Exertional heat stroke and the evolution of field care: A physician's perspective

Exertional heat stroke (EHS) is a medical emergency that requires immediate recognition and rapid reduction of body core temperature to reduce morbidity and mortality. What we know about EHS has changed dramatically in the last 40 years. Our expertise has been expanded by the clinical observations of medical providers and physiologists in heat stroke prone environments: road races, football fields, military training, and industrial settings.

My first experience with EHS was in 1982, when a 19-y old male Twin Cities Marathon finisher was triaged into the medical tent: confused, combative, perseverating, and unable to stand on his own. Despite the cool sunny day, his rectal temperature was 41.7°C (107.1°F). Based on my education and limited heat stroke experience, I thought he would die. We packed him in ice and transferred him to a nearby emergency department. Our aggressive whole body cooling dropped his rectal temperature to 36.1°C (97°F) by the time he arrived at the hospital, and he was released to home that same evening. This clinical course and outcome piqued my interest in EHS and rapid cooling.

Rapid cooling for EHS has not always been the gold standard that it is today, due to the concern for the body temperature dropping too fast. Wyndham in his presentation at the 1976 New York Academy of Sciences meeting dedicated to the science and medicine of the marathon stated, “Man should not be placed in a bath of ice-water as advocated by some heat physiologists. This method of treating heat stroke is dangerous because it is very difficult to control the extent of fall in rectal temperature and circulatory shock may occur, with fatal results during the rewarming phase.”¹ In 1990, Costrini published the outcomes of ice water tub immersion therapy in military personnel with EHS compared with other field methods and concluded ice water immersion was the best emergency therapy for EHS victims.² Costrini's article prompted the medical team at Falmouth Road Race to immerse the runners presenting to their medical tent with EHS in ice water tubs, resulting in a dramatic drop in prolonged hospitalizations. It appears, based on clinical outcomes of EHS victims at road races and in other settings, that early recognition and rapid cooling shuts off the cascade of events leading to organ systems failure and the often fatal systemic inflammatory response syndrome.

EHS was considered a rare medical event in the 1980s and 1990s. Road races reporting “no heat stroke” often did not use rectal body temperature measurement. As a “guest” physician at a marathon in the mid-1990s, I was placed in charge of the “hypothermia” response team and called to help treat a runner with a presenting tympanic membrane temperature of 35.5°C (96°F) who was already in the rewarming protocol. I asked for the rectal temperature and was told, “We don't use rectal temperatures, only tympanic.” I had my own thermometer with me, did a rectal measurement, and found that the runner had a rectal temperature of 40°C (104°F). We promptly changed from a warming protocol to a cooling protocol; the runner walked out of the tent under his own power about an hour or so later. It is difficult to speculate the outcome of this encounter had the warming protocol continued, but his recovery would certainly have been prolonged beyond his actual clinical course. Recognition of EHS is critical. Medical personnel have to consider it in the differential diagnosis, use the proper field measurement tool and measurement route to reach the correct conclusion, and to begin life-saving cooling.

The best approaches for estimating body core temperature have been controversial, with manufacturers extrapolating laboratory and hospital data to field situations without proper field testing. The old saying, “if it looks too good to be true…,” plays into this scenario, where the convenience of a new technology was enticing. Family circumstances placed me on Cape Cod in the 80s and 90s, and I was able to guest as a physician at the Falmouth Road Race. We field tested an excellent infrared tympanic thermometer at the race where rectal temperatures were the norm. The tympanic measures did not correlate with the rectal temperatures and those of us involved in the field study did not adopt the device for our races, and Falmouth Road Race continued to use the inconvenient, but reliable, rectal temperature measurement. When the temporal artery thermometer was introduced into clinical medicine, the device was quickly adopted in ERs, hospitals, and clinics. We studied the device in the medical tent at Twin Cities Marathon; our measurements showed a very poor correlation coefficient and found normal temporal artery device temperatures in runners with rectal temperatures of 42°C (108°F).³

The usual cause of EHS is working too hard in environments that do not facilitate metabolic body heat loss, usually hot and humid or unexpectedly hot and humid conditions. This is especially true for people who are not acclimated to the heat. EHS is “predictable” in certain circumstances, but it can also occur in what seem to be ideal running conditions.⁴ This is often associated with a recent illness and medical teams have to be vigilant and prepared in the best of conditions as documented in soon to be published data from the SAFER Study out of South Africa and in published case studies. EHS happens in 5k races and marathons; it is not uniformly associated with body fluid deficit, but EHS is clearly related to inability to transport heat created by muscle work to the shell and/or to remove heat from the shell. There is a “tipping point” when the environment becomes too stressful for the participants. The “race must go on” mentality has melted away and some events have set no competition limits and/or created work load restrictions to reduce the incidence of EHS.⁴

EHS is a multisystem disease, and “time is tissue” with greater cell and organ destruction as time elapses with elevated core temperatures. Immediate recognition and rapid cooling reduces, if not eliminates, morbidity and mortality associated with tissue damage of a prolonged elevated body temperature by interrupting the SIRS reaction. Again, Falmouth Road Race has played a role in demonstrating the effectiveness of onsite triage, rectal temperature measurement, and rapid immersion cooling.⁵ Despite high rectal temperatures well above the EHS threshold in several finishers each year, there have been no deaths and exceedingly few hospitalizations following their triage and immersion protocols.⁵ The outcomes speak to the efficacy of the medical team's approach to the problem.

My experience with EHS deaths is in the legal arena, where I have been asked to review cases. The common thread in the cases involves a series of omissions that usually result in prolonged exposure to elevated core temperatures in degree-minutes above a critical temperature threshold. Degree-minutes reflect the area under the body temperature versus time cooling curve and are affected primarily by delayed recognition, delayed cooling, and/or inadequate cooling. Rapidly reducing the core body temperature to less than 38.5–39°C (102–103°F) in <30–60 min is a key to survival and the area under the curve seems to be a better predictor of satisfactory outcome than the peak core temperature (see Fig. 1). The longer an EHS victim goes undiagnosed and untreated, the greater the degree-minutes under the cooling curve and the risk of prolonged morbidity or increased mortality.

Figure 1. The area under the cooling curve (degree-minutes) is a major factor in EHS outcomes. Late recognition and/or delayed cooling are/is associated with increased morbidity and mortality. The runner who was with promptly recognized and rapidly treated went home the same day. The football player who had a delayed diagnosis eventually died from multiple organ system failure.

While we know that EHS occurs sporadically in sport settings, the numbers are low and do not receive the attention of sudden cardiac death in road races and sporting events. EHS is also very environment dependent, so Falmouth Road Race run in hot, humid marine conditions has a relatively “stable” number of cases annually,⁵ while Twin Cities Marathon will have none in a usual year and increasing numbers with warmer than usual conditions.⁴ An international group of race medical directors will be meeting in May 2017 (Gothenburg, Sweden) to begin a multicenter surveillance study of “serious medical incidents,” like EHS, to address the issues of prevalence and incidence in running races.

I have been fortunate during my career to witness the evolution of EHS diagnosis and treatment, and privileged to have contributed some small building blocks to the wall of knowledge encompassing the field of heat injury and temperature regulation. Many lives have been saved and many athletes have been spared the EHS ordeal by applying what we have learned in the field to the care and reduction of EHS. This would not have been possible without the dedicated work of numerous scientists and the many collaborations between those in science and in medicine.