VA vs VV ECMO: Flow, Oxygenation and LV Loading
What is the difference between VA and VV ECMO?
VV ECMO returns oxygenated blood to the venous circulation, so the native heart still provides systemic pumping. VA ECMO returns blood to the arterial circulation and adds direct circulatory support. Neither pump flow nor one oxygen saturation value, alone, establishes effective oxygen delivery to every organ.
Key takeaways
- VV ECMO supports gas exchange while native cardiac function remains essential.
- VA arterial return does not guarantee left-ventricular unloading.
- Regional oxygenation can differ during femoral VA support.
In this video
- Good oxygen numbers and poor perfusion
- Gas exchange and systemic pumping
- The extracorporeal circuit and sweep
- VV return and the native heart
- Indirect effects on right ventricular load
- Mixing hemoglobin and oxygen supply
- Recirculation
- Oxygen content and membrane transfer math
- VA return and circulatory support
- Femoral VA and left ventricular load
- Two aortic streams and regional oxygenation
- A VV patient with falling native output
- Femoral VA and two oxygen readings
- CRNA interview reasoning
- Quick checks
Why does VV ECMO still depend on the heart?
The membrane exchanges gas across separate blood and gas streams. In VV support, the treated blood returns to the venous side and then passes through the native heart. The circuit is not a direct substitute for systemic cardiac pumping.
Mixing and recirculation affect how much of the circuit’s work contributes to systemic oxygen delivery. A displayed pump flow is therefore different from a complete measurement of the patient’s effective oxygen delivery.
How does VA support change circulation?
Arterial return provides a direct contribution to systemic flow. With femoral VA return, retrograde aortic flow can also increase left-ventricular afterload. Supporting systemic pressure is not the same as guaranteeing that the LV is unloaded.
Native ejection, loading conditions and the circuit interact. Simple flow addition requires explicit assumptions and a clear definition of what each measurement includes. It should not become a universal rule for every patient or circuit configuration.
Why can oxygenation differ between regions?
During femoral VA support, native ejection from poorly oxygenating lungs can meet the circuit’s retrograde stream in the arterial circulation. The balance can change, so lower-body oxygenation does not establish what reaches the coronary or cerebral circulation.
Different regional oxygen contents are not, by themselves, proof that regional tissue hypoxia occurred. This lesson explains the mechanism and the limitation of single measurements; it does not prescribe sampling, sweep or pump adjustments.
