A 57-year-old male was admitted with acute hypoxemic respiratory failure due to Influenza A pneumonia with ARDS. After an initial trial of prone ventilation failed, he was placed on veno-venous (V-V) ECMO. Initially, at low ECMO flows of 2 L/min, his pulse oximeter/peripheral oxygen saturation improved. However, as the flow was gradually increased to 4 L/min, his oxygen saturation levels worsened. A review of his chest X-ray (Figure 1) was conducted.

Figure 1: Chest Xray of the patient after cannulation
*Figure adapted from www.intensiveblog.com

Wrong Answer: A. Increasing the pump speed/flow will help improve pulse oximeter/peripheral saturation levels.

Right Answer: B. Measuring pre-oxygenator saturation levels will help diagnose the current condition.

Explaination:
During V-V ECMO, oxygenated blood from the ECMO circuit mixes in the right atrium with deoxygenated venous return that has bypassed the ECMO circuit before entering the right ventricle and pulmonary circulation.

Systemic arterial oxygenation depends on multiple factors, including:
● The balance between oxygenated ECMO blood flow and deoxygenated venous return
● The degree of pulmonary dysfunction
● Oxygen consumption
● Recirculation within the ECMO circuit
● Oxygenator efficiency

The issue described here is recirculation, where oxygenated blood from the return cannula is inadvertently drawn into the drainage cannula, preventing it from reaching systemic circulation. This occurs when the access and return cannulae are placed too close together (as depicted in Figure 1). Ideally, these cannulas should be spaced at least 10 cm apart to minimize recirculation. .


Figure 1: Chest Xray of the patient after cannulation (White arrow shows tip of the access cannula and black arrow shows tip of the return cannula)

Figure A showing normal scenario of V-V Ecmo where de-oxygenated blood flow is drained from right femoral and oxygenated blood flow is returned to right atrium.
Figure B showing re-circulation scenario of V-V Ecmo where some amount of oxygenated blood flow is diverted to inferior vena cava (IVC) and drained by right femoral drainage cannula
*Figure A and B adapted from www.criticalcarenow.com

Diagnosing Recirculation
A key indicator of recirculation is a decreasing pulse oximeter/peripheral oxygen saturation (SpO2), usually dropping below 86%. Notably, in V-V ECMO patients, SpO2 levels above 92% are rarely required, with 88% being generally acceptable.
Another confirmatory sign is an increased pre-oxygenator oxygen saturation (SpreO2), measured via a blood gas sample taken just before the oxygenator. A high SpreO2 suggests oxygenated blood is being drained back into the ECMO circuit rather than reaching systemic circulation.
The Role of ECMO Flow in Recirculation A common instinct in worsening V-V ECMO hypoxia is to increase pump flow, assuming this will enhance oxygen delivery. However, with significant recirculation, higher pump speeds often worsen the issue rather than resolving it. Recirculation can be quantified using the following formula:

Recirculation (%) = (SpreO2 – SvO2) / (SpostO2 – SvO2) × 100

SpreO2 = oxygen saturation of blood entering oxygenator (“pre-membrane gas”)

SpostO2 = oxygen saturation of blood exiting the oxygenator (“post-membrane gas”)

SvO2 = oxygen saturation of venous blood returning to the vena cavae just before being drained by the ECMO circuit

Where:
● SpreO2 = Oxygen saturation of blood entering the oxygenator ("pre-membrane gas")
● SpostO2 = Oxygen saturation of blood exiting the oxygenator ("post-membrane gas")
● SvO2 = Oxygen saturation of venous blood returning to the vena cava before drainage by the ECMO circuit

Clinical Pearls:
● Recirculation is unique to V-V ECMO and can cause worsening hypoxia.
● It occurs when oxygenated blood from the return cannula is drawn back into the access cannula instead of reaching systemic circulation.
● High pre-oxygenator oxygen saturation (SpreO2) and worsening peripheral SpO2 strongly suggest recirculation.
● Proper cannula positioning (ensuring at least 10 cm separation) is critical to minimize recirculation.
● Increasing ECMO flow does not necessarily improve oxygenation—beyond a certain point, it can actually exacerbate recirculation.
● Recirculation may also be influenced by positional factors, cardiac output, and intravascular volume status.
Thus, in the presence of worsening oxygenation despite increasing ECMO flow, the most appropriate next step is to measure pre-oxygenator saturation levels (SpreO2) to confirm recirculation, making option B the correct answer.

Wrong Answer: C. Re-positioning the cannula to bring them closer will help resolve this condition.

Wrong Answer: D. Increasing sweep gas flow will solve the issue.