Heart Action in Racehorses

Thoroughbred racehorses gallop, carrying a jockey. Standardbred racehorses either trot or pace while pulling a sulky and driver. Horses of both breeds work near the top limits of ability during races. Are there differences in cardiac performance between the two breeds? This was the question around which a study performed at the University of Guelph was designed.

Horses were fitted with heart monitors to record cardiac function from prerace preparations until the time the horses were returned to their stables after racing. The results showed that there were large variations between horses for heart rate and rhythm. For Standardbreds, little difference was found in results for trotters and pacers. There was a general trend for heart rates to increase as races progressed. Two-year-old horses had higher heart rates than older horses, averaging around 235 beats per minute while racing. There was a range of heart rates during races, from a low of just over 200 to a high of 275 beats per minute. After racing, horses had a wide range of recovery rates and times.

Abnormal heart rhythms were seen in 15% of Standardbred horses immediately following racing, most often as they slowed down after maximum effort. More than a quarter of the horses showed irregular function of the ventricular (lower) section of the heart during the night after a race. This varied from a few irregular beats to periods of extreme irregularity. Despite some extreme disturbances in heart rhythm, all horses continued to train and race in subsequent weeks and none showed any adverse effects.

For Thoroughbred racehorses, some results were similar to those found in Standardbreds, while others were quite different. Standardbred heart rates tended to rise gradually throughout a race and then quickly return to normal after the finish, while Thoroughbred hearts tended to rise quickly at the beginning of a race, stay at the same rate during the race, and taper more slowly to normal after the race.

Prerace heart rates for Thoroughbreds in the saddling paddock ranged from 60 to 100 beats per minute. In the starting gate just before a race, they ranged from 65 to 186 beats per minute. Postrace irregularities in heartbeat were found in Thoroughbreds, but correlations of their frequency or severity between Thoroughbreds and Standardbreds are not yet available.