LOWERING CORE BODY TEMPERATURE AND PERCEIVED EXERTION BY THREE COOLING METHODS

Transcription

1 LOWERING CORE BODY TEMPERATURE AND PERCEIVED EXERTION BY THREE COOLING METHODS By SUSAN MICHELLE WALKER A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN EXERCISE AND SPORT SCIENCES UNIVERSITY OF FLORIDA 2003

2 Copyright 2003 by SUSAN MICHELLE WALKER

3 ACKNOWLEDGMENTS I would like to take this opportunity to thank the many people who assisted in the completion of my thesis. First, I would like to thank the Gainesville Fire Department and Chief Northcutt for working with me, helping develop the idea of this project, and unselfishly volunteering to participate during their busy schedules. I would like to express thanks to Michael Powers and Lesley White for their many suggestions and recommendations to make this thesis better. Their help on my thesis committee was crucial. Many thanks go to Matt Morgan, my doctoral mentor, who gave generous amounts of time at any time of the day to help me finish this project. I thank Dr. Douglas McDonald for his interest in the study, his constant presence, and always listening. The guidance and friendship provided by Matt Walser and Kelli Frye over the past two years will also never be forgotten. Their understanding and patience were always appreciated. I would like to thank Dr. MaryBeth Horodyski, my chair and advisor, for all her assistance with the creation of this project, the data collection, the long analysis process, and making the finished document. Her help was invaluable and irreplaceable. Lastly, but not least, I thank my parents, who no matter the situation was never more than a phone call away. They both gave endless support and help with this paper and through all my years of continuing education. iii

4 TABLE OF CONTENTS page ACKNOWLEDGMENTS... iii LIST OF TABLES... vii ABSTRACT... viii CHAPTER 1 INTRODUCTION...1 Statement of the Problem...2 Hypotheses...2 Definition of Terms...3 Assumptions...5 Limitations...5 Significance of Study LITERATURE REVIEW...7 Environmental Conditions...7 Physiology...8 Thermoregulatory Adjustments...10 Dissipation...11 Sweating...11 Endocrine System...12 Immunity System...14 Heat Illness...14 Heat Cramps...15 Heat Syncope...16 Hyponatremia...16 Heat Exhaustion...17 Heat Stroke...18 Predisposing Factors...20 Acclimatization...22 Prevention...23 Hydration...24 Dehydration...25 iv

5 Fluid Replacement...26 Recommendations...28 Cooling Techniques...29 Cold Water Immersion...30 Fans and Misters...31 Cold Towels...32 Ice Bags...33 Precooling...34 Rehydration...34 Methods of Measurement...35 Perceived Exertion...35 Hydration Status...37 Temperature...38 Common Temperature Sources...39 Reliability of the Telemetric Pill MATERIALS AND METHODS...41 Subjects...41 Instrumentation...42 Procedures...44 Study Design and Data Analysis RESULTS...48 Subject Demographics...49 Environmental Heat Index...49 Core Body Temperature...50 Heart Rate...52 Rate of Perceived Exertion...53 Pre and Post Hydration Status...55 Weight...55 Urine Color...55 Specific Gravity DISCUSSION...57 Core Body Temperature...57 Heart Rate...62 Rate of Perceived Exertion...64 Hydration Status...67 Summary...69 Conclusions...70 Future Research...72 v

6 APPENDIX A INFORMED CONESENT FORM...73 B HEALTH QUESTIONNAIRE...78 C ANOVA WITH REPEATED MEASURES BY EACH DEPENDENT VARIABLE...80 REFERENCES...88 BIOGRAPHICAL SKETCH...94 vi

7 LIST OF TABLES Table page 4-1 Environmental Heat Index (degrees Celsius) by Treatment Day Comparison of Means for Core Body Temperature (degrees Celsius) Time Points TP1-TP Comparison of Means for Heart Rate (beats per minute) Time Points TP1-TP Comparison of Means for Rate of Perceived Exertion Time Points PE1-PE Comparison of Means for Weight (pounds) Loss Comparison of Means for Urine Color Comparison of Means for Specific Gravity...56 vii

8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science in Exercise and Sport Sciences LOWERING CORE BODY TEMPERATURE AND PERCEIVED EXERTION BY THREE COOLING METHODS By Susan Michelle Walker August, 2003 Chair: MaryBeth Horodyski Major Department: Exercise and Sport Sciences The purpose of this study was to evaluate three cooling techniques on core body temperature and hydration status in healthy individuals. Cooling methods were cold towels, fan, and cold water immersion compared to a control treatment. Dependent measures were core body temperature, heart rate, rate of perceived exertion, weight loss, urine color, and specific gravity. Twelve healthy subjects, nine males and three females (age: 22 ± 1.5 yrs.), completed twenty minutes of exercise, were treated with ten minutes of cooling, and then returned to exercise for twenty minutes. Exercising included stadium running and sprints and cooling time included treatment by one of the three cooling methods or control. Each subject participated in protocol four times to experience each cooling method. Telemetric ingestible temperature sensors, heart rate monitors, Borg s rate of perceived exertion viii

