Mgmnt of hypoxemia during OLV

One-lung ventilation is typically utilized for intrathoracic noncardiac surgery, although it has applications in some cardiac surgical procedures. Many of these procedures are performed in the lateral position with an open hemithorax. The goal of these anesthetics is to maximize atelectasis in the surgical lung to maximize surgical access while at the same time avoiding atelectasis in the nonoperative lung, which is usually the dependent lung due to positioning. Significant desaturation (SpO₂ < 90%) occurs in approximately 1%-10% of the surgical population during OLV despite a high FiO₂. A “safe” SpO₂ and PaO₂ varies from patient to patient depending on their comorbidities, but a general rule is an SpO₂ greater the 90% and a PaO₂ greater than 60 mm Hg is acceptable. Patients with comorbidities that may put organs at risk of hypoxia include those with coronary or cerebrovascular disease. A higher SpO₂ and PaO₂ may be necessary for these patients. The goal is to maximize pulmonary vascular resistance (PVR) to the nonventilated lung and minimize PVR in the ventilated lung. PVR is lowest at FRC and increases as lung volume either rises above, or falls below, FRC. A key factor in oxygenation during OLV is hypoxic pulmonary vasoconstriction, or HPV. HPV refers to the redistribution/shunting of pulmonary blood flow to the ventilated lung, away from the atelectatic areas that will not be ventilated. HPV is thought to be able to reduce the blood flow to the nonventilated lung by 50%. The surgical trauma itself, though, is thought to cause release of vasoactive metabolites, which may work against HPV by causing vasodilation in the nonventilated lung. All of the volatile anesthetics inhibit HPV in a dose related fashion. In doses less than 1 MAC the volatile anesthetics are weak inhibitors of HPV. The effect of volatile anesthetics on HPV is likely not clinically significant, and total intravenous anesthesia has not been shown to have a benefit in oxygenation versus volatile anesthetics. As OLV is initiated there is a fall in oxygenation over 20 to 30 minutes. At this time the saturation will stabilize and likely increase slightly over the next 2 hours as HPV is maximized. The treatment for hypoxemia during OLV is outlined below:

• For precipitous desaturations surgery should be stopped, 2 lung ventilation should be reinitiated, and the following should be performed (note that for less precipitous desaturations these steps can be performed without cessation of surgery or OLV but communication with the surgical team is always important):Increase FiO₂
• Reduce volatile anesthetic concentration to theoretically increase HPV
• Perform a recruitment maneuver
• Suction the endobronchial tube or bronchial blocker to clear secretions
• Diagnose causes of hypoxemia – correct tube position, decrease in cardiac output, need for inotropes/vasopressors, surgical manipulation of structures that impede venous return, pulmonary blood flow, or cardiac output
• Eliminate vasodilators such as nitroglycerin that may impede HPV
• Apply or increase PEEP to the ventilated lung to increase lung volume to FRC to maximize oxygenation
• Apply CPAP to the nonventilated lung – can try 5-10 cm H₂O without inflating the lung. Otherwise surgery will have to be interrupted to inflate the lung. 1-2 cm H₂0 may then be applied. 100% oxygen should be added to the CPAP.

CPAP works well in patients with COPD. PEEP is usually sufficient in patients with normal pulmonary mechanics. The nonventilated lung may need to be inflated to get any benefit from CPAP because the atelectatic operative lung requires a pressure of around 20 cm H₂O to inflate so the application of 5-10 cm H₂O of CPAP may not lead to recruitment of any alveoli. In a fully inflated lung, CPAP levels of 1-2 cm H₂O can be very effective at recruiting some alveoli while maintaining a low volume in the lung making surgical access adequate. Levels of CPAP of 5-10 cm H₂O can be tried in the deflated nonventilated lung. This may not be effective or this may lead to a lung that in too inflated to resume surgery. The benefit of CPAP is oxygen uptake by the nonventilated lung. Oxygen administration (100%) with the CPAP is, therefore, the most effective way to utilize CPAP to the nonventilated lung during OLV.

One can also intermittently switch to 2-lung ventilation. HPV may increase and become more effective with repeated hypoxic exposures.