Circulatory, respiratory and metabolic responses in Thoroughbred horses during the first 400 meters of exercise

Summary

These studies investigated circulatory, respiratory and metabolic responses in four Thoroughbred geldings during the first 400 metres of galloping (mean speed 14.4±0.38 m · s⁻¹), cantering (mean speed 10.0±0.61 m · s⁻¹) and walking (mean speed 1.58±0.05 m · s⁻¹) from a standing start. A radio-controlled device which collected blood samples anaerobically during each 100 m section of the exercise track allowed analyses of changes in and functional relationships of the variables measured. During the 400 m gallop, the mean heart rate (HR) increased from 125 to 201 beats · min⁻¹ and the haematocrit (Hct) from 0.513 to 0.589 l/l⁻¹. The haemoglobin [Hb], lactate [LA] and potassium [K⁺] concentrations increased significantly, while the pH and the partial pressure of oxygen (PaO₂) decreased significantly. The arterial partial pressure of carbon dioxide (PaCO₂) and the plasma bicarbonate concentration did not change significantly. There were significant correlations between HR and Hct, HR and [Hb], HR and PaO₂, HR and pH, HR and PvCO₂, HR and [LA], HR and [K⁺], pH and [K⁺], Hct and PaO₂, [Hb] and PaO₂, PaCO₂ and PaO₂, [LA] and PaO₂, pH and PaO₂, [K⁺] and PaO₂, stride frequency and PaO₂. With the exception of the PvCO₂ which increased significantly, changes in venous blood during the gallop were in the same direction as those of arterial blood. Thirty seconds before the start of the gallop, both HR and [Hb] were significantly higher than at rest, providing an approximate three-fold increase in oxygen delivery compared to that of the resting state. During the canter neither PaO₂ nor PaCO₂ changed significantly, whilst HR, [LA] and [K⁺] increased significantly. Changes in other variables were in the same direction as during the gallop but to a non-significant extent. During the walk the PaO₂ increased significantly. We concluded that exercise-induced hypoxaemia developed within <20 s of the start of a gallop at 14 to 15 m · s⁻¹ and that its development was significantly related to changes in cardiac, respiratory, acidbase and metabolic components of the physiological response to heavy exercise. A preliminary canter followed by a ten minute walk before a gallop ensured that horses began galloping with a greatly enhanced systemic oxygen delivery.