THE EFFECT OF CARDIOPULMONARY BYPASS ON PULMONARY GAS EXCHANGE

Abstract

Introduction. Cardiac surgery results in a predictable and consistent deterioration in gas exchange, thought to be due to the combined effects of anesthesia, surgery and cardiopulmonary bypass (CPB). CPB in particular has been regarded as one of the main causes, due to activation of inflammatory processes causing damage to lung parenchyma. [1] Postoperative lung dysfunction is easily quantifiable as a widening of the alveolar-arterial (A-a) oxygen gradient. In this study, we compared A-a gradients in two groups of patients randomised to undergo coronary artery bypass grafting (CABG) via median sternotomy, with or without CPB. Methods. Fifty two patients scheduled for first time CABG were randomised to receive either normothermic CPB, or surgery without CPB. Patients with known pulmonary disease or ejection fraction less than 30% were excluded. The anaesthetic and perfusion techniques were standardised. The pleura was opened for all cases in which the internal mammary artery was used. A-a gradients were measured (1) prior to anesthetic induction, while breathing air; (2) post induction, pre-incision; (3) postoperatively in the intensive care unit; (4) six hours following extubation. Measurements 2, 3 and 4 were each taken with FiO2 s of 0.35, 0.55 and 0.75. Results. Patient characteristics were similar in the two groups. There were no significant differences between the groups in the A-a gradients taken before or after induction of anesthesia. Thereafter, A-a gradients rose progressively in both groups (p<0.05, 3 way ANOVA). No differences were found between the groups at any stage during the perioperative period, with any of the FiO2 levels studied Comment. Our study suggests that the deterioration in pulmonary gas exchange following cardiac surgery is mainly due to factors other than the use of CPB, and that attempts to reduce lung dysfunction following cardiac surgery may be better concentrated on manoeuvres to reverse non-CPB mediated effects of anesthesia and surgery. Figure 1Figure 1: A-a gradients (mean +/- S.E.) measured at FiO2 0.35, 0.55 and 0.75): CPB (-), non CPB (-).