Winter excess mortality in intensive care in the UK: an analysis of outcome adjusted for patient case mix and unit workload.
OBJECTIVE
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.


METHODS
Cohort study in 115 adult, general ICUs in England, Wales and Northern Ireland.


METHODS
113,389 admissions from 1995 to 2000.


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.


CONCLUSIONS
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.
Intensive Care National Audit and Research Centre, Tavistock House, Tavistock Square, WC1H 9HR London, UK. david@icnarc.org
David A
DA
Harrison
Panuwat
P
Lertsithichai
Anthony R
AR
Brady
James R
JR
Carpenter
Kathy
K
Rowan
D018806:APACHE; D000328:Adult; D000368:Aged; D015331:Cohort Studies; D016638:Critical Illness / Q000145:classification; D005260:Female; D017052:Hospital Mortality; D006801:Humans; D007362:Intensive Care Units / Q000706:statistics & numerical data*; D008297:Male; D008875:Middle Aged; D010343:Patient Admission / Q000706:statistics & numerical data*; D011446:Prospective Studies; D012621:Seasons; D012720:Severity of Illness Index; D006113:United Kingdom / Q000453:epidemiology; D016526:Workload