Pulmonary interstitial pressure in intact in situ lung: transition to interstitial edema

In anesthetized rabbits (n = 25) subject to slow intravenous saline loading (0.4 ml.min-1.kg-1) for 3 h, we measured pulmonary interstitial pressure (Pip) in intact in situ lungs with glass micropipettes inserted directly into the lung parenchyma via a "pleural window." Measurements were done in apneic animals at the end-expiratory volume with O2 delivered in the trachea. Pip was -10 +/- 1.5 (SD) cmH2O in control and increased to 0.6 +/- 3.8 and 5.7 +/- 3.3 cmH2O at 66 and 180 min, respectively. The wet-to-dry weight ratio (W/D) of the lung was 5.04 +/- 0.2 in the control group and 5.34 +/- 0.7 at 180 min (+6%); the corresponding W/D for intercostal muscles were 3.25 +/- 0.03 and 4.19 +/- 0.5 (+28%). Pulmonary interstitial compliance was 0.47 ml.mmHg-1.100 g wet wt-1. Pulmonary arterial and left atrial pressures were 18.4 +/- 2 and 3 +/- 1 cmH2O in control and increased to 19.5 +/- 2.9 and 4.6 +/- 1.7 cmH2O at 180 min, respectively. Aortic flow (cardiac output) increased from 103 +/- 35 to 131 +/- 26 ml/min; pulmonary resistance fell from 0.17 +/- 0.06 to 0.14 +/- 0.05 cmH2O.min.ml-1 (-18%), suggesting that the increase in Pip did not limit blood flow. The pulmonary capillary-to-interstitium filtration pressure gradient decreased sharply from a control value of 10 cmH2O to 0 cmH2O within 60 min because of the increase in Pip and remained unchanged for < or = 180 min. Data suggest that the pulmonary interstitial matrix can withstand fluid pressures above atmospheric, preventing the development of pulmonary alveolar flooding.