From the Heart

In most horses, the heart weighs slightly less than 1% of the horse’s body weight, or up to about 8.5 pounds (3.9 kg). Among light breeds, the Thoroughbred’s heart is proportionally larger, usually weighing just over 1% of body weight. Arabian horses have hearts equal to about 0.76% of their weight. In draft horses, the heart is proportionally smaller, averaging about 0.6% of body weight.

Does heart size vary within a breed?
In Thoroughbreds, some bloodlines seem to produce larger hearts, and a study of over 7000 horses found evidence that young horses with the largest hearts often become above-average performers in their racing careers. Secretariat, Triple Crown winner and holder of several record race times, had a heart almost twice as large as that of the average Thoroughbred.

How fast does the horse’s heart beat?
The resting pulse rate in newborn foals is about 80 to 100 beats per minute. By the time the foal is several weeks old the rate is about 60 to 80 beats per minute, and this decline continues as the horse grows. A two-year-old’s heart beats about 45 to 65 times a minute, and an adult horse has a resting heart rate of around 30 to 50 beats per minute. Walking, trotting, and cantering raise the rate to around 80, 130, and 180, respectively, and the pulse of a galloping horse reaches a maximum of 220 to 240 beats per minute. Stroke volume (the amount of blood pumped out of the heart with each beat) is about a quart (0.95 liters). A resting horse’s heart pumps about 10 gallons (38 liters) of blood per minute, while at the most strenuous exercise levels this volume increases to 65 to 75 gallons (246 to 284 liters) of blood per minute.

What is the effect of exercise on cardiac function?
When the horse begins a period of moderate exercise, the heart rate rises steeply in the first 30 to 45 seconds and then tapers back, settling at a somewhat lower rate. At the highest level of exercise, the heart is beating about four times per second. This seems to be the maximum rate at which the heart chambers have time to refill between contractions. When an exercise period is finished, the heart rate decreases very rapidly for the first minute or two and then continues to drop slowly until it reaches the pre-exercise level. This return to resting rate can take as long as an hour. In the relaxed horse, not all red blood cells are in active circulation. Extra cells are stored in the spleen which contracts in response to exercise, increasing the circulating red cells by as much as 68% and therefore raising the oxygen-carrying capacity of the blood. Strenuous exercise changes the way blood is distributed throughout the horse’s body. Circulation to the leg muscles is increased as much as 75%. Blood supply to the diaphragm and adrenal glands also rises, while circulation to the kidneys and digestive organs is sharply reduced.

How does training affect the heart and blood vessels?
In humans, increased fitness tends to bring about a reduction in the resting heart rate, but this result is not seen to any significant degree in horses. Similarly, training has no noticeable impact on maximal heart rate in equines. Training does boost heart size, strength, and stroke volume. This takes place rather quickly, with a significant increase in the ratio of heart weight to body weight occurring after only a few weeks of training. Primarily because of these changes, a particular level of activity results in a slightly lower heart rate as training progresses. Another adaptation to exercise is the development of more capillaries in the muscles of locomotion, a process which is completed in about three to four months.

How is heart rate used as a measure of fitness?
Because the resting and maximal heart rates are not affected by changes in physical condition, they are not useful as indictors of fitness. One of the best measurements of a horse’s overall condition is recovery rate, or the speed with which the heart rate returns to resting level following exercise. In endurance riding where horses perform moderate exercise for long periods of time, most riders are well acquainted with the practice of measuring the heart rate at prescribed intervals during or after a workout or race. Records from the midpoint rest stop at one endurance ride showed that horses with a pulse rate of 60 or less after 30 minutes of rest showed less dehydration and muscle stress than those with higher rates. In the same type of competition, horses with rates over 65 after 30 minutes of rest often develop severe dehydration and exhaustion if allowed to continue. A second indicator of fitness is related to drift, or the slow increase of heart rate that occurs as a horse becomes fatigued. In general, horses at higher levels of fitness can maintain a particular heart rate longer than less fit horses exercising at a comparable level of exertion.

Besides fatigue, what factors affect heart rate during and after exercise?
Fear, excitement, or even the presence of a groom or handler can cause the heart rate to increase, and this effect must be taken into account as a horse’s pulse is recorded. This variation can be minimized somewhat by getting the horse accustomed to having the heart rate checked after exercise. In a horse whose recovery pattern is known, an increased heart rate after exercise can be an important signal of pain or discomfort. Trainers can use this sign as an early indicator of fever, lameness, or breathing difficulties, problems which should be ruled out before training is continued.

What is the relationship between feeding practices and heart rate?
One study showed that ponies fasted 24 hours before moderate exercise had lower heart rates than those given feed and alfalfa hay before performing the same exercise. Another study found lower heart rates in horses fed eight hours before exercise compared to those fed three hours before exercise. In a fat supplementation trial, horses that were given a high-fat heat-stabilized rice bran supplement as part of their ration had lower heart rates during exercise than horses that were given the same number of supplemental calories in the form of vegetable oil.