Thermoregulatory adaptations associated with training and heat acclimation.

The large metabolic heat load generated as a consequence of muscular 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 thermoregulatory system may be overwhelmed, resulting in the development 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 environmental conditions on exercise performance, it is clear that a thorough exercise training program together with a subsequent period of acclimatization 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: 1. A 2- to 3-month period of exercise training geared toward the specific athletic endeavor to be undertaken will result in substantial improvements in thermoregulatory capacity and is an absolute requirement for horses required to compete in hot ambient conditions. 2. Although physical training in a cool environment improves physiologic responses to exercise at high ambient temperatures, a 2-week 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 development of severe hyperthermia, regardless of the state of training or heat acclimation.