9 scale, a weight scale, urine color chart, and specific gravity reagent strips were tools used to measure dependent variables. Analysis of variance (ANOVA) with repeated measures revealed no significant difference within cooling treatments on core body temperature, rate of perceived exertion, urine color, and specific gravity. Cold water immersion did result in a significantly lower heart rate [F(3, 33)=4.467, p=.01] and urine color [F(3,33)=4.342, p=.01]. Cold water immersion resulted in significantly less weight loss from pre to post exercise [F(3, 33)=3.545, p=.025]. Core body temperature [F(10, 100)=40.880, p<.001], heart rate[f(10,110)= , p<.001], and rate of perceived exertion [F(13, 143)= , p<.001] significantly increased with exercise, decreased with cooling, and increased with return to exercise over the course of time during the testing session. Significant differences were noted pre and post activity for each hydration status measurement. Results suggest cooling methods may be effective in reducing thermal stress for preventive and emergency situations to decrease core body temperature, heart rate, and rate of perceived exertion. However this study did not indicate that any method is more effective than another or compared to the control. ix

10 CHAPTER 1 INTRODUCTION Education, advanced environmental monitoring, and hydration guidelines are just a few measures that health care professionals, including athletic trainers, have used to prevent severe heat exhaustion or heat stroke in individuals who exercise for competitive and noncompetitive reasons. Despite efforts to educate athletes, many types of athletes are still at great risk for heat illness and fatalities occur. With athletic teams and individuals practicing up to multiple times a day during the summer months, exercising in high temperatures and humidity intensifies the body s thermoregulatory response. This includes a shunting of blood away from internal organs, an increase in cutaneous blood flow, lactate production (28,12), core body temperature (64,28,3), heart rate because of a decrease in stroke volume (35,12), and sweating (35,12,64,3). These physiological responses are due to many factors including lack of heat acclimatization early in the training season, sedentary lifestyle or low fitness, being overweight, age, and dehydration (12). The potential for developing heat exhaustion and heat stroke are a daily reality as a result of these sometimes-dangerous physiological effects and predisposing factors on the body. To improve the body s thermoregulatory responses and reduce negative effects associated with elevated core body temperature, cooling methods should be employed for prevention and treatment for heat illness. Prompt recognition and treatment is crucial to prevent organ damage due to heat stroke is proportional to the length of time between core temperature elevation and initiation of cooling methods (4). Cooling methods that 1

11 2 are effective and can be used easily on the practice field or any exercise environment are necessary. With these techniques, health care professionals can efficiently and effectively combat heat illness, especially in the case of an emergency. Statement of the Problem Research reports on thermoregulation, treatment, and/or employed techniques not often used on the field of athletic practices or outside exercise facilities. Many studies have focused the effects of heat either on the physiological aspects, hydration status, or cooling techniques infrequently used. The cooling techniques typically explored can only be replicated in the laboratory setting, very few are reasonable to use on the athletic practice field or area. Little research has been done to validate the effectiveness of methods that are frequently used by medical professionals The purpose of this study is to test the efficacy of the three cooling methods that are currently used in the field setting to prevent and care for heat illness. It will investigate the effectiveness of cold towels, torso immersion in a cold water, and cooling fans on decreasing elevated core body temperatures due to environmental conditions. Perceived exertion, heart rate, and hydration status will also be assessed. Hypotheses Six hypotheses were identified for this investigation. 1. There will be a positive correlation between core body temperature and environmental heat index. Rationale: Warm temperature and high humidity will strain the body s natural physiologic responses and blunt cooling mechanisms causing an increase in core body temperature. 2. Using cold towels, cold water immersion, and cool fans individually as cooling methods will all significantly lower elevated core body temperature as measured by a ingestible telemetric temperature sensor compared to no treatment.

12 3 Rationale: Each cooling method will aid in improving the body s blunted cooling mechanisms and decrease the core body temperature. 3. Cold Water immersion will significantly decrease core body temperature and perceived exertion compared to control, cold towels, and fan treatments. Rationale: More surface area will be directly cooled by the contact of cold water compared to the other treatments and the contact is closest to the core body temperature. 4. Core body temperature will continue to drop initially once exercise is resumed but then return to previous elevated levels during the second exercise bout. Rationale: The blood that was cooled cutaneously will flow to the body s core even after the cooling method has been stopped and continue to decrease core body temperature. Yet as exercise is resumed the cooled blood will be warmed once again and core body temperature will gradually increase. 5. All three cooling techniques will significantly lower the athlete s rate of perceived exertion (RPE) immediately following cooling treatment. Rationale: The sensation of cold will be more prominent than the sensation of warmth, the decrease in physiological strain, and the decrease in core body temperature will all cause the subject to experience little perceived exertion. 6. Rate of perceived exertion will stay low initially when exercise is resumed but then return to previous elevated levels by the end of the second exercise bout. Rationale: There will be a carry over of cold sensation as the cooled blood continues to decrease core body temperature as well as the rest given to muscular activity following the cooling time and will keep the rate of perceived exertion low. As the core body temperature gradually increases and more demands are placed on the body physiologically because of the exercise, the rate of perceived exertion will rise to higher values. Definition of Terms It was necessary to define the following terms for the purposes of this investigation. 1. Acclimatization: body makes physiological and psychological adaptations to decrease the stress of a new environment (12,71) 2. Ambient Temperature: temperature or air that invests one s immediate environment (8) 3. Anaerobic Training: in the absence of oxygen, training that improves the efficiency of the anaerobic energy-producing systems and can increase muscular strength and tolerance for acid-base imbalances during high intensity effort (21).

