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The effects of prolonged positive pressure ventilation and overinflation on surface tension properties of the lung were studied in 24 dogs. No alteration of surfactant was observed in G animals ventilated at normal pressure and volume for periods up to G hours on a Morch respirator or in 2 nonventilated anesthesia controls. In 10 of 13 dogs subjected to overinflation to a degree sufficient to produce a paradoxical pulse there was absence of surfactant after 24 hours and gross atelectasis. In 3 animals studied immediately after overinflation there was normal surface tension activity. Overinflation of the lung results in depletion or alteration in surfactant which is essential to the maintenance of expanded alveoli at end-expiratory pressures. Prolonged positive pressure ventilation at normal pressure and volume does not alter surfactant and did not result in atelectasis in this study.

(C) 1964 American Society of Anesthesiologists, Inc.