Alveolar liquid pressure measured using micropipets in isolated rabbit lungs

The relationships among alveolar liquid pressure (Pliq), transpulmonary pressure (Ptp), and alveolar edema were studied in isolated rabbit lungs. Different amounts of edema were induced by instilling measured amounts of normal saline via the trachea. Micropipets were used in conjunction with a servonulling pressure measuring system to measure Pliq. Pliq was -1 cm H2O relative to alveolar air pressure (Palv) at Ptp of 3 cm H2O and decreased to -16 cm H2O at Ptp of 25 cm H2O. Pliq increased slightly as wet-to-dry weight ratio (W/D) of the lungs increased from 5.8 to 16. Alveolar surface tension estimated from these values of Pliq - Palv using an alveolar radius of curvature of 40 micron at Ptp of 25 cm H2O was consistent with the direct measurements of Schürch. In lungs made edematous by treating with oleic acid, values of Pliq were not different from those measured in lungs made edematous by normal saline instillation (control). Pressure-volume behaviour of the oleic acid treated lungs showed a marked reduction in lung volume at each Ptp value compared to the behavior in the control lungs. Most of this reduction in lung volume was removed by washing the airways with the bronchodilator, isoproterenol. We conclude that alveolar surface tension was not increased in the oleic acid treated lungs. The apparent increase in lung static recoil was most likely caused by the constriction of small airways.