13 4 4. Anhidrosis: the inability of the body to produce or deliver sweat to the skin (26) 5. Cardiac Output: the volume of blood pumped out by the heart per minute (21) 6. Cold Towel: terry cloth towel immersed in ice water and then placed on the body 7. Core Body Temperature: internal or deep body temperature monitored by cells in the hypothalamus, as opposed to the shell, or peripheral, temperature, which is registered by that layer of insulation provided the skin, subcutaneous tissues, and superficial portions of the muscle masses (21) 8. Dehydration: dynamic process of reduction from normal water stores to hypohydration (23, 38) 9. Endogenous Heat Production: internally produced such as basal metabolic rate, muscular activity, shivering and non-shivering thermogenesis (12) 10. Esophageal temperature: a measure of the aortic and heat blood (72) 11. Euhydrated: body has normal stores of body water (23,38) 12. Exogenous Heat Production: Externally produced such as ambient temperature, wind speed, humidity, solar radiation, ground thermal radiation, and clothing (12) 13. Heat Index: also known as the universal wet bulb globe temperature (WBGT), this index incorporates readings from a variety of thermometers and assesses the combined effect of ambient temperature (dry bulb thermometer, or DBT), humidity (wet bulb thermometer, or WBT), and radiation from the sun (globe thermometer, or GT). The formula for calculating the wet bulb globe temperature is (8): 14. WBGT = (0.1 X DBT) + (0.7 X WBT) + (0.2 X GT) 15. Hyperthermia: elevated body temperature (8) 16. Hypohydrated : a body fluid deficit (23,38) 17. Ingestible Telemetric Temperature Pill Sensor: means of measuring core body temperature using a temperature pill the subject swallows. The sensor transmits a continuous low frequency waveform that varies with temperature. This signal is transmitted through the body to a receiver unit and later downloaded to a computer after completion of data collection (40,47,55) 18. Maximal Oxygen Uptake (VO 2 Max): the maximal capacity for oxygen consumption by the body during maximal exertion (21) 19. Percent Body Fat: the composition of the body; fat free mass compared to fat mass (21)

14 5 20. Rectal Temperature: the amount of water vapor in the air, expressed as a percentage of the maximum amount that the air could hold at the given temperature (6) 21. Thermoregulation: the process by which the thermoregulatory center, located in the hypothalamus, readjusts body temperature in response to small deviations from the set point (21) Assumptions This study was conducted under several assumptions. 1. Subjects were physically in shape to finish testing during each session. 2. Subjects answered the perceived exertion scale honestly. 3. All changes observed are representative of changes experienced by subjects in similar practices and conditions. 4. Subjects were already heat acclimatized before testing. Limitations Several limitations were identified in relation to data collection of the study. 1. Temperature and humidity levels changed daily over the span of all testing sessions. 2. Solar radiation changed daily resulting in different radiation effects on the subjects. 3. The fitness level of each individual participating differed. 4. The intensity at which the athlete performed each test may not have been replicated each session. 5. Position of the sensor in the gastrointestinal tract during testing could not be determined exactly. 6. Equipment malfunctions impaired or delayed data collection. 7. Measurement of water consumption was not accurately recorded Significance of Study Heat illness treatment and prevention were brought to the forefront of the nation s attention following the death of 3 athletes during the summer of 2001 and one in From 1960 to 2001 there have been over 100 deaths resulting from heat stroke in the

15 6 sport of football alone (53). The need for more research focusing on heat illness became apparent during this time. Further research concerning heat illness could improve protocols for the immediate care of heat illnesses, treatment in non-threatening situations, and furthermore add to current knowledge of core body temperature changes during exercise. By evaluating which cooling methods are most efficacious in the field setting, heat illness may be prevented. Also, this study may reveal techniques that give the athlete a sense of relief from heat stress, but may not actually lower or maintain lower core body temperatures; therefore, increasing their risk of heat illness through a false sense of temperature.

16 CHAPTER 2 LITERATURE REVIEW Environmental Conditions Athletes practicing in environmental conditions such as high ambient temperatures, humidity, and solar radiation may experience a thermal stress increase on the body and the risk of heat illness. Common guidelines for health care professionals monitoring conditions for practice or competition time are set at specific ranges. Temperatures between 26.7 to 32.3 C with relative humidity under 70% indicate that athletes should be watched closely for any signs of distress. Yet with the same temperatures with relative humidity over 70% athletes should not only be monitored but take ten minute rests at least every hour. Temperatures ranging from 32.2 to 37.8 C with a relative humidity under 70%, the same recommendation should be heeded. Conditions with temperatures between 32.3 to 37.8 C or higher with a relative humidity over 70% should warrant a suspension in practice or a shortened practice time with little clothing (8). A study by Galloway under different ambient temperatures found a decrease in time to exhaustion, VO2max, and CHO oxidation at only 31 C. He also reported an increase in rectal temperature, sweat rate, rate of perceived exertion, and heart rate in the same condition compared to less stressful environmental conditions (30). Skin temperature is proportional to ambient temperatures; an increase can lower the mechanism threshold of sweating by core temperature (5). Ambient temperatures higher than 35 C with a relative humidity of 60% can cause a detriment in evaporation of sweat 7

