Predisposing Risk Factors on Susceptibility to Exertional Heat Illness: Clinical Decision-Making Considerations

Transcription

1 Journal of Sport Rehabilitation, 2007, 16, Human Kinetics, Inc. Predisposing Risk Factors on Susceptibility to Exertional Heat Illness: Clinical Decision-Making Considerations Michelle Cleary Objective: To present strategies for identifying predisposing conditions, susceptibility, and incidence reduction for the most common exertional heat illnesses (EHI): heat cramps, heat exhaustion, and most importantly, heat stroke. Data Sources: A comprehensive literature review of MEDLINE and 1996 to 2006, including all retrospective, controlled studies of EHI risk using the following keywords: exertional heat illness risk, exertional heat stroke risk, and exercise-induced dehydration risk. Search limits included the following: English language, published in the last 10 years, clinical trial, meta-analysis, practice guideline, randomized controlled trial, review, and humans. A manual review was conducted of relevant position statements and book chapters including the reference lists. Data Extraction: To evaluate the quality of the empirical studies to be included in this review, each study must have scored at least 17/22 or 77% of items included when reporting a randomized trial using the CONSORT checklist. Data Synthesis: Many cases of EHI are preventable and can be successfully treated if the ATC identifies individuals at increased relative risk and implement appropriate prevention strategies. The ability to objectively identify individuals at increased relative risk of EHI and to provide appropriate monitoring is critical in EHI prevention and reduction of repeated incidents of EHI. Conclusions: For any heat illness prevention program to be effective, greater attention and continued observation is needed for athletes at high risk for EHI. For many athletic teams or programs, because of the sheer numbers and associated catastrophic injury potential, health care professionals must implement a system by which high-risk individuals are monitored during activity with the highest level of vigilance for prevention of EHI. All physically active individuals are at some risk of exertional heat illness (EHI) such as heat cramps, heat exhaustion, and heat stroke while exercising in warm and hot environments. However, certain individuals have greater susceptibility and are at higher risk for EHI than others. Exercise in the heat causes many homeostatic changes in the body, including compensatory adjustments in the circulatory, thermoregulatory, and endocrine systems. Many interrelated physiological processes The author is with the Florida International University College of Nursing and Health Sciences in Miami, FL. clearym@fiu.edu. 204

2 Risk Factors 205 work together to sustain central blood pressure, maintain muscular function, and regulate body fluid balance. Sustaining exercise, especially intense exercise, in hot environments can overload the body s ability to appropriately respond to the imposed stress resulting in hyperthermia, dehydration, and deteriorated mental or physical performance leading to EHI. A paucity of data exists on the risk factors associated with EHI in the physically active population. To date, most efforts in preventing EHI have been directed at mitigating environmental risk factors that do not account for individual inherent risks predisposing certain individuals. Further efforts to establish sound criteria for identification of individual inherent (nonenvironmental) EHI risk factors and provide data upon which to establish safe clinical decision making associated with individual risk factors are needed. Coaches, athletic trainers, and physicians should place special emphasis on identifying individuals with inherent EHI risk factors that increase the relative susceptibility of these individuals to EHI prior to sport participation in conditions with high external risk. For many athletic teams or programs, because of the sheer numbers and associated catastrophic injury potential, health care professionals must set up a system by which high-risk individuals are monitored with the highest level of vigilance relating to hydration status, environmental conditions, equipment worn, and appropriate exercise intensity for the prevention of EHI. It is often obvious that certain individuals are at increased risk of various EHI such as the individual who has an acute illness, is obese, or is out of shape. The dangers associated with exercise in the heat have been well documented, but a lack of education, misunderstanding of inherent risks, and failure to modify existing policies have failed to prevent EHI from occurring. External factors such as environmental conditions (time of day or season), intensity, or duration of exercise, inappropriate work:rest ratios, or accessibility to fluid and shade are additional risk factors to be considered for modification, 1 particularly for individuals with precipitating factors that may or may not have been previously identified or addressed. The Certified Athletic Trainer s (ATC ) role in prevention of EHI is to proactively identify individuals who are at the highest risk so that special attention can be given to these individuals. 2 Many cases of EHI are preventable and can be successfully treated if the ATC identifies individuals at increased relative risk and implements appropriate prevention strategies. 1,3,4 Currently, little quantifiable evidence exists to allow the ATC to objectively identify EHI risk factors for each individual on a team of athletes. The ability to objectively identify individuals at increased relative risk of EHI (Table 1) and to provide appropriate monitoring is critical in EHI prevention. This paper addresses the predisposing conditions, susceptibility, and strategies for identifying risk factors for the most common EHI: heat cramps, heat exhaustion, and heat stroke. The purpose of this paper is to provide all members of the sports medicine team with sound evidence upon which to base best practice for identifying the inherent risks of EHI associated with physical activity in the heat so that a safe recovery and return to play may be accomplished. Several approaches are suggested for consideration as a standard of medical care taking into account individual differences as appropriate.

