Pulmonary clearance of blood-borne bacteria

Clearance of blood-borne bacteria has been attributed primarily to a fixed macrophages in the liver and spleen. Although many important nonrespiratory functions of the lung have been reported, a major role for this organ in the clearance of circulating bacteria has not been described. To investigate mechanisms of sepsis induced lung dysfunction and pneumonia, we measured lung clearance, tissue accumulation and morphologic effect of blood-borne. Pseudomonas aeruginosa in pigs. Single pass pulmonary clearance of 60 to 80 per cent occurred over a wide range of infusion concentrations. Tissue concentrations of viable, hematogenously delivered bacteria were greatest in the lungs and exceeded inflowing blood concentrations in the lungs, liver and spleen but were less than inflowing blood concentrations in the heart, kidney and skeletal muscle. Electron microscopy showed phagocytosis of bacteria predominantly by mononuclear cells located within small pulmonary vessels. Viable test organisms were recovered from the upper airways of pigs receiving high dose bacterial infusions but not from pigs in the control group. In this experimental model, an important nonrespiratory function of the lungs is clearance of blood-borne bacteria. If lungs in humans have a similar capacity, retention of circulating organisms may be one mechanism of sepsis induced pulmonary failure.