What is the mixing point in Extracorporeal Membrane Oxygenation (ECMO)?

What is the Mixing Point in ECMO?

The mixing point is the location in the aorta where poorly-oxygenated blood ejected from your failing heart meets well-oxygenated blood flowing backward from the ECMO machine. 1

How It Works

In peripheral VA-ECMO (the most common setup), two blood flows compete inside your aorta: 1

• Antegrade flow: Poorly-oxygenated blood pumped forward from your left ventricle (your native heart)
• Retrograde flow: Well-oxygenated blood flowing backward from the ECMO circuit through a cannula in your femoral artery

The point where these two flows meet is called the mixing point, and its location determines which parts of your body get well-oxygenated blood versus poorly-oxygenated blood. 2, 1

Why Location Matters

Good Scenario: Proximal Mixing Point

When the mixing point sits at or above the innominate artery (near the top of your aorta), your brain and heart receive well-oxygenated ECMO blood—this is what we want. 1

Dangerous Scenario: Distal Mixing Point

When the mixing point drops below the arch vessels (occurs in approximately 10% of patients), your upper body receives poorly-oxygenated blood from your failing lungs while your lower body gets good ECMO blood—this creates "differential hypoxemia" or "Harlequin syndrome." 2, 1 This can cause brain and heart damage from lack of oxygen. 1

How Doctors Monitor It

Your medical team watches the mixing point location by: 2, 1

• Right radial arterial line: Blood gases from your right wrist artery best represent what your brain is receiving 2, 1
• Pulse pressure assessment:
  • Narrow pulse pressure = mixing point is proximal (good—ECMO blood reaching your brain) 2
  • Wide pulse pressure = mixing point is distal (concerning—your heart is ejecting poorly-oxygenated blood to your brain) 2, 1

What Doctors Do If the Mixing Point Is Too Low

When your upper body isn't getting enough oxygen, immediate interventions include: 2, 1

1. Increase ECMO flow: Pushing more oxygenated blood backward moves the mixing point upward toward your brain 2, 1
2. Optimize the ventilator: Improving your lung function reduces the oxygen difference between the two blood flows 2, 1
3. Add a jugular cannula: In severe cases, inserting an additional return line in your neck vein (V-AV ECMO) delivers oxygenated blood directly to your upper body 2, 1

Key Clinical Point

The mixing point concept explains why ECMO patients need continuous monitoring from the right arm rather than other sites—it's the only way to know if your brain is getting adequate oxygen. 2, 1 Understanding this helps your care team prevent brain injury by catching and correcting differential hypoxemia early. 1, 3