Persistent hypoxemia on ECMO

Extracorporeal membrane oxygenation (ECMO) is used to support patients with hypoxemia due to severe respiratory failure. Hypoxemia can persist during ECMO as a result of reduction in mechanical ventilatory support as part of a lung protective strategy, following which gas exchange provided by the native lungs is reduced or absent. If tissue oxygen delivery is maintained, mild to moderate hypoxemia will be well tolerated, but if tissue hypoxia develops, then the cause of hypoxemia must be addressed and corrected. Patterns of hypoxemia are dependent on the mode of ECMO support. During veno-venous support for respiratory failure, hypoxemia is global, with all tissues receiving hypoxemic blood. When femoral veno-arterial support is used in the presence of respiratory failure, then differential hypoxemia can result, with hypoxemia limited to the upper part of the body. The approach to hypoxemia is dependent on whether veno-venous (VV) or veno-arterial (VA) ECMO is used. Adequate oxygen saturation on VV ECMO requires adequate hemoglobin, a membrane lung operating below its rated flow, low recirculation, and extracorporeal circuit flow that captures most of the cardiac output. The approach to hypoxemia during VV ECMO includes addressing each of these. Identifying and limiting recirculation will improve effective extracorporeal flow. Anemia has two detrimental effects. First, it limits oxygen transfer through the membrane lung. Second, it increases oxygen extraction resulting in low mixed venous oxygen saturation that increases the effective venous admixture with circuit blood. Transfusion to near normal levels will help improve oxygen saturation. Increasing effective extracorporeal flow to capture at least 60% of the cardiac output is necessary.1 If the extracorporeal flow fraction is less than 60% due to an increased cardiac output, then reducing cardiac output through reducing exogenous catecholamines, use of beta blockade,2 or hypothermia3 will help improve the extracorporeal flow fraction. Hypothermia has the added benefit of reducing oxygen consumption and therefore improving mixed venous saturation. A final option is to leverage the native lungs, such as through prone positioning, as long as lung protective strategies can be maintained.4 Oxygen saturation on femoral VA ECMO is typically normal (>90%). The upper part of the body is provided with blood from the left ventricle that has been saturated by normally functioning lungs. The lower part of the body is provided with fully saturated blood from the extracorporeal circuit. The two circulations meet at some point in the aorta that depends on the relative magnitude of flow rate in these two circulations. If there is pulmonary dysfunction, then the blood ejected by the left ventricle will be hypoxemic, and tissues perfused proximal to the mixing point will be perfused with hypoxemic blood, while the lower body will have a normal saturation. Management of this differential hypoxemia is through improvement of pulmonary gas exchange if possible, increasing extracorporeal flow to decrease pulmonary blood flow, or conversion of VA ECMO to VVA (hybrid veno-arterial and veno-venous).