Winter excess mortality in intensive care in the UK: an analysis of outcome adjusted for patient case mix and unit workload
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To investigate whether mortality in UK intensive care units is higher in winter than in non-winter and to explore the importance of variations in case mix and increased pressure on ICUs.
Design and setting
Cohort study in 115 adult, general ICUs in England, Wales and Northern Ireland.
Patients and participants
113,389 admissions from 1995 to 2000.
Measurements and results
Hospital mortality following admission to ICU was compared between winter (December–February) and non-winter (March–November). The causes of any observed differences were explored by adjusting for the case mix of admissions and the workload of the ICUs. Crude hospital mortality was higher in winter. After adjusting for case mix using the APACHE II mortality probability this effect was reduced but still significant. When additional factors reflecting case mix and workload were introduced into the model, the overall effect of winter admission was no longer significant. 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.
The excess winter mortality observed in UK ICUs can be explained by variation in the case mix of admissions. Unit occupancy was not associated with mortality.
KeywordsCritical care Intensive care units Mortality Seasons Workload Bed occupancy
We thank all the staff in the ICUs participating in the Case Mix Programme and those responsible for local funding.
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