edge of control confidence limits). In the first postnatal hours, the increase in P_ip favoured fluid reabsorption into pulmonary microcirculation in term cesarian delivered rabbits and in ventilated regions of premature rabbits. Conversely, in the atelectatic regions of premature rabbits the unchanged P_ip value in the postnatal hours favours fluid filtration from microcirculation into lung interstitium.