Thermoregulatory Adaptations Associated with Training and Heat Acclimation

FLUIDS AND ELECTROLYTES IN ATHLETIC HORSES

0749-0739/98 $8.00 + .00

THERMOREGULATORY ADAPTATIONS ASSOCIATED WITH TRAINING AND HEAT ACCLIMATION Raymond J. Geor, BVSc, MVSc, and L. Jill McCutcheon, DVM, PhD

In both human and equine athletes, it is well established that exces­ sive hyperthermia has a deleterious effect on exercise performance.18• 29 Furthermore, life-threatening metabolic and central nervous system dis­ turbances can develop as a result of severe exercise-induced hyperther­ mia (heat exhaustion). The risk for development of heat exhaustion and heat stroke is greatest during exercise in hot and humid ambient condi­ tions when the rate of heat dissipation may be inadequate to prevent an excessive rise in body temperature. 12 Horses that are inadequately conditioned for the required athletic task or those suffering from an impairment of the thermoregulatory system (e.g., anhidrosis) also are at risk for development of heat exhaustion.18 Despite recognition of the potentially deleterious effects of exercis­ ing in very hot weather, it is increasingly common that equine athletes are required to compete in warmer climates. In this context, concern for the safety of horses competing in Atlanta, Georgia during the 1996 Summer Olympic Games provided impetus for a number of investiga­ tions designed to promote further understanding of thermoregulatory mechanisms in the horse, particularly during exercise in adverse envi­ ronmental conditions. One of the key questions addressed by these

From the Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio (RJG); and the Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada (LJM)

V ETERINARY CLINICS OF NORTH AMERICA: EQUINE PRACTICE VOLUME 14 • NUMBER 1 • APRIL 1998

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studies was whether regular exposure of horses to hot, humid conditions results in physiologic adaptations conferring improved thermoregula­ tory ability (heat acclimation). The following article will discuss our current understanding of the thermoregulatory adaptations in the horse associated with exercise training and heat acclimation and will present management strategies for ameliorating the impact of hot environments on the health and performance of equine athletes. It must be emphasized that there have been very few studies of the adaptive responses in the thermoregulatory mechanisms of the horse to exercise training and heat acclimation. There­ fore, data from other species, particularly human subjects, will be pre­ sented to highlight the nature of the adaptive responses associated with exercise training and heat acclimation. Nevertheless, the reader should be aware that direct application of data across species is not always appropriate and that clarification of many issues concerning the nature and magnitude of thermoregulatory adaptive responses in the equine species must await further investigation. THERMOREGULATION DURING EXERCISE: A BRIEF REVIEW In homeotherms, internal body temperature is maintained within a narrow range by integrated neurophysiologic mechanisms regulating the balance between metabolic heat production and heat dissipation.41• 55 Heat is produced within contracting skeletal muscle, because the process of conversion of metabolic energy to mechanical work is only 20% to 25% efficient such that 75% to 80% of the chemical energy is converted to heat within the muscle cells.18• 21 During exercise, the rate of heat production is primarily a function of running speed, although the energy requirement for work is also affected by factors such as terrain, footing, and the weight of the rider and tack. The total heat produced is the product of running speed and duration of exercise at that intensity. 21 The thermoregulatory system, operating through the autonomic nervous system, controls various routes of heat flow in order to effect heat loss from the body. Coordination of this response is provided by the temperature regulating center in the hypothalamus.55 The four basic mechanisms of heat flow are radiation (energy-emitted or absorbed at the body surface), convection (heat transfer between two media of differ­ ent temperatures), conduction (direct transfer between surfaces in con­ tact), and evaporation (conversion of a liquid to a vapor with the cooling effect at the surface at which the change of state occurs).41• 55 During exercise, the primary physiologic mechanisms driving heat loss are an increase in the proportion of cardiac output directed toward the cutane­ ous circulation and an increase in the rate of sweat secretion.18• 41 The increase in cardiac output and blood flow to contracting muscle enables a substantial increase in convective heat transfer away from the muscle.

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The circulation carries the heat to the body core, resulting in an increase in core temperature. Increasing core temperature and, to a lesser extent, increasing skin temperature provides the afferent signal for reflex in­ creases in skin blood flow and sweating, thereby facilitating heat transfer to the skin surface and its dissipation to the environment.41, 55 Increased blood flow through the vascular beds of the skin allows heat to be lost to the environment via convection and direct radiation of heat from the skin surface. The efficacy of transfer of heat by convection and radiation varies according to the rate of air movement across the skin (wind speed) and the gradient of skin temperature to environmental temperature (see below).41 The principal mechanism of heat loss in the horse during exercise is evaporation, primarily in the form of sweating. Sweating rates of 35 to 50 mL/m- 2/min- 1 have been measured on the necks and backs of horses exercising on a treadmill in a laboratory. 19, 34, 36 Assuming a body surface area of 4.5 to 5.0 m2 for a 500-kg horse,19 these sweating rates correspond to fluid losses of 10 to 15 L/h. This estimate of hourly sweat fluid loss is in agreement with sweat rates calculated on the basis of the decrease in body mass during prolonged exercise under field conditions. 5 When expressed in terms of sweating rate per unit area of skin, these values are two- to threefold greater than those reported for human subjects. Insensible evaporation of water across the respiratory tract is another important mechanism for heat loss. Although there are limited data on the extent of heat loss through the respiratory tract, it has been estimated that 20% to 30% of the metabolic heat load may be dissipated via this route during low- to moderate-intensity exercise.1s, 19 At any given point in time during exercise, core body temperature (e.g., at the level of the right atrium) reflects the balance between heat production and dissipation. Soon after the onset of exercise, the rate of heat production greatly exceeds the rate of heat dissipation such that there is a rapid increase in muscle temperature.41, 55 The heat produced within the working muscle is distributed throughout the body by the blood that has perfused the muscle. During short-term, high-intensity work (e.g., racing), the rate of heat production will greatly exceed the rate of heat loss throughout exercise and body temperature will continue to increase until the cessation of exercise. 1s, 19 In this circumstance, a large proportion of the metabolic heat load will be dissipated during the recovery period. Conversely, during more prolonged low- to moderate­ intensity exercise, activation of heat dissipatory mechanisms progres­ sively attenuates the rate of rise of body temperature. Eventually, the rate of heat loss increases sufficiently to balance metabolic heat production, allowing a near steady-state core temperature to be achieved.41 When compared with human subjects, horses have mass-specific relative metabolic capacities that are 50% to 100% greater such that at any given work intensity, the rate of muscular heat production is higher in the horse. Furthermore, the surface area to body mass ratio is approxi­ mately 50% less than that in man. 1s, 21 Therefore, the horse has a smaller

