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February 14, 2023 - Heart disease is a major health concern not only for humans, but for our pets, too.

A Dynamic, Complicated Organ
A specialized muscle, the heart is designed to fill with blood and then contract, ejecting blood to begin its journey to all parts of the body. Inside the heart, valves help direct blood flow in the proper direction. An electrical system provides the signals to direct the heart muscle to contract in a regular and orderly fashion. And thousands of blood vessels, from the large and powerful aorta to the tiniest capillaries, serve as the conduits for blood as it travels throughout the body.

Over time, heart function can be compromised by a multitude of factors, including illness, genetics and other stresses. The heart can compensate to a point but eventually function will decline, leading to signs of heart failure. To understand how this happens, let’s start by looking at diseases of the heart muscle, and then move to diseases affecting the other main parts of the heart: valves, blood vessels and the electrical conduction system.

Cardiomyopathy is the general term applied to diseases affecting the heart muscle, and it can take several forms. For example, dilated cardiomyopathy is type of heart disease characterized by a thinning of the heart muscle, decreasing the heart’s ability to contract. In contrast, hypertrophic cardiomyopathy is a disease characterized by thickening of the heart walls. Although initially the heart is able to contract normally, as the muscle thickens, the heart is less able to contract. Less blood enters the heart, so output decreases. Eventually, the heart muscle fails. There are lots of intermediary and combinations of heart muscle disease, but all forms begin as abnormalities of the muscular structures of the heart.

Dilated Cardiomyopathy and Hypertrophic Cardiomyopathy
Dilated cardiomyopathy is most commonly seen in large-breed dogs, especially in certain breeds such as Doberman pinschers, Great Danes and Irish wolfhounds, although many breeds are known to develop the disease. The disease tends to affect middle-aged to older dogs and is treatable if caught early. Less commonly, dilated cardiomyopathy can be the result of certain chemotherapy drugs and nutritional abnormalities.

In cats, hypertrophic cardiomyopathy (HCM) is not only the most common type of heart muscle problem diagnosed, it is the most common heart disease of cats overall. As its name implies, hypertrophic cardiomyopathy is characterized by thickening of the heart muscle. Cats with HCM also are prone to forming abnormal blood clots, which block major blood vessels, cutting off blood supply.

There is a genetic component implicated in some cat breeds (for example, Maine coon cats), but many mixed-breed cats also suffer from this problem. As with all types of heart disease, early diagnosis is the key to successful treatment and longer, healthier lives.

Arterial Thromboembolism
The formation of large, abnormal blood clots, known as arterial thromboembolism, is a common problem in cats with cardiomyopathy.

The most common place for a large clot to lodge is in the distal aorta as it splits into the main vessels that supply each leg, often called a saddle thrombus. This results in a loss of blood flow to both rear legs, resulting in extreme pain and loss of movement in the rear legs.

Other organs also can be affected by abnormal blood clots, such as the kidneys and brain. Although treatable, prevention of clot formation through specific treatment of the clot and treatment of the underlying heart problem is critical.

Heart Valves and Abnormal Blood Flow
Heart valve problems are another type of heart disease that affects cats and dogs. There are four valves in the heart, two on the right side of the heart (the pulmonic and tricuspid valves), and two on the left (the aortic and the mitral valves). Disease affecting the valves results in abnormal blood flow in the heart (valve disease is a common cause of murmurs), which places strain on the heart as it tries to compensate for abnormal flow through thickening of the muscles and dilation of the chambers. Eventually the heart can no longer compensate and begins to fail.

Myxomatous Degeneration of the Mitral Valve in Dogs
Myxomatous degeneration of the mitral valve accounts for nearly 75% of all heart problems seen by veterinarians, making it the most common heart disease diagnosed in dogs. Affecting mostly middle-aged to older small-breed dogs, the disease is a common cause of heart murmurs detected in dogs.

The mitral valve separates the left atrium and the left ventricle. As the valve degenerates, blood can flow backwards through the valve, resulting in volume overload on the heart and a decrease in blood flow out of the heart into the body. Mitral valve disease is treatable when identified early, and many dogs with the diseases have an excellent quality of life.

Subvalvular Aortic Stenosis
Subvalvular aortic stenosis is the most common congenital heart defect in dogs. The disease is a combination of muscular and valve problems. Thickening of the tissue just below the aortic valve causes a narrowing of the outlet between the muscular left ventricle and the aorta, the main artery leading blood away from the heart. The disease often is hard to detect, and many dogs have no signs of heart disease. Unfortunately, the disease can cause sudden death in severely affected dogs. Breeds predisposed to this defect include Newfoundlands, golden retrievers, Rottweilers and boxers, although it has been reported in many other breeds. Genetic testing is available as a screening tool.

Interestingly, cats rarely develop primary heart valve problems, although the valves can be affected by other heart diseases.

Arrhythmias or Irregular Heartbeats
The heart’s rhythm also is an important component of adequate function. If we think of the heart as having an electrical system that sends signals to coordinate heart muscle contraction and relaxation, any short circuit in the system can lead to an irregular heartbeat and then decreased function. When the heart begins to beat irregularly (arrhythmia), the heart may not fill properly with blood and not eject blood consistently or completely. Different areas of the heart can start generating their own electrical activity, compounding the problem. These compensatory mechanisms eventually will fail.

Many arrhythmias in our pets are secondary to other abnormalities, such as electrolyte abnormalities, hyperthyroidism in cats, and shock or trauma. Others are secondary to underlying heart disease, such as dilated cardiomyopathy. Treating the underlying problems often resolves an irregular heartbeat, but occasionally medication specifically aimed at correcting the abnormal heartbeats is needed.

Coronary Artery Disease
Finally, a quick word about coronary heart disease. Dogs and cats rarely get coronary artery disease, which is a huge problem in people: the classic heart attack. Although myocardial infarctions (blockages of the coronary arteries) have been reported in the veterinary medical literature, they are rare.

Regular Veterinary Visits Essential!
Routine veterinary visits can help detect emerging heart problems, and owners can watch for common signs of heart disease such as fatigue, decreased appetite and increased respiratory rate and effort. Many heart diseases are treatable, but only when caught early and treated appropriately.

What We're Doing to Help
Morris Animal Foundation has been funding heart disease studies in dogs and cats since 1960. We’ve funded many studies on heart disease that are making a difference now to help our pets live longer, happier lives.

In 2002, we funded a study looking at radiofrequency catheter ablation to treat life-threatening arrythmias in dogs. The technique is now in use in clinics with an astounding 95% success rate, saving countless lives.

A recently completed study might be a game changer for cats with hypertrophic cardiomyopathy. Early detection of HCM can be challenging in cats. Definitive diagnosis requires a heart ultrasound that can be costly and requires specialized training. A Foundation-funded team developed a quick, two-minute focused cardiac ultrasound procedurethey taught to general practitioners. The team found that these practitioners were 93% successful at diagnosing cats with moderate heart disease and 100% successful at diagnosing severe heart disease in cats with no clinical signs!

Another exciting Foundation-funded study led to the development of a diagnostic test for subvalvular aortic stenosis (SAS) in Newfoundland dogs. The test has impacted breeders and lovers of these gentle giants and paved the way to keep heart disease out of the breed. The Foundation recently funded another study that will expand on these initial findings and could help not only Newfoundland dogs, but other breeds prone to SAS as well.

We’re passionate about helping dogs and cats with heart disease live longer, healthier lives. Learn more about all our cardiovascular studies and what you can do to help.

Take a Listen to Learn More!
Our Fresh Scoop podcast is a great place to find more information about heart disease in dogs and cats - check out these episodes: