The impact of pediatric intensive care unit volume on mortality: A hierarchical instrumental variable analysis*

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To evaluate the relation between annual pediatric intensive care unit (PICU) admission volume and mortality.


Nonconcurrent cohort design.


Pediatric patients included in the most currently available research database from the Pediatric Intensive Care Unit Evaluations (PICUEs).


A total of 34,880 consecutive pediatric admissions to a contemporary volunteer sample of 15 U.S. PICUs.

Measurements and Main Results:

We conducted an instrumental variable analysis and adjusted for similarities between patients admitted to different PICUs using mixed-effects, hierarchical techniques. Case mix and severity of illness was adjusted for using patient-level data and the Pediatric Risk of Mortality, version III (PRISM III). On average, admission to higher-volume PICUs was associated with lower severity-adjusted mortality (odds ratio = 0.68 per 100 patient increase in volume; 95% confidence interval: 0.52–0.89) when volume was analyzed as a linear term; however, when PICU volume was analyzed as a quadratic term, we found the lowest severity-adjusted mortality rates among PICUs with annual admission volumes between 992 and 1,491. Furthermore, lower severity-adjusted mortality rates were primarily found among patients with less than a 10% PRISM III predicted risk of mortality.


Although there is an association between lower severity-adjusted mortality among higher volume PICUs, our data suggest that best outcomes are among mid- to large-sized PICUs. These data support minimum annual admission criteria for PICUs but raise the concern that PICUs with very high annual admission volumes may operate beyond an ideal capacity.

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