Aircraft Speed and Time to Definitive Care

In the United States the leading cause of death is cardiovascular disease, and the leading cause of disability is stroke. In addition, there are almost 200,000 deaths and 2.5 million hospitalizations each year due to trauma, making this the leading cause of death in those age 1–44.1 

Time to definitive medical care has become an important measure in these disease processes, such as time to laparotomy in trauma, door-to-balloon times, door-to-needle times, and even first-medical-contact-to-balloon times in strokesand STEMI.2–5The reason for the push for faster care is that some diseases benefit from rapid intervention more than others. In severe abdominal trauma, every three-minute delay causes a 1% increase in mortality; STEMI sees a 7.5% mortality increase for every 30-minute delay; and strokes are estimated to cause around 2 million brain cells to die each minute.4–6These time-critical diseases require rapid access to definitive care.

The need to transport these patients rapidly from the field led to the creation of helicopter emergency medicine services (HEMS) in the 1970s and today a vast network of care across the country. Today many of these flights are from local hospitals that may lack interventional resources to care for these time-critical patients. Currently the optimal aircraft frame and design have yet to be determined. There are no studies examining how aircraft cruise speeds affect the time to definitive care.

The goal of our study was to see if there is a statistically and medically relevant delay in time to definitive care based on aircraft frame for the 20 most common flight routes in a regional hospital-run HEMS. For this analysis we searched for a difference of three minutes or more. That was our threshold for significance, as it would indicate a potential 1% mortality increase for trauma patients, for whom the best data on time-critical disease exists.

Methods

This was a theoretical evaluation assessment of the most common flight routes used by Ohio-based Mercy Life Flight and Transport. As there was no patient data, IRB approval was waived for this study. Inclusion required the seven most common aircraft frames and our 20 most common flight routes. These flight routes were analyzed based on the manufacturer’s rated cruise speed of the most common HEMS airframes on the market. We assessed time to the scene and time to the hospital as surrogates of time to definitive care. We divided the distance of individual flight routes by the rated cruise speed of each aircraft to project flight times. These flight times were compared for a statistically and medically relevant difference of three minutes and more. We then assessed each aircraft to determine a potential mortality benefit for patients based on aircraft speed.

Results

It was noted there were large differences in the flight times depending on the distance the helicopter traveled to the receiving hospital (Table 1). The slope of the line is steeper for slower aircraft, leading to increasing times relative to faster aircraft with increasing distance (Figure 1). For distances less than 20 nautical miles (NM), there was less of an impact of a faster aircraft; however, over long distances some sites showed differences of almost 12 minutes (Tables 2, 3). Compared with the similar-performing AS365, the distance required to see a difference with the Agusta was 203 NM, well beyond our longest route of 82 NM. The next fastest aircraft, the EC145, required a distance of almost 63 NM to show a medically relevant delay to the Agusta. Only 8 of the 20 flight routes were longer than this distance. The Agusta showed a cut-point of 43 NM with the EC135 and was significantly faster in 12 of 20 flight routes evaluated. The three-minute difference between the Agusta and the EC130 was even lower at almost 35 NM and encompassed all but our shortest route. Compared to the slowest aircraft in the analysis, the Bell 206, the Agusta was significantly faster at a distance of 21 NM and encompassed all 20 flight routes (Table 3). The difference in speed between these two airframes has the potential mortality benefit of 3% in 9 of our 20 common flight routes (Table 2).

