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The relationship between hospital volume and mortality in mechanical ventilation: an instrumental variable analysis
Jeremy M Kahn1, Thomas R Ten Have, Theodore J Iwashyna
1Division of Pulmonary, Allergy & Critical Care, Center for Clinical Epidemiology & Biostatistics, University of Pennsylvania School of Medicine, 723 Blockley Hall, Philadelphia, PA 19104, USA. jmkahn@mail.med.upenn.edu
Objective:
To examine the relationship between hospital volume and mortality for nonsurgical patients receiving mechanical ventilation.
Data Sources:
Pennsylvania state discharge records from July 1, 2004, to June 30, 2006, linked to the Pennsylvania Department of Health death records and the 2000 United States Census.
Study Design:
We categorized all general acute care hospitals in Pennsylvania (n=169) by the annual number of nonsurgical, mechanically ventilated discharges according to previous criteria. To estimate the relationship between annual volume and 30-day mortality, we fit linear probability models using administrative risk adjustment, clinical risk adjustment, and an instrumental variable approach.
Principle Findings:
Using a clinical measure of risk adjustment, we observed a significant reduction in the probability of 30-day mortality at higher volume hospitals (>or=300 admissions per year) compared with lower volume hospitals (<300 patients per year; absolute risk reduction: 3.4%, p=.04). No significant volume-outcome relationship was observed using only administrative risk adjustment. Using the distance from the patient's home to the nearest higher volume hospital as an instrument, the volume-outcome relationship was greater than observed using clinical risk adjustment (absolute risk reduction: 7.0%, p=.01).
Conclusions:
Care in higher volume hospitals is independently associated with a reduction in mortality for patients receiving mechanical ventilation. Adequate risk adjustment is essential in order to obtained unbiased estimates of the volume-outcome relationship.
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