Although one lung ventilation (OLV) is frequently used for facilitating thoracic surgical procedures, arterial hypoxemia can occur while using one lung anesthesia. Continuous positive airway pressure (CPAP) in 5 or 10 cmH₂O to the non-ventilating lung is commonly recommended to prevent hypoxemia. We evaluated the effects of incremental CPAP to the non-ventilating lung on arterial oxygenation and pulmonary shunt without obstruction of the surgical field during OLV.

Twenty patients that were scheduled for one lung anesthesia were included in this study. Systemic and pulmonary hemodynamic data and blood gas analysis was recorded every fifteen minutes according to the patient's positions and CPAP levels. CPAP was applied from 0 cmH₂O by 3 cmH₂O increments until a surgeon notifies that the surgical field was obstructed by the expanded lung. Following that, pulmonary shunt fraction (Q_S/Q_T) was calculated.

There were no significant differences of Q_S/Q_T between supine and lateral positions with two lung ventilation (TLV). OLV significantly decreased arterial oxygen partial pressure (PaO₂) and increased Q_S/Q_T compared to TLV. PaO₂ and Q_S/Q_T significantly improved at 6 and 9 cmH₂O of CPAP compared to 0 cmH₂O. However, there were no significant differences of PaO₂ and Q_S/Q_T between 6 and 9 cmH₂O CPAP. In 18 patients (90%), surgical fields were obstructed at 9 cmH₂O CPAP.

This study suggests that 6 cmH₂O CPAP effectively improved arterial oxygenation without interference of the surgical field during OLV when CPAP was applied from 0 cmH₂O in 3 cmH₂O increments.