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Shortness of breath

Increased respiration rate or increased intensity of breathing

Loss of condition

Increased fatigue during exercise

Weakness that can result in collapse or fainting

Fluid accumulation in the abdomen or beneath the skin of the lower thorax

HOW THE EQUINE HEART WORKS

Geneticists believe the large heart gene from Eclipse is passed from dam to colt on the "X" chromosome. Breeds, like the Quarter Horse and Standardbred, can also have the large heart gene due to the influx of Thoroughbred blood.

When someone says a horse has "heart", what do they mean? Sometimes they are talking about a horse's courage or braveness. A horse that has come through an ordeal, like being trapped or surviving a tragic injury, is said to persevere because they have heart. At other times, this reference is made about a horse that is naturally competitive. For instance, the racehorse that finds that extra bit of energy to pull ahead of the pack during the last few strides of a race is said to have a lot of heart.

So when we refer to a horse having "heart", we are talking about the horse's strength of character or resolve and not about the actual organ. However, the equine heart is what sets horses apart from other large farm animals. In a comparison between horses and cattle of similar weight, researchers found that horses have a larger heart than that of steers. While the average bovine heart weighs less than five pounds, the horse's heart averages between 8.5 to 12 pounds depending upon the breed.

The size of the heart is significant when it comes to athletic performance. The heart is essentially a pump and the bigger it is, the larger volume of blood it can move. Blood not only delivers oxygen and energy to the cells and tissues of the body, but it also carries away waste products and heat. The more efficiently these tasks are performed, the faster and farther the animal can run. Certainly one of the reasons we use horses, and not cows, for athletic events is due to the size of their heart.

THE X-FACTOR

Due to the influx of Thoroughbred blood into Quarter Horses and Standardbreds, individuals of these breeds can also have the large heart gene. The Arabian, one of the oldest and purest of horse breeds, has a proportionally smaller heart. On average, a 1200 pound Arabian would be expected to have a heart that only weighs 9 pounds or 0.76% of their body weight. Likewise, Draft horses have an even smaller heart to body weight ratio at 0.6%. A draft horse with a 9 pound heart would be expected to weigh about 1500 pounds.

When a horse begins to exercise, their heart rate increases in order to supply more oxygen to the working muscles. The heart rate of a galloping horse can reach a maximum of about 220 to 240 beats per minute. It is believed that 4 beats per second is about the maximum rate the heart can beat and still have time to refill the chambers between contractions.

As heart rate increases, the cardiac output of the heart increases. This, in turn, also increases the stroke volume. During strenuous exercise, the stroke volume can reach 1.7 liters per beat.

Genetics and conditioning are two variables that can influence the size of a horse's heart and therefore, how much blood it can pump. Centuries of selectively breeding horses to run faster and be more athletic has resulted in some breeds having a larger heart than others. Most horse's have hearts that weigh about 1% of their body weight. Among light horse breeds, Thoroughbreds have the largest heart size-to-weight ratio. Coming in at just over 1% of their body weight, the heart of modern-day Thoroughbred racehorses averages about 10 to 12 pounds.

Even among Thoroughbreds, there can be individual variation in the size of the heart. A study that measured the heart size of young horses found a correlation between increased heart size and an above-average racing career. It is estimated that the racing legend, Secretariat, had a 22 pound heart, ten pounds heavier than that of the average Thoroughbred. Since a horse's heart continues to grow until it is four years old, the early training and conditioning that racing Thoroughbreds receive can be one cause for this variation.

Another reason Thoroughbreds have larger hearts traces back to Eclipse, an 18th century British racehorse. Eclipse not only won every race he entered, he was so fast that he was forced into retirement after only 17 months of racing as it became impossible to find suitable competition that would generate wagers against him. Today, almost all Thoroughbreds can trace their lineage to Eclipse.

Upon his death, it was discovered that Eclipse had a 14 pound heart. While this may not seem notable by today's standards, in Eclipse's era the average horse heart only weighed about six pounds. Eclipse's large heart is believed to be a sex-linked trait that is carried on the "X" chromosome. This means the "large heart" gene is passed from generation to generation through the mare's lineage, since stallions can only pass a Y chromosome on to their sons. Stallions with the large heart gene received it from their dam.

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How the Equine Cardiac System
Influences Performance

Straight From the Heart

Like the human heart, the equine version has four chambers. The two atria receive blood into the heart, while the two ventricles pump it back out. The left side of the heart collects the oxygen-rich blood from the lungs and pumps it out to the body. The right side receives the blood returning from the body and sends it to the lungs.

The stroke volume is the amount of blood pumped out of the heart each time it beats. Stroke volume is in direct relation to ventricle size. For horses, stroke volume is just under a liter or about a quart of blood per beat. The average at-rest heart rate for an adult horse is between 28 and 40 beats per minute. So in one minute, each ventricle will pump approximately 28 to 40 liters of blood. The measure of blood pumped per minute is referred to as the cardiac output.

The heart can also increase in size due to coronary disease. As horses age, changes in the heart can make it make it more difficult for one or more of the heart chambers to keep up with the flow of blood. The body initially adapts to this problem by increasing both the muscle mass of the heart and the size of the chambers. Since a larger heart can move more blood, this mechanism works for a while. But eventually, the heart becomes so large that its size simply adds to the stress the heart is experiencing.

Fortunately, horses seldom suffer from heart problems. Both heart attacks and degenerative heart disease in horses are rare. When these do occur, they are often the result of a secondary issue, such as heart damage from migrating internal parasites. Unfortunately, when heart disease is present in horses, performance can be adversely affected.

Heart disease in horses can be caused by either congenital heart defects or acquired heart disease. Congenital heart defects are present at birth and are usually discovered within the first few months of life. They are most often detected by the presence of a heart murmur, which is an unusual or extra sound that is heard during a stethoscope examination.

Acquired heart disease is slightly more common in horses and is the result of degenerative changes to the heart due to aging. Diagnosis usually occurs when heart rhythm irregularities or heart valve leakage are detected by a veterinarian. There are, however, symptoms of heart disease that can be observed when the horse is undergoing moderate or strenuous exercise. If heart problems are severe, these symptoms may also be present during rest or normal activity.

Outward symptoms of heart disease in horses include:

EQUINE HEART DISEASE

At a gallop, the equine heart can beat four times per second. During strenuous exercise, the horse's heart can pump about 65 gallons of blood per minute.

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