These robot-assisted cardiac surgeries usually require single-lung ventilation (SLV) to facilitate surgical exposure. SLV creates ventilation/perfusion mismatch and shunt (Qs:Qt) through the collapsed lung and leads to hypoxemia. Pulmonary gas exchange often deteriorates after cardiopulmonary bypass (CPB) because of ischemic tissue damage. In some cases, severe hypoxemia may require the cessation of surgical procedures and the initiation of double-lung ventilation to improve oxygenation. In this study, the investigator applied the continuous positive airway pressure (CPAP) or the high-frequency low-volume ventilation (HFLVV) to the non-dependent lung (differential ventilation) during the weaning from CPB. The investigator hypothesized that the differential ventilation would produce the least interference with the surgeon's exposure and better oxygenation. The investigators evaluate the airway pressure, shunt fraction, PaO2/FiO2, cerebral oximetry, surgical field condition and the length of stay in intensive care unit of patients underwent the robot-assisted cardiac surgery.
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Changes of arterial PaO2
Timeframe: 5 min after induction of anesthesia during DLV, 5 min after SLV, 5 min after HFLVV, 5 min after CPB flow reduced to 1/3, 5min after CPB flow reduced to 2/3, 15min after resuming of DLV]
Changes of PaO2/FiO2 ratio
Timeframe: 5 min after induction of anesthesia during DLV, 5 min after SLV, 5 min after HFLVV, 5 min after CPB flow reduced to 1/3, 5min after CPB flow reduced to 2/3, 15min after resuming of DLV]