Effects of hyperhydration on cardiorespiratory and metabolic responses to exercise in horses during a simulated 2nd day of the 3-day-event

Abstract

We hypothesised that hyperhydration before prolonged exercise would increase and maintain plasma volume (PV), assisting thermoregulation and cardiorespiratory function. In a cross-over design, 7 horses received ≈26 L (6% bodyweight) of isotonic fluid or nothing (control). Horses undertook 98 min of treadmill exercise at intensities eliciting 20-60% V·O2 max (24.5°C and 60% RH) to simulate 2nd day of a 3-day event. Results (mean±sem, P<0.05) at the end of the test in the treated group when compared with control showed: a lower TP (69±2 vs 77±2 g/L) indicating PV expansion; a higher HR but no differences in Q̇ and SV; and lower venous pH, HCO3-. Core temperature remained unchanged despite higher bodyweight losses (25.7±1.7 vs 17.1±0.9 kg), mainly as sweat. Hyperhydration does maintain PV during prolonged exercise but does not assist thermoregulation and may adversely affect acid-base balance.