The Heat Tolerance Test: An Efficient Screening Tool for Evaluating Susceptibility to Heat

Transcription

1 Journal of Sport Rehabilitation, 2007, 16, Human Kinetics, Inc. The Heat Tolerance Test: An Efficient Screening Tool for Evaluating Susceptibility to Heat Daniel S. Moran, Tomer Erlich, and Yoram Epstein Context: Individuals in the population who are not able to sustain heat and whose body temperature will start rising earlier and at a higher rate than that of others, under the same conditions, are defined as heat intolerant. Objectives: The applicability of the heat tolerance test (HTT) in identifying individuals tolerance/intolerance to heat is presented. Setting: HTT is performed according to the following protocol: 120 minutes exposure to 40 o C and 40% relative humidity in a climatic chamber while walking on a treadmill, dressed in shorts and T-shirt, at a pace of 5 km/h and 2% elevation. Rectal temperature and heart rate are continuously monitored, and sweat rate is calculated. Results and Conclusion: The HTT that is based on controlled exposure to an exercise-heat stress is an applicable and an efficient tool in differentiating between a temporary and permanent state of heat susceptibility. Keywords: diagnosis, heatstroke, exercise-physiology, case-reports Body temperature is determined by the balance between heat accumulation, ie, the total sum of heat generated by physical activity (metabolic heat) jointly with heat absorbed from the environment (environmental heat) and heat dissipation. Heat storage is the result of either excessive heat accumulation or reduced ability to dissipate body heat. It follows that either overloading of the normal thermoregulatory mechanism or predisposing factors that affect normal heat dissipation cause body temperature to rise. The mechanism of heat dissipation is complex and comprises both vasomotor and sudomotor activity. Efficient heat dissipation requires a functional thermoregulatory center, an adequate function of the cardiovascular system, and the integrity of the skin and dermal appendages. 1,2 Tolerance to sustain heat stress varies due to individual variation. Under extreme conditions of exercise-heat stress, even healthy, well acclimated, and physically fit individuals will ultimately store heat at a rate that will cause body temperature to rise. Individuals who are not able to sustain heat and whose body temperature will start rising earlier and at a higher rate than that of others, under the same conditions, are defined as heat intolerant. 3 The authors are with the Heller Institute of Medical Research, Sheba Medical Center, Tel Hashomer, Israel. Phone: ; [email protected]. 215

2 216 Moran, Erlich, and Epstein The reasons underlying heat intolerance are ample. 3 Congenital factors reduce the ability to thermoregulate efficiently and will result with permanent heat susceptibility, while others are acquired and could be considered as temporary, not affecting the thermoregulatory center or the heat dissipation mechanisms. 3,4 Of interest for the present discussion is a temporary heat intolerance that may occur following exertional heat stroke (EHS). Noteworthy, there are some acquired circumstances, such as large scarred burnt skin area, that will affect the heat dissipation mechanism to a degree that will result with a permanent condition of heat intolerance. 5 To identify an individual s susceptibility to exercise-heat stress, a Heat Tolerance Test (HTT) was developed by Shapiro et al 3 decades ago. 6 This test, which consisted of stepping for 3 hours on a 30 cm high bench under controlled hot environment, was later adapted to be performed on a treadmill. 7 Noteworthy, other tests to predict tolerance to heat were suggested. 8,9 These tests, which are performed under controlled temperate environment, however, were ultimately found to be inadequate to differentiate between heat tolerant and intolerant individuals. 10 The HTT in the Heller Institute of Medical Research is performed according to the following protocol 6,7 : 120 minutes exposure to 40 o C and 40% relative humidity in a climatic chamber while walking on a treadmill, dressed in shorts and T-shirt, at a pace of 5 km/h and 2% elevation. During the exposure, rectal temperature and heart rate are continuously monitored, and sweat rate is calculated from differences in weight (±10 g) and corrected for fluid intake. 5,6 Based on preliminary data collection conducted in our laboratory, it is expected that under the specified conditions of exercise-heat stress, at the end of the 120 minute exposure, rectal temperature of heat tolerant individuals will be 38.0 ± 0.3 o C, their heart rate will be 120 ± 15 bpm, and their sweat rate, 780 ± 160 g/h. Accordingly, heat intolerance is ascertained when throughout the exposure to heat, rectal temperature exceeds 38.5 o C or heart rate exceeds 145 bpm. 1 The larger the deviation from the expected values in a healthy individual, the more pronounced the state of heat intolerance. Recently, Moran et al reported that the dynamics of changes in rectal temperature and heart rate will tend to plateau, while in the intolerant subjects a continuous rise is evident. 10 EHS is a state of extreme hyperthermia that occurs when excess heat, generated by muscular exercise, exceeds the body s ability to dissipate it at the same rate. Occurrence of EHS is sporadic, and thus it resembles a state of heat intolerance. EHS is a medical emergency that might be fatal and, therefore, study of risk factors for the management and prevention is warranted. Accordingly, it may be suggested that former EHS patient should undergo a HTT to study whether the underlying cause for heat stroke was temporary or permanent. The present report describes the applicability of the heat tolerance test in identifying individuals tolerance to heat. Case Report #1 A 21-year-old infantry soldier (weight: 75 kg, height: 185 cm) collapsed because of heat exhaustion on the 4th day of a continuous physical exercise. The soldier participated in this exercise despite a known condition of sinusitis for which he was treated with antibiotics. Eight weeks after the episode, he underwent a HTT. The dynamics of rectal temperature and heart rate changes during the HTT are depicted

