Does helicopter transport impact outcome following trauma?

Abstract

Helicopter transport (HT) has evolved from military roots into a critical component of trauma systems throughout the world. Concerns over cost and safety continue to challenge the role of HT in the civilian setting. Despite this, recent evidence has demonstrated a survival advantage for trauma patients undergoing HT. For patients transported from the scene of injury, improved survival has been shown in several multicenter studies as well as evaluation of large national databases. Issues of overtriage, however, remain problematic for scene HT and represent a prime area for future research in helicopter emergency medical systems (EMS). Patients undergoing inter-facility transfer have also been shown to have improved outcomes over ground transport in terms of shorter transfer times and increased survival particularly in more severely injured patients. The benefits seen are likely a result of a combination of rapid transport, advanced medical capabilities, and accessibility to remote terrain. Several subgroups of patients undergoing HT have been the subject of study as well. Patients with severe head injury have consistently been shown to have superior outcomes over ground ambulance, attributable to improvements in airway management early in the course of their injury. Conversely, HT for urban and penetrating injury has not seen similar benefits, likely due to proximity of trauma centers and recent advancements in urban EMS systems. The benefits of including physicians in helicopter crews are less clear and vary by region and system. Helicopter transport for trauma does appear to improve outcomes for trauma patients, and optimizing utilization of this valuable resource will be key as the role of helicopter EMS continues to develop within trauma systems.

Keywords

Helicopter emergency medical services outcomes prehospital air medical aeromedical

Introduction and history

Helicopter transport (HT) of the injured patient is an important component of trauma care in many parts of the world. Rapid transport of the traumatized patient to a facility capable of providing definitive care is a fundamental part of trauma system design and delay in this setting is a well-documented cause of mortality (Sampalis et al., 1999). In this context, HT has been shown to improve timely access to regionalized trauma centers for many in the United States (Branas et al., 2005). Despite this, HT in the civilian trauma population remains controversial. Concerns over safety as well as appropriate utilization of this costly and limited resource continue to challenge the role of helicopters in modern trauma care (Bledsoe et al., 2006; Bledsoe and Smith, 2004).

Like many advances in trauma care, HT has its roots in military experience. Although dedicated medical evacuation helicopters emerged during the Korean War, the first medical HT occurred in April of 1944 during World War II, less than five years after the inaugural flight of the modern rotorcraft. The Korean conflict saw a large-scale implementation of helicopters for the dedicated transport of injured troops. This resulted in a reduction of the casualty evacuation time from the previously reported six hours during World War II to two hours, with a reduction in mortality rate from 5.8% to 2.4%. HT for medical evacuation was further expanded and popularized during the Vietnam War with care initiated during flight for the first time. This reduced casualty evacuation times to one hour and further reduced the mortality rate to 1.7% (Holmes, 1997).

Development of HT for injured civilian patients quickly followed this impressive military wartime experience. The Swiss Air Rescue Association began using HT in 1952. Military and commercial helicopters were adapted for civilian HT of the injured in Belgium in 1963 and in the United States in 1965. In 1967, the first dedicated commercial HT service was established in Westland, Michigan. By 1969, the Maryland State Police had established a statewide HT system in conjunction with the University of Maryland Trauma Program. In Germany, a national system was developed in 1970 with the first permanent civil air ambulance helicopter. Flight for Life began in 1972 in Denver, marking the first hospital-based civilian medevac helicopter (Holmes, 1997). The civilian sector has continued to expand with both aviation and medical advances. Currently there are an estimated 400,000 HT missions flown annually as well as approximately 840 dedicated aeromedical programs in the United States alone (Association of Air Medical Services, n.d.; Federal Aviation Administration, 2010).

Scene transport

The amount of time that elapses between injury and definitive care is a critical factor in the survival of severely injured patients. This principle drives the perceived benefit of HT from the scene of injury. Because helicopters are capable of higher speeds over longer distances without regard to difficult terrain, they can potentially afford an injured patient a survival advantage compared to ground transport (GT).