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surface area over which to dissipate a relatively larger metabolic heat load. Although these disadvantages may be partially offset by higher rates of cutaneous and respiratory evaporative heat loss, it is apparent that exercise presents a considerable challenge to the thermoregulatory system of the horse, particularly during prolonged exercise. There are a number of circumstances in which the thermoregulatory system may be overwhelmed such that the horse is at risk for develop­ ment of life-threatening hyperthermia. The greatest demands on the thermoregulatory system occur during prolonged exercise in hot and, particularly, hot and humid ambient conditions.12• 18 Any impairment to the horse's thermoregulatory mechanisms (e.g., anhidrosis, dehydration) may result in an excessive rise in body temperature during exercise.12• 18 It is important to emphasize that these risk factors are additive with respect to the likelihood for development of severe heat stress. For example, dehydration associated with a high rate of sweat fluid loss during exercise in hot ambient conditions will further impair heat dissi­ pation and exacerbate the increase in body temperature.29• 43 The effectiveness of heat loss mechanisms is profoundly influenced by ambient temperature and humidity.12• 15• 18• 41 As ambient temperature rises, the thermal gradient between skin and the external environment is diminished, and sensible heat loss (i.e., convective and radiative heat transfer) is impaired. When ambient temperature exceeds skin temperature (>35°C), the gradient for heat transfer is reversed and the body gains heat from the external environment.41• 55 If humidity is low, a decrease in sensible heat loss can be offset by an increase in sweating rate and evaporative heat loss. As humidity rises, the gradient between skin and ambient dew point is reduced and evaporative heat loss is impaired. The decrease in sweat evaporation is manifested by excessive wetting of the skin surface and drippage of sweat from the body, 12, 36 Sweat that drips from the body only removes 5% to 10% of the heat that can be dissipated by evaporation of sweat.27 Therefore, during exercise under ambient conditions of high heat and humidity, the rate of heat dissipation may be inadequate to prevent a progressive rise in body temperature. The important consequence of this impairment to heat dissipation is that the time to attainment of the horse's critical core temperature, a point at which exercise must be discontinued to ensure the horse's safety, occurs sooner.12 The impact of the environment on the rate of rise of core body temperature was demonstrated in a recent study in which horses completed a standardized exercise test under each of three environmental conditions: cool, dry (room temperature [T] = 20°C; relative humidity [RH] = 45%-55%); hot, dry (T = 32°-34°C, RH = 45%-55%); and hot, humid (T = 32°-34°C, RH = 80%-85%).15 Horses were exercised at a workload equal to 50% of their maximum oxygen consumption (Vo2max) until attainment of a pulmonary artery blood temperature of 41.5°C. In the hot, humid condition, the rate of rise of pulmonary artery blood temperature was more than twofold higher when compared with that of the cool, dry condition (Fig. 1).

THERMOREGULATORY ADAPTATIONS

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Figure 1. Blood temperature of five horses during exercise at 50% of maximal oxygen consumption in cool, dry (room temperature [Tl � 20° C; relative humidity [RH] = 45% to 55%), hot, dry (T = 32° to 34° C, RH = 45% to 55%), and hot, humid (T = 32° to 34° C, RH = 80% to 85%) ambient conditions. Exercise was discontinued when pulmonary artery blood temperature reached 41.5° C. Note greater rate of rise in blood temperature during exercise in hot, dry and in particular hot, humid conditions when compared with cool, dry ambient conditions. (Adapted from Gear RJ, Mccutcheon LJ, Ecker GL, et al: Thermal and cardiorespiratory responses of horses to submaximal exercise under hot and humid conditions. Equine Vet J Suppl 20:125, 1995; with permission.)

THERMOREGULATORY ADAPTATIONS ASSOCIATED WITH TRAINING AND HEAT ACCLIMATION Methodologic Considerations

There are several important methodologic considerations that affect interpretation of results from studies examining thermoregulatory adap­ tations during training and heat acclimation. First, it is necessary to define carefully the environmental and exercise (duration and intensity) conditions used by researchers.2, 54 The extent of the "heat stress" will depend not only on the ambient conditions (external heat load) but also on the duration and intensity of exercise performed under a particular set of environmental conditions (internal heat load). In most investiga­ tions, exercise tests have been designed to mimic the demands of specific athletic endeavors (e.g., endurance exercise, speed and endurance test of a 3-day event) or environmental conditions (cool versus hot ambient temperatures). The advantage of such specificity is that tests modeling real-world conditions are more likely to be useful in predicting physio­ logic responses under field conditions than tests that do not. However, wide differences in experimental design often limit direct comparison of results.

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The nature and magnitude of the adaptive responses also are influ­ enced by the duration of daily heat exposure. Many conclusions regard­ ing physiologic changes have been drawn from studies performed in climatic chambers, in which there is a restricted period of daily heat exposure (usually a period ranging from 2 to 4 hours). The adaptive response pattern demonstrated under these circumstances can differ from that occurring during a period of uninterrupted heat exposure as would occur when an individual moves from a cool to a hot climate.64 In this context, the terms acclimatization and acclimation are often used interchangeably to refer to the adaptive changes that occur when an individual undergoes prolonged or repeated exposure to a stressful environment and changes that reduce the physiologic strain produced by such an environment. As defined by the International Union of Physiological Sciences, however, acclimatization refers to the adaptive responses produced by a change in the natural environment, whereas acclimation is defined as the adaptations produced in controlled labora­ tory conditions. 1• 54 The reader should be aware that the physiologic adaptations resulting from a period of laboratory heat acclimation are not necessarily the same as those responses occurring during a similar period of heat acclimatization. The acclimation responses to repeated dry heat exposure can differ from adaptive responses to humid heat exposure.44-46, 54 In human subjects, increases in sweating rate and evaporative heat loss are key factors in the improvement of thermoregulatory capacity and exercise performance after acclimation to dry heat.45• 54 In contrast, although similar physiologic adaptations occur in response to humid heat, the biophysical limitations to sweat evaporation in a hot, humid environ­ ment are such that only small improvements in exercise performance are measured after a similar period of acclimation.44 Interpretation of heat acclimation responses can also be confounded by alterations in the basal level of cardiorespiratory and metabolic fitness occurring during the protocol.1· 2• 16• 17• 54 In general, physical training in a cool environment improves physiologic responses during exercise at high ambient temperatures (see below). Thus, the physiologic responses of subjects who are only partially conditioned for the exercise to be undertaken during a heat acclimation protocol may be greater than those in thoroughly trained subjects. Conversely, high-intensity training of 8 to 12 weeks' duration appears to hasten the process of heat acclimation in human subjects.2• 16 Therefore, the duration and intensity of exercise training undertaken prior to a heat acclimation protocol must be consid­ ered when interpreting heat adaptive responses. PHYSICAL TRAINING AND EXERCISE-HEAT TOLERANCE In human athletes, it is generally well accepted that physical train­ ing in a cool environment improves physiologic responses during exer-

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cise at high ambient temperatures (exercise-heat tolerance) and speeds the process of heat acclimation.1· 2· 16· 17· 54 Moreover, highly trained subjects retain the benefits of heat acclimation longer when compared with poorly trained individuals.1· 54 It should be noted that in the context of this discussion, exercise-heat tolerance refers to the ability to with­ stand the stress of exercise in the heat rather than to the capacity to sustain higher core body temperatures during exercise. In other words, the principal benefit of exercise training is enhancement of thermoregu­ latory mechanisms such that the rate of rise in body temperature is attenuated and the time until attainment of critical upper limits in core temperature is delayed.2· 16 Studies in human and animal subjects have clearly demonstrated that fatigue during prolonged exercise in the heat occurs at a similar core body temperature, regardless of the state of training (or heat acclimation).2· 43· 45 The important clinical implication is that highly trained subjects are not more resistant to thermal injury than their untrained counterparts.16 The extent to which training improves exercise-heat tolerance is dependent on the magnitude of stimulation of heat dissipatory effector mechanisms and the duration of the traij:ling stimulus.2 At low to moder­ ate training intensities (30%-40% Vo2max), there is minimal improve­ ment in thermoregulatory capacity. 2· 16 Under these circumstances, core body temperature is not maintained at a sufficiently elevated level for long enough to modify the mechanisms responsible for improved cardiovascular stability and sweating efficiency. In contrast, higher inten­ sity exercise that elevates core temperature by 1.5° to 2.0°C for 30 to 60 min/ d significantly improves exercise-heat tolerance.2· 16· 17 The duration of training also is important for optimization of physiologic adaptation. Whereas detectable improvements in thermoregulatory mechanisms are present subsequent to 1 to 2 weeks of training in human subjects, optimal enhancement occurs after 8 to 12 weeks of regular training.2· 16 The primary thermoregulatory adaptations to physical training in a cool environment include increased plasma volume, increased stroke volume, a lowered threshold temperature for the onset of sweating, and decreased rectal temperature in response to a given level of submaximal exercise.2· 16 Although these adaptations are qualitatively the same as those occurring during heat acclimation (see discussion below), the magnitude of the physiologic responses under each of these two circum­ stances will reflect the extent of the thermal stimulus driving the adapta­ tions.2· 16· 54 Therefore, when compared with a period of training in cool to moderate conditions, the larger thermal stimulus associated with repeated exercise in hot ambient conditions would be expected to invoke proportionally greater thermoregulatory adaptations. Few studies have specifically addressed the issue of thermoregula­ tion and exercise training in horses. Nevertheless, several studies have assessed training-induced alterations in resting plasma volume and car­ diovascular function during exercise.3· 26· 38· 47· 5D--52 Given the important role of the cardiovascular system for heat dispersal, training-induced improvements in cardiovascular stability during exercise would be ex-

