Calculating mixed venous saturation during veno-venous extracorporeal membrane oxygenation

Introduction: Recirculation (R), the shunting of arterial blood back into to the venous lumen, commonly occurs during veno-venous extracorporeal membrane oxygenation (VV-ECMO) and renders the monitoring of the venous line oxygen saturation no longer reflective of patient mixed venous oxygen saturation (S_VO₂). Previously, we failed to prove the hypothesis that, once R is known, it is possible to calculate the S_VO₂ of a patient on VV-ECMO. We hypothesize that we can calculate S_VO₂ during VV-ECMO if we account for and add an additional correction factor to our model for dissolved oxygen content. Therefore, the purpose of this study is to derive a more accurate model that will allow clinicians to determine S_VO₂ during VV-ECMO when ultrasound dilution is being used to quantify R. Methods: Using an extracorporeal circuit primed with fresh porcine blood, two stocks of blood were produced; (1) arterial blood (AB), and (2) venous blood (VB). To mimic recirculation, the AB and VB were mixed together in precise ratios using syringes and a stopcock manifold. Six paired stock AB/VB sets were prepared. Two sets were mixed at 20% R increments and 4 sets were mixed at 10% R increments. The partial pressure of oxygen (pO₂ ) and oxygen (O₂) saturation of the stock blood and resultant mixed blood was determined. The original model was modified by modeling the residual errors with linear regression. Results: When using the original model, as the partial pressure of arterial oxygen (P_aO₂) of the stock AB increased, the calculated S_VO₂ was higher than actual, especially at higher R levels. An iteration of the original model incorporating the P_aO₂ level (low, medium, high) and R was derived to fit the data.