title: Outcomes after helicopter versus ground emergency medical services for major trauma--propensity score and instrumental variable analyses: a retrospective nationwide cohort study.
abstract: BACKGROUND
  Because of a lack of randomized controlled trials and the methodological weakness of currently available observational studies, the benefits of helicopter emergency medical services (HEMS) over ground emergency medical services (GEMS) for major trauma patients remain uncertain. The aim of this retrospective nationwide cohort study was to compare the mortality of adults with serious traumatic injuries who were transported by HEMS and GEMS, and to analyze the effects of HEMS in various subpopulations.
  
  
  METHODS
  Using the Japan Trauma Data Bank, we evaluated all adult patients who had an injury severity score ≥ 16 transported by HEMS or GEMS during the daytime between 2004 and 2014. We compared in-hospital mortality between patients transported by HEMS and GEMS using propensity score matching, inverse probability of treatment weighting and instrumental variable analyses to adjust for measured and unmeasured confounding factors.
  
  
  RESULTS
  Eligible patients (n = 21,286) from 192 hospitals included 4128 transported by HEMS and 17,158 transported by GEMS. In the propensity score-matched model, there was a significant difference in the in-hospital mortality between HEMS and GEMS groups (22.2 vs. 24.5%, risk difference -2.3% [95% confidence interval, -4.2 to -0.5]; number needed to treat, 43 [95% confidence interval, 24 to 220]). The inverse probability of treatment weighting (20.8% vs. 23.9%; risk difference, -3.9% [95% confidence interval, -5.7 to -2.1]; number needed to treat, 26 [95% confidence interval, 17 to 48]) and instrumental variable analyses showed similar results (risk difference, -6.5% [95% confidence interval, -9.2 to -3.8]; number needed to treat, 15 [95% confidence interval, 11 to 27]). HEMS transport was significantly associated with lower in-hospital mortality after falls, compression injuries, severe chest injuries, extremity (including pelvic) injuries, and traumatic arrest on arrival to the emergency department.
  
  
  CONCLUSIONS
  HEMS was associated with a significantly lower mortality than GEMS in adult patients with major traumatic injuries after adjusting for measured and unmeasured confounders.
authors.affiliation: Department of Clinical Epidemiology & Health Economics, School of Public Health, Graduate School of Medicine, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 1130033, Japan. asuka-t@umin.ac.jp.|Department of Emergency and Critical Care Medicine, National Hospital Organization Mito Medical Center, 280, Sakuranosato, Ibarakimachi, Higahi-Ibarakigun, Ibaraki, 3113193, Japan. asuka-t@umin.ac.jp.
authors.forename: Asuka
authors.initials: A
authors.lastname: Tsuchiya
authors.affiliation: Department of Emergency and Critical Care Medicine, National Hospital Organization Mito Medical Center, 280, Sakuranosato, Ibarakimachi, Higahi-Ibarakigun, Ibaraki, 3113193, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Tsutsumi
authors.affiliation: Department of Clinical Epidemiology & Health Economics, School of Public Health, Graduate School of Medicine, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 1130033, Japan.
authors.forename: Hideo
authors.initials: H
authors.lastname: Yasunaga
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D017732:Air Ambulances / Q000706:statistics & numerical data*; D000401:Aircraft; D016208:Databases, Factual; D004632:Emergency Medical Services / Q000379:methods*; D005260:Female; D005500:Follow-Up Studies; D017052:Hospital Mortality / Q000639:trends; D006801:Humans; D015601:Injury Severity Score; D007564:Japan / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D009104:Multiple Trauma / Q000401:mortality / Q000628:therapy*; D012189:Retrospective Studies; D014193:Trauma Centers / Q000706:statistics & numerical data*; D055815:Young Adult
keywords: Ground emergency medical service; Helicopter emergency medical service; Instrumental variable; Japan Trauma Data Bank; Mortality; Propensity score; Trauma