17 8 and maintenance of steady state temperatures (38). Nadel s study had subjects exercise at different intensities at different temperatures. The results demonstrated a significant decrease in cardiac output and skin blood flow in a temperature of 36 C. A 7% decrease in plasma volume at a moderate intensity was seen at 36 C and dropped to 17% with an increase in intensity similar to an anaerobic burst (51). These studies show that even in hot environmental conditions that thermoregulatory strain is increased significantly with implications toward decreased performance and an increased risk of heat illness. Physiology The body makes many adjustments in the circulatory system to compensate during exercise. When exercising in the heat, more adjustments are made to protect and maintain normal function. Exercise causes an initial increase in blood flow to the skin and active muscles with a correlation in cardiac output to meet these demands. Skin temperature during exercise is a reflection of the amount of heat reaching the skin from deeper heated tissues. Blood goes to the skin after being warmed by exercising muscles or increased body core temperature in an effort to dissipate heat through convection and radiation. (28). Dissipation can only occur if the ambient temperature is lower than the skin temperature, otherwise heat is gained instead of dissipated. The amount of blood to the muscle changes with the intensity of the exercise. Receptors from the skin and core temperatures regulate the amount of blood through the hypothalamus. Consequently there is a decrease in blood to the kidneys, stomach, and other abdomen organs to help supply the increase to the skin (12). Vasodilatation to the skin, arms, legs, and trunk are affected by the sympathetic nervous system s response to an increase in body temperature. With the stress of an

18 9 increase in blood flow to the skin in an effort to decreases the thermal load and supply oxygen to the active muscles, the cardiovascular system can not keep up with the demands (12). A redistribution of blood is needed because of the increased body temperature. Blood pressure is more vital compared to the skin trying to cool the body or the performing muscles, as a result hyperthermic and metabolic inefficiencies occur (64). The hyperthermic and metabolic inefficiency response is especially relevant to athletes in the heat during practice. With this inefficiency comes an increase in lactate production as a result of a decrease in hepatic blood flow and an increase in muscle temperature, which can eventually lead to fatigue. In an attempt to help waste removal, delivery, and buffering capacity; blood vessels in the muscles try to vasoconstrict but are unable to because of the decrease in blood flow. Thus in turn causing more inefficiency and fatigue (28). Athletes during practice train the majority of time in an upright position, which results in 70% of blood volume to be below the heart. With increasing core body temperature, adequate venous return is needed to prevent blood pooling in the lower extremity, causing a decrease in cardiac return (64). With the decrease in venous return, baroreceptors in the heart sense the pressure change and alert the cardiovascular control center to stop skin and muscle vasoconstriction in an effort to maintain blood pressure and cardiovascular function (12). During an increase in core temperature a convective heat exchange between the arteries and veins in the extremities happens. Venous blood returning from the skin precools the arterial blood, allowing for maximum heat loss once it reaches the skin (35).

19 10 Even though heart rate increases with exercise, stroke volume decreases because of the decrease in cardiac filling. The decrease in stroke volume is largely due to the decease in blood flow to the skin and muscles and dehydration. The body reacts by increasing the heart rate even more in an attempt to compensate for the decreased stroke volume. Eventually, VO 2, performance, and fatigue are effected and a decrease in exercise tolerance occurs in the heat (35). Thermoregulatory Adjustments Regulation of body temperature involves both the shell and core temperatures. The shell consists of the skin, subcutaneous tissues, and limbs. The core involves the contents of the skull, thorax, and abdomen. There can be variations in body temperature from day (97.3 F) to night (98.8 F), which is called the diurnal temperature rhythm (35). Positive heat exchange occurs when there is heat transferred from the body to the environment, where as negative heat exchange is the opposite, heat from the environment is transferred to the body. Heat gain must equal heat dissipation in order to maintain normal function with exercise. If not, body temperatures can rise to life threatening levels within minutes. Nielson, a scientist in 1938, was the first to describe the linear relationship between increased core temperature and exercise (cited in Fortney and Vroman [28]). Heat gain is affected by exogenic factors such as ambient temperature, wind speed, humidity, solar radiation, ground thermal radiation, and clothing. Ambient temperature and humidity are the most influential factors. The body loses its main resource of heat loss through evaporation when humidity increases resulting in an increase in core body temperature (62). Endogenic heat production comes from basal metabolic rate, muscle activity, nonshivering thermogenesis, and shivering thermogenesis (12).

20 11 Dissipation Heat is dissipated four ways: convection, conduction, evaporation, and radiation. Convection is when the air in contact with the surface of the body becomes heated and airflow carries it away. It is regulated by the temperature gradient change between the peripheral blood vessels and skin. Increased air force over the body, such as wind or fans, disrupts the still air next to the skin that is being heated and removes heat quicker. Conduction results in direct contact of the body surface with another surface at a lower temperature such as cold metal or water. In sport participation conduction rarely plays a role in heat dissipation. Radiation, heat comes from the body surface and is absorbed from the environment such as a snow bank or the sky (35,12). Evaporation is the most effective continuos heat dissipation process. Water is evaporated from the respiratory tract during breathing, diffusion through the skin surface and sweat glands. Ambient temperature, if increased, causes a decrease in radiation and convection. Thus, the body depends on evaporation for cooling (35,12,28). Evaporation can account for 98% of cooling in a hot dry environment but drops to 80% when the conditions are hot and wet (12). When heat is gained from the environment because the air temperature is above body temperature, the body is exposed to high radiating heat loads such as direct sunlight. When this happens, evaporative heat loss is the only means of dissipation (35,12,28). Sweating The evaporative cooling response is the most important response of the body to cool itself during exercise. Sweat glands produce salty fluids, when moisture evaporates, heat is released from the body at the rate of 2.14 kj x ml -1 at normal body temperatures and low humidity levels. Apocrine glands are in the hair follicles found in the axilla and pubic areas, they react mostly to emotional stimulus. Eccrine glands are more widely

