Mixing Points in VA-ECMO: Clinical Significance and Management

Understanding the Mixing Point Concept

The mixing point is the location in the aorta where poorly-oxygenated blood ejected from the left ventricle meets well-oxygenated blood flowing retrograde from the ECMO circuit, with an innominate mixing point occurring at or proximal to the innominate artery, and a distal mixing point occurring further down the aorta, beyond the arch vessels 1, 2
In peripheral VA-ECMO with femoral arterial cannulation, two competing blood flows create what is termed "dual circulation", with antegrade flow of poorly-oxygenated blood from the left ventricle and retrograde flow of well-oxygenated blood from the ECMO circuit 1, 2

When the mixing point is at or proximal to the innominate artery, the upper body, including the brain and heart, receives well-oxygenated ECMO blood, which is the desired scenario for adequate cerebral and coronary perfusion 1, 2
A distal mixing point, occurring in approximately 10% of peripherally cannulated VA-ECMO patients, creates differential hypoxemia, where the upper body receives poorly-oxygenated blood from the failing native lungs, while the lower body receives oxygenated ECMO blood 1, 2, 3
Clinically, a distal mixing point is recognized by a wide pulse pressure on right radial arterial line monitoring, indicating significant left ventricular ejection reaching the upper body 1, 2, 3

Arterial blood gases should be obtained from a right radial arterial line to best represent cerebral oxygenation, as this site approximates ascending aortic and innominate artery blood, reflecting what the brain receives 1, 2, 3
Pulse pressure assessment from the right radial line provides real-time indication of mixing point location 1, 2, 3

Immediate intervention is required to prevent cerebral and myocardial ischemia when a distal mixing point causes upper body hypoxemia, including increasing ECMO flow to move the mixing point proximally toward the innominate artery 1, 2, 3
Optimizing mechanical ventilation to improve native lung oxygenation, reducing the oxygen gradient between the two circulations, is also crucial 1, 2, 3
Consideration of V-AV ECMO configuration by inserting an additional oxygenated return cannula in the jugular vein to directly deliver oxygenated blood to the upper body may be necessary 1, 2