A new formula for determining arterial oxygen saturation during venovenous extracorporeal oxygenation

Venovenous extracorporeal membrane oxygenation (VV-ECMO) is used to treat severe forms of acute respiratory distress syndrome (ARDS). VV-ECMO management may be confusing due to the lack of information about the interplay between the determinant parameters and their impact on oxygenation. We found a relationship between arterial oxygen saturation (SaO(2)) and its relevant parameters. The aim of this study was to assess the validity of this model.

We report our experience in 17 patients under VV-ECMO for severe ARDS. We compared, at two different levels of pump flow, SaO(2) and the oxygen saturation measured in the pulmonary artery (SpaO(2)) with the predicted saturation using the formula: SpaO(2) = (EF/CO)SmO(2) + (1 - EF/CO)SvO(2) + 10(-2)PmO(2), where PF is pump flow, R is recirculation, EF is effective flow [= (1 - R)PF], SmO(2) is saturation of the oxygenator outgoing blood, CO is cardiac output, SvO(2) is saturation of mixed venous blood, and PmO(2) is oxygen partial pressure of the oxygenator outgoing blood.

There was no significant difference between predicted and measured SpaO(2): the mean predicted and measured SpaO(2) values were 90.7 ± 2.8 % and 90.4 ± 2.7 % , respectively (p = 0.696, r = 0.966). Bland-Altman analysis showed good agreement between predicted and measured SpaO(2). Predicted SpaO(2) and SaO(2) was well correlated (r = 0.80).

We have presented an explicit relationship between SaO(2) and its direct determinants during VV-ECMO. Good agreement was found with the measured values of SaO(2), but the model remains to be fully validated before its use in clinical practice.