• horse;
  • exercise;
  • heat stress;
  • aerobic metabolism


The purpose of this study was to investigate the effect of a hot and humid environment on the pattern of breathing and the gas exchange in heavily exercising horses. Five healthy fit Standardbred horses were investigated twice at 8 day intervals, once in temperate conditions (TC) (ambient temperature: 15°C; relative humidity: 55%) and once in hot and humid conditions (HHC) (ambient temperature: 30°C; relative humidity: 75%). The standardised treadmill exercise test consisted of 8 min warm-up and 8 min exercise (1 min at 1.7, 4, 7, 8, 9 and 10 m/s and 2 min at 4 m/s with a 6% slope). Running in HHC induced a significant decrease in peak expired minute volume (V̇E), oxygen uptake (V̇O2), V̇CO2 and oxygen pulse, while RQ remained unchanged and PACO2 increased. The changes in skin temperature (ST), i.e. the difference between ST at rest and at 10 m/s, was dramatically higher in HHC. The test induced changes, i.e. the differences between the values before and after the test, in Cortisol, β-endorphins and plasma lactate (LA) values after the test were significantly higher. This study showed that performing a strenuous exercise in hot, humid conditions in unacclimatised horses induced a reduction of the aerobic capacity, i.e. decrease of the V̇O2 and increase of LA production. A relative hypoventilation could be partly responsible for this observation, but all the steps of the oxygen transport from the pulmonary ventilation to the muscle metabolism could also potentially be impaired in such conditions.