Authors: Busana M et al.
Anesthesiology, March 23, 2026, 10.1097/ALN.0000000000006032
Summary
This in vivo porcine study evaluates a practical and long-standing problem in V-V ECMO: quantifying recirculation and understanding its impact on oxygenation. Recirculation—where oxygenated blood from the return cannula is immediately drained back into the ECMO circuit—reduces effective systemic oxygen delivery and contributes to refractory hypoxemia, yet is difficult to measure at the bedside.
The investigators used a novel approach, trans-ECMO thermodilution (TET), injecting cold saline into the circuit and measuring temperature changes across drainage and return cannulas. By analyzing the area under the temperature-time curves, they quantified recirculation and incorporated those measurements into mass-transfer equations to estimate mixed venous oxygen content and saturation non-invasively.
Recirculation varied widely across measurements, with a median of 7.4%, but with substantial variability. About half of the measurements showed minimal recirculation (<10%), while the rest demonstrated significant recirculation (median ~27%). The key physiologic driver identified was the ratio of ECMO blood flow to cardiac output (ECSF). This ratio strongly correlated with recirculation (r = 0.67 overall and 0.81 in high-recirculation states), highlighting an important clinical insight: increasing ECMO flow does not necessarily improve oxygenation and may actually worsen efficiency due to increased recirculation.
Interestingly, cannula tip distance—often emphasized clinically—was not significantly associated with recirculation in this model. Increased recirculation was modestly associated with lower mixed venous PO₂, reinforcing its physiologic relevance.
Perhaps the most clinically impactful finding is that TET-derived calculations allowed highly accurate non-invasive estimation of mixed venous oxygen content and saturation (both r = 0.98), suggesting a potential alternative to pulmonary artery catheterization in ECMO patients.
Overall, this study introduces a feasible bedside technique to quantify recirculation and better understand oxygenation inefficiency during V-V ECMO, while also challenging some common assumptions about flow optimization.
Key Points
- Recirculation during V-V ECMO is highly variable and often clinically significant.
- The ECMO flow-to-cardiac output ratio (ECSF) is the primary determinant of recirculation.
- Increasing ECMO flow can paradoxically worsen oxygenation efficiency due to increased recirculation.
- Cannula position (distance between tips) was not a significant factor in this model.
- Trans-ECMO thermodilution enables accurate non-invasive estimation of mixed venous oxygenation.
What You Should Know
This is a concept-shifting paper for ECMO management. Many clinicians instinctively increase ECMO flow when oxygenation is poor, but this study reinforces that more flow is not always better—it may simply increase recirculation and reduce efficiency. The ability to quantify recirculation at the bedside and estimate mixed venous oxygenation without a PA catheter could be a major advance if translated clinically. For now, the takeaway is to think in terms of flow relative to cardiac output—not just absolute ECMO flow—and to recognize recirculation as a key, often hidden, driver of hypoxemia.
Thank you to Anesthesiology for allowing us to summarize this article.