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Volume: outcome associations are well established in the surgical literature. In 1995, ICNARC first investigated whether a potential volume: outcome association existed in critical care but found no evidence. Since then, other international studies have investigated this both for all admissions and for admissions receiving mechanical ventilation. Most of these studies have found an association. This study re-investigates the volume: outcome association for admissions to critical care units in the UK, now using a much larger, more representative sample of critical care units.
Data were extracted from the Case Mix Programme Database (CMPD) for 672 626 admissions to 199 adult, general critical care units from 1995 to 2008. The critical care units were split into quartiles by volume of admissions over a two-year period from 1/1/06 to 31/12/07 (units with less than two years’ data were scaled up). Multilevel logistic regression was performed to investigate the association between ultimate acute hospital mortality and quartile of volume, adjusted for case mix and hospital type. This analysis was then repeated solely for admissions receiving mechanical ventilation.
Between 1/1/06 and 31/12/07 there were 154 905 admissions to 172 units. For all admissions and for mechanically ventilated admissions, crude ultimate acute hospital mortality decreased as volume increased across each quartile. The decrease in mortality was explained by case mix with lower severity of illness of admissions in units with higher volume. The results of a multilevel logistic regression analysis for all admissions found no evidence of an association between ultimate acute hospital mortality and quartile of volume, adjusted for case mix and hospital type (p = 0.126). However, odds ratios for ultimate acute hospital mortality for mechanically ventilated admissions did decrease as volume increased across each quartile, but the association was not statistically significant (p = 0.182).
For all admissions and for mechanically ventilated admissions to adult, general critical care units in England, Wales and Northern Ireland, this study found no evidence of an association between ultimate acute hospital mortality and the volume of admissions to the critical care unit.