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Atelectasis is a major cause of decreased arterial oxygenation after cardiopulmonary bypass (CPB). There is a close relationship between atelectasis and inflammatory responses. We therefore tested the hypothesis that neutrophil number and the concentrations of proinflammatory cytokines and elastase in plasma and bronchoalveolar lavage fluid correlate with changes in arterial oxygenation. Bronchoalveolar lavage was performed just after the induction of anesthesia and at the end of surgery in 80 patients undergoing CPB. Peripheral blood was sampled simultaneously. Arterial oxygenation was quantified by Pao2/fraction of inspired oxygen (Fio2) and intrapulmonary shunt (Qs/ Qt). Pao2/Fio2 and Qs/ Qt decreased significantly at the end of surgery, whereas neutrophil number, interleukin (IL)-6, IL-8, tumor necrosis factor-α, and elastase concentrations in the lavage fluid increased significantly. The increase in neutrophil count from the lavage fluid correlated significantly with the increases in IL-8 and elastase concentrations. The increase in neutrophil number and IL-8 and elastase concentrations in the lavage fluid correlated significantly with Pao2/Fio2 and Qs/ Qt at the end of surgery. In contrast, none of the plasma values correlated with these variables. Significant correlation between immune mediators and decreased arterial oxygenation suggests that inflammatory responses in the distal airway are strongly related to a decrease in arterial oxygenation after CPB.The increases in neutrophil number, interleukin-8, and elastase concentrations in bronchoalveolar lavage correlated significantly with decreases in arterial oxygenation. Our results suggest immunologic responses in the distal airway are closely related to pulmonary gas change.