3 Table 1 Inherent Risk Factors for EHI and How Identified Inherent Risk Factor How Identified Objective Criteria Measure Baseline hydration level* and current (acute) hydration level 1 At-risk: Urine specific gravity > μg, urine color <4,%body mass loss > 3.0% Obtain a baseline urine specific gravity measure using a clinical refractometer (preferably) or color chart, and use body mass changes pre and post exercise to identify acute dehydration. Body mass index 4 Body Mass Index = (body mass in kg/height in m x height in m) = kg m -2 At-risk: BMI > 22 kg m -2 Physical fitness level 4 At-risk: 12 min run-time of < 1.5 miles VO 2 max Run Test (12 min walk/run), graded exercise test, or other test with norms for comparison Recent illness or digestive problems such as vomiting or diarrhea (may be subjective) At-risk: Febrile condition (> 101 o F) or dehydration (see above) Questionnaire, personal interview, or chart review (if available, more objective), elevated body temperature (>101 o F) Previous history of exertional heat illness (may be subjective) At-risk: Previous incident(s) of heat cramps, heat exhaustion, or exertional heat stroke Questionnaire, personal interview, or chart review (if available, more objective) 206

4 Inherent Risk Factor How Identified Objective Criteria Measure Heat acclimatization (may be subjective) At-risk: Insufficient level of aerobic activity in hot, humid environment within past 3 mo. Questionnaire of training history and volume performed in heat, chart review (if available, more objective) Subjective Criteria Assessment Products consumed (including medications, drugs, or sports drinks) Questionnaire or personal interview At-risk: Poor nutrition or hydration status Level of motivation or zealousness At-risk: Athletes with the never give up or warrior mentality Questionnaire or personal interview Duration and quality of sleep (ie, in air conditioned room) At-risk: Sleep deprived athletes or who are exposed to heat/humidity throughout night Questionnaire or personal interview All subjective items are limited to the athlete s honest response and/or recall ability. *Note. Baseline hydration level must be an accurate euhydrated body mass confirmed over several days and verified by urine analysis (color or refractometer). 207

