The Relationship between Hospital Volume and Mortality in Mechanical Ventilation: An Instrumental Variable Analysis

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.