The growing air medical transport sector represents a highly visible concentration of resources. A 2007 publication estimated that in the United States, 753 helicopters (and 150 dedicated fixed-wing aircraft) were in EMS service, providing about 3% of all ambulance transports.¹ By 2011, the Association of Air Medical Services (AAMS) placed the number of rotor-wing (ie, helicopter) transport vehicles at about 900. Individual helicopter EMS (HEMS) programs’ mission profiles and crew configurations vary widely. Varying programs have varying mission breakdowns (as well as differing aircraft and crew configurations), but a typical US HEMS program performs 54% interfacility transports, 33% scene runs, and 13% “other” mission types (eg, neonatal, pediatric, transplant related).²

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  Discuss the utilization and integration of air medical services in field response.

  
  
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  Discuss the unique role of air medical transport for interfacility transfers.

  
  
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  Describe the physiologic changes and medical limitations associated with air medical transport.

  
  
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  Describe the utilization of fixed-wing versus rotor-wing transport.

  
  
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  Discuss different rotor-wing aircraft types and give specific examples.

  
  
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  Discuss unique safety considerations.

  
  
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  Describe operations for establishing an LZ and for safe landing/take-off of rotor-wing aircraft in the field and at the hospital.

  
  
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  List governmental and professional agencies that set standards for ­aircraft and air medical operations.

  
  
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  Discuss the licensure and certification implications of operations across state boundaries.

While there is occasional utility in HEMS deployment for nontrauma ­situations (eg, time-critical diagnoses such as stroke),^5–6 most of the applicable use and evidence for scene response deals with HEMS dispatch for injured patients. The availability of rotor-wing response is variable throughout the country based on the location of air bases in relation to populations they serve (Figure 18-1).

EVIDENCE SUPPORTING INCORPORATION OF AIR MEDICAL ASSETS INTO SYSTEM SCENE RESPONSE

An independent 2007 review of all studies dating from the year 2000, ­conducted by the Institute of Health Economics for the Canadian health ministry in Alberta, concluded: “Overall, patients transported by ­helicopter showed a benefit in terms of survival, time interval to reach the healthcare facility, time interval to definite treatment, better results, or a benefit in general.”⁷ This finding was endorsed in a recent review of the worldwide HEMS scene response literature.⁸ Since the landmark 1983 JAMA paper from Baxt and Moody,⁹ which suggested roughly 50% ­mortality improvement with HEMS, the preponderance of subsequent evidence has identified lesser—but significant—outcomes improvement in the range of 20% to 30% better survival. ^11–15 As outlined elsewhere,^11–15 evidence supporting HEMS use varies in methodology and quality, but the overall state of the data support a benefit when HEMS is properly used.

POSSIBLE MECHANISMS BY WHICH HEMS SCENE RESPONSE MAY IMPROVE OUTCOME

Analysis of over 250,000 scene trauma transports from the National Trauma Data Bank (NTDB)¹⁹ found that HEMS (as compared to ground EMS) reduced mortality by 22%. However, methodology did not focus on mechanism for benefit, leaving this question open. Several different proposed characteristics may explain this benefit (Box 18-1). Box 8-1 Benefit of Air Medical Transport in Field Response

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  Earlier arrival of advanced-level prehospital care

  
  
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  Streamlined prehospital times and direct transport to high-level care

  
  
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  Extension of advanced level of care throughout a region

EARLIER ARRIVAL OF ADVANCED-LEVEL PREHOSPITAL CARE

It is both fashionable and foolish to dismiss the idea that, at least occasionally, HEMS benefit is in its speed. In some situations there is undoubtedly an important time advantage. Particularly in rural regions, the only readily available ground prehospital care may be BLS level.² Data focusing on patients with severe trauma including head injuries suggest that the HEMS crews’ early provisions of ALS-level airway and hemodynamic support (ie, intravenous access and fluid management) are the mechanism for improved overall outcome and better neurological function.²⁰ Better functional outcome in HEMS near-drowning patients has been theorized as explainable by deployment of HEMS to areas lacking ALS coverage (by ground EMS).²¹