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pected to aid in temperature regulation. Indirect evidence for training­ induced alterations in thermoregulatory function also has come from studies reporting attenuation of the increase in core body temperature during a standardized exercise test after a period of exercise training.3- 50 Cardiovascular Adjustments

It has been demonstrated that exercise training results in an expan­ sion of plasma volume in the resting horse. McKeever et al38 reported a 4.7-L, or 29% increase in resting plasma volume after only 2 weeks of very low-intensity (walking) exercise training. Knight and coworkers26 reported a slower and more modest increase (10%-12%) in plasma volume in horses undergoing a 6-week period of low- or high-intensity exercise training. As packed cell volume was unchanged throughout training in the latter study, these results imply a concurrent increase in red cell mass. More recently, Geor and McCutcheon 12 examined the thermoregulatory responses of horses to submaximal exercise in the heat (T = 33 °-35 °C, RH = 45%-55%) during an 8-week period of exercise training in moderate ambient conditions (T = 2O°-22 °C). This study demonstrated a 12% to 15% increase in the resting plasma volume of horses after 8 weeks of combined low- and high-intensity training.12 The specific mechanisms for this training-induced expansion in plasma volume have not been clearly elucidated.6- 7, 10 In both humans and animals, plasma osmolality and electrolyte concentrations remain unchanged in spite of the increase in plasma volume.6- 10 This implies that the mass of osmotic components within the vascular compartment is increased (i.e., protein and electrolyte contents). Therefore, water accu­ mulation in the vascular space might be due to intravascular retention of both proteins and electrolytes (mainly sodium).6- 7, 1 0 In particular, an increase in plasma albumin content may play a critical role in the training-induced expansion of plasma volume. The most likely source of additional proteins is de novo synthesis.40 Fluid-regulating hormones (aldosterone, arginine vasopressin, and atrial natriuretic peptide) also contribute to the training-induced hyper­ volemia.6- 10, 53 In human subjects, the plasma volume expansion is associ­ ated with progressive retention of sodium and water, particularly during the initial 6 to 10 days of exercise training.8- 1 0- 56 This response is attribut­ able, in part, to repeated elevations in plasma renin activity, arginine vasopressin, and aldosterone. In a recent study in human subjects, plasma volume expansion was significantly attenuated by administra­ tion of spironolactone (a competitive antagonist of aldosterone) during a 2-week period of exercise training. In the study by McKeever et al,38 changes in resting plasma aldosterone and arginine vasopressin concentrations or in renal conservation of sodium were not detected. Nevertheless, although the rate of daily water intake did not change during the training period, 24-hour urine output and urea, potassium, and osmotic clearance were significantly decreased. Furthermore, the

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infrequency of measurements (1-week intervals) may have precluded detection of changes in sodium excretion during the first days of train­ ing. Therefore, it is probable that in horses as in man, activation of fluid-regulatory mechanisms resulting in retention of sodium and water contributes to the training-induced hypervolemia.37 As mentioned previously, the cardiovascular system must simulta­ neously provide sufficient blood flow to working skeletal muscle to support metabolism and sufficient skin blood flow to dissipate the heat released from the contracting muscle.48 During exercise in the heat, the large increase in skin (thermoregulatory) blood flow displaces blood volume from the central circulation, thus reducing cardiac filling and stroke volume and requiring a higher heart rate to maintain cardiac output.41• 48 Furthermore, the extent of cardiovascular strain can be com­ pounded by a decrease in plasma volume. The decrease in plasma volume occurs as a result of fluid movement between the plasma com­ partment and the tissues and loss of extracellular fluid by sweating.40• 41 Therefore, a training-induced expansion of plasma volume may be an important mechanism for improved cardiovascular stability and thermo­ regulatory capacity (increase in skin blood flow) during exercise, The increase in venous return associated with 'an expanded plasma volume increases ventricular filling pressure and stroke volume.48 Some50• 51 but not all3• 52 studies have demonstrated modest (10%-12%) increases in stroke volume in horses during submaximal exercise after low-intensity training, Cardiac output during exercise may be unchanged, however, reflecting a decrease in heart rate during exercise at any given submaxi­ mal workload subsequent to training, Nonetheless, training-induced increases in plasma volume and exercise stroke volume may allow the equine athlete to maintain cardiac output during prolonged exercise, particularly in warm ambient conditions. Consistent with this hypothe­ sis, Gear and McCutcheon11 recently demonstrated increased stroke vol­ ume and cardiac output in horses during submaximal exercise in the heat after 8 weeks of exercise training, As these cardiovascular adapta­ tions were accompanied by significant decreases in muscle and core body temperatures in response to exercise, 11 it is reasonable to speculate that the increase in stroke volume and cardiac output provides an improved capacity to transfer heat from the body core to the periphery. Sweating Responses

In human subjects, exercise training results in an increase in sweat­ ing sensitivity (i.e., the increase in sweating rate per unit increase in core body temperature) and a reduction in the core temperature threshold for the onset of sweating.2• 4• 16 Whereas the sweat gland has increased sensitivity to central drive (change in body temperature) and increased secretory capacity, total body sweat loss decreases after even short peri­ ods of physical training.2• 4 There are several possible explanations for this decrease in total body sweat loss. First, when compared with the

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untrained state, a lower core body temperature is maintained during exercise in the trained state.2• 16 Therefore, the central drive for sweat production is decreased. Potential mechanisms for the maintenance of a lower core temperature include a decrease in metabolic heat production and increased heat transfer to the periphery via improvements in cardio­ vascular function. Second, there may be improvements in "sweating economy," wherein there is an increase in total cutaneous evaporative heat loss, despite a decrease in total body sweating.58 In part, this improvement in the efficiency of sweating reflects alterations in the regional distribution of sweating. In some areas of the body, there is a decrease in local sweating rate, whereas sweat output increases in other regions with recruitment of previously inactive glands.2• 58 The net result is a more even distribution of sweat secretion over the body's surface and a decrease in "wastage" (i.e., sweat that drips from the body before evaporation). The decrease in sweat loss has important implications for maintenance of body fluid volumes and central cardiovascular function during prolonged exercise.2• 49 • 54 There also is evidence for alteration in the sweating responses of horses during conditioning in a temperate environment. McCutcheon and Geor 32 measured sweating rate, sweat composition, the relationship between pulmonary blood temperature and sweating rate, and total sweat fluid loss in horses during exercise in hot conditions before, during, and after an 8-week period of exercise training. Similar to the findings in human subjects, training resulted in greater sweat sensitivity but a decrease in total sweat fluid and ion (particularly sodium) losses.32 The decreased sodium loss in sweat may have contributed to the ob­ served improvement in maintenance of plasma volume and cardiac output during the latter stages of exercise (R.J. Geor, BVSc, MVSc, and L.J. McCutcheon, DVM, PhD, unpublished observations, 1996). HEAT ACCLIMATION