3 Heat Tolerance Test 217 in Figures 1 and 2. At the conclusion of the HTT rectal temperature was 37.9 o C, heart rate was 116 bpm, and sweat rate was 685 g/h. The soldier was released as heat tolerant. The causes for his collapse were identified as an acute infectious disease and the treatment with antibiotics. Case Report #2 A 19-year old male, physically fit infantry soldier (weight: 78 kg, height: 174 cm) collapsed with EHS during a 5 km obstacle course run with full gear. He participated in the exercise without reporting sick, although he suffered from diarrhea for 2 days. The soldier was referred to perform a HTT six weeks after the event. At the conclusion of the HTT rectal temperature was 37.8 o C, heart rate was 106 bpm (Figures 1 and 2), and sweat rate was 710 g/h. He was released as tolerant to heat. The collapse was attributed to the underlying gastric illness. Case Report # 3 Following a 4 km march under moderate heat load, an 18-year old soldier (weight: 64 kg, height: 168 cm) collapsed suffering from EHS. Eight weeks after the event he was referred for a HTT. At the conclusion of the HTT rectal temperature was Figure 1 Dynamics of rectal temperature response during 120-minute exposure to exercise-heat stress. Subjects 1 to 3 response is within the normal range, while that of subject 4 reveals a state of intolerance to heat. In any event the test should be discontinued when the safety limit of 39 o C is reached.

4 218 Moran, Erlich, and Epstein Figure 2 Dynamics of heart rate response during 120-minute exposure to exercise-heat stress. Subjects 1 to 3 response is within the normal range, while that of subject 4 reveals a state of intolerance to heat. In any event the test should be discontinued when the safety limit of 180 bpm is reached o C, heart rate was 121 bpm (Figures 1 and 2), and sweat rate was 650 g/h. The soldier was released as heat tolerant. Based on the laboratory examination in the ER and the conclusion of the investigation committee, the underlying cause for his collapse was identified as dehydration. Case Report #4 An 18-year old Caucasian male, apparently a healthy soldier (weight: 61 kg, height: 184 cm), complained about severe generalized malaise and fatigue following exercise in the heat. Consequently, he was referred for a HTT. The subject sustained the test for 60 minutes before his rectal temperature reached 39.0 o C and heart rate was 190 bpm. Calculated sweat rate was 270 g/h. He was identified as heat intolerant. Further investigation of the sweating mechanism (pilocarpine iontophoresis) resulted with anhidrosis with a skin biopsy that revealed intact eccrine sweat glands. The underlying cause for his susceptibility to heat is a state of idiopathic anhidrosis. Comments In 1979, Shapiro et al suggested a controlled test for evaluating tolerance to heat. 6 This test aims to identify those individuals in the population that are more susceptible to heat than others and thus are more prone to be exposed to the hazards of

