Introduction
Extracorporeal membrane oxygenation (ECMO) is a form of temporary cardiopulmonary support used in severe cardiovascular or respiratory failure. Using large cannulae placed in major blood vessels, deoxygenated venous blood is circulated through an extracorporeal circuit, which includes an oxygenator that conducts efficient gas exchange. This circulation is supported by pressure generated by a pump. The oxygen-rich blood is then reinfused back into the body via another large blood vessel.
Veno-venous (VV) ECMO is used primarily for respiratory support whereby oxygen-rich blood is reinfused into a large vein. Veno-arterial (VA) ECMO provides hemodynamic support by reinfusing this oxygen rich blood into the arterial circulation through a large cannula usually placed in a major distributary of the aorta. It is not uncommon for patients who primarily have respiratory failure to also have significant cardiovascular compromise. The latter usually improves once adequate gas exchange is restored on VV ECMO. However, not infrequently there is concomitant and significant cardiovascular and respiratory failure. For example, patients with severe respiratory failure on VV ECMO may develop cardiomyopathy requiring significant cardiovascular support. Conversely, patients with primarily cardiovascular failure on VA ECMO may have or develop concomitant severe respiratory failure due to pulmonary edema from elevated left atrial pressure or a primary injury to the lungs. In the absence of significant cardiovascular recovery, oxygenation of the proximal aorta and its distributaries is largely from the oxygen rich blood reinfused by the ECMO circuit. In patients on VA ECMO, recovery of heart function with persistent severe lung injury leads to “differential hypoxemia” where ejection of poorly oxygenated blood from the left ventricle competes with retrograde oxygenated blood flow from the ECMO circuit, leading to myocardial and cerebral ischemia.(1-8) This is detected by comparing oxygenation from a right radial arterial sample with one obtained from the left. As LV function recovers, the right sided arterial blood sample will have relatively lower oxygen content reflecting native lung dysfunction. With further recovery, the “mixing point” of blood may move further down the aortic arch with lower oxygen content of both compared to the femoral arteries.
To address these perfusion requirements, clinicians have developed “hybrid” configurations by placing additional cannulae.(9) The most common of these is venoarterio-venous (VA-V) ECMO, which consists of a single venous drainage cannula and both arterial and venous reinfusion limbs. In patients with secondary heart failure (on VV ECMO), a femoral, subclavian, or axillary artery cannula is added to provide adequate circulatory support. In differential hypoxemia (on VA ECMO) an internal jugular or subclavian vein catheter is added to deliver oxygenated blood to the pulmonary circulation.(4) Using an adjustable clamp on the venous reinfusion limb, arterial and venous reinfusion flows are titrated until adequate hemodynamics and oxygenation are obtained.(10, 11)
The number of patients being treated with ECMO has dramatically increased over the last decade, and its use in our ICU has followed a similar trend.(4, 12) While a few case series have provided initial descriptions of patients requiring VA-V ECMO, there remains a paucity of literature on the subject.(10, 11, 13, 14) We report our hospital’s experience with VA-V ECMO to further characterize the patient population, indications, and outcomes associated with this hybrid configuration.