A Continuous Noninvasive Method to Assess Mixed Venous Oxygen Saturation: A Proof-of-Concept Study in Pigs.

ABSTRACT

BACKGROUND: Mixed venous oxygen saturation (Svo2) is important when evaluating the balance between oxygen delivery and whole-body oxygen consumption. Monitoring Svo2 has so far required blood samples from a pulmonary artery catheter. By combining volumetric capnography, for measurement of effective pulmonary blood flow, with the Fick principle for oxygen consumption, we have developed a continuous noninvasive method, capnodynamic Svo2, for assessment of Svo2. The objective of this study was to validate this new technique against the gold standard cardiac output (CO)-oximetry Svo2 measurement of blood samples obtained from a pulmonary artery catheter and to assess the potential influence of intrapulmonary shunting. METHODS: Eight anesthetized mechanically ventilated domestic-breed piglets of both sexes (median weight 23.9 kg) were exposed to a series of interventions intended to reduce as well as increase Svo2. Simultaneous recordings of capnodynamic and CO-oximetry Svo2 as well as shunt fraction, using the Berggren formula, were performed throughout the protocol. Agreement of absolute values for capnodynamic and CO-oximetry Svo2 and the ability for capnodynamic Svo2 to detect change were assessed using Bland-Altman plot and concordance analysis. RESULTS: Overall bias for capnodynamic versus CO-oximetry Svo2 was -1 percentage point (limits of agreement -13 to +11 percentage points), a mean percentage error of 22%, and a concordance rate of 100%. Shunt fraction varied between 13% at baseline and 22% at the end of the study and was associated with only minor alterations in agreement between the tested methods. CONCLUSIONS: In the current experimental setting, capnodynamic assessment of Svo2 generates absolute values very close to the reference method CO-oximetry and is associated with 100% trending ability.