Surface forces in the lung, atelectasis, and transpulmonary pressure.

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Abstract
We studied the rate at which lung compliance changed in anesthetized, paralyzed rabbits and newborn lambs ventilated at a constant tidal volume for 20 min. following 4 inflations to an intratracheal pressure of 25-30 cm H2O. The compliance of rabbits and lambs decreased by 53 and 40% of the initial value when the end-expiratory transpulmonary pressure (PTP) was unaltered, But by only 21 and 12% when the PTP was increased by 2.1 and 2.0 cm H2O. This difference was not due to airway collapse since a) no trapped nitrogen was detected when the animal had breathed air, and (b) lungs frozen with liquid propane had patent airways. Intravenous isoproterenol, ventilation with 100% O2, or ventilation with CO2 and air had no effect. Probably the alveolus cannot maintain a low surface tension for long without occasional large inflations which replenish the surface with surface-active material. An increasing surface tension in the air space may lead to its reconfiguration, compliance decrease, and eventual atelectasis. Small changes in the PTP may greatly affect the rate of decrease of the lung surface area.