STREAMLINED PREHOSPITAL TIMES AND DIRECT TRANSPORT TO HIGH-LEVEL CARE

Studies conducted from regions as disparate as California and the Netherlands clearly demonstrate HEMS mortality benefit, yet often find similar scene-to-trauma center times for ground and HEMS transports.^31–32 Authors focusing on logistics support the notion that time to definitive care (ie, arrival of advanced crew on scene) is an appropriate primary endpoint, writing that “correlation between length of time to definitive care and outcome has been well established in the literature, so the premise that faster transport is better seems justifiable.”³⁵ For areas in which there is no trauma center, air medical scene response for direct transport of injured patients to the trauma center is often the best course.³⁷ On the nontrauma front, suggestion of potentially growing indications for HEMS “scene” transports of noninjured patients is provided by an evolving literature consisting of both case series (eg, for primary percutaneous intervention) and sporadic reports (eg, scene transport to neurological centers for lytic therapy for ischemic stroke).^5,38,39

EXTENSION OF ADVANCED LEVEL OF CARE THROUGHOUT A REGION

HEMS may allow an EMS system to provide for early ALS in isolated and/or difficult-to-reach areas which otherwise would be poorly ­covered. In pointing out that HEMS can cover roughly the geographic area of seven ground ALS ambulances, Hankins² has written that: “This kind of coverage, in many areas of the country, provides advanced care where it is not otherwise available.” Others considering the US trauma system as a whole have agreed that at least in some areas of the United States the extension of trauma regional care provided by HEMS is critical.¹⁹

As compared to scene response, there are fewer data directly addressing outcomes benefits associated with interfacility air medical transport. This section discusses secondary missions.

EVIDENCE SUPPORTING INCORPORATION OF AIR MEDICAL ASSETS INTO SYSTEM INTERFACILITY TRANSPORT

Reports outlining extension of percutaneous coronary intervention (PCI) to community hospitals include incorporation of HEMS into ­systems planning as a necessary backup in cases where urgent CABG is required.⁴⁴ It is increasingly well known that time savings can be helpful: each 30 minutes’ additional ischemia time increases mortality by 8% to 10%.⁴⁵ Similar to the situation with integration of HEMS into cardiac care systems is the rapidly solidifying role for air transport in stroke care. NAEMSP recommends air transport of stroke patients if the ­closest fibrinolytic-capable facility is more than an hour away by ground.⁴⁷ The American Stroke Association Task Force on Development of Stroke Systems⁴⁸ identified HEMS as an important part of stroke systems, with helicopters to be used to streamline times. There are data addressing interfacility HEMS trauma transport. Brown et al identified a significant benefit for secondary HEMS trauma transport when Injury Severity Score (ISS) exceeded 15.⁵² A Canadian study, notable for its similarity in acuity between air and ground transport cohorts (ground patients were those in whom HEMS was requested but was unavailable), also found that air transport significantly improved mortality.⁵⁴

POSSIBLE MECHANISMS BY WHICH HEMS INTERFACILITY RESPONSE MAY IMPROVE OUTCOME

Many of the putative mechanisms for air transport’s salutary outcomes effects in scene patients are reproduced for interfacility transports. HEMS crews may in fact have more comfort with high-acuity patients than even physicians at a referring hospital.^1,344 Furthermore, data demonstrate HEMS utility for time savings (and mortality advantage) in interfacility trauma transport.⁵² Loss of HEMS availability has been recognized as a potentially important factor causing increased trauma mortality in patients presenting to non-Level I centers.⁵⁵ A logistics study from the University of Wisconsin⁴⁹ demonstrated the importance of time ­savings accrued by HEMS for interfacility transports of patients with time-­critical cardiac or neurological conditions. In assessing average transport times from their 20-hospital network, the investigators found that for all hospitals, the average HEMS total transport time over the study period was at least as good as the best ground transport time—this finding occurred despite the fact that for many hospitals ground EMS was on-site at the time of transport request. The Wisconsin group found clinically significant time savings for all institutions: patients at close-by hospitals accrued an average of 10 minutes’ savings, while those from ­further-out hospitals’ HEMS transport times were up to 45 minutes shorter than those achievable by ground. For interfacility transfers, an additional time interval of “out-of-hospital” time is reduced. Obstetrics transports serve as an excellent example of a situation in which out-of-hospital time is best minimized. HEMS has also long been known to allow for fetal/maternal outcome benefits occurring when HEMS or fixed-wing aircraft allow high-risk obstetrics transports that simply would not have occurred (due to prolonged transport times) in the absence of air transport.⁵⁶

Since even the harshest critics of air transport acknowledge its potential for occasional benefit, the question of “appropriate utilization” forms the crux of the HEMS debate. (Probably because of its use in situations in which ground EMS is simply not an option, fixed-wing utilization tends to garner less attention.)

