| Feb 10, 2015
By Glen P. Kenny, Ph.D.
The human body is quite inefficient at using the energy derived from metabolic processes to create external work, with approximately 70 to 95 percent of energy as heat— this does, of course, vary with the physical task. The human body has an amazing capacity to handle the large amount of heat released during physical activity. To offset the large increase in metabolic and environmental heat gain [high ambient air temperature, radiant heat sources (sun, fires, kiln, etc.), the human thermoregulatory system must adjust the rate of heat loss by increasing skin blood flow and sweating. Under circumstances where the body is unable to increase heat loss sufficiently to offset the increase in heat production/gain, core temperature continually rises. If left unchecked, core temperature can continue to increase to dangerously high levels— placing individuals at high risk of developing exertional heat stroke (EHS). The risk of EHS is always present when military personnel, laborers, athletes and others perform physical activity in the heat, especially when protective equipment is worn. Key to the survival of victims of EHS is the early recognition of the condition. Even if EHS is promptly recognized at the time of the incident, an individual can still succumb if extreme hyperthermia is not rapidly reduced. The severity and reversibility of multisystem organ failure associated with EHS is related to the duration of temperature elevation.
Cold water immersion (2°C circulated ice-water bath, CWI) is the gold standard treatment for victims of EHS, as it has been shown to produce the highest core cooling rates to date (0.35°C/min). Recent work by DeMartini and colleagues showed that the use of CWI resulted in a 100 percent survival rate for all 274 recorded cases of EHS at the Falmouth Road Race over the last 18 years. Yet, despite the obvious clinical advantages of CWI in the treatment of EHS, some experts advocate the use of temperate water immersion (26°C) on the basis that CWI may negatively impact the health of the EHS patient as a result of the potentially undesirable side effects, such as cold shock response, excessive shivering, etc. – see the Casa et al. perspectives column in MSSE for an expanded discussion. However, as our recent research report published in MSSE showed, the benefits of CWI far outweigh the short lived discomfort that the patient may experience. More importantly, CWI provided core cooling rates 2.7 faster than temperate water immersion. A critical oversight in many discussions pertaining to the treatment of EHS patients relates to the physical characteristics of the patient. We showed that when it comes to treating victims of EHS size matters. Individuals with a low body surface area-to-lean body mass ratio (AD/LBM) experience longer cooling times irrespective of the choice of water temperature (i.e., temperature, 26°C versus cold water immersion, 2°C) compared to those individuals with high AD/LBM. However, immersion in cold water dramatically reduced the differences between groups such that the effects of the physical differences on core cooling rates were minimized.
Exertional heat-related illness is a risk to individuals of all sizes and shapes irrespective of the nature of activity performed. While the pathway to hyperthermia may be different for each EHS patient, our study clearly demonstrates that the path to recovery must always be immediate CWI treatment for all individuals. Viewpoints presented on the ACSM Blog reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.
Dr. Glen P. Kenny is a professor of exercise physiology at the University of Ottawa, holds a university research chair in environmental physiology, and is a member of ACSM. His research has been directed at characterizing the physiological control mechanisms governing human temperature regulation during heat stress. An area of special focus in his work is investigation of the physiological effects of heat stress in subpopulations with conditions rendering them particularly vulnerable to heat injury, such as aging, obesity and diabetes.
This commentary presents Dr. Kenny’s views on the topic related to a study which he and colleagues recently completed. Their research report appears in the September 2014 issue of Medicine & Science in Sports & Exercise® (MSSE).