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The reduction in functional residual capacity (FRC) after anesthesia and thoracic surgery may result in atelectasis, hypoxia, and respiratory failure. Mechanical ventilation reverses the FRC reduction but may also decrease cardiac output and increase the pulmonary vascular resistance index (PVRI) in some patients.The cardiopulmonary effects of stopping mechanical ventilation after open-heart surgery were studied in 17 children. FRC, arterial pH, and Pao2 were significantly reduced, while Paco2, oxygen consumption, and right ventricular stroke work index significantly increased. Mean FRC on spontaneous respiration was below normal despite continuous positive airway pressure. PVRI increased significantly in patients whose FRC fell below 22 ml/kg on spontaneous respiration. The PVRI increase was most marked in patients with pre-existing pulmonary vascular disease. These results confirm the value of appropriate mechanical ventilation in the early postoperative management of children undergoing open-heart surgery, particularly those with pulmonary vascular disease.