5 208 Cleary Intervention Strategies for Mitigation of EHI Risk For full recovery and successful return to play, correcting the inherent risk factors and mitigating the external risk factors to the extent possible is necessary. Environmental-specific conditioning should be performed with a gradually increasing emphasis on exercise intensity and exposure to a hot, humid environment. Many athletes rarely complete a comprehensive off-season conditioning program to the extent necessary to be fully acclimatized for preseason practice. Since off-season conditioning is rarely effective in fully acclimating athletes to the severe environmental conditions typical of preseason practice or summer events, individuals recovering from EHI and other high EHI risk individuals should be identified using a proactive approach by documenting the criteria from Table 1 and strictly monitoring these individuals during high-risk activities. Although time consuming, it is imperative that a greater emphasis be placed on identifying risk factors listed in Table 1 during preparticipation physical exams. For any successful recovery and heat illness prevention program to be effective, greater attention and continued observation is needed for individuals at high risk for heat stroke and heat intolerance such as those who have had previous EHIs. For many athletic teams or programs, because of the sheer numbers and associated catastrophic injury potential, ATCs must set up a system by which high-risk individuals are monitored at the highest levels of vigilance. 5 Successful recovery and prevention of future EHI requires a sound understanding of the relationship between the risk factors and the physiologic response to exercise heat stress. Correcting inherent risk factors provides an opportunity for implementation of preventive measures, before release to full activity in extreme environments. Effective intervention for the high-risk individuals may greatly decrease the chances of repeated incidents and severity of EHI during intense exercise in hot or warm environments. Several intervention strategies may be possible, including limiting individuals at highest risk to practicing in the cooler environments such as air-conditioned gymnasium or before/after the hottest parts of the day (10 AM to 4 PM). If practicing during the hottest time of day is necessary, then special attention must be provided for those at highest risk, which ought to include a slower conditioning schedule, which postpones more strenuous activities until 2-3 weeks of acclimatization is accomplished. In the course of all practices/events, ATC s must pay close attention to practice schedules (work-rest cycles, time of day, and speed of progression of exercise intensity and duration), hydration, clothing, access to shade, and immediate treatment (cooling and rehydration) of EHI cases. 4 Every athlete has inherent qualities that may increase the predisposition to EHI and, combined with external risk factors, may lead to an adverse event. Identifying and/or correcting inherent risk factors is critical to mitigating risk associated with exercise in a hot, humid environment. Inherent Risk Factors for Exertional Heat Illness Dehydration When athletes do not replenish fluids lost via sweat, urine, and respiratory losses, they will become dehydrated and their ability to exercise in the heat diminishes.

6 Risk Factors 209 As the level of dehydration increases, the athletes performance can be adversely affected as the capacity for thermoregulation is hindered. 1 Although the thirst mechanism may be present, it is not uncommon for individuals to voluntarily dehydrate by 2 to 8% of their body weight during acute bouts of exercise-heat stress, despite the availability of fluid to rehydrate. 6 This voluntary dehydration is a commonly observed mismatch between water output and water intake in which insensible water loss (water lost through respiration, feces, and skin other than sweat); obligatory water loss (urine); and sweat production exceed the individual s capacity to rehydrate. At rest, insensible water loss is usually about 0.8 L day -1 and sweat volume is about 0.1 L day -1 but both routes of water loss vary greatly, primarily depending on ambient temperature/humidity, clothing, and exercise intensity/duration. 7 Individuals exercising in hot humid environments have sweat rates ranging from 1.0 to 2.5 L hr -1, and fluid losses through sweat can be enormous, often reaching 5.0 L day -1 during prolonged heavy exercise. 7 Sweat rates vary greatly and often exceed an individual s ability to rehydrate to match sweat losses during exercise resulting in a fluid deficit. Rehydration during exercise when fluid is readily available is primarily limited by gastric emptying. Gastric emptying rate ranges from 0.9 to 1.2 L hr -1 and may be reduced by approximately 20 to 25% when an individual is dehydrated by 5% body weight. 8 During exercise-heat stress, ad libitum drinking frequently fails to match sweat output, resulting in deleterious changes in hormonal, circulatory, thermoregulatory, and psychological status. Body-weight changes are the preferred method to monitor for dehydration in the field and urine specific gravity measured using a clinical refractometer and urine color using an appropriate urine color chart are other accurate methods (Table 1). 1,3 Dehydrated individuals have a urine specific gravity greater than µg. Dehydration is commonly identified by monitoring urine color or body weight changes before, during, and after a practice or an event and across successive days. 1,3 Collegiate football players have demonstrated a loss of 0.7 to 2.0% of their body mass during consecutive days of two-a-day preseason training. 9 The players had significantly decreased body mass compared to baseline before and after morning and afternoon practice sessions on days 2 through 8 of the preseason practice. Urine specific gravity was elevated from baseline on 12 of 20 measures over the 8-day period ranging from ± µg to ± µg. 9 High Body Mass Index Obese individuals are at an increased risk for heat illness because the fat layer decreases heat loss. Obese persons are less efficient and have a greater metabolic heat production during exercise. Muscular athletes with high lean body mass have increased metabolic heat production and a lower ratio of surface area to mass, contributing to a decreased ability to dissipate heat (Table 1). 3 The risk for developing EHI (heat stroke, heat exhaustion, and heat cramps) has been demonstrated during military basic training to increase with an increase in body mass index (BMI) as measured on arrival. Recruits at highest risk for developing EHI had a BMI of 22 kg m -2, and they had an eightfold higher risk for developing EHI during basic training when compared with those with BMI less than 22 kg m -2. Only 18% of male recruits met the criteria for high risk, but they accounted for nearly half (47%) of the EHI cases occurring during the 12-week basic training course. Army recruits

