Heart Murmurs in Horses

Q. Is a heart murmur in horses the same as a heart murmur in people? What signs will a horse with a heart murmur exhibit? How can a heart murmur be diagnosed?

A. The average 1,000-pound horse has approximately 45 liters (about 11 gallons) of blood that the heart pumps throughout the body. The main purpose of the cardiovascular system is to pass the blood through the lungs, where it picks up oxygen, then to move it on to all parts of the body, where the oxygen is released and utilized. The heart is the center of this system and functions as a very basic pump with a series of valves.

The average-sized horse has a heart that is approximately the size of a basketball. The horse's heart is comprised (as in all species) of very dense muscle that differs slightly from the other muscles of the body. The heart is divided into four chambers with four valves separating the various chambers, as well as the entry and exit points. The heart is divided into the "right heart" and the "left heart," with two chambers on each side, one on the top and one on the bottom.

The top chambers are called the atria, and the lower chambers are called the ventricles. Blood that is poor in oxygen travels on the right side of the heart, and oxygen-rich blood (that has traveled through the lungs) travels on the left side of the heart. Blood (oxygen poor) enters the heart from both the front and hind directions relative to the heart. The large cranial (front) vena cava and caudal (hind) vena cava are the blood vessels through which all of the body's oxygen-depleted blood flows. So, the oxygen-poor blood enters the right atrium from the cranial and caudal vena cava. At the same time that the right atrium is filling with poorly oxygenated blood returning from the body, the left atrium is filling with oxygen- rich blood coming from the lungs.

The contraction cycle of the heart has two major components: 1) contraction of the atria, which gets an extra amount of blood into the ventricles, and 2) contraction of the ventricles, which moves blood out of the heart and into the lungs or the body. As the atria contract, the blood is forced into the ventricles (which have passively filled with blood), pushing just a little more blood into the larger chambers of the heart. The ventricles then contract with the pressure created, causing the main valves (the tricuspid on the right and the mitral on the left) to slam closed between the atria and the ventricles, thus causing the "lub" of the "lub-dub" sound heard when listening to the heart with a stethoscope.

The poorly oxygenated blood in the right ventricle is moved into the lungs, where it will pick up oxygen, and the well-oxygenated blood in the left ventricle is moved into the main artery of the body (the aorta), where it supplies the whole body. Just after the ventricles finish their contractions and the blood has moved out of the heart, the valves on the heart's exit points (the pulmonic valve on the right and the aortic valve on the left), slam shut, preventing the blood from flowing back into the heart. This causes the "dub" sound of the "lub-dub."

Then, the whole cycle starts over again and continues about 40 times a minute for the life of the horse.

The presence of a heart murmur is typically an indication of a valve abnormality. The two main heart sounds are caused by the valves slamming shut under back pressure of the blood as it is pumped into and out of the heart. These sounds, as mentioned before, are heard as the "lub-dub" sound, with "lub" being caused by the closure of the tricuspid and mitral valves and the "dub" being caused by the closure of the pulmonic and aortic valves. Murmurs are caused by turbulent blood flow caused by either an obstruction of blood as it is being pumped out of the heart, or the leakage of blood back into the heart caused by a "leaky" valve.

The contraction cycle of the heart is defined by the time the heart is passively filling with blood, or the time the heart is contracting and pumping blood. The passive filling time is called "diastole," and the contraction/pumping time is called "systole." The common blood pressure measurements "diastolic" and "systolic" refer to the blood pressure during each of these cycles of the heart.

Heart murmurs are characterized by the location (which valves are loudest), the character of the sound that they make, and whether they occur during diastole, systole, or both. They are graded based on loudness on a scale of 1 to 5 or 1 to 6.

The most common heart murmur that horses get is an aortic insufficiency murmur, which is caused by a leaky aortic valve. Because the murmur is caused by a leakage of blood back into the heart from the aorta, it occurs during diastole--just after the heart relaxes, the blood backflows into the heart, causing turbulence and thus the murmur.

The murmur is obviously loudest over the aorta and is typically loud and very "musical" in sound quality.

To contrast this, a much less common murmur is an aortic "stenotic" murmur. Stenosis means "narrowed." If the aorta is narrowed where it attaches to the heart, there will be turbulence as the blood is pumped through the narrowed area--in this case, the murmur would be during systole.

The aortic insufficiency murmur is caused by the development of an irregular edge to the aortic valve; the murmur is thought to be an "old-age" change and generally does not cause a physical problem. The murmur can be disturbing because it typically is very loud, but if it is solely aortic insufficiency, there is little overall effect on the health of the horse. However, the heart should be evaluated thoroughly to ensure that there are no other problems.

A more serious murmur in horses is a mitral insufficiency murmur. The mitral valve is the valve between the left atria and left ventricle and slams closed when the left ventricle contracts, moving oxygenated blood out of the aorta and to the body. The murmur is during systole and typically causes a more serious problem. This murmur can be caused acutely by the rupture of some of the support structures of the valve. The mitral valve has leaflets with cord-like tissue that attach to the free edge of the valve and to the heart muscle in the ventricle (the valve is almost like a parachute with the cords). If the cord tissue ruptures, the valve will flap backwards into the atria and become leaky.

The signs of acute mitral valve rupture are extreme exercise intolerance (many pull up while working and are extremely prostrate), collapsing, tremors, high heart rate, rapid breathing, blue gums, colic, and/or death. A more chronic mitral valve murmur might have no clinical signs or just exercise intolerance. The aortic insufficiency and mitral insufficiency murmurs are two of the relatively common heart murmurs that horses develop, but there are others that can develop that are caused by turbulence of blood flow within the heart for a variety of reasons.

The diagnosis of heart murmurs obviously is based on hearing them with the stethoscope. A common clinical sign, if the murmur is causing a problem, is exercise intolerance. Once a murmur is detected, the most accurate and sensitive way to evaluate the heart and the murmur's extent is by using diagnostic ultrasound. The use of ultrasound will allow the direct imaging of the heart--the chamber size, muscle thickness, valve structures, and a number of other details that will aid in determining the significance of a heart murmur.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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