Quantification of recirculation during Veno-Venous Extracorporeal Membrane Oxygenation: in vitro evaluation of a thermodilution technique.

Objective: During Veno-Venous Extracorporeal Membrane Oxygenation (vv-ECMO) a certain amount of extracorporeal oxygenated blood can flow from the reinfusion cannula back into the drainage one without delivering oxygen to the patient. Recognition and quan tification of this dynamic phenomenon, defined recir culation, are pivotal to maximize ECMO efficiency. This study aimed to quantify the recirculation fraction (RF) by means of a thermodilution technique. Methods: We assembled an in-vitro circuit simulating a patient undergoing vv-ECMO with a recirculation bridge to achieve different known RFs. A cold bolus of 20 ml was injected downstream the ECMO pump, tem perature was recorded on both the reinfusion and drainage lines. RF was computed as the ratio of the area under the temperature-time curves (AUC) of the drain age over reinfusion. We tested three ECMO flows (1.5, 3, and 4.5 L/min) with recirculation fractions ranging from 0% to 50% (5% incremental steps) performing three different thermodilutions for each step. Results: A Bland-Altman analysis, see figure, showed an overall bias of 1.77%. Test-retest reliability showed an excellent intraclass correlation coefficient of 0.997 while the linear mixed model showed a significative effect of ECMO flow according to the following equation RF(%) (-5.078- 1.454 x set ECMO flow) (1.086 - 0.039 x set ECMO flow) x AUC ratio(%). Conclusions: The thermodilution technique tested in this in-vitro study proved to be feasible and measure the RF of a vv-ECMO with high accuracy and precision. In vivo studies are needed to confirm these results.