21 12 distributed over the body (28). Sweat in the glands contains sodium, chlorine, lactic acid, glucose, potassium, calcium, magnesium, iron, and water-soluble vitamins. Sweat production and loss are measure by weight loss. The sympathetic response effects levels of norepinephrine that regulate sweating. Peripheral and central thermal receptors provide afferent input into the hypothalamus thermoregulatory center, which initiates and maintains sweat rate (35,5). Yet, it can be affected by outside factors such as high humidity and dehydration. In a dehydrated state, the sweat rate can decrease in an effort to maintain blood pressure and central venous filling. Sweat rate can reach maximum values within one hour of thermal stress but then gradually decrease even though core body temperature is still increasing (28). Most athletic practices last two or more hours, making this an increased risk. Plasma tonicity and blood volume are the primary variables that modify thermoregulatory responses. Extracellular fluid volume decreases as sweat increases because of dehydration. A decreased blood flow with intense exercise is a result of decreased blood volume caused by sweating and fluid movement from the vascular compartment into the interstitial spaces surrounding exercising muscles. This only happens in hot environments because in cooler environments convection and radiation help retain fluids. Evaporation decreases in hot and humid environments because of the small water vapor pressure gradient. If these responses to initiate heat dissipation decrease, then a rapid increase in core temperature occurs (64). Endocrine System Hormones play a major role in the redistribution of blood and body fluids during exercise. The adrenal glands release a variety of hormones from both the adrenal cortex and the adrenal medulla that contribute to fluid and electrolyte regulation. Additional

22 13 hormones are released from neurosecretory cells in the posterior pituitary gland. The adrenal medulla produces, stores, and secretes the catecholamines epinephrine and norepinephrine. These hormones stimulate the sympathetic nerve response, causing increased heart rate, increased force of cardiac muscle contraction, elevated blood pressure, increased breathing rate, and decreased activity in the digestive system (67). A hormone directly involved with helping regulate blood osmolality is the antidiuretic hormone (ADH), also known as Arginine Vasopressine. The posterior pituitary gland, which is just inferior to the hypothalamus, stores the hormone until it is stimulated. This can be done by triggering osmoreceptor neurons in the hypothalamus because of an increase in blood osmotic pressure or by sensory stretch receptors found in the left atrium of the heart. A common cause of an increase in plasma osmolality is by dehydration or increased sodium intake. The thirst mechanism then increases, resulting in more water intake. This stimulates the sensors, increasing ADH levels, which act on the kidneys to promote water retention. A decrease in urine output therefor increases blood volume and dilutes the sodium in the plasma (29). If there is in increase in water and a decrease in sodium instead, the blood volume increase, dilutes the blood more, and decreases the plasma osmolality. Therefor, the secretion of ADH is decreased and less water is reabsorbed by the kidneys and instead is urinated out. ADH binds to the membrane receptors on the collecting ducts in the kidneys to make them more or less permeable to water so as to either reabsorb or let water pass (29). Hyperthermia can increase ADH levels despite osmolality levels as well as hypohydration and high exercise intensity, which directly effect hydration status, the athlete s performance, and recovery (48).

23 14 Aldosterone, is a mineralocorticoid, or a type of corticosteriods. It is produced and secreted by the adrenal cortex. It is known to regulate sodium and potassium balance. The adrenal cortex is directly stimulated by an increase in potassium levels in the blood or indirectly by low plasma sodium and a decrease in blood volume and pressure. The low sodium causes a chain reaction to stimulate the release of aldosterone. The enzyme Renin is secreted from granular cells, which causes a conversion of angiotensionogen into agiotension I which once again is changed to angiotension II and stimulates the cortex. Once stimulated, the aldosterone promotes reabsorbtion of sodium from the corticol collecting ducts in the kidneys in proportionate amounts so as not to dilute the blood as does ADH. The high plasma osmolality with low water concentration or low plasma volume can be caused while exercising in the heat, excessively sweating, and not adequately replacing fluids (29). Immunity System The immune system is also stressed by heat. Decreases in leukocytes, epinephrine, norepinephrine, dopamine, growth hormone and cortiosl have been seen in extreme conditions while exercising in the heat. A decrease in cortisol can cause a decrease in monocyte migration to tissues that are injured. During moderate sustained exercise though, an increase in epinephrine, norepinephrine, cortisol, and growth hormone can positively effect immune function (48). Heat Illness Heat stress can take many forms. The most common are heat cramps, heat syncope, heat exhaustion, and heat stroke. A fifth condition termed hyponatremia has also begun to be explained and examined. Hyperthermia has been defined as the body s rate of heat production exceeding heat dissipation. Environmental conditions, caloric intake, fluid

