Effect of prior high-intensity exercise on exercise-induced arterial hypoxemia in Thoroughbred horses.

This study examines how previous intense exercise in Thoroughbred horses affects the occurrence of low oxygen levels in their arteries (arterial hypoxemia) during a later bout of similar exercise. The research suggests that, contrary to expectations, the physical changes in the horse’s blood-gas barrier induced by the first exercise round do not exacerbate arterial hypoxemia during the second. Instead, it actually reduced its severity.

Methodology of the Research

• The study involved two different types of experiments carried out on seven healthy Thoroughbred horses. These experiments were performed one week apart in random order; one involving two bouts of intense exercise (double-exercise experiments), and the other involving just one bout (single-exercise bout experiments).
• In the double-exercise experiments, the horses performed two rounds of strenuous exercise, with each round lasting 120 seconds. The two rounds were separated by a six-minute interval. The exercise involved galloping uphill at a rate of 14 m/s, on a 3.5% incline.
• In the single-exercise bout experiments, the horse performed the same exercise as above, but only once.

Findings of the Research

• The strenuous exercise resulted in arterial hypoxemia within just 30 seconds of the horses starting to gallop. This was accompanied by desaturation of haemoglobin, an increase in arterial PCO2 (partial pressure of carbon dioxide in the blood), and acidosis as the exercise extended from 30 to 120 seconds.
• All horses in the single-exercise bout experiment developed an exercise-induced pulmonary haemorrhage (EIPH), confirming that the stress of the exercise had affected their pulmonary capillaries.
• Despite these stress-induced changes to the blood-gas barrier, the arterial hypoxemia experienced during the second bout of exercise in the double-exercise experiment was milder than that from the first round of the same experiments. The arterial PO2 (partial pressure of oxygen) values during the second round were significantly higher than those from the first bout of exercise.
• These findings suggest that previous bouts of intense exercise do not increase the severity of arterial hypoxemia during subsequent strenuous activity, but actually decrease it.
• The decrease in the severity of exercise-induced arterial hypoxemia in the second bout was linked to an associated increase in alveolar PO2 and significantly lower arterial PCO2 than in the first run.
• The study, therefore, concluded that structural changes related to stress failure of pulmonary capillaries due to intense exercise do not contribute to the occurrence of exercise-induced arterial hypoxemia in horses.