As previously mentioned, in hot compared with cool environments, exercise capacity is reduced. Nevertheless, numerous studies in human subjects have demonstrated that the negative effects of high ambient temperature can be substantially reduced by a period of acclimatization (for review, see references 1 and 54). Regular exposure to hot conditions results in a number of physiologic and biochemical adaptations that improve an individual's ability to thermoregulate and perform in these conditions. 1 • 1 6• 54 Even heat exposure without exercise (passive heat acclimation) will induce physiologic adaptations that confer an improved ability to dissi­ pate heat. 1 • 54 In the natural environment, some degree of acclimatization occurs during the change from winter to summer. Furthermore, as noted above, exercise training in a cool, dry environment will induce adapta­ tions that improve physiologic responses to exercise at high ambient temperatures. Regular exercise in the heat (active heat acclimation) is

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necessary for maximum heat acclimation effects, however.11, 16, 54 The magnitude of the adaptation to heat that occurs is closely related to the degree of heat stress to which the individual is exposed. In this context, it is important to emphasize that at least in hot, dry environments, the degree of heat acclimation is directly related to the specific set of environmental and exercise conditions to which the athlete has been exposed. In other words, rather than being an absolute state, there are relative degrees of heat acclimation.16, 54 The physiologic changes occurring during heat acclimation have been extensively studied in human subjects.9, 16, 39, 42, 44--45, 57 The process of acclimation begins within a few (3-5) days of regular exposure to and exercise in the heat, with most adaptations complete within a 14-day period. The most notable changes are an increase in plasma volume, a reduction in heart rate and core body temperature during exercise, an increase in sweating rate and initiation of sweating at a lower body temperature, and redistribution of cardiac output such that there is an increase in blood flow to capillary beds of the skin (Table l).1, 54 In general, the cardiovascular adaptations are complete during the first week of acclimation, whereas alterations in sweating responses require 10 to 14 days of repeated heat exposure.1 Heat acclimation also may result in an improved efficiency of biochemical energy transformation in contracting muscles, thereby attenuating heat production in the accli­ mated versus unacclimated state.22 To date, there have been few detailed investigations into the nature and magnitude of adaptive responses in horses during a period of repeated heat exposure. In July 1994, Marlin and coworkers conducted a study in which eight horses were transported from the moderate climate of Northern Europe to Atlanta, Georgia, where the summer ambient conditions are typically hot and humid (daytime T = 28°-35°C, RH = 50%-75%). These horses undertook daily exercise training over a 3-week period in preparation for a one-star 3-day event competition. At the time of writing, the results of this investigation have not been

Table 1 . PHYSIOLOG IC ADAPTATIONS TO DAILY EXERCISE IN TH E H EAT Physiologic Response Cardiovascular Reduction in heart rate at same exercise intensity I ncrease in plasma volume I ncreased cardiac output I ncreased blood flow to capillary beds in skin Sweating responses Increase in sweating rate I n itiation of sweating at a lower core body temperatu re Core body temperatu re Reduction in core body temperature at same exercise intensity (slower rate of rise of core body temperatu re)

Time Course for Adaptation to Occur

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reported. Nevertheless, all horses successfully completed the one-star 3day event at the end of the period of training in hot, humid conditions. 25 More recently, laboratory heat acclimation studies have been per­ formed at the University of Guelph in Ontario, Canada and at the Animal Health Trust in New Market, England.27 In the former study, horses were exposed to hot, humid conditions (T = 33°-35°C, RH = 80%-85%) for 4 h/d over a 21-day period. During the daily heat expo­ sure, horses completed 1 hour of light to moderate exercise (30%-65% Vo2max). At regular intervals during the 21-day study, horses undertook a standardized submaximal (50% Vo2max) exercise test during which measurements were made of plasma volume and ion contents, heart rate, core body temperature, sweating rate, and sweat composition. At the Animal Health Trust, Marlin and colleagues27 exercised (moderate­ to high-intensity exercise) horses in hot, humid conditions (T = 30°C, RH = 80%) for 40 min/d over a 14-day period. Before and after the 14day protocol, horses undertook a simulated speed and endurance phase of a 3-day event. It is important to note that in each of these studies, the horses had undergone a period of exercise training (3 months or more) prior to commencement of active heat acclimation. Furthermore, in the study at the University of Guelph the basal level of fitness as indicated by Vo2max was not different before and after the heat acclimation protocol.13 Although not all data from these studies have been reported, sufficient information is available to provide an indication of the extent to which horses adapt to exercise under these hot and humid ambient conditions. A further study by Hyyppa and coworkers20 examined the effects of repeated training sessions in hot and humid conditions (T = 28°C, RH = 55%-60%) on the restoration of water and electrolyte balance in horses after exercise. The exercise sessions in the heat were separated by 2 weeks, however, with horses training in cool ambient conditions (T = 7°-l2°C) during the interim periods. As discussed later, at least in human subjects, many of the benefits of short-term heat acclimation are lost after 7 days without heat exposure.1, 54 Similar to the previous discussion regarding physiologic adaptations associated with exercise training, heat acclimation responses can be subdivided into two major components: cardiovascular, fluid-regulatory adaptations that confer improved ability to maintain transfer of heat from production sites to the body's surface and peripheral heat dissipa­ tory mechanisms (primarily sweating) that enhance heat transfer from the body to the external environment, Cardiovascular Adjustments One of the early heat acclimation adaptations is expansion of resting plasma volume.1, 42, 54 In human subjects, the time course and amount of expansion are variable, however. In general, a 10% to 25% increase in plasma volume occurs within 1 week of initiation of heat acclimation

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and is highly correlated with a decrease in exercise heart rate.54 Despite the reduction in heart rate, cardiac output is maintained or even in­ creased when compared with the unacclimated state. This is because the expanded plasma volume contributes to an increase in stroke volume during exercise. 1• 54 As is the case in exercise training, the change in plasma volume is, in part, mediated by the addition of protein to the vascular space; however, the mechanism of this response has not been elucidated. Plasma volume expansion during heat acclimation apparently is a temporary response; most studies in human subjects have demonstrated that plasma volume returns to preacclimation levels after 8 to 14 days of repeated heat exposure. 1• 54 Even after plasma volume has returned towards "normal," however, there is an improved ability to regulate vascular volume during exercise such that cardiac output and skin blood flow are maintained.54 Qualitatively, similar changes in resting plasma volume have been observed in horses during heat acclimation. Lindinger and coworkers28 reported an approximately 7% to 10% increase in resting plasma volume during the first week of acclimation. Thereafter, plasma volume tended to decrease and was not \'.fifferent from control values at the end of the experimental period. Similar to the findings in human subjects, exercise heart rate decreased early in the heat acclimation period (Fig. 2). This lower heart rate response to exercise, which is attributed to reduced circulatory strain in human studies, was main­ tained throughout the remainder of the heat acclimation protocoL 13· 14 Another potentially important cardiovascular adjustment is a redis­ tribution of cardiac output, with an increase in skin blood flow in particular. 12• 57 Increasing the volume of blood in the skin circulation is the major mechanism for heat transfer from the body core to the periph­ ery. During acute heat exposure in human subjects, there is a large increase in skin blood flow, particularly to limb skin, which is an area that has a high surface area to body mass ratio and therefore offers the greatest potential for heat exchange.54 By use of the color-labeled microsphere technique, McConaghy and colleagues30 demonstrated a threefold increase in skin blood flow in resting, unacclimated ponies exposed to a hot environment (T = 41°C, RH = 34%). This adjustment was achieved by a twofold increase in cardiac output (increase in stroke volume and heart rate) with no reduction in blood flow to tissues not involved in heat dissipation. In human subjects, the relative proportion of cardiac output directed to the skin at a given core and skin tempera­ ture is higher after heat acclimation; this change parallels alterations in the core temperature threshold for sweating.42• 54 Nevertheless, actual skin blood flow may be unchanged, because core and skin temperatures during exercise are lower following acclimation.54 Unfortunately, there are no data on skin blood flow in horses during heat acclimation. Given the reliance on sweating for heat loss, however, it seems likely that in horses, as in human subjects, adjustments in the distribution of cardiac output and control of skin circulation are important components of the heat acclimation response.

