Mechanical and cellular bacterial clearance in lung atelectasis.

A pig model with aerosolized pneumococcal bacteria was used to establish that bacterial clearance in a collapsed lung in the perioperative period was decreased compared with the opposite, aerated lung. Cannulation of the right lymphatic duct revealed a significant increase in both lymph flow and ratio of lymph to plasma protein, indicating the development of a high-permeability edema in the collapsed, infected lung. Only 22% of the efferent lymph and blood was positive for the infecting organism. Examination of the T cells obtained by bronchopulmonary lavage showed an initial fall in the numbers of alveolar macrophages at 6 hours after collapse and infection, relative to the opposite, control lung. However, at 24 hours, the collapsed lung had replenished its alveolar macrophage population to such a degree that it was greater than the control. Electron microscopy revealed that the macrophages in the collapsed lung were more activated with increased lysosomal and pseudopodial activity. The in vitro chemotactic function of the macrophages appeared depressed, but phagocytosis and intracellular bactericidal activity were increased in the atelectatic lung. We conclude that there is a decreased bacterial clearance capacity in atelectasis. This finding indicates that impaired mucociliary clearance plays the dominant role in susceptibility to infection.