Lactate and ScvO₂ combinations do not predict mortality in ICU patients

Introduction

Lactate (LCT) is used to identify critically ill patients at risk of death. Low central venous oxygen saturation (SvO₂) is also associated with increased mortality. It is physiologically plausible that patients with low SvO₂ and high LCT do worse versus those with only one of SvO₂ or LCT abnormal, or with both normal. However, the prognostic value of combining LCT and SvO₂ is unknown. We studied the association between ICU mortality and combinations of SvO₂ and LCT abnormality.

Methods

We used a retrospective single-centre cohort methodology using data obtained from the comprehensive electronic clinical information system (CareVue™). All patients who underwent LCT and SvO₂ measurement (from internal jugular or subclavian vein) during the first 24 hours after ICU admission (2004 to 2009) were included. Baseline (demographic, physiological), daily follow-up (physiological, SOFA) and 30-day mortality data were recorded. Worst admission values were used to combine SvO₂/LCT into four groups (SL groups: 0 to 3) dichotomised by mean (SvO₂) and median (LCT): 0 - N/N; 1 - L/N; 2 - N/H; 3 - L/H. Descriptive analysis used standard statistical techniques. Variables individually associated (P < 0.20) with SvO₂ and/or LCT, and mortality were included in a multivariate logistic regression model (with mortality as the dependent variable) using forward stepwise inclusion. Variables with adjusted P < 0.05 remained in the final model.

Results

A total of 1,544 patients were included. Mean (SD) SvO₂ was 63.6 (11.3) and median (IQR) LCT 3.85 (4.3). Overall mortality was 20.2%. Univariate analysis showed a statistically significant association between SL group and mortality (P < 0.001). This association did not remain significant in multivariate analysis: only LCT (not SvO₂) was associated with mortality when adjusted for other variables.

Conclusions

In this population LCT but not SvO₂ predicts 30-day ICU mortality. These data does not support the hypothesis that patients with low SvO₂ and high LCT do worse compared with when only one of SvO₂ or LCT is abnormal, or if both are normal.