5 Heat Tolerance Test 219 EHS. The cases described provide example of the ability of a HTT to differentiate between a permanent and temporary state of susceptibility to heat. In most of the cases, the physiological response to the controlled 120-minute exercise-heat stress is expected to be within specified limits (rectal temperature: 38.0 ± 0.3 o C, heart rate: 120 ± 15 bpm, and sweat rate: 780 ± 160 g/h). Only in rare cases is heat intolerance defined as permanent. The physiological responses of the four cases that are graphically depicted in Figure 1 (rectal temperature) and Figure 2 (heart rate), which clearly depict the difference in the response of subjects 1-3 from that of subject 4. The response of the former is that of tolerance to heat while that of the latter depicts the response of intolerance to heat. It is obvious that under the specified conditions of the HTT, sweat rate of subject 4 is notably lower than that of the others, being only about 40% of the expected sweat rate. Noteworthy, this test can be applied for other populations, as was recently been published in determining the tolerance to heat of schizophrenic patients. 11 Whether excessive heat storage reflects a permanent or temporary state of heat intolerance has been previously described. 3 It was concluded that the reasons underlying heat intolerance are ample and there is no single cause or consistent scenario. Heat intolerance can be inherent or acquired, as well as permanent or temporary; however, in a young active population (eg, athletes, soldiers, and adventurers), in most cases, a single predisposing factor reduces temporarily the ability of an individual to tolerate heat. 3,4 In this population, the common underlying factors that were identified to reduce the ability to tolerate heat are low physical fitness, lack of acclimatization to heat, overweight, dehydration, and infectious diseases. 4 Motivation to accomplish a mission, while disregarding early signs and symptoms for disability to perform efficiently in the heat, is a major contributing factor that exacerbates the situation. 4,12 Under extreme conditions of exertion in the heat, even acclimated and physically fit individuals will ultimately store excessive heat, resulting in an elevation in core temperature. In most cases it is an uneventful condition. HTT appears to be a valuable tool to screen individuals for susceptibility to heat. We recommend that after an episode of heat exhaustion or EHS, that a HTT be conducted within 6-8 weeks of the hyperthermic episode. These guidelines are recommended because extreme elevated body temperature (> 40 o C), which is evident during EHS, may compromise to some extent the thermoregulatory center resulting in temporary heat intolerance. 3 The results of the HTT will provide insight if the condition developed because of a permanent or temporary underlying cause. If following testing there is an abnormal HTT response, with no apparent cause (eg, anhidrosis), it is common practice in our institute to repeat the test in 4 to 8 weeks to refute or support the diagnosis of heat intolerance. In conclusion, the HTT that is based on controlled exposure to an exercise-heat stress is an applicable and an efficient tool to differentiate between a temporary and permanent state of heat susceptibility. It is advisable that any former EHS individual or subjects who are suspected to be heat susceptible consider this test to evaluate his/her tolerance/intolerance to heat.

6 220 Moran, Erlich, and Epstein References 1. Shapiro Y. Pathophysiology of hyperthermia and heat intolerance. In: Hales JRS, Richards DAB, eds. Heat stress, physical exertion and environment. Amsterdam: Experta Medica; 1987: Sawka MN, Wenger CB. Physiological responses to acute exercise-heat stress. In: Pandolf KB, Sawka MN, Gonzalez RR, eds. Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. New York: Benchmark; 1988: Epstein Y. Heat intolerance: predisposing factors or residual injury? Med Sci Sports Exerc. 1990;22: Epstein Y, Moran DS, Shapiro Y, Sohar E, Shemer J. Exertional heat stroke: a case series. Med Sci Sports Exerc. 1999;31: Shapiro Y, Epstein Y, Ben-Simchon C, Tsur H. Thermoregulatory responses of patients with extensive healed burns. J Appl Physiol. 1982;53: Shapiro Y, Magazanik A, Udassin R, Ben-Baruch G, Shvartz E, Shoenfeld Y. Heat intolerance in former heatstroke patients. Ann Int Med. 1979;90: Epstein Y, Shapiro Y, Brill S. Role of surface area-to-mass ratio and work efficiency in heat intolerance. J Appl Physiol. 1983;54: Shvartz E, Shibolet S, Meroz A, Magazanik A, Shapiro Y. Prediction heat tolerance from heart rate and rectal temperature in temperate environment. J Appl Physiol. 1977;43: Kenney LW, Lewis DA, Anderson RK, Kamon E. A simple exercise test for the prediction of relative heat tolerance. Ann Ind Hyg Assoc J. 1986;47: Moran DS, Heled Y, Still L, Laor A, Shairo Y. Assessment of heat tolerance for post exertional heat stroke individuals. Med Sci Monit. 2004;10:CR Hermesh H, Shilo R, Epstein Y, Manaim H, Weizman A, Munitz H. Heat intolerance in patients with chronic schizophrenia maintained with antipsychotic drugs. Am J Psychiatry. 2000;157: Shibolet S, Lancaster MC, Danon Y. Heat stroke: a review. Aviat Space Environ Med. 1976;47: Appendix The Heller Institute HTT protocol Target population Preliminary investigation Preliminary instructions (prior to the test) Suspected for heat susceptibility Cases of EHS or heat exhaustion (6-8 weeks after recovery) The test is limited to the ages yrs. Complete medical history (including family history about heat susceptibility) Medical check-up (preclude any illness) At least 6 hrs night sleep Normal diet Euhydration (not over hydrated) No physical exercise 48 hrs prior to the test

7 Heat Tolerance Test 221 HTT set-up HTT safety limits (test to be discontinued) Criteria for heat intolerance 120 minutes exposure to 40 o C and 40% relative humidity, while walking on a treadmill at a pace of 5 km/h and 2% elevation (VO 2 ~1 L/min) Continuous monitoring of rectal temperature and heart rate Calculating sweat rate Supervision: a physiologist and a physician Rectal temperature 39 o C and/or heart rate bpm Dizziness or faintness Individual s wish Physician decision Rectal temperature >38.5 o C and/or heart rate >145 bpm HTT is repeated in any case of abnormal response (4-8 weeks)