Several early studies of HT from the scene of injury demonstrated reductions in mortality. Baxt and Moody (1983) found a 52% mortality reduction when comparing HT and GT cohorts using predicted and actual deaths with TRISS methodology. The same group also noted a mortality benefit using probability of survival in a multicenter study of HT (Baxt et al., 1985). Additional studies have reported that injury severity impacts the potential benefit of HT; with enhanced benefit in more severely injured patients when stratified by injury severity score (ISS) or trauma score (Brathwaite et al., 1998; Cunningham et al., 1997; Jacobs et al., 1999). A more recent multicenter trial by Thomas et al. (2002) reported a 24% mortality reduction for blunt HT patients compared with GT after adjusting for injury and hospital factors. Biewener et al.(2004) demonstrated a nearly 50% mortality reduction in HT patients, attributing the benefit to direct transport to a trauma center in patients that would have otherwise been transported by ground ambulance to a non-trauma center. An Australian study found a similar 50% reduction of expected deaths in the HT group, which also required fewer interventions on arrival to the emergency department (Bartolacci et al., 1998).

Two studies examined the impact of scene HT in the United States using a national database for analysis.

In a population of nearly 42,000 patients identified using the National Trauma Databank (NTDB®), Brown et al. (2010) found a 22% survival advantage for patients transported from the scene by helicopter when compared to those transported by ground. Despite having more serious injuries and requiring more hospital resources, patients transported by helicopter in this study were more likely to be discharged home following injury than the GT cohort. In a similar study, Sullivent et al. (2011) examined 10,049 scene HT patients from the National Sample Program of the NTDB® and found that the odds of death were reduced by 39% in patients aged 18–54.

However, other authors have not been able to demonstrate improved outcomes for HT, and recent studies raise the concern that many HT patients are not severely injured. Brathwaite et al. (1998) compared HT with advanced life support GT in a statewide system using logistic regression and failed to identify HT as an independent predictor of survival, although the data suggested that patients with ISS 16–60 may benefit. Cunningham et al. (1997) similarly examined a statewide data set where regression analysis did not demonstrate HT to be a predictor of survival, despite identifying a benefit for a subset of patients based on ISS and trauma score. Some authors have suggested that a significant percentage of patients undergoing HT have relatively minor injuries as a result of overtriage (Brathwaite et al., 1998). A meta-analysis by Bledsoe et al. (2006) of 22 studies reported that one in four HT patients are discharged within 24 h of arrival to the trauma center, and between 60% and 70% had non-life threatening injuries by ISS, trauma score or TRISS probability of survival criteria.

Overtriage is an issue that continues to plague scene HT. Early reports demonstrated a relatively high ISS in HT patients (mean ISS 36). However, more recent studies have reported a mean ISS of HT patients between 16 and 19 (Brathwaite et al., 1998; Brown et al., 2010; Cunningham et al., 1997; Schwab et al., 1985). Further, overtriage was found to increase with time over a nine-year period in one study, and the authors suggested competition among HT services plays a role as the industry expands (Towsend et al., 1996). Others have added that perceived benefits of HT by emergency medical service (EMS) providers lowers the threshold for scene personnel to request HT based solely on mechanism of injury criteria rather than actual injuries or physiologic criteria (Shantey et al., 2002). Additionally, some have suggested that mechanism criteria should be eliminated from the decision to utilize HT at the scene (Cook et al., 2001). The Air Medical Physician Association (2001) points out the limitations of retrospective injury scoring systems to determine appropriate aeromedical helicopter use, as it fails to acknowledge regional, environmental and situational factors that play into the decision to use HT.

