Background: Helicopter emergency medical systems (HEMS) have been reported to improve trauma survival. This study seeks to determine HEMS effect on survival across different subpopulations in relation to injury severity, degree of physiologic derangement, and transportation time (TT).
Methods: The 2007 National Trauma Data Bank adult patients transported directly to hospitals by ground ambulance or HEMS were compared in relation to their survival with discharge and other possible confounders. Multivariate models were constructed to determine the adjusted odds ratios (OR) of survival for the entire cohort and across subpopulations stratified by different Injury Severity Score (ISS), hospital Revised Trauma Score (RTS), and TT.
Results: A total of 192,422 patients with complete data were analyzed. HEMS patients (15.3%) experienced lower survival rates than those transported by ground (93.8% vs. 96.1%, p < 0.001). Multivariate analysis revealed a survival advantage for HEMS in the entire cohort (OR, 1.78 [1.65-1.92]). Adding TT to the model did not affect HEMS effect on survival. HEMS effect was present across all ISS levels but was limited to those with RTS of less than 6 (n = 15,427; OR, 2.28 [2.10-2.49]). In contrast, those with RTS of 6 or greater experienced lower adjusted survival when transported by HEMS (n = 176,995; OR, 0.83 [0.74-0.94]). Stratification by RTS and ISS did not affect the results. Substratification by TT revealed no effect of HEMS on survival among patients with RTS of less than 6, ISS of less than 16, and TT of 60 minutes or greater. Remaining associations were not affected by TT substratification.
Conclusion: HEMS beneficial effect on survival seems to be limited to patients with physiologic instability. Physiologically stable patients seem to have a worse outcome when transported by air.
Level of evidence: Epidemiologic/prognostic study, level III.