Table 1: Est. Travel Time to Definitive Care for Common Life Flight Routes With Various Helicopters

Aircraft                           Agusta 109     AS365     EC145     EC135     EC130     Bell 206 

Cruising speed (knots)       145                140          130          125         120           108

Total distance (NMs)     Travel time (mins.)
32.0                                   13.24             13.71       14.77       15.36      16.00        17.78
36.0                                   14.90             15.43       16.62       17.28      18.00        20.00
41.0                                   16.97             17.57       18.92       19.68      20.50        22.78
42.0                                   17.38             18.00       19.38       20.16      21.00        23.33
43.0                                   17.79             18.43       19.85       20.64      21.50        23.89
44.0                                   18.21             18.86       20.31       21.12      22.00        24.44
48.0                                   19.86             20.57       22.15       23.04      24.00        26.67
51.0                                   21.10             21.86       23.54       24.48      25.50        28.33
56.0                                   23.17             24.00       25.85       26.88      28.00        31.11
58.0                                   24.00             24.86       26.77       27.84      29.00        32.22
59.0                                   24.41             25.29       27.23       28.32      29.50        32.78
63.0                                   26.07             27.00       29.08       30.24      31.50        35.00
65.0                                   26.90             27.86       30.00       31.20      32.50        36.11
68.0                                   28.14             29.14       31.38       32.64      34.00        37.78
71.0                                   29.38             30.43       32.77       34.08      35.50        39.44
74.0                                   30.62             31.71       34.15       35.52      37.00        41.11
76.0                                   31.45             32.57       35.08       36.48      38.00        42.22
78.0                                   32.28             33.43       36.00       37.44      39.00        43.33
80.0                                   33.10             34.29       36.92       38.40      40.00        44.44
82.0                                   33.93             35.14       37.85       39.36      41.00        45.56

(Assume times to take off, reach cruising speed, and land are the same for all helicopters and that travel is at a constant speed. Time [minutes] = 60 minutes x [total distance (nautical miles)/aircraft cruising speed (knots)].) 

Table 2: Difference in Mortality Between the Fastest and Slowest Helicopters

Dist. to def. care    Agusta 109 @145 knots    Bell 206@108 knots    Add'l travel time       Mort. increase*

32                                        13.24                              17.78                         4.54                         1.51%
36                                        14.90                              20.00                         5.10                         1.70%
41                                        16.97                              22.78                         5.81                         1.94%
42                                        17.38                              23.33                         5.95                         1.98%
43                                        17.79                              23.89                         6.10                         2.03%
44                                        18.21                              24.44                         6.24                         2.08%
48                                        19.86                              26.67                         6.80                         2.27%
51                                        21.10                              28.33                         7.23                         2.41%
56                                        23.17                              31.11                         7.94                         2.65%
58                                        24.00                              32.22                         8.22                         2.74%
59                                        24.41                              32.78                         8.36                         3.00%
63                                        26.07                              35.00                         8.93                         2.98%
65                                        26.90                              36.11                         9.21                         3.07%
68                                        28.14                              37.78                         9.64                         3.21%
71                                        29.38                              39.44                        10.07                        3.36%
74                                        30.62                              41.11                        10.49                        3.50%
76                                        31.45                              42.22                        10.77                        3.59%
78                                        32.28                              43.33                        11.06                        3.69%
80                                        33.10                              44.44                        11.34                        3.78%
82                                        33.93                              45.56                        11.62                        3.87%
 
(Assume a 1% increase in mortality for every three-minute increase in travel time.)

Table 3: Min. Distance at Which the Difference in Total Travel Time Between Two Helicopters is Three Minutes

                                             AS365     EC145    EC135     EC130     Bell 206 

                  Speed (knots)       140          130          125          120          108

Agusta               145             203.00      62.83      45.31       34.80       21.16
AS365               140                  -           91.00      58.33       42.00       23.63
EC145               130                                  -         162.50      78.00       31.91
EC135               125                                                 -          150.00      39.71
EC130               120                                                                   -         54.00

Discussion

When choosing airframes, there are several aspects management and medical providers should consider. All HEMS agencies look at initial costs and operating costs of the aircraft. Some agencies may have a limited choice of aircraft frames. Our analysis focused on the speed of an aircraft and how this may affect patient outcomes. As there were some time differences of almost 12 minutes, it is conceivable to hypothesize that a faster aircraft could show a 4% reduction in mortality (Table 2). A longer flight may contribute to a delay in definitive care and potential increase in mortality. Even between the ultrafast and fast helicopters, there is still a potential difference in mortality when the distance is greater than 63 NM (Table 3). 

