Respiratory responses to exercise in the horse

Horses are elite athletes when compared with other mammalian species. In the latter, performance is limited by cardiovascular or musculoskeletal performance whereas in athletic horses it is the respiratory system that appears to be rate limiting and virtually all horses exercising at high intensities become hypoxaemic and hypercapnoeic. This is due to both diffusion limitation and a level of ventilation inadequate for the metabolic level that enables horses to exercise at these intensities. In conjunction with these blood gas changes, total pulmonary resistance increases and the work of breathing rises exponentially and airflow eventually plateaus despite increases in inspiratory and expiratory intrapleural pressures. Horses breathe at comparatively high frequencies when galloping due to the tight 1:1 coupling of strides to breathing. Whether this effects gas exchange and, if so, to what extent, has not been fully elucidated.