Influence of tidal volume and pulmonary artery occlusion on arterial oxygenation during endobronchial anesthesia

Arterial blood gases, intrapulmonary right-to-left shunt, airway pressures, and systemic and pulmonary arterial pressures were studied in 20 patients ventilated through a double lumen endobronchial tube during thoracotomy. Inspired oxygen concentration was 98% to 100%. Blood samples for measurement of blood gases and oxygen content were obtained during ventilation of both lungs (VT 15 ml/kg), one lung (VT 15 ml/kg and 8 ml/kg), and after occlusion of the opposite pulmonary artery. Mean oxygen tensions fell significantly, from 310 mm Hg during two-lung ventilation to 155 during one-lung ventilation, and rose again to 280 after pulmonary artery occlusion. Corresponding mean shunt values rose from 25% to 34% of cardiac output and fell again to 25%. When VT was reduced, patients who had had low oxygen tensions (less than 150 mm Hg) at the higher volume showed an increase in PaO2, and conversely. Shunts changed accordingly. Even with this high FIO2, some patients had oxygen tensions of less than 80 mm Hg during lung collapse. PaO2 values during one-lung ventilation were significantly related to patients' preoperative oxygen tensions, as well as to those during two-lung ventilation. In view of the low arterial oxygen tensions found in some patients and the lack of absolute correlation of the PaO2 with tidal volume, an FIO2 of close to 1.0 is recommended during one-lung ventilation, along with frequent or continuous monitoring of the arterial PaO2.