RESUMO
BACKGROUND: An increased alveolar-arterial oxygen tension difference is frequent in anesthetized patients. In this study, we evaluated the effect on the lung of anesthesia, muscle paralysis, and a brief course of mechanical ventilation. METHODS: Lung diffusion for carbon monoxide (DLCO), including pulmonary capillary blood volume (Vc) and conductance of the alveolar-capillary membrane (DM), and pulmonary surfactant protein type B (a marker of alveolar damage) were measured in 45 patients without pulmonary disease undergoing extrathoracic surgery. RESULTS: Anesthesia, muscle paralysis, and mechanical ventilation led to impairment of gas exchange, with a reduction of DLCO values immediately after anesthetic induction due to a concomitant reduction of both DM and Vc. While changes in DM were due to the reduction of lung volume, changes in Vc were not limited to volume loss, since the Vc/alveolar volume ratio decreased significantly. Although DLCO and its components decreased immediately after induction, none of the values decreased further at 1 and 3 hours. Surfactant protein type B, however, was unchanged immediately after anesthesia but increased at 1 hour after induction and further increased after 3 hours of anesthesia. The level of alveolar damage correlated with the reduction of lung perfusion and lung dynamic strain (i.e., ratio between tidal volume and end-expiratory lung volume). CONCLUSIONS: A brief course of anesthesia and controlled ventilation leads to: (1) alveolar damage, which is correlated with lung strain and perfusion, and (2) impaired gas exchange mainly due to volume loss but also to reduced aerated lung perfusion.
