Dynamics of cardiorespiratory function in Standardbred horses during different intensities of constant-load exercise

Six Standardbred horses were used to evaluate the time course of pulmonary gas exchange, ventilation, heart rate (HR) and acid base balance during different intensities of constant-load treadmill exercise. Horses were exercised at approximately 50%, 75% and 100% maximum oxygen uptake (\(\dot V_{{\text{O}}_2 } \) max) for 5 min and measurements taken every 30 s throughout exercise. At all work rates, the minute ventilation, respiratory frequency and tidal volume reached steady state values by 60 s of exercise. At 100%\(\dot V_{{\text{O}}_2 } \) max, the oxygen consumption (\(\dot V_{{\text{O}}_2 } \)) increased to mean values of approximately 130 ml/kg·min, which represents a 40-fold increase above resting\(\dot V_{{\text{O}}_2 } \). At the low and moderate work rates,\(\dot V_{{\text{O}}_2 } \) showed no significant change from 30 s to 300 s of exercise. At the high work rate, the mean\(\dot V_{{\text{O}}_2 } \) at 30 s was 80% of the value at 300 s. The HR showed no significant change over time at the moderate work rate but differing responses at the low and high work rates. At the low work rate, the mean HR decreased from 188 beats/min at 30 s to 172 beats/min at 300 s exercise, whereas at the high work rate the mean HR increased from 204 beats/min at 30 s to 221 beats/min at 300 s exercise. No changes in acid base status occurred during exercise at the low work rate. At the moderate work rate, a mild metabolic acidosis occurred which was nonprogressive with time, whereas the high work rate resulted in a progressive metabolic acidosis with a base deficit of 16 mmol/l by 300 s exercise. It is concluded that the kinetics of gas exchange during exercise are more rapid in the horse than in man, despite the relatively greater change in\(\dot V_{{\text{O}}_2 } \) in the horse when going from rest to high intensity exercise.