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Walking recovery

5 10 15 20 25 30 0 5 10 15 20 25 30 35 Time (min)

Figure 2. Heart rate of six horses during a 35-m inute standardized exercise protocol and subsequent 30-minute walking recovery during 3 weeks of daily exercise training in hot, humid (T = 33 ° to 35 ° C , RH = 80% to 85%) ambient conditions. Heart rate responses of the same horses during the exercise protocol in cool, d ry (CD; T = 20° c , RH = 45% to 55%) ambient conditions also are plotted. Val ues during exercise on day one of training in the hot, humid conditions (H H 1 ) were significantly (P < 0.05) higher when compared with the other trials. W = walk ( 1 8% to 24% Vo2 max) , T = trot (45% to 50% of Vo2 max) , C = canter (60% to 65% Vo2 max) . (Adapted from Geor RJ , McCutcheon LJ , Lindinger M l : Adaptations t o daily exercise in hot and humid ambient conditions i n trained Thoroughbred horses. Equine Vet J Suppl 22:63, 1 996; with permission .)

Sweating Responses Sweating rate elevations following heat acclimation in human subjects have been consistently reported with exercise in both dry and humid heat.1• 39• 42• 44• 45 • 54 Alterations in sweating mechanisms are linked closely to the improved cardiovascular capacity enabling a larger heat transfer to the skin. Similar to the case in exercise training, there is an increase in sweating sensitivity and a decrease in the core temperature threshold for onset of sweating.42• 54 In hot, dry environments, in which exercise performance is limited by hyperthermia, this enhancement in the sweating response is one of the key factors for performance improve­ ment following acclimation.45 The increase in sweating rate together with a more even distribution of sweat over the body's surface results in a substantial increase in evaporative capacity. The increased rate of evaporative heat loss coupled with a redistribution of cardiac output ameliorates the rate of rise in core and peripheral body temperatures and therefore reduces circulatory strain.45 • 54 In humid heat, however, the rate of sweating exceeds that of evaporation even in the unacclimated state, and although whole-body sweat rate increases during repeated

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exposure to humid heat stress, the effect on thermoregulation and exer­ cise performance is negligible, because there is little or no change in evaporative heat loss.44 In horses, sweating rates greatly exceeded evaporative capacity throughout a 21-day period of humid heat exposure (as evidenced by profuse drippage of sweat from the body). 13, 35 In contrast to human subjects, however, whole-body sweat loss was attenuated with repeated heat exposure. This decrease in sweat loss was due to lower sweating rates during exercise and, in particular, to a more rapid abatement of sweating during the recovery period following exercise.35 The mecha­ nisms responsible for this altered sweating response are unknown. As the rate of rise of pulmonary artery blood temperature during exercise was unchanged throughout the 21-day protocol, 1 4 the decrease in sweat­ ing rate may represent a decreased central drive for sweating, perhaps in association with mechanisms for fluid and ion conservation (see below). In this context, it is important to emphasize the wasteful over­ production of sweat in these humid conditions. Given the limited evapo­ rative capacity of the environment, this overproduction of sweat pro­ vides no advantages for thermoregulation and could potentially impair body fluid regulation. Equine sweat is isotonic to slightly hypertonic relative to plasma with sodium and chloride concentrations of 100 to 150 mmol/L and 130 to 160 mmol/L, respectively.5, is, 36 Similar to the findings in man, the sodium concentration in sweat increases with the sweating rate.34, 36 Therefore, the higher sweating rates demonstrated during exercise in the heat are accompanied by relatively larger sweat ion losses. During a 21day period of heat acclimation, the mean concentration of sodium in sweat decreased significantly (from 119.1 to 99.2 mmol/L).35 This decline in sweat sodium concentration could reflect the reduction in sweating rate measured over the same period. A classic described feature of heat acclimation in human subjects is a decrease in the salt concentration of sweat at any given sweating rate.23, 5 This salt-sparing effect of heat acclimation is, in part, dependent on activation of the renin-angiotensin­ aldosterone system. Aldosterone seems to be the hormone primarily responsible for conservation of sodium; administration of spironolac­ tone, an aldosterone antagonist, increases the sodium concentration in the thermal sweat of acclimating subjects. The reduction in sweat sodium and chloride concentrations is dependent on the level of dietary salt intake, however. 1, In studies in which the level of salt supplementa­ tion is increased in relation to the sodium losses, there is less evidence of a decline in sweat sodium and chloride concentrations. Under these circumstances, there is diminished activation of the renin-angiotensin­ aldosterone system, and the stimulus for sodium retention is reduced. 1 4

54

54

Fluid and Ion Regulation

Regardless of the state of training or heat acclimation, exercise in hot ambient conditions represents a substantially greater challenge to

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the body's mechanisms for fluid and ion regulation when compared with exercise in a cool environment.29, 48 In hot ambient conditions, total body fluid loss (respiratory and cutaneous water losses) can exceed 10 to 15 L/h, 5, 34 Clearly, failure to adequately replace these losses on a daily basis could result in persistent dehydration and ion deficits, The benefit of heat acclimation is the reduction in sweat fluid losses at a given thermal load. This diminution of fluid and ion losses could result in an improved ability to regulate plasma volume during exercise in the heat. In fact, Lindinger and coworkers28 demonstrated that during a 21day period of heat acclimation, there was a progressive attenuation of the exercise-induced decrease in plasma volume, 90% of which was attributable to the improved maintenance of plasma sodium content. Respiratory Responses The respiratory tract provides an important and responsive mecha­ nism for heat loss in the horse, 19, 31 Reliance on the respiratory system for selective brain cooling during rest and exercise has been demonstrated in the horse, 31 During acute heat exposure (T = 41°C, RH = 34%), there may be a four- to fivefold increase in respiratory frequency, 31 Following repeated bouts of heat exposure, there appears to be increased thermal sensitivity of the respiratory center such that by 5 days of daily heat exposure, horses increased their respiratory rate by 55% to 60% within 1 hour,1 3 As a result, an increase in rectal temperature observed during the first hour of passive heat exposure on the initial day was not evident on the fifth and subsequent days (Fig. 3).13 Heat Acclimation and Exercise Performance Evidence of heat acclimation is primarily manifested by an improve­ ment in exercise performance in the hot ambient conditions, In particu­ lar, an enhancement in thermoregulatory mechanisms should result in an attenuation in the rate of rise of core body temperature, Despite the effects of heat acclimation, however, the rate of rise of core body temper­ ature during exercise in hot conditions will remain substantially greater when compared with the increase in cool ambient conditions (see Fig. 3). Although the extent of physiologic adaptations will reflect the severity of the environmental conditions, duration and intensity of work in the hotter ambient conditions, and training status of the individual, evidence to date would suggest that most acclimation-related changes in the horse occur within a 14-day period of repeated active heat exposure. Decay of Physiologic Adaptations A number of studies and observations of people working in hot environments have reported on the rate of decay of the physiologic

