Winter excess mortality in UK critical care units: an analysis of outcome adjusted for patient case mix and unit workload

Introduction

Mortality in the UK is known to be higher in winter than in nonwinter, but the comparative importance of variation in case mix and increased pressure on hospitals is not clear. We explored this issue using data from the national audit of critical care admissions, the ICNARC Case Mix Programme.

Methods

Using data from 113,389 admissions to 115 adult, general critical care units in England, Wales and Northern Ireland from 1995 to 2000, we investigated whether hospital mortality following admission to critical care was higher in winter (defined as the first working day in January–31 January) than in nonwinter (March–November inclusive) and explored the causes of any observed differences in terms of the case mix of admissions and the workload of the units.

Results

Crude hospital mortality was higher in winter than in nonwinter (odds ratio [OR] 1.18, 95% confidence interval [CI] 1.11–1.25). After adjusting for case mix using the APACHE II mortality probability, this effect was reduced but still significant (OR 1.11, 95% CI 1.04–1.18). When additional factors reflecting case mix and workload were introduced into the model, the overall effect of winter was no longer significant (P = 0.08). Factors reflecting both the case mix of the individual patient and of the patients in surrounding beds were found to be significantly associated with outcome. After adjustment for other factors, the occupancy of the unit (proportion of beds occupied) was not significantly associated with mortality.

Conclusions

The excess winter mortality in UK critical care units can be explained by the variation in the case mix of admissions. Unit occupancy was not associated with mortality.