This study looked exclusively at aircraft from a single location. However, with such variation in aircraft speed, it is conceivable that a slower aircraft on scene could still take longer to deliver a patient to definitive care, even if a faster aircraft has not arrived on scene. When a reduction in time to definitive care (e.g., laparotomy or arterial recanalization) can improve patient mortality, the aircraft frame and speed has a great potential to improve outcomes. This should be taken into consideration when statewide dispatch protocols require an agency to call the closest helicopter asset rather than one slightly farther away that could get the time-critical patient to definitive care faster.7

Limitations

There are several limitations to this study. First, this is a theoretical assessment that does not consider variability of wind, temperature, pressure, or weight of crews and patient. Variations of start-up, climb-out, approach, landing, and shutdown times are difficult to measure, as they depend on avionics installed and other situational variabilities. For example, required cruise altitude, pilot familiarity with the landing zone, the level of engine automation, and whether the flight is VFR or IFR all impact flight time. Lastly, trauma, STEMI, and stroke care have multiple factors that can affect mortality outcomes. Time spent in flight is only one component of these calls, with time spent at the patient’s side performing procedures and preparing for flight also being a factor. Since there are multiple variables, we are focusing only on one aspect that can potentially be altered at a system level.

Conclusion

When patients suffer time-critical threats such as trauma, STEMI, or stroke, the speed of their transporting aircraft has the potential to improve survival by reducing the time to definitive treatment. As this was only a theoretical HEMS analysis, field comparison studies will be required to further assess these differences in time to definitive care.

References

1. Centers for Disease Control and Prevention, National Center for Injury Prevention and Control. Key Injury and Violence Data, www.cdc.gov/injury/wisqars/overview/key_data.html.

2. Rathore SS, Curtis JP, Chen J, et al. Association of door-to-balloon time and mortality in patients admitted to hospital with ST elevation myocardial infarction: national cohort study. BMJ, 2009; 338: b1,807.

3. Koul S, Andell P, Martinsson A, et al. Delay from first medical contact to primary PCI and all-cause mortality: a nationwide study of patients with ST-elevation myocardial infarction. J Am Heart Assoc,2014 Mar 4; 3(2): e000486.

4. Kurz MW, Kurz KD, Farbu E. Acute ischemic stroke—from symptom recognition to thrombolysis. Acta Neurol Scand Suppl, 2013; 196: 57–64.

5. Clarke JR, Trooskin SZ, Doshi PJ, Greenwald L, Mode CJ. Time to laparotomy for intra-abdominal bleeding from trauma does affect survival for delays up to 90 minutes. J Trauma, 2002; 52(3): 420–5.

6. De Luca G, Suryapranata H, Ottervanger JP, Antman EM. Time delay to treatment and mortality in primary angioplasty for acute myocardial infarction: every minute of delay counts. Circulation,2004; 109(10): 1,223–5.

7. Megerian C, Friedman M. N.J. approves controversial change to dispatching medical helicopters. NJ.com, www.nj.com/news/index.ssf/2010/09/nj_approves_controversial_chan.html.

William Krebs, DO, RDMS, is medical director for the Mercy Health Life Flight Network and an adjunct assistant professor of emergency medicine at Ohio State University. 

Allen Kao is a pilot with the Mercy Health Life Flight Network. 

Jeffery Schorsch is a pilot with the Mercy Health Life Flight Network. 

Kevin Beaulieu is a pilot with the Mercy Health Life Flight Network. 

Madalyn Popil is a medical student at Ohio University. 

Gregory Turissini is a medical student at Ohio University.

Steven Zohn, MD, is assistant medical director for the Mercy Health Life Flight Network. 

Scott Swickard, DNP, PhD, ACNP, is director of clinical operations for the Mercy Health Life Flight Network.