TRIAGE AND APPROPRIATE UTILIZATION

Whether indications relate to chance for patient improvement or time-distance factors and “protection” of ALS coverage for a given region, HEMS dispatch should occur only when there is potential advantage over alternatives. No guidelines for dispatch are ideal, and authorities on the subject recognize the inevitability—and necessity—of some degree of HEMS overtriage.⁷ Some have identified gross deficiencies (eg, regions lacking HEMS triage guidelines)⁶⁵ and others have identified low ISS and high rates of early hospital discharge.⁶⁶ However, the most recent nationwide data (assessing over 250,000 ground and air scene responses) find that HEMS patients are in fact far more acutely injured, and require far more hospital resources, than ground EMS patients.¹⁹ The triage question has to incorporate distance, since rural regions may have fewer non-HEMS options for transport of injured patients. Even the ISS question seems to be related to distance; while 57% of HEMS patients in the nationwide NTDB study had ISS <15, this average ISS fell below this critical value only for those patients with transport times of over 2 hours.¹⁹

Some authors suggest that ground transport times of at least 30 ­minutes are consistent with need for HEMS transport for head trauma patients⁷¹; others recommend using 45 miles⁷² or 110 km (62 miles).⁷³ Although there is not likely a uniform exact distance relating to proven patient benefit, the range of approximately 45 miles from the trauma center is endorsed by those executing large-scale studies of HEMS trauma scene response.¹⁹ Logistics decisions are also informed by the consideration that depending on the specific aircraft, HEMS units can cover a radius of roughly 150 to 200 miles from their base. Another approach uses time rather than mileage. One group has suggested that HEMS be reserved for cases where ground transport to appropriate trauma centers exceeds 45 minutes.⁷⁴ This group pointed out that time benefits of air transport are optimal only if the referring and receiving hospitals have ready access to helipads.^75,76

The authors of one discussion addressing triage⁷⁴ conclude that: “The decision to use a helicopter is not straightforward, and a number of important geographical, physiological, and pathological factors need to be considered.” The question is how to use prospectively available information, rather than retrospectively calculated scores (such as ISS) to maximize triage sensitivity while maintaining acceptable positive predictive value. The many previous studies of field triage have been overviewed in a review by Lerner.⁷⁰ Highlights of the literature include consistent findings that, to achieve sensitivity in the 95% or higher range, positive predictive value falls to under 10%.⁸⁶ In an air transported population, the ACS triage criteria were associated with an admirable 97% sensitivity—but at a cost of specificity of 8%.⁸⁷ In Australia, a study found that even the most seasoned paramedics (air crews) were able to achieve acceptable triage sensitivities only with high levels of overtriage.⁶⁹ Until trauma triage is better understood, HEMS will be fraught with overtriage.

Although many studies use an ISS cutoff of 15 to define “major” injury, there is not uniform agreement on what ISS defines the possible need for HEMS.⁶⁵ Since data have shown HEMS-associated outcomes improvement (W of 8.8, or about 9 lives saved per 100 transports) with air transport use for ISS at least 11,⁸⁹ setting the cutoff for HEMS appropriateness at an ISS of 15 is suboptimal. There is solid evidence on some points: (1) rapid transport to trauma centers saves lives⁹²; (2) much of the US population can only reach Level I centers in timely fashion by HEMS^36,49; and (3) trauma transport decision making is heterogeneous and inconsistent even when physicians (at community/rural hospitals) are doing the triage.^55,93 Both sides of the HEMS debate agree that future research efforts should focus on refinement of triage.66,94 Until such ideal triage data exist, those involved in HEMS have a duty to utilize whatever data are available to generate sensible guidelines for helicopter dispatch. Though any system of guidelines will have flaws, the alternative (of haphazard HEMS dispatch without regional cooperative planning) is clearly an inferior option. NAEMSP generated updated Guidelines for Air Medical Dispatch in 2003.⁹⁵ These guidelines have also been endorsed by the Air Medical Physician Association (AMPA) and the Association of Air Medical Services (AAMS), as well as the American Academy of Emergency Medicine (AAEM) (BOX 18-2).

