A Continuous Noninvasive Method to Assess Mixed Venous Oxygen Saturation

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.