What are distal and innominate (innominate artery) mixing points in Extracorporeal Membrane Oxygenation (ECMO)?

Mixing Points in VA-ECMO: Distal vs. Innominate

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; an innominate mixing point (proximal) occurs when this intersection is at or proximal to the innominate artery, while a distal mixing point occurs when it is located further down the aorta, beyond the arch vessels. 1, 2

Understanding the Mixing Point Concept

In peripheral VA-ECMO with femoral arterial cannulation, two competing blood flows create what is termed "dual circulation":

• Antegrade flow: Poorly-oxygenated blood ejected from the left ventricle (if native cardiac function persists and lungs are failing) travels forward through the aorta 1, 2
• Retrograde flow: Well-oxygenated blood from the ECMO circuit travels backward up the aorta from the femoral artery cannula 1, 2
• The mixing point: Where these two flows intersect, determining which body regions receive which blood supply 2

Clinical Significance of Mixing Point Location

Innominate (Proximal) Mixing Point

When the mixing point is at or proximal to the innominate artery, the upper body (including brain and heart) receives well-oxygenated ECMO blood 1:

• This is the desired scenario as it ensures adequate cerebral and coronary perfusion 1
• Clinically recognized by a narrow pulse pressure on right radial arterial line monitoring 1
• The narrow pulse pressure indicates minimal left ventricular ejection reaching the upper extremities 1

Distal Mixing Point

When the mixing point is distal to the arch vessels, the upper body receives poorly-oxygenated blood from the failing native lungs 1:

• This creates differential hypoxemia (Harlequin or North-South syndrome) 1
• Occurs in approximately 10% of peripherally cannulated VA-ECMO patients 1
• Clinically recognized by a wide pulse pressure on right radial arterial line monitoring, indicating significant left ventricular ejection reaching the upper body 1
• The upper body (brain, heart) receives deoxygenated blood while the lower body receives oxygenated ECMO blood 1, 3

Determinants of Mixing Point Location

The mixing point location depends on the relative strength of native cardiac output versus ECMO flow 2:

• Stronger native cardiac function with preserved stroke volume pushes the mixing point distally (toward the legs) 3, 2
• Higher ECMO flow rates push the mixing point proximally (toward the head) 1, 2
• Weaker native cardiac function allows ECMO flow to dominate, naturally positioning the mixing point more proximally 2

Monitoring Strategy

Arterial blood gases should be obtained from a right radial arterial line to best represent cerebral oxygenation 1:

• This site approximates ascending aortic and innominate artery blood, reflecting what the brain receives 1
• Femoral arterial samples would only reflect lower body oxygenation and miss differential hypoxemia 1
• Pulse pressure assessment from the right radial line provides real-time indication of mixing point location 1

Management of Distal Mixing Point (Differential Hypoxemia)

When a distal mixing point causes upper body hypoxemia, immediate intervention is required to prevent cerebral and myocardial ischemia 1:

1. Increase ECMO flow to move the mixing point proximally toward the innominate artery 1
2. Optimize mechanical ventilation to improve native lung oxygenation, reducing the oxygen gradient between the two circulations 1
3. Consider V-AV ECMO configuration by inserting an additional oxygenated return cannula in the jugular vein to directly deliver oxygenated blood to the upper body 1

Critical Pitfall

Do not rely solely on lower extremity arterial monitoring or assume adequate oxygenation based on ECMO circuit parameters alone 1:

• Computational modeling demonstrates that even with adequate ECMO flow, patients with preserved stroke volumes (>28 mL in one model) can have all arch vessels perfused by poorly-oxygenated native blood 3
• The brachiocephalic artery is particularly vulnerable to receiving predominantly left ventricular blood in nearly all scenarios with residual cardiac function 3
• Always monitor right radial arterial blood gases to detect this potentially catastrophic complication 1

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