Agreement of central venous saturation and mixed venous saturation in cardiac surgery patients

Comparison of the bias and the limits of agreement (LOA; 2 SD) of the central venous saturation (S_cvO₂) before, during and after coronary artery bypass graft surgery with a simultaneous measurement of the mixed venous saturation (S_vO₂).

Prospective controlled study in a university hospital department of anaesthesiology.

60 patients with coronary artery bypass surgery, 300 paired measurements of S_vO₂ and S_cvO₂.

S_cvO₂ and S_vO₂ were analysed after induction of anaesthesia 15 min after cardiopulmonary bypass and 1, 6 and 18 h after admission to the intensive care unit. Regression analysis for the pooled measurements of S_cvO₂ and S_vO₂ showed a correlation R ² = 0.52. After induction of anaesthesia 15 min after weaning from cardiopulmonary bypass and 6 h after admission to the intensive care unit the correlation coefficient was R ² = 0.46, on admission to the intensive care unit it was R ² = 0.42, and at 18 h it was R ² = 0.38. Bland–Altman analysis for the measurements of S_cvO₂ and S_vO₂ showed a mean bias and LOA of 0.3% and −11.9 to +12.4%. In patients with a low S_cvO₂ there was a trend to overestimate the S_vO₂ by using the S_cvO₂. The only factor that influenced the ΔS_vO₂ − S_cvO₂ was the oxygen extraction rate (R ² = 0.16). In patients with S_cvO₂ below 70% this association was more pronounced (R ² = 0.60).

Our findings demonstrate that oxygen extraction rate is the major factor in the difference between S_vO₂ and S_cvO₂. Under certain circumstances S_cvO₂ differed substantially from S_vO₂. Therefore in selected patients both parameters should be monitored to exclude general or focal hypoperfusion.