Cardiorespiratory dynamics during successful and unsuccessful trials of weaning from mechanical ventilation following cardiac surgery

The purpose of this study was to examine the effects of a weaning trial with pressure support ventilation (PSV) on the cardiorespiratory dynamics in 20 cardiac surgery patients. In the patients who failed a weaning trial (failure group,n=6), the mean duration of cardiopulmonary bypass was 270±83 min and the mean postoperative lung-thorax compliance was 38±5 ml·cmH2O−1, whereas in successfully weaned patients (success group,n=14) they were 145±30 min and 55±10 ml·cm H2O−1 respectively (mean±SD). Significant differences were recognized in those values between the two groups. Preoperative cardiac function, intraoperative blood loss, and postoperative fluid balance were similar in both groups. Cardiac index (CI) increased similarly in both groups. Pao2/FIo2 and percentage intrapulmonary shunt were constant in the success group, and these variables worsened in the failure group. Oxygen consumption (\(\dot VO_2 \)) increased and mixed venous O2 tension (\(P\bar vO_2 \)) decreased in the failure group, whereas\(\dot VO_2 \) remained constant and\(P\bar vO_2 \) increased in the success group. These data suggest that prolonged cardiopulmonary bypass might have produced acute lung injury. Decreased lung compliance may be responsible for rapid shallow breathing and an increase in oxygen consumption during a weaning trial, and may lead to weaning failure from mechanical ventilation.