Heart Murmurs and Valvular Heart Disease in the Horse
Dr. Melissa Mazan
The heart is the most important muscle in the body – without the heart, none of the other muscles would be able to get the fuel that they need – oxygen. The job of the heart is to pump blood that receives oxygen in the lung to the entire body – and certain areas of the body, especially the brain, cannot survive without the oxygen in the blood for more than a few minutes. The right side. The right side of the heart receives blood from the body that is depleted of oxygen and is full of carbon dioxide (waste product) and pumps it to the lungs. The lungs take this deoxygenated blood, and remove carbon dioxide, while adding oxygen from inspired air.
The heart has two sides or halves – the left and right.
The left side. The left side of the heart receives the oxygen-rich, or oxygenated blood, and pumps it to every part of the body.
It is very important that oxygenated and deoxygenated to be kept separate so that the body can get plenty of pure oxygen for fuel. For this reason, the heart has four one-way valves that function to keep the blood flowing in the right direction – which is critical for the survival of any individual.
The right heart has the tricuspid and pulmonic valves. The left heart has the mitral and the aortic valves. Deoxygenated blood flows from the body to the right heart, where it goes into a holding chamber called the right atrium. It then goes through the tricuspid valve and into the pumping chamber called the right ventricle. The blood then flows through the pulmonic valve and into the lungs.
Once the blood circulates through the lungs it becomes oxygenated blood and returns to the heart. Here it goes into the left atrium, through the mitral valve, into the left ventricle, and then out through the aortic valve and to the rest of the body, where it is distributed into body as fuel. It is this series of valves that keep the blood organized and flowing in the right direction.
Valvular heart disease interferes with the normal rate and smoothness of blood flow through the heart. Valvular disease in horses usually leads to congestive heart failure (CHF), and, indeed, is the most common cause of CHF in horses.
If a sufficient amount of blood flows in the wrong direction, the affected individual will be oxygen-deprived. The heart chambers that receive the extra blood (flowing in the wrong direction) will also get overloaded – a leaky valve can produce a futile cycle for the heart. No matter how hard it tries to pump the blood out, more blood leaks back, increasing the work load further. Eventually, the heart becomes deformed (dilated or hypertrophied) and may fail.
Valvular heart disease can be due to problems that were present at birth (congenital disease), or it can be acquired. The most common congenital causes of cardiac disease include stenosis (narrowing) of one of the valves and leaky valves.
Regurgitation is a backward flowing of blood through a defective heart valve, and the most common types of valvular heart disease in the horse include
What to Watch For
Decreased cardiac output. The most important feature of CHF is markedly decreased cardiac output.
Murmur. Murmurs are audible through a stethescope. The signs may only be seen when the horse is exercising strenuously.
Exercise intolerance. As the disease progresses, you may note that your horse has exercise intolerance, even at slow paces.
Venous distention. He will eventually develop generalized venous distention, as well as edema of the legs, or under the belly.
Cough. Some horses may cough and, if the cardiac failure is sufficiently profound, then he may have froth coming from his nostrils – this is a manifestation of pulmonary edema.
Increased heart rate. With severe CHF, the heart rate will increase substantially in an attempt to push enough blood around the body.
Although a common symptom of valvular heart disease in the horse is a heart murmur, it is important to remember that a heart murmur does not necessarily mean that your horse has CHF or even heart disease.
A heart murmur, whether in man, or horse, or any other species, simply tells us that there is turbulent blood flow somewhere in the cardiovascular system. Murmurs denote the release of vibratory energy into the fluid and tissue that surround this disturbance, which is sometimes compared to the sound produced by a violin string. For example, a valve that is vibrating will make a noise like a murmur.
In some cases, the heart murmur is an indication of a malfunctioning heart. In other cases, it has no real health significance at all. Even when there is cardiac disease (that is, there is some anatomical or physiological abnormality associated with the heart), the horse may not show any signs of cardiac failure.
Because horses have very large hearts and exceptionally high blood flow, they are more prone to having audible heart murmurs without having actual heart disease. Nonetheless, cardiac dysfunction is thought to be a very important cause of poor performance in the horse, ranking only behind musculoskeletal problems and respiratory disease as a cause of exercise intolerance.