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Rationale The efficacy of veno-venous extracorporeal membrane oxygenation is limited by the phenomenon of recirculation, which is difficult to quantify. Existing measurement techniques using readily available equipment are unsatisfactory. Objectives 1) To compare the accuracy of measurements of recirculation made using equations comparing blood oxygen content or saturation alone at different points in an ex vivo circuit; 2) to validate a new step-change technique for quantifying recirculation in vivo. Methods Anesthetized greyhound dogs cannulated for veno-arterial support were connected to a circuit that allowed the creation of a known level of recirculation ex vivo and blood oxygen content/saturation monitoring. In two dogs, the accuracy of measurements derived from oxygen content and oxygen saturation were compared. The potential of a new technique for measuring recirculation in vivo by comparing the oxygen content of blood sampled during oxygenator bypass to that following a step-change in circuit oxygenation was demonstrated in a veno-venous pilot study and validated in a three-dog veno-arterial study. Results Measurements made using oxygen content versus oxygen saturation showed superior correlation with true recirculation (r2=0.87 vs. 0.64, p<0.0001) and less proportional measurement bias (10.3% vs. 49.8%, p=0.0045). Measurements of recirculation made using a step-change in circuit oxygenation and comparing oxygen content as is required for measuring in vivo recirculation overestimated by only 18.6% (95% CI: 3.9–33.2%) and had excellent correlation with true values (r2=0.89). Conclusions 1) Measurement of recirculation using oxygen content is superior to that using oxygen saturation alone, which demonstrates significant measurement bias; 2) the novel step-change technique is a sufficiently accurate technique for the measurement of recirculation in animal models.

Background: Veno-venous extracorporeal membrane oxygenation has several advantages over veno-arterial support for patients with severe reversible respiratory failure. However, recirculation can limit oxygen delivery as pump flow increases. This could be ameliorated by placing the return catheter in the right ventricle instead of the central veins. We compared recirculation in veno-right ventricular support with that in conventional veno-venous support and its relationship with pump flow. Methods: Five greyhound dogs were sequentially cannulated percutaneously for both veno-venous and veno-right ventricular support. Recirculation was measured by comparing oxygen levels in the circuit drainage and return lines before and immediately after a sudden increase in circuit oxygenation at pump flows between 0.5 L/min and 4 L/min for both modalities. Results: Recirculation was reduced in veno-right ventricular support compared with conventional veno-venous support at 4 L/min pump flow (8.4% versus 37.9%, p=0.0076) and increased less with increases in pump flow (2.9% per 1 L/min vs. 11.1% per 1 L/min, p<0.0001). Conclusions: Recirculation can be dramatically reduced by returning blood into the right ventricle, which improves oxygen delivery to the lungs and the systemic circulation. The design of specialized catheters may facilitate percutaneous ventricular cannulation, improve safety and further reduce recirculation.

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