7 210 Cleary with the highest risk (rates) for EHI weighed more than kg and were more than cm tall, corresponding to a BMI >30 kg m -2 (about 2% of the recruit population). This study reported that the anthropometric relationship to EHI risk was independent of other risk factors when expressed as BMI. Recruits in the highest quartile for BMI had about a threefold increase in risk compared with those in the lowest quartile, regardless of physical fitness level. 4 Poor Physical Condition Individuals who are untrained are more susceptible to heat illness than are trained athletes. Certain times of the year are especially problematic in that individuals are unprepared for the imposed physical demands and often the environmental conditions are extreme. For example, the first few weeks of preseason football practice where athletes report to practice in early August, one of the hottest and most humid months of the year. As the maximal oxygen uptake (VO 2 max) of an individual improves, the ability to withstand heat stress improves independent of acclimatization and heat adaptation. High-intensity work can easily produce 1000 kcal hr -1 and elevate the core temperature of at-risk individuals (those who are unfit, overweight, or unacclimatized) to dangerous levels within 20 to 30 minutes. 3 Individuals who are physically fit are at lower risk for developing EHI and less fit athletes practicing with a team are likely to exert themselves beyond their physical limits. Team practices will strain less fit athletes at higher relative workload intensities to keep up with more fit teammates, leading to higher relative core body temperature for the less fit athletes. Cardiovascular fitness, expressed as maximal oxygen uptake per unit body mass, is a strong predictor of individual thermoregulatory effectiveness during exercise. 4 Researchers have quantified the risk for EHI according to 1.5-mile run time during the first week of military basic training and found that recruits in the slowest quartile (a 1.5-mile run time of 12 minutes or more) have about a threefold increase in risk if EHI compared with those in the fastest quartile (1.5-mile run time under 10 minutes), regardless of BMI. 4 Good cardiovascular fitness provides increased cardiac reserve, allowing greatly increased blood flow to the skin and muscles necessary for thermoregulation during exercise. Heat Acclimatization Heat acclimatization (HA), an adaptive response to a new temperature, climate, or environment, reduces physiological strain, optimizes performance, and lessens thermal injury risk during occupational or athletic activities. 4 HA occurs a result of continuous or repeated exposures to naturally occurring climatic heat stress. 10 Little is known about time course or magnitude of high intensity-based adaptive responses in real-life conditions of heat exposure. This is pertinent as higher intensity exercise in real life conditions is more applicable than low intensity protocols to many exercise or athletic settings. Collegiate football players have been demonstrated to become sufficiently heat acclimatized with enhanced exercise-heat tolerance as a result of a progressive and gradual increase in exercise heat exposure. 10 Practical implications are that many individuals who regularly exercise, work, or compete in athletic events in the heat must become acclimatized to reduce the possibility of an EHI and improve exercise performance in the heat. 4

