Discussion
In normal awake supine subjects, Cst,rs amounts to 108 ±7 ml/cm HaO. In our patients, lower values were obtained (60 ±2 ml/cm H20), in line with a previous report by Marvel et al. This reduction in Cst,rs can be explained by reduced lung volume postoperatively, altered surfactant function due to residual effects of anesthesia, reduced chest wall compliance due to surgery, and atelectasis.
Postoperative pulmonary atelectasis was first described by Pasteur in 1908. The incidence of atelectasis, particularly in the left lower lobe, after open heart surgery is well documented.The pathogenesis, however, is not well understood. Reduced lung volume and residual effects of anesthesia are possible causes. Further insight concerning atelectasis stems from recent work of Brismar et al and Hedenstierna. In elegant experiments with computed tomography (CT scan), they showed that in supine subjects, atelectasis occurred within 5 min of induction of anesthesia in the dependent parts of both lungs. The atelectasis was most pronounced in the caudal segments of the lung.