24 15 consumption, clothing worn, amongst a slough of other factors, can influence an individual s ability to tolerate heat and the severity of heat illness (4). Exercise intensity significantly effects core body temperature since it increases dramatically as a function of the maximal oxygen uptake percentage. High intensity activity, without recovery to allow heat dissipation, can allow rates of heat gain as high as 0.15 C per minute leading to hyperthermia (46). Lack of recognition of signs and symptoms can cause the assumption of laziness, so forced continuation of exercise occurs and inevitably results in the form of heat illness (66). In the early 1900 s survival rate of severe heat illness was 20%, today it is between % with more recognition of signs and symptoms (63). Heat Cramps Heat cramps are painful spasms of the skeletal muscle. Common causes are lack of adequate fluids and muscle fatigue (49,25). Body temperature and blood pressure are usually normal (25). It has been theorized that a NaCl deficit during sweat loss and nonreplacement of fluids causes a decrease in plasma NaCl. A change in the water expansion also attributes because of a difference in the sodium/potassium pump. The action potential changes across the cell membrane, which increases the level of calcium and the release of calcium from sarcoplasmic reticulum. All these changes cause random muscle contractions (12). The muscle contractions do not necessarily effect the entire muscle, individual bundles can contract in a sporadic manner (12,25). Common cramps are in the arms, legs, and abdominal muscles (62). Cramps usually occur after performing for several hours, which is often in multiple practices a day in any sport or competitive athlete. Acclimatized and conditioned athletes seem to be the most commonly affected by these disabling muscle contractions (62,25).

25 16 Heat Syncope Heat syncope has been described as an orthostatic syncopal episode (63) that usually happens after exercise. The athlete experiences a feeling of weakness and tiredness (49). Fainting or syncope occurs often in conjunction with heat exhaustion (4). In incidences of heat stress or none at all, causes can be vasodilatation accompanied with dehydration (63). Hyponatremia Hyponatremia has been explained as a chronic sodium loss with an excessive fluid intake, which causes an electrolyte imbalance (36). Individuals experiencing this condition have usually exercised between seven and seventeen hours (3). Hyponatremia is categorized on the basis on their fluid status: hypovolemic, euvolaemic, or hypervolemic. The onset of this condition can be acute or chronic (36). Possible reasons for the condition are sodium losses during the long exercise period, which are normal, but because of the increased volume of excessive fluids, the plasma sodium concentration is critically reduced. The great amount of fluid intake can overwhelm or confuse fluidelectrolyte control mechanisms. The fluid retention could possibly be held in the gastrointestinal tract during exercise and then enter the extracellular fluid later causing symptoms to be delayed (3). Hyponatremia can have effects on the nervous system, cardiovascular system, musculoskeletal system, and renal systems. Because it can effect so many organ systems, signs and systems widely vary. Headache, nausea, vomiting, weakness, and muscle cramps are common. Disorientation, coma, increased intracranial pressure, seizures, pulmonary edema, decreased reflexes, and cardiac arrests are also possible amongst many more serious conditions (36,3). Treatment depends on the acuity,

26 17 symptoms, sodium level, and fluid volume status. Water restriction, diuretic therapy, and hypertonic saline intravenously are the usual course of action (36). Heat Exhaustion Heat exhaustion is the inability to continue exercise in the heat (63,4); it is the most common heat illness among athletes (4,12). Two types of heat exhaustion have been proposed. Water depleted exhaustion usually occurs shortly after exercise starts, where as salt depleted exhaustion does not occur until several days of exposure in hot and humid conditions without adequate recovery (12). The inability of the cardiovascular system to sustain and respond to exercise workloads, external temperatures, and dehydration along with a decrease in blood volume, (49) can be contributory causes of heat exhaustion (4). With the continuation of exercise, cardiac output is limited. Therefore, muscle and skin blood flow needs are not met, causing the collapse and termination from exercise (12). Temperatures between C along with postural hypotension are causes cited by Eichner (25). There are no known chronic harmful effects after experiencing heat exhaustion (4). There are numerous signs and symptoms of heat exhaustion, many overlapping with heat stroke: headache, weakness, dizziness, vertigo, chills, goose bumps, heat feelings on the head and neck, cramps, vomiting, nausea, hyperventilation, muscular decordination, agitation, irritability, impaired judgement, and confusion (4,49). Symptoms are sudden and the duration of the collapse that follows can be quick. The athlete looks ashen and has a decreased blood pressure and an increased heart rate (4). People who are susceptible usually exert themselves at or near maximum, are dehydrated,

27 18 not physically prepared, have a significant loss of water and electrolytes (12), and/or are not acclimatized to the heat (12,4). Key factors for classifying between the more severe heat illnesses are core body temperature and mental status (62). There are other noticeable differences between heat exhaustion and heat stroke. A heat exhaustive person usually is profusely sweating, only has a slightly elevated body temperature (49) and their mental change is minimal comparatively (63). Heat stroke can present the complete opposite signs. Heat stroke can happen quickly after heat exhaustion if treatment is not started immediately (12). Heat Stroke Heat stroke is the most severe heat illness because of its extreme hyperthermia; it is considered a medical emergency with the possibility of fatality. There are two types of heat stroke that have been explained. Classical heat stroke is a common disorder in the elderly, children, and people with chronic diseases during heat waves (66,63). Within this classical stroke, hyperpyrexia, anhidrosis, and mentality changes are common. Its occurrence is purely the result of exogenous heat loads (63). Exertional heat stroke, which we will discuss in this chapter, occurs sporadically and the onset is sudden (66). Exogenous and endogenous heat loads cause it, which predisposes young active individuals (63,66). The excessive elevation in body temperatures can cause damage to tissues and organs in the body (4). The longer the athlete tries to continue exercising and handle the rise in temperature, the higher the temperature can rise before collapse (37). Levels as high as 40.6 C have been seen in conscious athletes and 43 C in unconscious (4). Temperatures above 41 C can cause disturbances in the nervous system and cause hot