8 Risk Factors 211 Gradual increases to exercise heat exposure produces progressive changes in thermoregulation that involve sweating, skin circulation, thermoregulatory set point, cardiovascular alterations, and endocrine adjustments. Rate of acclimatization is related to aerobic conditioning and fitness; more conditioned athletes acclimatize more quickly. 3 For adults, the negative effects of heat and humidity can be attenuated by a period of HA, usually requiring between 8 to 14 days, 11,12 but maximum acclimatization may take 2 to 3 months. 3 Prior dehydration has a negative effect on HA even on exercise of short duration where sweat losses are small. The athlete must begin exercise in a fully hydrated condition and regular ingestion of fluids is necessary when the exercise duration exceeds 40 minutes. High intensity HA protocols produce similar heat tolerance capabilities in highly fit older males (mean age 45.8 ± 1.9 years) and young adult males (mean age = 20.4 ± 0.8 years). 13 Acclimatization has been demonstrated to induce physiological adaptations, including increased resting plasma volume, reduced exercise heart rate, a 26% increase in sweat rate and sweat sensitivity, lower sweat sodium concentrations, a 6% reduction in exercise oxygen consumption, reduced rating of perceived exercise intensity, and increased thermal comfort. 11 HA occurs more rapidly with fit individuals performing high intensity exercise than with less fit individuals, low intensity exercise, or resting in the heat. 14 Other Inherent EHI Risk Factors In extreme environmental conditions, intense exercise cannot be preformed without some risk of EHI, regardless of fluid replacement or HA, 13 and athletes with other inherent predisposing factors may be at additional risk (Table 1). Athletes who are currently or were recently ill may be at an increased risk for heat illness because of fever or dehydration. Individuals with a history of previous exertional heat illness are at greater risk for recurrent heat illness, 3 particularly when the precipitating factors have not been identified and addressed. Additional risks include conditions of electrolyte imbalance (particularly important in heat cramp cases), presence of gastrointestinal illness, and ingestion of certain medications (ie, antihistamines, diuretics) or some dietary supplements. Athletes are at higher risk of EHI if they are overzealous, highly motivated to push oneself during exercise, have a warrior mentality, or are described as the never give up athlete. 1 If they are reluctant to report problems, issues, or illness, they are also at higher risk of EHI. External Risk Factors Environmental Conditions Before, During, and After Exercise Environmental temperature exceeding skin temperature results in absorption of heat from the environment and depends entirely on evaporation for heat loss. High relative humidity attenuates heat loss from the body through evaporation. 3 Environmental factors that influence the risk of heat illness include the ambient air temperature, relative humidity, air motion, and radiant heat from the sun. Relative risk of heat illness can be estimated using the wet bulb globe temperature (WBGT). WBGT is not air temperature. It indicates wet bulb globe temperature, an index of climatic heat stress that can be measured on the field by the use of a psychrometer.

9 212 Cleary This apparatus, available commercially, is composed of three thermometers. One (wet bulb [WB]) has a wet wick around it to monitor humidity. Another is inside a hollow black ball (globe [G]) to monitor radiation. The third is a simple thermometer (temperature [T]) to measure air temperature. The heat stress index is calculated as WBGT = 0.7 WB temp G temp T temp. It is noteworthy that 70% of the stress is due to humidity, 20% to radiation, and only 10% to air temperature. 2 High WBGT indicates extreme risk of heat-related problems and appears to be one of the best predictors of heat illness. Exposure to heat and humidity in days preceding exercise heat stress has been associated with increased risk of EHI. 3 Athletes who sleep in cool or air-conditioned rooms appear to be at less risk. 1,3 High temperature/humidity/sun exposure increases the degree of EHI risk, particularly when work:rest ratios are not based on environmental conditions, exercise intensity, and amount of protective equipment. 1 The combination of extreme environmental conditions and intense physical activity, often while wearing vapor-impermeable equipment, require strict adherence to the recommended guidelines. 1,3 One of the highest risk activities in North America is preseason football, which begins in early August, usually the time of year with highest ambient heat and humidity. This follows three months of off-season training, usually in an airconditioned gym. Although regulations at some levels exist, exercise during these periods significantly exposes athletes to risk of heat illness and possible death and is made even more dangerous while training in a full American football uniform. Individual risk factors are exacerbated when combined with environmental stress. A climatological analysis revealed that no environmental combinations existed at any time periods during the month of August for any locale in Alabama that would be considered safe for outdoor practice in a full football uniform. 15 During August in Alabama, only two time periods existed during which players could practice outdoors with extreme caution. The periods were at 9 AM and 6 PM in the north-central and northern portions of the state. The National Collegiate Athletics Association has adopted specific regulations concerning the preseason practice period for American football in an effort to prevent or decrease the incidence of heat illness, to ensure safety and recovery of players, and to maximize performance. These regulations appear to adequately prepare football players for exercise heat exposure through proper heat acclimatization, appropriate time between practices, the phasing of equipment, and limiting the number of practices. 10 In general, if football practice is to be conducted outdoors in the Southeastern U.S. during August and September, clear regulations are needed at all levels: time limited with adequate breaks between practices for recovery, scheduled early in the morning or late in the evening, cool water consumed prior to practice, frequent water breaks scheduled throughout practice with amounts of fluid matching sweat losses, gradual increase of equipments over time, and careful monitoring of players for signs of EHI. 15 Equipment as a Barrier to Thermoregulation The uniforms for sports such as football or lacrosse represent a precarious balance between musculoskeletal protection related to the rigors of the sport and potential harm from the thermal environment as both dry and evaporative heat loss are impeded. 16 Heat loss from the body occurs through the process of thermoregulation, a complex interaction among the central nervous system, the cardiovascular

