What is the treatment for hypoxemia during one-lung anesthesia?
Ventilation of the dependent lung with large tidal vol-umes (10–12 mL/kg) is recommended during one-lung anesthesia to prevent atelectasis in the dependent lung.
Increased airway pressures also reduce transudation of fluid from pulmonary capillaries. However, too much of an increase in airway pressure may increase dependent lung pulmonary vascular resistance (PVR) and divert blood flow to the nondependent lung.
Ventilatory rate is adjusted to maintain the PaCO2 near 40 mmHg. This is usually achieved with a rate similar to that employed during two-lung ventilation. One hundred percent oxygen is administered during one-lung ventila-tion. Large tidal volumes can prevent absorption atelecta-sis, which tends to occur with the use of high FIO2 levels. The benefits of 100% oxygen generally outweigh the possi-ble risks of its use. Patients at risk for oxygen toxicity should receieve the lowest FIO2 compatible with adequate oxygenation.
Temporarily reinflating and ventilating the nondepen-dent lung with 100% oxygen rapidly corrects sudden and precipitous drops in arterial saturation. Possible causes for hypoxia should be sought and corrected. These etiologies include malposition of the double-lumen endobronchial tube, kinking of the tube, secretions, pneumothorax of the dependent lung, bronchospasm, low cardiac output, low FIO2, and hypoventilation.
In the absence of an identifiable cause for hypoxemia, shunting of blood through the nondependent lung is likely to be responsible. Therefore, 5–10 cm H2O of continuous positive airway pressure (CPAP) should be applied to the nondependent lung. This maneuver has been shown to increase PaO2 during one-lung ventilation. CPAP to the nondependent lung opens alveoli so that they can partici-pate in gas exchange and allows oxygenation of blood pass-ing through the nondependent lung. Ten centimeters of water pressure expands the lung by only 100 mL, a relatively small volume. Insufflation of oxygen at zero airway pressure does not improve PaO2, and using greater than 10 cm H2O CPAP may lead to interference with surgical exposure. CPAP is usually effective in restoring the PaO2 to a safe level.
If functional residual capacity (FRC) is low, PEEP to the dependent lung may improve oxygenation by returning FRC toward normal and by lowering PVR in the depend-ent lung. Further increase of FRC, however, may increase dependent lung PVR and decrease blood flow to the dependent lung. If CPAP to the nondependent lung is inef-fective in improving PaO2, 5–10 cm H2O of PEEP to the dependent lung can be used in addition.
Other treatment modalities include intermittent ventila-tion of the nondependent lung with oxygen and clamping the nondependent pulmonary artery to eliminate shunting.