Atelectasis and gas exchange impairment after cardiopulmonary bypass : An experimental study

Abstract: Despite of the improvement in management and equipment for cardiopulmonary bypass (CPB), postoperative pulmonary complications remain a major problem after cardiac surgery. In the present thesis, using a pig model, atelectasis has been tested as a cause of the increase in intrapulmonary shunt commonly seen after CPB. Also techniques to avoid formation of atelectasis has been tested as well as their safety.After 90 minutes of CPB in a pig model, atelectasis were found to a much larger extent than during general anaesthesia alone or even than after the same surgery but without CPB. The intrapulmonary shunt was also found to be correlated to the amount of atelectasis.A vital capacity manoeuvre (VCM), i.e. inflating the lungs to 40 cmH2O pressure during 15 seconds just before the end the CPB, was found to be effective in preventing atelectasis formation and gas exchange impairment. The efficacy of a VCM remained up to six hours after the end of the CPB and there was no further advantage by repeating the VCM every hour regarding gas exchange. On the other hand, lung mechanics had returned to baseline when the VCM was repeated every hour which was not the case when only one VCM had been applied.Animals subjected to repeated VCM, with or without CPB, showed an increase in the alveolo-capillary lung membrane permeability associated to some overdistension of the upper lobe. These findings were not associated with any clinical consequences. On the other hand, when only one VCM was applied at the end of the CPB, lung permeability had returned to baseline after 6 hours and no morphological differences were seen compared to control groups.Maintaining a continuous positive airway pressure (CPAP) during the CPB did not prevent atelectasis formation. Moreover, CPAP had some negative effects on post-CPB cardiac function.In conclusion, atelectasis explains most of the marked post-CPB increase in shunt and hypoxemia. One VCM applied at the end of the CPB seems to be the most effective and safest method to prevent occurrence of the atelectasis.