Common Heart Disease in Dogs and Cats
December 14, 2014
When we think of heart disease, cholesterol and heart attacks usually come to mind. In our dogs and cats, however, that type of heart disease is extremely rare. When it comes to heart disease, in dogs we commonly see a type of heart valve disease called mitral valve disease (MVD, sometimes also called mitral valve insufficiency or mitral valve regurgitation) while in cats we most commonly see a type of heart muscle disease called hypertrophic cardiomyopathy (HCM).
To understand these diseases, it is helpful to understand how blood circulates through the heart. The heart is made of four chambers: Right atrium, right ventricle, left atrium, and left ventricle. In real time, the smaller atria contract together and then the larger ventricles contract together so there is a constant flow of blood to, within, and from the heart; this also causes the classic “lub-dub” sound that is heard when listening to the heart with a stethoscope. However, it is easier to think about how the heart works by following the path of blood flow through the heart one chamber at a time.
First, de-oxygenated (“used”) blood returns to the right atrium from the body through large veins known as the superior and inferior vena cava. The right atrium then passes the blood along to the right ventricle, which pumps the blood through the pulmonary artery to a network of capillaries (tiny blood vessels) in the lungs where the blood gets “recharged” with oxygen. Blood returns from the lungs through the pulmonary vein into the left atrium, which passes the blood to the left ventricle. It is the powerful left ventricle which pumps the blood to the rest of the body through a large blood vessel called the aorta, which splits into smaller and smaller branches.
Mitral Valve Disease
There are one-way valves found within the heart and blood vessels that normally form a tight seal to ensure that blood always flows in one direction only. This increases efficiency and decreases the work load on the heart. Specifically, the mitral valve is found between the left atrium and the left ventricle inside the heart. In MVD, the mitral valve becomes “leaky,” usually due to degenerative aging changes, and allows a small amount of blood to squirt back in the wrong direction from the left ventricle to the left atrium when the ventricles contract. This is known as “regurgitation.”
In early stages, the leaky valve does not typically cause any symptoms at all. However, because the left ventricle is so strong, this regurgitation causes significant back pressure into the left atrium and by extension into the capillaries of the lungs. Over time, this chronically increased back pressure causes the left atrium to enlarge in an attempt to compensate against the increased pressure. As the atrium enlarges, dogs may begin to show symptoms such as a soft cough (typically after getting up from lying down, when excited, or after exercise) or exercise intolerance (tiring easily). Eventually, in late stages of the disease the back pressure may overwhelm the atrium’s ability to compensate; when this happens we see hypertension within the lungs themselves, which squeezes fluid out of the capillaries into the lung tissue. This is called congestive heart failure, and symptoms include an increased respiratory rate (breathing fast), labored breathing, and/or a wet cough.
MVD is most commonly diagnosed in middle aged, medium to small breed dogs. Cavalier King Charles Spaniels are especially at risk, as MVD is found in a high percentage of individuals in this breed and they are often affected at a younger age than other breeds. Typically, the first sign of a problem is that a new murmur is heard on a routine yearly examination; the regurgitation causes turbulent blood flow within the heart that creates the abnormal sound we call a murmur. The age, breed, and lack of prior murmur or symptoms are usually very suggestive of mitral valve disease; however, to definitively diagnose MVD an ultrasound of the heart called an echocardiogram must be performed as part of a cardiac evaluation. Some dogs live with compensated MVD throughout their entire life without ever going on to develop congestive heart failure, while other dogs’ disease will eventually progress; typically this happens slowly over the course of years. Congestive heart failure is diagnosed by clinical signs with a previous diagnosis of MVD, and chest x-rays which will show fluid in the lung tissue (called pulmonary edema).
Humans with heart valve disease can receive artificial valve replacements; however, this is not a practical or affordable solution in dogs. It used to be believed that starting certain medications as soon as a murmur was confirmed to be caused by MVD could slow the progression to congestive heart failure, but studies have since shown that this is not true. Typically, no treatment is started until dogs show signs of congestive heart failure; at that point, the disease can be managed with a combination of medications to remove fluid from the lungs and help the heart pump more efficiently. Specific medications can be discussed with one of our doctors. While every dog is an individual, many dogs who have experienced an episode of congestive heart failure can be maintained with medication for months to years.
Hypertrophic cardiomyopathy
While MVD is a disease of the heart valves, HCM is a disease of the muscles of the heart wall. In HCM the muscles of the heart, particularly the left ventricle, become extremely thickened. This has several harmful effects: 1.The walls become stiffer and less flexible; this means they are both less able to relax between beats and cannot contract strongly enough to move blood out normally. In particular, the left ventricle cannot move blood effectively to the rest of the body, which causes back pressure into the left atrium and lungs (similar to the back pressure caused by MVD). This can eventually lead to congestive heart failure. In addition, when the weakened left ventricle is unable to completely empty itself, the blood “stuck” there may form blood clots that can lead to dangerous complications. 2. The thickened walls reduce the size of the heart chamber and therefore the volume of blood the heart can pump to the body. This means that organs and tissues may not receive the oxygen they need. 3. The thickened walls can also interfere with the normal function of nerves in the heart, preventing the signals that are needed to coordinate beats and control heart rate. This can lead to irregular heartbeats (arrhythmias); in extreme cases, the heart can suddenly stop beating.
HCM can sometimes be secondary to other diseases such as hyperthyroidism (overactive thyroid), systemic hypertension (high blood pressure, usually secondary to kidney disease in cats), or congenital heart defects. HCM that is not secondary to another disease is inherited in cats. A specific mutation in Maine Coon and Ragdoll cats can be detected with a genetic test. Many other purebred cats are predisposed to HCM, but any cat of any lineage can be affected. Purebred cats are often affected younger (by 5 years old) than non-purebred cats, who are usually affected in middle age.
Symptoms of HCM vary widely. Many cats show no symptoms at all. In some cats a murmur, an abnormally fast heart rate, or an irregular heartbeat might be heard on a physical exam. Because cats are so good at masking signs of illness, early symptoms may be vague, go unnoticed, or be attributed to aging: For example, decreased appetite or activity, increased respiratory rate, or tiring easily. In rare cases, sudden death may occur without previous symptoms. If a cat forms blood clots as described above, the clots can eventually be passed into the general circulation where they can cause a blockage called a thromboembolism. Affected cats usually experience sudden, painful paralysis of the back legs, but this can also happen in the lungs and cause sudden death.
Like MVD in dogs, an echocardiogram as part of a full cardiac evaluation is the best way to diagnose HCM in cats. Like MVD, HCM is not curable but it can be managed with medications. The prognosis for HCM is extremely variable. Mildly affected cats have a relatively good chance of being managed for months to years, while more severely affected cats’ disease may progress rapidly; each cat must be treated as an individual. General goals of treatment are to slow the heart rate (which may help the walls of the ventricle relax and the chamber to fill more fully), correct any abnormal heart rhythms, reduce the risk of blood clots, and/or treat congestive heart failure. Specific medications are tailored to each individual and should be discussed with our doctors.
As usual, please contact the clinic to speak with one of our doctors if you have any questions about your pet’s symptoms, diagnosis, or management.
