Cardiac output and oxygen consumption in exercising Thoroughbred horses.

This study characterizes the effects of exercise on the cardiac output (Q) and the metabolic rate (VO2) of trained Thoroughbred racehorses. Heart rate, Q, and arteriovenous (a-v)O2 difference were measured at rest and at three levels of submaximal treadmill exercise (1.6 m/s walk and 3-4 m/s trot at 6% incline, and 6.5 m/s horizontal canter). Heart rate and (a-v)O2 difference were also measured during maximal exercise (12.5 m/s gallop, 5% incline) to obtain an estimate of maximum O2 uptake (VO2max). The walk, trot, and canter represented 25, 45, and 55% VO2max. Mean heart rate went from 48.9 (rest) to 197 beats/min (gallop). Q ranged from 106 (rest) to 571 ml.min-1.kg-1 (canter), and stroke volume went from 1.34 (rest) to a maximum of 1.58 liters (walk). Thoroughbreds were able to bring hematocrit from 38 (rest) to 63% (gallop), and this adjustment allowed them to reach an impressive (a-v)O2 difference of 23 vol%, which represents a fivefold increase over resting values. These outstanding athletes probably support an aerobic scope of 40-fold.