Identifying patients that would benefit from HT remains a challenge and an area for further investigation. There are few uniform triage criteria specifically for HT. The National Association of EMS Physicians published guidelines for helicopter dispatch following trauma in 2003 (Thompson and Thomas, 2003). These guidelines outline several physiologic, anatomic and mechanism of injury criteria that should prompt consideration of helicopter dispatch and are similar to the field trauma triage criteria published by the Centers for Disease Control and American College of Surgeons Committee on Trauma (Sasser et al., 2009). This has left trauma systems to develop regional criteria for scene HT with little evidence. Despite efforts to standardize and promulgate such regional criteria, significant variation and poor adherence remains an issue (Tiamfook-Morgan et al., 2008).

Rhodes et al. (1986) were able to demonstrate in a sample of HT patients that abnormal vital signs and entrapment predicted severe injury and had a significantly lower observed to expected mortality. They also noted that a GT time of greater than 20 min was a useful criterion. A review of regional protocols in a rural system determined that a structured prehospital assessment of physiology, anatomic injury and mechanism could identify major trauma victims and bypass medical control for HT dispatch (Purtill et al., 2008). More recently, King et al. (2009) determined that prehospital heart rate variability could be used to predict base excess and lifesaving operating room procedures better than standard trauma triage criteria, concluding that heart rate variability could be used to avoid unnecessary HTs. Overtriage and safety concerns make effective triage essential (Bledsoe et al., 2006; Bledsoe and Smith, 2004; Hinkelbein et al., 2008). This includes not only the usual physiologic, anatomic, and mechanism factors, but must also account for distance and logistical factors. Regional protocols should be individualized to reflect the trauma system capabilities and resources over the entire geographical area, with regular quality improvement review.

Despite these concerns, the available evidence points to a survival advantage for trauma patients transported by helicopter from the scene of injury. There are several potential explanations for this. The first is the perceived benefit of rapid transport to definitive care in time-sensitive clinical situations. Diaz et al. (2005) were able to show that HT was faster than GT if both a helicopter and ground ambulance were simultaneously dispatched to an injury scene 10 miles from the trauma center. Additionally, the authors demonstrated that if the helicopter was requested by ground EMS after they had arrived at the scene, HT was faster than GT if the scene of injury was 45 miles from the trauma center. Since simultaneous launch is likely to lead to significant overtriage, critically injured patients initially evaluated by ground EMS providers in the field are likely to benefit from HT when distances from the trauma center exceed 45–50 miles (Brown et al., 2010).

Another potential benefit of HT is that aeromedical crews may provide a higher level of care in some systems than ground ambulance crews. Helicopter providers may be trained and authorized to perform potentially lifesaving interventions such as rapid sequence intubation (RSI), cricothyrotomy or blood product administration that ground crews cannot (Norton et al., 1996). Additionally, helicopter crews are more likely to have exposure to a larger volume of trauma patients, affording them more experience and comfort with managing a severely injured patient. Thomas et al. (2002) demonstrated this principle with a 98% success rate of airway interventions performed by aeromedical providers over a six-year period. This potential benefit is also borne out in earlier studies. Nardi et al. (1994) in Italy found a nearly 3-fold reduction in mortality while demonstrating significantly higher rates of intubation, thoracic decompression and intravenous resuscitation in their HT patient population. One review compared an advanced German HT system to the emerging HT system in the US, showing better outcomes in Germany related to more aggressive prehospital care (Schmidt et al., 1992). Finally, some authors have shown a mortality benefit when helicopter crews arrive and provide interventions to patients that subsequently are transported by ground (Frankema et al., 2004; Oppe and De Charro, 2001).

Lastly, HT may have utility in terrain which would be difficult to access by ground ambulance. This is a less common situation in which HT may confer benefit, however given the scope of operating helicopter services throughout the world, there are certainly areas on each continent where this potential benefit may play a role (Malacrida et al., 1993).

