Effects of oral fluid on cardiorespiratory and metabolic responses to prolonged exercise

Summary Administration of oral fluids prior to exercise may provide a fluid ‘buffer’ supporting cardiorespiratory and thermoregulatory function during prolonged exercise. Six Standardbred horses, previously acclimatised to a treadmill and a respiratory gas collection mask, were used in a crossover study and were given ∼17.5 litres hypotonic commercial electrolyte mixture via a nasogastric tube, 90 min before exercise. Total fluid was equivalent to 4% bwt, administered in 3 equal doses, 20 min apart, the last 50 min before exercise resulting in a mean ± s.e. pre‐exercise plasma total protein (TPP) of 57.6 ± 0.6 g/l, compared with 63.5 ± 0.5 g/l (P < 0.01) when no fluid was given. One week later, each horse received the alternate treatment. The exercise test consisted of 90 min at 30% V̇O 2max with measurements taken throughout exercise. Exercise increased temperature, haematocrit, TPP, [glucose], [lactate] and pH and decreased plasma [K] and [Cl], respiratory exchange ratio and PaCO 2 . There was no effect of exercise duration on heart rate, cardiac output, stroke volume, V̇O 2 V̇CO 2 , [HCO 3 ] or PaO 2 . Fluid administration resulted in lower TPP, plasma [Cl], [HCO 3 ] and [Na] during exercise. Although fluid administration before exercise did not improve cardiorespiratory and thermoregulatory function, the fluid treated group had a higher plasma volume which may be advantageous to horses exercising in hot and humid environments.