Comparative effectiveness of helicopter emergency medical services compared to ground emergency medical services

The use of helicopter emergency medical services (HEMS) for the transportation and treatment of trauma patients, while commonplace in most developed nations, remains controversial. The purported beneficial effects of HEMS compared to ground emergency medical services is likely to be some combination of speed, crew expertise, and the fact that HEMS is part of an organized trauma system. When the HEMS literature is assessed as a whole, considerable heterogeneity of effects and study methodologies preclude an accurate estimate of composite effect. However, when the outcome of mortality is studied using advanced multivariable regression techniques to control for multiple known confounders, an improved odds of survival has been repeatedly demonstrated. Future HEMS research must rely on robust observational study designs and assessments of a variety of patient outcomes. Questions about the role of speed, distance, and other potentially beneficial elements of HEMS remain.

trauma registry maintained by the German Society for Trauma Surgery.
HEMS are capable of transporting patients with serious injury over greater distances signifi cantly faster than ground emergency medical services (GEMS), and the speed benefi t is more pronounced as the distance from a trauma center increases. HEMS crews are typically staff ed by experienced providers and exist as an integral part of regional trauma systems. Th us, any benefi cial eff ect of HEMS compared to GEMS is likely to be some combination of speed, crew expertise, and the fact that HEMS, as part of an organized trauma system, may aff ord seriously injured patients timely access to trauma centers [2,3] (Figure 1).
Th e question of which elements of HEMS are most benefi cial for patients has not been fully answered, and the present study by Andruszkow and colleagues attempts to evaluate features of HEMS that may provide benefi ts for patients with major trauma. Crew expertise is likely to be an important contributing factor for improved survival in HEMS patients. HEMS in Germany and Europe is exclusively physician-staff ed, unlike many HEMS systems in the US. In the present study, HEMS patients were statistically signifi cantly more likely to be intubated, have a chest thoracostomy tube placed, receive sedation, or be treated with vasopressors [1]. HEMS patients were more frequently admitted to Level I trauma centers compared to GEMS patients even though a subgroup analysis demonstrated improved survival for HEMS patients independently of Level I admission status. Indeed, the accurate identifi cation and triage of patients most likely to benefi t from HEMS remains an elusive goal. Even when staff ed by well-trained physicians, the accuracy of suspected diagnoses during resuscitation was problematic; future studies are required to accurately predict which groups of patients are most likely to benefi t from HEMS.
To date, most HEMS comparative eff ectiveness studies have dealt with the outcome of survival, and less is

Abstract
The use of helicopter emergency medical services (HEMS) for the transportation and treatment of trauma patients, while commonplace in most developed nations, remains controversial. The purported benefi cial eff ects of HEMS compared to ground emergency medical services is likely to be some combination of speed, crew expertise, and the fact that HEMS is part of an organized trauma system. When the HEMS literature is assessed as a whole, considerable heterogeneity of eff ects and study methodologies preclude an accurate estimate of composite eff ect. However, when the outcome of mortality is studied using advanced multivariable regression techniques to control for multiple known confounders, an improved odds of survival has been repeatedly demonstrated. Future HEMS research must rely on robust observational study designs and assessments of a variety of patient outcomes. Questions about the role of speed, distance, and other potentially benefi cial elements of HEMS remain. Duration of ventilation, ICU length of stay, and overall hospital length of stay were signifi cantly increased for HEMS patients in the present study. Future HEMS outcomes research must consider assessments for additional dimensions, such as health-related quality of life, rather than the traditionally studied endpoints of morbidity and mortality [4].
When the HEMS literature is assessed as a whole, considerable heterogeneity of eff ects and study methodolo gies preclude an accurate estimate of composite eff ect [3]. In the present study, a multivariable regression analysis found an improved odds of survival for HEMS compared to GEMS patients when 11 confounding variables were included in the model. Indeed, when the outcome of mortality is studied using advanced multivariable regression techniques to control for multiple known confoun ders, an improved odds of survival has been repeatedly demonstrated (Table 1) [1,[5][6][7][8][9]. Th e use of advanced methods, such as propensity scores and instrumental variables, should be considered for future HEMS studies to minimize biased eff ect estimates [3,10].
Th e trauma and injury severity score (TRISS) and Revised Injury Severity Classifi cation (RISC) were used to compare mortality between HEMS and GEMS. As the authors acknowledge, there are several limitations that must be considered when these methodologies are used to compare study groups. Although used in several previous HEMS studies, TRISS has been shown to have a high misclassifi cation rate, particularly in patients with severe trauma [11]. Additionally, since TRISS probability of survival is compared to a historical cohort, survival probabilities may be incomparable due to advances in trauma care. Th e authors did not report M and W statistics for the TRISS-based analysis. Reporting of these statistics is important because the M statistic for non-US populations may be below the cutoff for non-standardized TRISS analyses, thereby rendering comparisons invalid [12]. Th e RISC, while a more contemporary and potentially more accurate system for comparing outcomes [13], did not reveal a statistically signifi cant survival benefi t for HEMS compared to GEMS.
Given the infeasibility of conducting a randomized controlled trial comparing HEMS versus GEMS, future studies to estimate treatment eff ects for trauma patients will need to rely on robust observational study designs. Questions about the role of speed, distance, and other potentially benefi cial elements of HEMS remain. Improvements in statistical techniques and the continued development of large databases, with minimization of missing data, are warranted to advance the science of aeromedical critical care [14].   Figure 1