Inter-facility transport

Despite the improved access to trauma center care afforded by HT, many injured patients undergo initial assessment and stabilization at non-trauma facilities with subsequent transfer to a trauma center for definitive care. The immediate benefits of HT over conventional GT in the setting of inter-facility transfer following injury are unclear (Arfken et al., 1998; Boyd et al., 1989; Karanicolas et al., 2006; Schwartz et al., 1989).

Several reports suggest that HT is beneficial when compared to ground ambulance. Moylan et al. (1988) demonstrated a 29% improvement in survival for HT patients with a trauma score between 5 and 10, noting more airway and resuscitation interventions in the HT group. Svenson et al. (2005) found that HT offered a 10–45 min time advantage over GT, depending on the location of the referring hospital, concluding that HT should be considered for time-critical injuries or delayed GT. The same authors also showed that HT can reduce time spent in the referring hospital emergency department by 30–60 min if HT is utilized (Svenson, 2008). Mann et al. (2002) conducted a pre- and post-analysis of rural inter-facility transfers following the loss of HT capabilities, finding an increase in transfer times. Moreover, multivariate regression identified the loss of HT as a significant predictor of mortality, which was four times higher in the post-period. In a comparison of HT and ground ambulance, Boyd et al.(1989) reported a 25% reduction in mortality for HT patients with a probability of survival < 90%. A review of 14,771 inter-facility HT patients from the NTDB® showed that HT shortened transport and overall prehospital times, and was an independent predictor of survival in patients with an ISS > 15 (Brown et al., 2011).

Some reports have not borne out an advantage for inter-facility HT. Karanicolas et al. (2006) reported that time from transfer decision to arrival at the trauma center was shorter for GT and were unable to show a mortality difference between HT and GT transfers, or optimal distance for HT. Arfken et al. (1998) compared patients transferred by helicopter and those deemed to require HT but did not actually undergo air transport. In multivariate regression, the authors found that transport modality was not a significant predictor of 30-day mortality.

The utilization of helicopters for inter-facility transfer of the injured patient does appear to provide benefit, particularly in reduction of transport times. Further, most reports noted some survival benefit in more severely injured patients. When determining the method of transfer, the decision should be based on objective measures of injury severity as well as the impact on current system resources. Evaluations done at referring hospitals can include assessments of injury severity not available to providers in the prehospital setting and can facilitate a more informed decision regarding the appropriate utilization of HT for such transfers. The benefits described are probably due to the decrease in pre-trauma center time and could be related to the specialized care available while undergoing HT.

Head injury

HT for patients with severe head injury has received much attention. Baxt and Moody (1987a) noted a 9% mortality reduction and better Glasgow outcome scores in patients with severe head injury transported by helicopter, probably attributable to advanced airway intervention by HT crews. A similar early analysis noted an intubation rate of 80% in HT patients compared with 10% in ground patients, as well as a 34% lower mortality for HT patients (Celli et al., 1997). Di Bartolomeo et al. (2001) compared HT and GT patients, reporting improvements in outcome for patients requiring urgent neurosurgical intervention. Again these authors noted a higher rate of airway interventions in the HT patients. Davis et al. (2005a, 2005b) reported improved survival and functional outcome for HT patients with severe brain injury, and improvement in outcomes for RSI performed by HT providers. In addition, a large multicenter review by Wang et al. (2004) found that prehospital intubation in patients with severe head injury was associated with worse survival and functional outcome in their overall population, but was associated with improved outcomes when performed by helicopter crews.

Mortality and functional outcomes have routinely been superior in HT patients with severe head injury. Management of airway and ventilatory status, particularly early in the course of injury, is known to be of critical importance. Several authors have shown that advanced airway management is directly linked with outcomes in severely head injured HT patients, providing some insight into why HT may confer benefit in this population.