28 19 dry skin. Sweat glands can still be active though and sweating can been seen during evaluation of a collapsed person (66). People susceptible to heat stroke have skin diseases, sunburns, are dehydrated, alcohol and drug abuse, are obese, sleep deprived, poorly physically fit, insufficiently heat acclimatized, elderly, and have previous history (4). Heat production of 1,033 Kcal.h -1 can cause a heat stroke within 15 minutes (37). In an active person, energy metabolism increases 10 times. In well-trained athletes, it can reach 20 times, of which 80% is released as heat (72). Metabolic requirements and the attempt of the muscle and skin cooling can be overwhelmed by temperature, humidity, and other environmental conditions. In turn the ability to dissipate heat is overwhelmed (12). The body tries to maintain blood pressure, which is the main priority, and as result core temperature is increased. Heat stroke can occur suddenly with no other signs of heat distress (49) or dehydration (12). Common signs are collapse, unconsciousness, stupor, delirium, irritability and aggressiveness if conscious, the skin is hot and dry, and a significant increase in body temperature is noted (49). A decrease in blood pressure, vomiting, coma, convulsions (12), fecal incontinence, flaccidity, and hemiplegia can also be seen (66). Central nervous system disturbances are evident in all cases of heat stroke because the brain is highly sensitive. Cerebella problems can cause ataxia and dysarthria. Pupillary changes are common such as constriction. Cardiovascularly, sinus tachycardia is common because of the circulatory demands. ECG s will show transient disturbances with a flat or inverted T wave. If hypoperfusion results, shock may happen (66). Shock and delirium have been seen in temperatures ranging from C (25).

29 20 Hyperventilation is a sign of pulmonary problems and can result in respiratory alkalosis. Decreased blood flow, dehydration, and vasodilatation can all cause kidney tissue damage or clotting, renal failure is seen in about 25% of severe cases (66). If 90% of the body s total body fluids are depleted, the brain shuts downs and sweating slows in an attempt to stop the loss. Once the sweating has stopped, body temperature has the ability to reach 41 C in twenty minutes. A temperature over 41 C for even a few minutes can cause irreversible damage to the liver, kidney, and brain cells (49). Some causes of misdiagnoses include if core body temperature is not measured properly because of a delay in measurement or unreliable measurements such as a falselow reading from an oral temperature. Lack of history of events leading to the collapse, mild environmental conditions, and presence of sweating also can cause misdiagnosis. A coma that persists even after cooling unfortunately is a sign of poor prognosis, yet people with comas longer than twenty four hours with seizures have recovered with no central nervous problems before (66). Predisposing Factors Heat intolerant people have been described as individuals unable to sustain heat and whose body temperature will start rising earlier and at a higher rate than the normal population. The main reasons behind this can be non- acclimatization, sedentary life or low fitness, being overweight, and age. Dehydration, lack of sleep, pyrexial illness, cardiovascular dysfunction, dystrophy of skin appendages, and drug abuse are other reasons. Ethnic origin has no influence and gender influences have minimal support. Women may have a better ability to deal with humidity, where as men cope better in hot dry conditions (26).

30 21 Anhidrosis is the inability of the body to produce or deliver sweat to the skin; this dysfunction can be because of the sweat gland itself or its innervation. Heat rashes can be a sign of intolerance. It is caused by the obstruction of the sweat gland ducts by kerotin debris. It can be resolved within eight to twenty one days by repeated exposures to heat, which decreases sodium secretion and also repetitive showers. People who are not physically fit have a lower tolerance of heat than people who are athletically conditioned. This is not a direct result of VO2max, but instead the physiological adaptations that come with being physically in shape, such as an increase in cardiovascular function, blood volume, and sweating response. These adaptations result in efficient heat dissipation. Work efficiency is the ratio of external or mechanical work to metabolic rate. Therefore, if a person has a higher work efficiency, the lower the metabolic rate and heat production, which in turn helps the person better tolerate heat (26). Dehydration is a factor affecting intolerance. It decreases sweat efficiency, peripheral circulation, and heat transfer from the core to the skin. Infectious diseases can also be a factor, causing chemotaxis of prostaglandins, changing the body s set point temperature to shift slightly higher. The disease itself can have lasting effects on heat tolerance from eight to twelve weeks (26). Morphology has an effect on intolerance especially in reference to obese people. They have a decreased tolerance of heat and higher heat production during exercise because of the increased energy needs due to body weight (26,31). Their heart rate is increased during rest and activity compared to someone leaner causing the cardiovascular systems to begin already compromised, and they have a lower surface area to body mass ratio which can decrease heat dissipation. People with labeled body types as mesomorphy have an inverse relationship between

