Cardiac Compression of Lung Lower Lobes after Coronary Artery Bypass Graft with Cardiopulmonary Bypass

Background: Atelectasis is a major cause of hypoxemia after coronary artery bypass grafting (CABG) and is commonly ascribed to general anesthesia, high inspiratory oxygen concentration and cardiopulmonary bypass (CPB). The objective of this study was to evaluate the role of heart-induced pulmonary compression after CABG with CPB. Methods: Seventeen patients without pre-operative cardiac failure who were scheduled for coronary artery bypass graft underwent pre-and postoperative thoracic computed tomography. The cardiac mass, the pressure exerted on the lungs by the right and left heart and the fraction of collapsed lower lobe segments below and outside of the heart limits were evaluated on a computed tomography section 1 cm above the diaphragmatic cupola. Results: In the postoperative period, cardiac mass increased by 32% (117 +/- 31 g versus 155 +/- 35 g, p<0.001), leading to an increase in the pressure that was exerted on the lungs by the right (2.2 +/- 0.6 g.cm(-2) versus 3.2 +/- 1.2 g.cm(-2), p<0.05) and left heart (2.4 +/- 0.7 g.cm(-2) versus 4.261.8 g.cm(-2), p, 0.001). The proportion of collapsed lung segments beneath the heart markedly increased [from 6.7% to 32.9% on the right side (p<0.001) and from 6.2% to 29% on the left side (p<0.001)], whereas the proportion of collapsed lung segments outside of the heart limits slightly increased [from 0.7% to 10.8% on the right side (p<0.001) and from 1.5% to 12.6% on the left side (p<0.001)]. Conclusion: The pressure that is exerted by the heart on the lungs increased postoperatively and contributed to the collapse of subjacent pulmonary segments.