10 Risk Factors 213 system, and the integumentary system to maintain a core body temperature of 37 C. Thermoregulatory responses include cutaneous vasodilatation, increased sweating, increased heart rate, and increased respiratory rate. 17 Football uniforms contribute significantly to the heat load on a player by decreasing thermoregulatory capacity, 16 and heavy or dark-colored clothing or equipment may cause a greater absorption of heat from the environment. 1 Such a situation defines a condition of uncompensable heat stress, where the body continues to store heat and core temperature continues to rise to dangerously high levels. The classic description of the cardiovascular response to heat stress involves a progressive increase in the redistribution of the central blood volume to the cutaneous circulation to increase convective and evaporative heat loss, resulting in a lowered venous return, stroke volume, central venous pressure, and mean arterial pressure. 18 To our knowledge, only one study has provided published data for football uniforms. 16 Data on the thermal and evaporative resistance of football uniforms should be incorporated into so-called rational approaches to environmental safety and can be used in the solution of heat balance equations to predict physiological responses of football players. Football uniforms contribute significantly to the heat load on a player as the amount of protective gear worn during preseason practice progressively increases, so does the risk of EHI. 16,18 Until recently, core body temperature has been impractical to measure during in vivo football practice sessions. The incorporation of ingestible telemetry systems has provided valuable information on core body temperature during actual real life situations. Collegiate football players have been demonstrated to have core body temperatures exceeding 39.0 o C on six of eight days of preseason practice. Further, core body temperature has been recorded to be significantly elevated during afternoon practice sessions with full football equipment compared to morning sessions in half pads with environmental conditions of 34.5 o C/43.0% relative humidity and 28.4 o C/64.9% relative humidity, respectively. 9 As a result of HA, core body temperature, heart rate, and signs/symptoms of heat illness are expected to progressively decrease across days despite progressive increases in exercise intensity, exercise volume, and protective gear. Clinical Implications Identification of risk factors for EHI is a critical step required prior to performance of physical activity in hot, humid environments. Using objective, quantitative criteria to determine the inherent risk factors for each individual prior to activity is a proactive approach that will prevent EHI or correct the predisposing factors that contributed to the incident of an EHI. In order to prevent future incidents of EHI, individuals with a history of EHI must correct the predisposing factors and minimize risk prior to returning to full activity in the heat. Using a proactive approach to minimize risk and to identify and address other precipitating factors are critical to successfully preventing EHI and to prevent repeated incidents of EHI. References 1. Casa DJ, Almquist J, Anderson S, et al. Inter-Association Task Force on Exertional Heat Illness Consensus Statement. NATA News. 2003;8:24-29.

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