Refinement of a Novel Empirical Formula to Determine the “True” Mixed Venous Saturation in Patients Undergoing Veno-Venous Extracorporeal Life Support

Veno-venous extracorporeal life support (vvECLS) is an important treatment methodology for adult respiratory failure and complex airway reconstruction. However, the supplemental oxygen provided to the venous circulation artificially raises the observed mixed venous saturation (SvO(2)), thereby eliminating its utility in evaluating the match between oxygen delivery and consumption in the critically ill patient. We previously developed an equation to calculate the "true" or native SvO(2) (NSvO(2)) but recently recognized that the prior model failed to account for both coronary sinus venous return and asymmetric return of venous sources to either the right heart or the ECLS circuit. A more complete model of venous return in the setting of vvECLS was developed to illustrate flow sources and vectors. From these diagrams, a series of equations for blood flow and oxygen content were developed. Using algebraic rearrangements, a more precise formula for the calculation of NSvO(2) was determined. Assuming the following representations: Fc = circuit flow, SatV(5) = circuit arterial oxygen saturation (expressed as a fraction of 1), CSvO(2) = circuit venous oxygen saturation, PAPO(2) = partial pressure of oxygen in the pulmonary artery, PO(2)V(5a) = partial pressure of oxygen in the circuit outflow, and CO = cardiac output,(Equation is included in full-text article.)Refinement of a previously published equation for the calculation of native mixed venous saturation in the setting of veno-venous extracorporeal life support may allow improved evaluation of the value of interventions to improve oxygen delivery.