The a posteriori follow-up of HEMS utilization is critical to ­determining whether the ongoing utilization of HEMS in a particular region is ­optimal. After criteria have been generated and implemented, ongoing air ­medical optimization must take into account that regions will have ­varying degrees of compliance with “agreed-upon” protocols for air transport. Broad ­variability in HEMS use, in areas operating under the same triage guidelines, has been demonstrated in many areas.63,97 Well-executed studies reporting years of experience demonstrate that the correct use of these triage guidelines, even in the most rural settings, can result in optimal HEMS triage even without requiring base station contact.⁹⁸ There can be little doubt that when viewed from a large-sample, post-transport perspective, HEMS overutilization is rampant if overutilization is defined as execution of missions for which transport mode appears retrospectively to have had no impact on outcome. In fact, this definition, though often used, is flawed: post hoc determination that air transport did not impact outcome is a necessary, but not sufficient, requirement for defining a flight as unnecessary. On a case-by-case basis, HEMS overtriage should be defined as occurring when the aircraft use offered insufficient potential advantage (logistical, clinical, etc) to justify resource expenditure.

Exploration of the broader “regional coverage view” has resulted in at least one economic study concluding that HEMS is less expensive than same-area coverage through development of a wide-ranging fleet of ground EMS vehicles.⁹⁹ Unfortunately, the job of assessing HEMS’ incremental (as compared to ground transport) cost-effectiveness is made difficult by the limited amount of information on cost-effectiveness of ground EMS itself.¹⁰⁰

SYSTEM-BASED DATA ASSESSING HEMS COST-EFFECTIVENESS

One study, calculating cost -benefit for the entire spectrum of HEMS transports in Norway, concluded: “The analysis indicates that the benefits of ambulance missions flown by helicopters exceeds the costs by a factor of almost six.”¹⁰¹ Another group estimates that HEMS contributes to the cost-effectiveness of primary PCI.¹⁰² In a study conducted in Finland, authors calculated that the cost of HEMS, per beneficial mission, was roughly $30,000.¹⁰³ In a report of the British Department of Health,¹⁰⁴ HEMS’ cost per QALY (quality adjusted life year) was $10,000 to $30,000, within the UK’s “acceptance threshold” of about $35,000. In the Netherlands, HEMS’ cost-effectiveness (depending on models) was $10,000 to $50,000 per QALY.^34,105

COST-EFFECTIVENESS FOCUSING ON NONTRAUMA TRANSPORTS

Though most extant information addresses use of HEMS for trauma, it should be pointed out that cost-effectiveness calculations are increasingly being applied to other patient populations. One of the studies is that of Silbergleit et al,¹⁰⁸ who demonstrate HEMS cost-effectiveness for patients with acute ischemic stroke (for thrombolytic therapy). Cardiac patients may be the next population for HEMS cost-effectiveness ­studies. A 2010 study revealed that centralization of cardiac catheterization resources, with appropriate buildup of EMS transfer systems, is significantly more cost-effective than construction of multiple cardiac catheterization centers; the relevance of this to the HEMS issue is emphasized when the same authors note that 20% of Americans live more than an hour away (by ground) from a cardiac catheterization center.¹⁰⁹

Parameters of altitude interest include elevation above mean sea level (MSL—Denver is much higher at “ground altitude” than is Boston), pressurization level for fixed-wing aircraft (which rarely pressurize to MSL), and equipment in use on a given transport.

GAS LAWS

Perhaps the most important of the gas laws is Boyle’s law, which states that a gas’ volume is inversely proportional to the pressure exerted upon it. As altitude increases (and atmospheric pressure decreases), the molecules of gas move farther apart and the gas volume expands. Expansion and contraction of gases within the body can occur with altitude changes. “Squeeze” on descent, and “reverse squeeze” on ascent occur when decrease in ambient barometric pressure leads to an increased volume of the air trapped within physiologic spaces. Depending on the particular space in the body, exertion of pressure on adjacent structures may cause, for example, sinus pain or enlargement of a pneumothorax. Medical equipment containing closed air spaces (eg, tubes for balloon tamponade of bleeding from esophageal varices) can also be affected. Intravenous flow rates, the pressure in air splints and in pneumatic antishock garment suits, and endotracheal tube cuff volumes may be altered with altitude.¹¹⁰