Our aim was to assess the association between mixed venous oxygen saturation (SvO₂) and cardiac index (CI) among cardiac surgical patients during the initial 4 hours in the intensive care unit (ICU).

A single-center retrospective observational study.

A tertiary-level university hospital.

Adult cardiac surgical patients (N = 4,958) operated on during 2007 to 2020.

Pulmonary artery catheter (PAC) measurements of SvO₂ and CI were taken at ICU admission and 4 hours later. Linear regression was used to analyze the association between these variables.

Paired CI and SvO₂ values were available from 4,958 patients. The median (interquartile range) SvO₂ was 68% (63-72%) at ICU admission and 66% (61-71%) 4 hours later. CI was 2.34 L/min/m² (2.03-2.79) at ICU admission and 2.48 L/min/m² (2.15-2.86) 4 hours later. In the entire cohort, a 10% change of SvO₂ coincided with a CI change of 0.36 L/min/m² (95% confidence interval 0.34-0.38) at ICU admission and a change of 0.33 L/min/m² (0.31-0.36) at 4 hours. In patients with SvO₂ less than 60%, the association between CI and SvO₂ weakened further.

Factors affecting oxygen consumption and demand weaken the association between SvO₂ and CI values in the early ICU recovery phase after cardiac surgery. Therefore, SvO₂ and CI values should not be relied upon in clinical decision-making as reliable predictors of one another.