Validation of the capnodynamic method to calculate mixed venous oxygen saturation in postoperative cardiac patients

Cardiac output and mixed venous oxygen saturation are key variables in monitoring adequate oxygen delivery and have typically been measured using pulmonary artery catheterisation. The capnodynamic method measures effective pulmonary blood flow utilising carbon dioxide kinetics in ventilated patients. Combined with breath-by-breath measurements of carbon dioxide elimination, a non-invasive approximation of mixed venous oxygen saturation can be calculated.

This study primarily investigated the agreement between mixed venous oxygen saturation calculated using the capnodynamic method and blood gas analysis of mixed venous blood sampled via a pulmonary artery catheter in 47 haemodynamically stable postoperative cardiac patients. Both measurements were synchronised and performed during alveolar recruitment by stepwise changes to the level of positive end-expiratory pressure. Simultaneously, we studied the agreement between effective pulmonary blood flow and thermodilution cardiac output. The Bland-Altman method for repeated measurements and calculation of percentage error were used to examine agreement. Measurements before and after alveolar recruitment were analysed by a paired t test. The study hypothesis for agreement was a limit of difference of ten percentage points between mixed venous oxygen saturation using the capnodynamic algorithm vs. catheter blood gas analysis.

Capnodynamic calculation of mixed venous saturation compared to blood gas analysis showed a bias of -0.02 [95% CI - 0.96-0.91] % and limits of agreement at 8.8 [95% CI 7.7-10] % and - 8.9 [95% CI -10-- 7.8] %. The percentage error was < 20%. The effective pulmonary blood flow compared to thermodilution showed a bias of - 0.41 [95% CI - 0.55-- 0.28] l.min^-1 and limits of agreement at 0.56 [95% CI 0.41-0.75] l.min^-1 and - 1.38 [95% CI - 1.57--1.24] l.min^-1. The percentage error was < 30%. Only effective pulmonary blood flow increased by 0.38 [95% CI 0.20-0.56] l.min^-1 (p < 0.01) after alveolar recruitment.

In this study, minimal bias and limits of agreement < 10% between mixed venous oxygen saturation calculated by the capnodynamic method and pulmonary arterial blood gas analysis confirmed the agreement hypothesis in stable postoperative patients. The effective pulmonary blood flow agreed with thermodilution cardiac output, while influenced by pulmonary shunt flow.