title: The relationship between hospital volume and mortality in mechanical ventilation: an instrumental variable analysis.
abstract: OBJECTIVE
  To examine the relationship between hospital volume and mortality for nonsurgical patients receiving mechanical ventilation.
  
  
  METHODS
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
authors.affiliation: Division 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
authors.forename: Jeremy M
authors.initials: JM
authors.lastname: Kahn
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Ten Have
authors.forename: Theodore J
authors.initials: TJ
authors.lastname: Iwashyna
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001509:Bed Occupancy / Q000706:statistics & numerical data*; D015982:Bias; D015986:Confounding Factors, Epidemiologic; D003644:Death Certificates; D005260:Female; D006302:Health Services Research; D017052:Hospital Mortality; D006801:Humans; D016014:Linear Models; D008297:Male; D008498:Medical Record Linkage; D008875:Middle Aged; D015999:Multivariate Analysis; D017063:Outcome Assessment, Health Care; D010343:Patient Admission / Q000706:statistics & numerical data*; D010351:Patient Discharge / Q000706:statistics & numerical data*; D010414:Pennsylvania / Q000453:epidemiology; D012121:Respiration, Artificial / Q000401:mortality*; D012189:Retrospective Studies; D020379:Risk Adjustment; D012720:Severity of Illness Index
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