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--+__..,_ �-

41

37

0

60

Pre-exercise

113

HH 1 HH lO HH 20 CD

0 5 10 15 2� 25 30

Exercise Time (min)

5 1 0 15 20 25 30

Walking recovery

Figure 3. Temperatu re in the rectum of six horses 1 hour before, during, and 30 minutes after a standardized exercise protocol during 3 weeks of daily exercise training in hot, humid (T = 33° C to 35 ° C, RH = 80% to 85%) ambient conditions. The temperatu re responses of the same horses during the exercise protocol in cool, dry ambient (T = 20° c , RH = 45% t o 55%) conditions are also plotted. (Adapted from Geor RJ , Mccutcheon LJ , Lindinger M l : Adaptations to daily exercise in hot and humid ambient conditions in trained Thoroughbred horses . Equine Vet J Suppl 22:63, 1 996; with perm ission . )

adaptations attributed to heat acclimation. Depending on the experimen­ tal design, the reported rate of decay of heat acclimation varies from a few days (7-10) to several weeks. 1 • 54 Most importantly, physically fit individuals retain the benefits of heat acclimation longer than untrained individuals.54 This observation serves to emphasize the importance of training to maximizing the benefits of heat acclimation. GUIDELINES FOR TRAINING AND COMPETITION IN HOT CONDITIONS

When considering management strategies for horses competing in hot climates, riders, trainers, and veterinarians must recognize that a thorough training program is of greatest importance in the overall prepa­ ration of the horse. Untrained horses should not be subjected to exercise training in the heat until an adequate level of fitness has been achieved in cool conditions. Although there may be an advantage to training under hot, dry conditions for a period of 10 to 14 days prior to compet-

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ing in the heat, training in conditions that combine high temperature and high humidity conveys little additional benefit and is potentially quite dangerous. As mentioned, the combination of high ambient tem­ perature and humidity compromises the horse's ability to dissipate heat during exercise. The added thermal strain associated with exercise in hot, humid conditions results in an earlier onset of critical hyperthermia without providing a measurable improvement in exercise-associated heat tolerance. When moving from a cool, dry to a hot, humid (or from a hot, dry to a hot, humid) climate, it is advisable to conduct training sessions, particularly higher intensity work, in the cooler air temperature of the early morning. In general, there should be a gradual increase in the quantity of exercise undertaken and only low-intensity training should be performed on very hot days.33 In circumstances in which the horse is being transported from a cooler to a warmer environment, the horse should be allowed enough time to recover from transport and to become accustomed to exercising in the hotter climate. In the context of the 1996 Summer Olympic Games, the International Equestrian Federation recommended that horses arrive approximately 3 weeks in advance of the events. Although transport to a hotter climate 3 weeks in advance of a competition should provide a sufficient period for acclimatization, there should not be the expectation of conducting the major portion of the horse's training and preparation in the adverse environmental conditions. During strenuous exercise in hot weather, signs of fatigue or heat­ related illness can develop rapidly, and particularly during the initial days of active heat acclimatization, the horse must be closely monitored to ensure safe adaptation to the environment. Collection of detailed clinical data should begin 1 to 2 weeks before travel to the hotter climate. These data will provide baseline information against which to compare clinical responses during the initial days of training in the hot conditions. In addition, evaluation of this information during the period of heat acclimatization will provide an objective assessment of how well the horse is adapting to the hot (and humid) environment. Water and feed intake should be measured on a daily (or twice-daily) basis. Heart rate, respiratory rate, and rectal temperature should be recorded before and after exercise training sessions. The intensity of work efforts can be estimated by use of a heart rate meter. When possible, daily weighing helps to establish the horse's normal weight and the extent of weight and fluid losses associated with travel and training. Ideally, an indication of the magnitude of fluid losses associated with training bouts can be obtained by measurement of pre- and postexercise body weight. Assuming similarity in the duration and intensity of exercise bouts undertaken in both the cool and hot ambient conditions, the exercise­ induced increase in rectal temperature will serve as an indicator of the added thermal burden associated with exercise in the hotter conditions. Importantly, monitoring of post-exercise rectal temperature is critical to early detection of severe heat stress. As there is a lag in the rise in rectal temperature, particularly after heavy exercise in hot and humid

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conditions, 1 it is very important to continue to monitor rectal tempera­ ture during the first 5 to 10 minutes after exercise. A rectal temperature that exceeds 42°C (l08°F) when first measured after exercise is cause for concern and indicates the need for immediate and vigorous cooling. Furthermore, this level of hyperthermia may signal the need for a reduction in the intensity or duration of exercise training in the hot ambient conditions. There may be a two- to threefold increase in resting respiratory rate following the transition from cool to hot ambient conditions. 13 • 15 This increase reflects the thermoregulatory role of the respiratory system. Similarly, the postexercise respiratory rate is typically 20% to 30% higher following exercise in hot ambient conditions than in cool to moderate ambient conditions. 1 5• In very hot and humid conditions (T = 33 °-35 °C, RH = 75%-85%), particularly when the horse is not aggressively cooled, the respiratory rate may remain markedly increased (100-120 breaths per minute) for 1 hour or more following cessation of exercise. 13• 15 When the respiratory rate remains high after the horse is placed in a cooler environment or is cooled vigorously, the increased respiratory rate may indicate that body temperature is still ma,rkedly elevated. In concert with the monitoring of rectal temperature, the sweating response of the horse should be assessed. Of particular concern for horses required to train and perform in a hot and humid environment is the potential for development of anhidrosis. Anhidrosis refers to the inability to produce an adequate sweating response when appropriate stimuli are applied.12• 18 Horses that have been raised in cooler regions and then are transported to tropical climates appear to be the most susceptible to the development of anhidrosis. This impairment of the sweating response results in an excessive elevation in core body temper­ ature during exercise. Although a greater level of hyperthermia in re­ sponse to a given exercise regimen is to be expected in a hotter environ­ ment, a decreased sweating response, high rectal temperature, reduced exercise capacity, and delayed recovery following exercise could reflect the onset of anhidrosis. Maintenance of fluid and electrolyte balance is crucial to the success­ ful adaptation to a hot environment. 1 The first concern is the fluid deficit resulting from the period of transport to the new location. Even when access to food and water is maintained during travel, horses typically incur a substantial loss of weight (approximately 3 kg/h of transport). Fluid losses will be even higher during road transport in hot weather. It can therefore be anticipated that horses will be dehydrated following transport. As many horses drink poorly during the initial days in a new environment, this dehydration may persist for 3 to 4 days after arrival. 33 Oral or intravenous fluid support is frequently provided to recently transported horses in an attempt to hasten correction of fluid balance. Regardless of how quickly transport-associated fluid deficits are cor­ rected, horses should be allowed a 4- to 5-day period of recovery follow­ ing prolonged transport. The horse may be lightly exercised during this 2

