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 (≥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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