Urban transport

HT in the urban setting has also received attention, as it is unclear that the potential benefits of HT apply in this environment. Fischer et al. (1984) first touted urban HT as civilian medevac programs were getting started based on advanced invasive procedures required in 57% of patients and congested traffic in the Houston area; however, no measurable improvement in outcomes was reported. HT in the urban setting received early criticism, when Schiller et al. (1988) noted longer prehospital times for HT compared to GT and slightly higher mortality, although adjustment for injury severity was not performed. Subsequent studies in the urban setting have also illustrated a lack of benefit for HT patients, citing advanced prehospital systems and shorter transport times for GT encountered in more populated areas. Shantey et al. (2002) found that less than 23% of urban HT patients potentially benefited from the service. Cocanour et al. (1997) evaluated penetrating trauma in the urban setting and found that HT crews performed only three interventions beyond the scope of ground crews over 12 months.

Urban use of scene HT appears to lack benefit. Proximity to the receiving trauma center nullifies any benefits based on rapid transport. Many urban ambulance ground crews have increasingly advanced prehospital capabilities, often with the availability of EMS physicians when necessary. This may mitigate the benefits of advanced prehospital care attributed to HT. Some logistic and geographic circumstances may favor HT in an urban setting such as severe traffic congestion, however urban HT is unjustified in most cases.

Physician crew members

The composition of medical helicopter crews varies and is influenced by regional trends. Currently, physician crews are more common in Europe than in the United States (Butler et al., 2010). Debate continues whether advanced care capabilities are central to the benefits of HT. Several studies have reported mortality reduction when comparing physician staffed HT to standard ground EMS; however, few studies evaluate crew differences for HT programs (Bartoloacci et al., 1998; Davis et al., 2005a; Frankema et al, 2004; Schwartz et al., 1990).

Early work by Baxt and Moody (1987b) demonstrated that the presence of a physician on the HT crew reduced predicted mortality when compared to a paramedic-only crew. This may be due to interventions performed by physicians that would be outside the scope of paramedic practice. An Australian study evaluated two HT programs with and without a physician crew member, noting a lower predicted mortality in the physician program with no difference from predicted mortality in the non-physician HT program for blunt trauma patients (Garner et al., 1999).

Conversely, several studies have shown little benefit of physician over non-physician HT crews. Hamman et al. (1991) reported a 30% predicted mortality reduction for physician HT crews compared with 47% reduction for paramedic HT crews, concluding that experienced non-physician providers with advanced skills may be comparable to prehospital physician providers for HT. A recent Finnish study found no difference in survival or quality of life three years after injury for physician staffed crews and historical controls (Iirola et al., 2006). Another study comparing a physician and nurse crew to a dual nurse crew found no differences between crew types in mortality, intensive care unit length of stay or hospital length of stay (Burney et al., 1992).

Early evidence supported physician involvement in HT for trauma. These studies attributed the benefit seen to invasive interventions offered at the time only by physicians (advanced airway control, chest decompression). As the aeromedical field has evolved, the scope of interventions for flight paramedics and nurses has expanded significantly in many systems. In modern aeromedical care, there is little convincing evidence that physician staffing provides a significant benefit to patients when non-physician providers are capable and experienced with these advanced therapies. Thus, the benefit of a physician crew will vary widely by regional system and be dictated by the scope of practice for non-physician helicopter providers as well as the comfort of physicians practicing in the prehospital setting.

Conclusions

HT of injured patients has evolved as a critical component of modern trauma systems throughout the world. Current evidence demonstrates that many trauma patients benefit from HT. A survival advantage is consistently described for patients transported by helicopter directly from the scene of injury and for those with severe injuries undergoing inter-facility HT. These benefits are probably attributable in some combination to rapid transport, advanced medical capabilities and the advantages of helicopters in areas of inaccessible or remote terrain.

Patients with severe head injury have particularly been shown to benefit from HT, primarily due to improved airway management. The benefits of urban HT and physician crews are less clear, especially as the capabilities of both ground and aeromedical providers expand. Attention to concerns regarding safety, cost and overutilization of this valuable resource will be essential as the field of helicopter EMS continues to develop.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Conflict of interest

None declared.

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