24

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period, but the normal level of exercise should not be resumed until the trainer or veterinarian is satisfied that the horse has fully recovered. In human subjects, dehydration of as little as 2% of body weight not only impairs thermoregulation and exercise performance but also prevents the expansion in plasma volume that typically occurs during the early phase of heat acclimation. 1 • 29• 49 Hyyppa and colleagues20 re­ cently demonstrated that a 1.2% deficit in body weight in horses can impair performance during high-intensity exercise. These observations illustrate the importance of maintaining hydration during regular train­ ing in the heat and demonstrate that measurement of body weight is useful for monitoring daily changes in fluid balance. In the authors' experience, replacing fluid losses as quickly as possible after exercise hastens full recovery. Furthermore, water consumption tends to be great­ est in the period immediately after exercise. To take advantage of this early opportunity for fluid replacement, cool water should be made available as soon as possible after completion of exercise. During several studies in hot and humid conditions, horses routinely consumed 10 to 15 L of water within 3 to 5 minutes of completing exercise. 13• 1 5• 35 No harmful effects have been associated with consuming cool water shortly after exercise. Given the increased fluid losses associated with a period of exercise training in a hot climate, an increase in daily water consumption is to be anticipated. Recent studies have demonstrated a 30% to 50% increase in 24-hour water consumption in horses undergoing heat acclimation; this change occurred within 10 days of heat exposure and largely re­ flected an increase in water consumption in the immediate post-exercise period.13 Some horses will not voluntarily rehydrate following exercise. If the extent of fluid losses is substantial (> 15-20 L) and the horse fails to voluntarily replace at least a portion of these losses, rehydration will require administration of fluids. This fluid replacement can be accom­ plished by provision of a balanced electrolyte solution via nasogastric tube or by intravenous administration of isotonic fluids. The reader is referred to other articles in this issue for further discussion of fluid support for horses during training and competition. Similarly, topics surrounding replacement of sweat ion losses are discussed elsewhere (see the article on fluid and ion losses and replacement due to sweating by L.J. McCutcheon and R.J. Geor in this issue). SUMMARY

The large metabolic heat load generated as a consequence of muscu­ lar work requires activation of thermoregulatory mechanisms in order to prevent an excessive and potentially dangerous rise in body temperature during exercise. Although the horse has highly efficient heat dissipatory mechanisms, there are a number of circumstances in which the thermo­ regulatory system may be overwhelmed, resulting in the development

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of critical hyperthermia. The risk for development of life-threatening hyperthermia is greatest when (1) the horse is inadequately conditioned for the required level of physical performance; (2) exercise is undertaken in hot and particularly, in hot and humid ambient conditions; and (3) there is an impairment to thermoregulatory mechanisms (e.g., severe dehydration, anhidrosis). Both exercise training under cool to moderate ambient conditions and a period of repeated exposure to, and exercise in, hot ambient conditions (heat acclimation) will result in a number of physiologic adaptations conferring improved thermoregulatory ability. These adaptations include an expanded plasma volume, greater stability of cardiovascular function during exercise, and an improved efficiency of evaporative heat loss as a result of alterations in the sweating response. Collectively, these adjustments serve to attenuate the rise in core body temperature in response to a given intensity of exercise. The magnitude of the physiologic adaptations occurring during exercise training and heat acclimation is a reflection of the thermal load imposed on the horse. Therefore, when compared with a period of training in cool conditions, the larger thermal stimulus associated with repeated exercise in hot ambient conditions will invoke proportionally greater thermoregulatory adaptations. Although it is not possible to eliminate the effects of adverse envi­ ronmental conditions on exercise performance, it is clear that a thorough exercise training program together with a subsequent period of acclima­ tization will serve to ameliorate the impact of the environment. Based on our current understanding of the nature and extent of thermoregulatory adaptations in the horse, the following conclusions can be made: l. A 2- to 3-month period of exercise training geared toward the specific athletic endeavor to be undertaken will result in substan­ tial improvements in thermoregulatory capacity and is an abso­ lute requirement for horses required to compete in hot ambient conditions. 2. Although physical training in a cool environment improves phys­ iologic responses to exercise at high ambient temperatures, a 2week period of moderate exercise training in these more adverse conditions is necessary for optimization of thermoregulatory function and physical performance. 3. Heat acclimation does not reduce the need for close monitoring of horses during training and competition in the heat. This is particularly true in hot, humid ambient conditions, where the biophysical limitations to sweat evaporation can result in devel­ opment of severe hyperthermia, regardless of the state of training or heat acclimation. References 1. Armstrong LE, Maresh CM: The induction and decay of heat acclimatisation in trained athletes. Sports Med 12:302, 1991

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2. Armstrong LE, Pandolf KB: Physical training, cardiorespiratory fitness and exercise­ heat tolerance. In Pandolf KB, Sawka MN, Gonzalez RR, et al (eds): Human Perfor­ mance Physiology and Environmental Medicine at Terrestrial Extremes. Indianapolis, Benchmark, 1988, p 199 3. Bayly WM, Gabel AA, Barr SA: Cardiovascular effects of submaximal aerobic training on a treadmill in Standardbred horses, using a standardized exercise test. Am J Vet Res 44:544, 1983 4. Buono MJ, Sjoholm NT: Effect of physical training on peripheral sweat production. J Appl Physiol 65:811, 1988 5. Carlson GP: Thermoregulation and fluid balance in the exercising horse. In Snow DH, Persson SGB, Rose RJ (eds): Equine Exercise Physiology. Cambridge, Granta Editions, 1983, p 291 6. Convertino VA: Blood volume: Its adaptation to endurance training. Med Sci Sports Exerc 23:1338, 1991 7. Convertino VA, Greenleaf JE, Bemauer EM: Role of thermal and exercise factors in the mechanism of hypervolemia. J Appl Physiol 48:657, 1980 8. Costill DL, Branam G, Finck W, et al: Exercise-induced sodium conservation: Changes in plasma renin and aldosterone. Med Sci Sports Exerc 8:209, 1976 9. Febbraio MA, Snow RJ, Hargreaves M, et al: Muscle metabolism during exercise and heat stress: Effect of heat acclimation. J Appl Physiol 76:589, 1994 10. Fellmann N: Hormonal and plasma volume alterations following endurance exercise: A brief review. Sports Med 13:37, 1992 11. Geor RJ, McCutcheon LJ: Influence of training on exercise-associated heat tolerance in Thoroughbred horses. J Sports Sci 14:347, 1996 12. Geor RJ, McCutcheon LJ: Thermoregulation and clinical disorders associated with exercise and heat stress. Compend Contin Educ Pract Vet 18:436, 1996 13. Geor RJ, McCutcheon LJ, Lindinger MI: Adaptations to daily exercise in hot and humid ambient conditions in trained Thoroughbred horses. Equine Vet J Suppl 22:63, 1996 14. Geor RJ, McCutcheon LJ, Lindinger MI: Exercise heat-tolerance in Thoroughbred horses associated with a 21-day period of heat acclimation. J Sports Sci 14:347, 1996 15. Geor RJ, McCutcheon LJ, Ecker GL, et al: Thermal and cardiorespiratory responses of horses to submaximal exercise under hot and humid conditions. Equine Vet J Suppl 20:125, 1995 16. Gisolfi GV: Influence of acclimatization and training on heat tolerance and physical endurance. In Hales JRS, Richards DAB (eds): Heat Stress: Physical Exertion and Environment. Amsterdam, Elsevier Science, 1987, p 355 17. Gisolfi GV: Work-heat tolerance derived from interval training. J Appl Physiol 35:349, 1973 18. Hodgson DR, Davis RE, McConaghy FF: Thermoregulation in the horse in response to exercise. Br Vet J 150:219, 1994 19. Hodgson DR, McCutcheon LJ, Byrd SK, et al: Dissipation of metabolic heat in the horse during exercise. J Appl Physiol 74:1161, 1993 20. Hyyppa S, Saastamoinen M, Poso AR: Restoration of water and electrolyte balance in horses after repeated exercise in hot and humid conditions. Equine Vet J Suppl 22:108, 1996 21 . Jones JH, Carlson GP: Estimation of energy costs and heat production during a three­ day event. Equine Vet J Suppl 20:23, 1995 22. Jooste PL, Strydom NB: Improved mechanical efficiency derived from heat acclimation. South African Journal of Research on Sport, Physical Education and Recreation 2:45, 1979 23. Kirby CR, Convertino VA: Plasma aldosterone and sweat sodium concentrations after exercise and heat acclimation. J Appl Physiol 61:967, 1986 24. Kohn CW, Hinchcliff KW: Physiological responses to the endurance test of a 3-day event during hot and cool weather. Equine Vet J Suppl 20:31, 1995 25. Kohn CW, Hinchcliff KW, McCutcheon LJ, et al: Physiological responses of horses competing at a modified one-star 3-day event. Equine Vet J Suppl 20:97, 1995 26. Knight PK, Sinha AK, Rose RJ: Effects of training intensity on maximum oxygen

