Factors determining end-expiratory alveolar pressure after cardiac surgery

Intrinsic positive end-expiratory pressure can occur when the sternum is closed following cardiac surgery, causing anaesthetic and surgical problems. We measured intrinsic positive end-expiratory pressure after sternal closure in two studies of patients undergoing coronary artery bypass grafting with the same anaesthetic regime. In one, patients were ventilated at 10 breaths min-1, in the other they were randomized to be ventilated at 10 or 20 breaths min-1. After sternal closure, intrinsic positive end-expiratory pressure increased significantly, especially in patients in whom it had been present before sternotomy. Multiple linear regressions with the pooled data from the studies showed that intrinsic positive end-expiratory pressure was positively correlated with ventilatory rate (P < 0.005), age (P < 0.005) and Body Mass Index (P < 0.0005), and negatively with FEV1, as percentage of predicted (P < 0.01). There was no correlation between intrinsic positive end-expiratory pressure and a history of smoking. As there was no difference in expiratory resistance between patients ventilated at the two different rates, any effect on intrinsic positive end-expiratory pressure of the higher ventilatory rates could have been due only to the shorter expiratory time.