Association of direct helicopter versus ground transport and in-hospital mortality in trauma patients: a propensity score analysis

Objectives: Helicopter emergency medical services (HEMS) transport of trauma patients has been used for decades. Its use, however, is still a subject of debate, including issues such as high costs, increasing numbers of crashes, and conflicting results regarding effectiveness in reducing mortality. The aim of this study was to examine whether mode of transport (HEMS vs. ground EMS) is independently associated with mortality among trauma patients transported directly from the scene of injury to definitive care.

Methods: All trauma patients transported directly to a Level I or Level II trauma center by either air or ground EMS over a 4-year period were selected from the Oklahoma State Trauma Registry. Multivariable logistic regression was used to develop propensity scores based on variables measured at the scene of injury. The propensity scores represented the predicted probabilities of a patient being transported by HEMS given a specific set of characteristics and were used as a composite confounding variable in subsequent models of the association of mortality and mode of transport. Along with the propensity scores, Injury Severity Scores (ISS), initial Revised Trauma Score (RTS), and distance from the trauma center were included in a Cox proportional hazards model of the association of mode of transport and 24-hour and 2-week mortality.

Results: Overall, the hazard ratio (HR) for 2-week mortality in patients transported by HEMS was 33% lower (HR = 0.67, 95% confidence interval [CI] = 0.54 to 0.84) than in patients transported by ground EMS from the scene of injury, after adjustment for the propensity score and other covariates. In subanalyses, the apparent association of a reduction in the hazard of early mortality among patients transported by HEMS was most evident for patients with an RTS based on injury scene vital signs of 3 to 7 (HR = 0.61, 95% CI = 0.46 to 0.82). The point estimate of the HR was similar (HR = 0.65 95% CI = 0.34 to 1.2) in the 75% of cases who had normal vital signs at the scene of injury, although it was no longer statistically significant because crude mortality was very low (1.7%) in this group. Among those with a RTS of 3 or less at the scene, crude mortality was 58%, and mode of transport was not associated with mortality (HR = 1.02, 95% CI = 0.68 to 1.6).

Conclusions: Helicopter EMS transport was associated with a decreased hazard of mortality among certain patients transported from the scene of injury directly to definitive care. Refinements in scene triage and transport guidelines are needed to more effectively select patients that may benefit from HEMS transport from those unlikely to benefit.