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uptake. In Persson SGB, Lindholm A, Jeffcott LB (eds): Exercise Physiology 3. Davis, CA, ICEEP Publications, 1991, p 77 Lindinger MI, Marlin DJ: Heat stress and acclimation in the performance horse: Where are we and where are we going? Equine Vet Educ 7:256, 1995 Lindinger MI, Geor RJ, Ecker GL, et al: Improved regulation of plasma volume and plasma sodium content during exercise with 21 days of heat acclimation in horses. Physiologist 39:A-29, 1996 Maughan RJ, Lindinger MI: Preparing for and competing in the heat: The human perspective. Equine Vet J Suppl 20:8, 1995 McConaghy RR, Hodgson DR, Rose RJ, et al: Redistribution of cardiac output in response to heat exposure in the pony. Equine Vet J Suppl 22:42, 1996 McConaghy RR, Hales JRS, Rose RJ, et al: Selective brain cooling in the horse during exercise and environmental heat stress. J Appl Physiol 70:1849, 1995 McCutcheon LJ, Geor RJ: Influence of training-associated thermoregulatory adaptations on sweating rate and sweat composition in Thoroughbred horses. J Sports Sci 14:347, 1996 McCutcheon LJ, Geor RJ: Management of horses during training and competition in hot, humid conditions. Compend Contin Educ Pract Vet 19:102, 1997 McCutcheon LJ, Geor RJ: Sweat fluid and ion losses during training and competition in cool vs. hot ambient conditions: Implications for ion supplementation. Equine Vet J Suppl 22:53, 1996 McCutcheon LJ, Geor RJ, Lindinger MI: Sweating rate and sweat composition during heat acclimation in Thoroughbred horses. J Sports Sei 14:347, 1996 McCutcheon LJ, Geor RJ, Hare MJ, et al: Sweating rate and sweat composition during exercise and recovery in ambient heat and humidity. Equine Vet J Suppl 20: 153, 1995 McKeever KH, Hinchcliff KW: Neuroendocrine control of blood volume, blood pres­ sure and cardiovascular function in horses. Equine Vet J Suppl 18:77, 1995 McKeever KH, Schurg WA, Jarrett SH, et al: Exercise training-induced hypervolemia in the horse. Med Sci Sports Exerc 19:21, 1987 Mitchell D, Senay LC, Wyndham CH, et al: Acclimatization in a hot, humid environ­ ment: Energy exchange, body temperature, and sweating. J Appl Physiol 40:768, 1976 Nadel ER: Importance of albumin in the plasma volume expansion process. Physiolo­ gist 39:A-65, 1996 Nadel ER: Temperature regulation and prolonged exercise. In Lamb DR, Murray R (eds): Perspectives in Exercise Science and Sports Medicine, vol 1 . Carmel, IN, Bench­ mark, 1988, p 125 Nadel ER, Pandolf KB, Roberts MF, et al: Mechanisms of thermal acclimation to exercise and heat. J Appl Physiol 29:36, 1970 Naylor JRJ, Bayly WM, Gollnick PD, et al: Effects of dehydration on thermoregulatory responses of horses during low intensity exercise. J Appl Physiol 75:994, 1993 Nielsen B, Strange S, Christensen NJ, et al: Acute and adaptive responses in humans to exercise in a warm, humid environment. Pfliigers Arch 434:49, 1997 Nielsen B, Hales JRS, Strange S, et al: Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment. J Physiol 460:467, 1993 Pandolf KB, Cadarette BS, Sawka MN, et al: Thermoregulatory responses of middle­ aged and young men during dry heat acclimation. J Appl Physiol 65:65, 1988 Rose RJ, Evans DL: Cardiovascular and respiratory function in the athletic horse. In Gillespie JR, Robinson NE (eds): Equine Exercise Physiology 2. Davis, CA, ICEEP Publications, 1987, p 1 Rowell LB: Cardiovascular adjustments to thermal stress. In Shepherd JT, Abboud FM (eds): Handbook of Physiology: The Cardiovascular System: Peripheral and Organ Blood Flow. Bethesda, American Physiological Society, 1983, p 967 Sawka MN, Pandolf KB: Effects of body water loss on exercise performance and physiological functions. In Gisolfi GV, Lamb DR (eds): Perspectives in Exercise Science and Sports Medicine: Fluid Homeostasis During Exercise, vol. 3. Indianapolis, Bench­ mark, 1990, p 1 Sexton WL, Erickson HH, Coffman JR: Cardiopulmonary and metabolic responses in

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the Quarter horse: Effects of training. In Gillespie JR, Robinson NE (eds): Equine Exercise Physiology 2. Davis, CA, ICEEP Publications, 1987, p 77 Thomas DP, Fregin GF, Gerber NH, et al: Effects of training on cardiorespiratory function in the horse. Am J Physiol 245:R160, 1983 Thornton J, Essen-Gustavsson B, Lindholm A, et al: Effects of training and detraining on oxygen uptake, cardiac output, blood gas tensions, pH and lactate concentrations during and after exercise in the horse. In Snow DH, Persson SGB, Rose RJ (eds): Equine Exercise Physiology. Cambridge, Granta Editions, 1983, p 470 Wade CE, Hill LC, Hunt MM, et al: Plasma volume and renal function in runners during a 20-day road race. Pflugers Arch 54:456, 1985 Wenger CB: Human heat acclimatization. In Pandolf KB, Sawka MN, Gonzalez RR, et al (eds): Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Indianapolis, Benchmark, 1988, p 153 Werner J: Temperature regulation during exercise: An overview. In Lamb DR, Murray R (eds): Perspectives in Exercise Science and Sports Medicine, vol 1. Carmel, IN, Benchmark, 1988, p 49 Williams ES, Ward MP, Milledge JS, et al: Effect of seven consecutive days of hill­ walking on fluid homeostasis. Clin Sci (Colch) 56:305, 1979 Wyndham CH, Rogers GG, Senay LC, et al: Acclimatization in a hot, humid environ­ ment: Cardiovascular adjustments. J Appl Physiol 40:779, 1976 Yamauchi M, Matsumoto T, Ohwatari N, et al: Sweating economy by graded control in well-trained athletes. Pflugers Arch 433:675, 1997 Address reprint req uests to

R.J. Geor, BVSc, MV Sc Department of Veterinary Clinical Sciences College of Veterinary Medicine The Ohio State University 601 Vernon L. Tharp Street Columbus, OH 43210