Effect of cardiopulmonary bypass on respiration function in man

Abstract

The change in pulmonary function which follows cardiopulmonary bypass (CPBP), as practised with modern techniques, was evaluated in 10 patients undergoing coronary vein-graft operations. Measurements were made during the week before operation and on 5 occasions postoperatively. The study was preceded by an attempt to establish normal values for pulmonary blood-gas exchange, in the supine position. It proved impracticable to predict the normal range of alveoLar-to-arteriaL oxygen tension difference (AAsPq^) and venous admixture (Qva/Qt), when supine, according to age. The relationship of closing volume (CV) to expiratory reserve volume (ERV) was important in determining these and was not closely related to age. In the CPBP study, mean Qva/Qt breathing air, increased from 9.59% to a maximum of 13.71%, 22 hr postoperatively. Mean Qva/Qt, breathing oxygen, increased from 7.02% to a maximum of 15.71%, 48 hr postoperatively. Most of the deterioration in gas exchange appeared to be due to increase in regions of lung with no ventilation or with critically low ventilation/perfusion ratio (V/Q). These changes were no greater than those reported to follow upper abdominal operations without CPBP though this comparison is difficult to make because of technical and other differences. None of the preoperative tests afforded a reliable forecast of the postoperative venous admixture. Arterio-venous oxygen content difference (AavCt^) had risen significantly by 22 hr and was still raised at 48 hr. This probably indicates an abnormal cardiac output in this period. Ten days postoperatively Qva/Qt and AsvCq^ had returned to the preoperative level. Despite an attempt to select subjects whose lung function was normal before operation, there was variation not only in their preoperative clinical state but also in their postoperative clinical course. These variations may have accounted for some of the differences in physiological behaviour. Conclusions drawn from the mean values above should, therefore, be applied with caution in other circumstances. The group is, however, likely to be representative of the fittest patients having cardiac operations with CPBP at Green Lane Hospital.