31 22 sweat gland density and percent body fat, so a given heat load will increase tissue temperature more in obese people (26). An epidemiological study by the Marine Corps found that recruits with Body Mass Index s greater than 22 kg.m -2 were at higher risk for heat illness (31). Drug abuse of a wide range of drugs can effect tolerance of heat. Examples of drugs are beta-blockers, diuretics, antihistamines, tricylic antidepressants, vasoconstrictors, amphetamines, alcohol, cocaine, LSD, and opiates. Previous history of heat stroke can slightly increase a persons risk for repeat incidence, yet evidence suggests it would be caused by a preexisting condition (26). A long term follow up between six months and four years of Marines who experienced heat stroke at a recruitment depot between the years of was done. Compared to Marines who had no incidence of heat illness a non significant difference was indicated. A slightly lower incidence of hospitalization rates or heat illness was actually seen (57). Acclimatization When the body makes physiological and psychological (72) adaptation to decrease the stress of a new natural environment, the term is known as acclimatization. Acclimation is the same changes, only within a controlled environment. With acclimatization there is a decrease of thermal and cardiovascular strain. As a result heart rate and rectal temperatures drop (12). Sweat rate increases along with an earlier activation of sweating. Work time till exhaustion is significantly increased in acclimatization. Improvement in muscle glycogen, increase in cutaneous blood flow, and red cell filterability have also been suggested as adaptations (72). Decreased heart rate, increased plasma volume, and perceived exertion changes within three to six days, rectal temperatures and electrolyte concentrations within six to nine days, and increased

32 23 sweating response in 14 days (12). Physical training contributes 50% of the total adjustment and 75% of these total changes are usually within five days (26), a period between seven to twelve days can be sufficient (12). Chronic dehydration can delay acclimatization. Nadel did a study looking at acclimatization with ten days of exercise acclimatization and then ten days of heat acclimatization in 45 C with 16Torr and 36 C at 35 Torr. Both exercise and heat acclimatization enhanced sweating. With heat acclimatization, the people were also better able to dissipate heat (51). Prevention There are many ways to prevent events leading up to heat illness. Having a very thorough pre-participation and environmental questionnaire can provide useful information on possible susceptible individuals (12,26,14). Motivating athletes to start conditioning before practices can help early acclimatization (12). Encouraging healthy habits such as decreasing life stresses, sleeping well, avoiding rapid weight loss, avoiding people who are sick, and staying well hydrated can help prevent heat illness. Also, putting an emphasis on eating a nutritionally balanced diet on a regular basis and especially twenty four hours and immediately before any exercise event can have lasting effects (4). Having mandatory weigh ins before and after practice can help monitor hydration and susceptible people (14). Monitoring environmental conditions each day before practice using a psychrometer should be mandatory. Evaluating Wet Bulb Globe Temperature information on air temperature, relative humidity and solar radiation (71) and then following guidelines to decide status of practice can also be beneficial in assuring the safety of athletes.

33 24 Establishing preset rehydration procedures and protocols to balance sweat loss and the negative effects on the thermoregulatory system is extremely important (12). Making protocols specific to the sport is necessary to the type of exercise and the breaks available (44). Providing ample supply of beverages such as water and sports drinks in individual containers, for accessibility, is useful (14) during numerous short breaks or rest periods (12,14). Use of the shade during breaks can help decrease solar radiation heat gain (44). Change practice times to avoid the hottest hours of the day (71,44) or cancel in extreme conditions. Have cooling supplies easily available for breaks or emergencies (12). A preset plan of action in case of an emergency (12), and proper medical coverage at all times should be determined in advance (14). Education of athletes, coaches, and support staff on the signs and symptoms of heat illness and dehydration can be the biggest course of action (14,44). Encourage athletes to speak up if they are not feeling well to stop symptoms early (44). Hydration Historically fluid restriction was common practice, drinking was a sign of weakness. Today, research and obvious benefits support the importance of plenty of fluid intake. The human body is comprised of 60% water. Lean body mass alone is made up of 72% water (38,41). Water maintains homeostasis and equilibrium inside and between cells and blood volume (38). A disturbance in body water levels can effect cellular and systemic function and decrease exercise tolerance (4). Adequate blood volume is needed for delivery of oxygen and nutrients to muscle and transport heat from the working muscles to the skin (41). Euhydration is when the body has normal stores of body water. Dehydration is the dynamic process of reduction from these normal stores to hypohydration, a body fluid deficit (38,41). The rate of fluid loss can be influenced by

34 25 exercise intensity, individual differences, environmental conditions, acclimatization state, clothing, and baseline hydration status (44). Dehydration Hypohydration has been known to increase core temperature, decrease exercise tolerance in the heat, decrease blood flow to the skin, and increase cardiovascular strain. Cardiovascular strain is due to a decrease in blood volume, in cardiac filling, and in stroke volume, so the heart can not meet the demands of exercise (64,38,4). In high ambient temperatures, if blood volume is decreased due to hypohydration or by high sweat volume loss, oxygen and substrates can not be carried at the high rate needed to supply exercise and also to the skin to aid in heat dissipation through convection (69). Hyperthermia, reduction in energy stores, accumulation of metabolic by-products, alterations in muscle fiber recruitment and disturbances in the central nervous system all are results of dehydration and ultimately effect exercise performance (38). Dehydration effects intracellular and extracellular compartments (4) which can change plasma osmotic pressure and sodium concentrations causing a decrease in sweat rate. Dehydration of 1-2% can impair heat dissipation (4) causing an elevation in core temperature. The thirst mechanism does not begin until a 2% loss (38). Dehydration greater than 3%, magnifies the effects and increases the risk of heat illness (14). At 4% dehydration, rehydration and the rate of replenishing the stores becomes more difficult, as well with gastrointestinal discomfort (14). At the extreme levels of 10-15%, mortality is significant. Nielson et al. showed a 45% decrease in exercise performance with a 2.5% loss of fluids (70). Common signs of dehydration are thirst, discomfort, complaining, flushed skin, weariness, apathy, and cramps (14).