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      Effect of high ambient temperature on physiological responses during incremental exercise in Thoroughbred horses

      1 , 1 , 1 , 1
      Comparative Exercise Physiology
      Brill

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          Abstract

          Several reports have suggested that the risk of exertional heat illness (EHI) in Thoroughbred racehorses increases in high ambient temperatures. Heat dissipation in horses during exercise becomes less efficient when the body temperature and ambient temperature are close. Therefore, we hypothesised that exercise at 40 °C may increase body temperature, oxygen consumption, and cardiac output during incremental exercise tests compared to 20 and 30 °C. Six trained Thoroughbred horses were studied in a randomised, crossover design at three ambient temperatures with a 6-day washout period. Using a 3% inclined treadmill, horses performed incremental exercise tests at 1.7, 3.5, 6, 8, and 10 m/s for 90 s at ambient temperatures of 20, 30, and 40 °C. The effects of ambient temperature at 10 m/s on physiological variables were analysed using mixed models (P<0.05). Pulmonary arterial temperature and rectal temperature at 40 °C were higher than those at 20 °C (P<0.001) and 30 °C (P<0.001). Similarly, oxygen consumption (vs 20 °C, P=0.009; vs 30 °C, P=0.006) and cardiac output (vs 20 °C, P=0.001; vs 30 °C, P=0.001) at 40 °C were higher than those at 20 and 30 °C. Arterial O 2 partial pressure, O 2 saturation, and pH at 40 °C were lower than those at 20 and 30 °C. Arterial CO 2 partial pressure at 40 °C was higher than that at 20 and 30 °C. No differences were observed in arterial-mixed venous O 2 concentration difference (P=0.391) and plasma lactate concentration (P=0.134) at different ambient temperatures. These results indicate that exercise at 40 °C causes excessive high body temperature, decreased running economy, and increased cardiac output compared to exercise at 20 and 30 °C. We strongly suggest that trainers and veterinarians should anticipate the occurrence of increased thermal stresses when ambient temperature is extremely high even in dry conditions and prepare to mitigate the risk of EHI from the perspective of equine welfare.

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          Most cited references38

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          National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses.

          To present best-practice recommendations for the prevention, recognition, and treatment of exertional heat illnesses (EHIs) and to describe the relevant physiology of thermoregulation.
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            The cardiovascular challenge of exercising in the heat.

            Exercise in the heat can pose a severe challenge to human cardiovascular control, and thus the provision of oxygen to exercising muscles and vital organs, because of enhanced thermoregulatory demand for skin blood flow coupled with dehydration and hyperthermia. Cardiovascular strain, typified by reductions in cardiac output, skin and locomotor muscle blood flow and systemic and muscle oxygen delivery accompanies marked dehydration and hyperthermia during prolonged and intense exercise characteristic of many summer Olympic events. This review focuses on how the cardiovascular system is regulated when exercising in the heat and how restrictions in locomotor skeletal muscle and/or skin perfusion might limit athletic performance in hot environments.
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              Exercise under heat stress: thermoregulation, hydration, performance implications and mitigation strategies

              A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat, by examining the benefits of heat acclimation, cooling strategies and hyperhydration. Finally, contemporary controversies are summarized and future research directions provided.
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                Author and article information

                Journal
                Comparative Exercise Physiology
                Comparative Exercise Physiology
                Brill
                1755-2540
                1755-2559
                April 03 2023
                April 03 2023
                : 19
                : 2
                : 159-167
                Affiliations
                [1 ]Sports Science Division, Equine Research Institute, Japan Racing Association, Shiba1400-4, Shimotsuke, Tochigi 329-0412, Japan.
                Article
                10.3920/CEP220018
                8d764f3e-5675-4b3a-82f4-7579c8acb048
                © 2023
                History

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