Peripheral venous-arterial (VA) ECMO, typically via the common femoral vein and artery, can be utilized as a bridge to recovery or decision in critically ill patients; however, this cannulation strategy can result in unique physiologic derangements like a watershed phenomenon.
In this situation, perfusion occurs via antegrade cardiac output (blood leaving the heart in the usual fashion) and retrograde flow returning from ECMO via the femoral artery. These flows can abut each other creating a watershed zone. This zone’s location depends on vascular resistance and which is stronger – the antegrade (normal) or retrograde (ECMO) flow. As a patient’s heart function changes (resolving pathology, the addition of inotropes, etc.) and/or the ECMO flow changes (viscosity, pump speed, vascular resistance, etc.), this “watershed zone” can move in the aorta. For example, if the heart function improves and the ECMO flow decreases, this zone will migrate distally.
In the CT angio, a femoral venous cannula extending to the right atrium is visualized in the IVC. However, there is a stark transition of contrast in the aorta suggestive of thrombus (celiac/SMA vessels patent on Doppler exam). This is in fact a watershed region created by the competing antegrade and retrograde flows resulting in a flow-related artifact.
Strategies to mitigate the watershed phenomenon include circuit optimization (tweaking the cannula size/positioning and ECMO pump settings) or reconfiguring the ECMO (V-AV ECMO, central cannulation, etc.)
