Reprinted with the author's permission

Courtesy Of DOGWORLD/1990

Heart disease, though not the killer it is in human medicine, nevertheless constitutes a major cause of disease and, ultimately, death in pet animals. Heart disease can be broadly classified into congenital, or developmental, and acquired.
Most congenital disease is generally structural in nature, in which some portion of the heart or its surrounding structures fails to develop properly and is, therefore,
defective. Acquired disease, on the other hand, can affect any part of the heart, including the muscle, the inner or outer lining of the heart, or the valves, and can
be due to a host of factors, including infectious, nutrition and parasites.

CONGENITAL DEFECTS

Because these are caused by defects occurring during fetal development, they are usually discovered early in life, and may even be responsible for neonatal death.

Patent Ductus Arteriosus: This is the most common congenital defect seen in the dog and, except for a relatively uncommon form of the defect, is probably the
most amenable to surgical treatment if diagnosed early. Ordinarily, the oxygen depleted venous blood is brought to the right ventricle, from which it is pumped
to the lungs for a re-infusion of oxygen. But during fetal development, the lungs are not functioning and oxygen rich blood is brought to the fetus via the umbilical
cord. Therefore, passage through the lungs is unnecessary, and blood is diverted through a small duct between the arteries called the ductus arteriosus. This duct
closes shortly after birth, as the lungs take over the function of supplying oxygen to the body. If the duct remains open, or patent, some of the blood will continue
to pass through it, placing a strain on the circulation and reducing the total amount of oxygen available to the body. This is what is known of as a patent ductus arteriosus. Fortunately, the diagnosis can be made very early in life with nothing more than a stethoscope, as the condition produces a very distinctive sound called a "machinery murmur". Surgical correction is curative and, if caught early enough, the pup can lead a full and normal life.

Abnormalities in the Heart Muscle: Among the most serious defects affecting heart muscle development is incomplete closure of the wall, or "septum", separating the chambers of the heart. Remembering that there are two upper chambers, or atria, and two lower chambers, or ventricles, an opening, or septal defect, between the two ventricles is by far the more common and more serious.
Because the opening allows blood to pass freely between the two ventricles, oxygenation of blood is interfered with and the circulation does not proceed normally. If the opening is small enough, it will have very little effect on the circulation, and the patient can live a reasonably normal life. Larger defects, however, result in poor oxygen supply to the tissues and brain so that the mucous membranes darken, the patient becomes lethargic, tires easily, and may develop heart failure. Septal defects will occasionally heal spontaneously, but surgical repair is usually required if the animal is to lead a normal life.

Tetralogy of Fallot: This is a somewhat unusual abnormality involving the heart valves, heart muscle, the major arteries and the septa dividing the chambers of
the heart. Because of the complex nature of the problem, the type and extent of the abnormalities must be carefully evaluated before a treatment program can be
established. Treatment, which may be surgical, medical, or a combination of both, depending on the severity of the changes within the heart, may not be successful, and many patients die young.

Heart Valve Abnormalities: These consist of a variety of defects, including malformations, valves that are too narrow and those that do not close properly.
The diagnosis and evaluation requires the use of the stethoscope, and some combination of X-rays, cardiac catheterization and ultrasound. Any abnormality of the valves ultimately interferes with the circulation and, depending on the severity, must be treated or corrected, if the patient is to survive.

ACQUIRED HEART DISEASE

This is, by far, the largest group of heart diseases, and constitute a host of problems which develop in the older puppy and mature dog, including such entities as heart murmurs, cardiomyopathy, alteration in the heart rhythm and heart failure. Dogs rarely suffer from coronary heart disease, probably because of the large numbers of arteries supplying blood to the heart. However, they also have high levels of HDLs or high density lipoproteins which, in human medicine, are recognized as the so- called "good cholesterol" that helps to prevent fatty deposits from accumulating in the coronary arteries, thus protecting the heart.

Diseases of the Heart Valves: This is the most common group of heart diseases in the dog, as well as a major cause of heart failure. Any of the valves connecting
the chambers of the heart can be affected, and more than one valve can be affected at the same time. Valves, of course, do the same thing in the heart as they do in any other fluid conduction system - they operate to keep fluids moving in one direction. In the heart, they open when it contracts, to allow blood to pass from one heart chamber to another, and close to prevent the backward flow of blood when the heart muscle relaxes. There are many types, classifications, and sub-classifications of valvular disease. Essentially, however, the changes will be due to the valves being either too narrow (stenotic), or defective so that they don't close completely. When listening to the heart, varying types of "splashing" sounds, called murmurs, are heard. The character and intensity of the murmur, as well as the clinical signs that develop, depends on the valve involved and the type and severity of the defect. Clinical signs can vary from negligible to so severe as to have fluid accumulation in the abdomen, legs, lungs and tissues, cough, shortness of breath, debility and heart failure. The diagnosis depends on electrocardiographic findings, X-rays, laboratory tests, ultrasound or combinations of these procedures. An accurate diagnosis is essential if proper treatment is to be given. If the condition is found and diagnosed, and treatment begun early enough, even severely affected dogs can live long and happy lives.

Abnormalities of the Heart Rate and Rhythm: This constitutes a large number of conditions which can arise in different regions of the heart, due to a wide range
of causes. The impulse that initiates the heart beat is an electric current that arises in a specialized type of nerve tissue in the atrium, passes down a conduction path
through the walls separating the chambers of the heart, and then follows a meshwork of fibers that spread throughout the ventricles. The electric current stimulates the muscle fibers to contract, forcing blood out of the chambers and into the circulation. Following contraction of the heart muscle, there is a brief pause or rest, after which the electrical impulse initiating the contraction fires all over again. Normally, the dog has an irregular heart beat called a sinus arrhythmia, in which the heart rate variably speeds up or slows down. The heart rate can occasionally get so slow as to sometime seem as though it had skipped a beat. Since there are conduction disturbances that can interfere with the normal spread of electrical current and make the heart rate abnormally slow, it is important for the veterinarian to carefully measure the rate and rhythm, to be sure that it's a normal sinus pattern and not an abnormal, or diseased one. There are many factors affecting the rate and rhythm of the heart, including anything that interferes with the conduction fibers within the heart, damage or loss of the pacemaker - that piece of specialized tissue that actually establishes the rate and rhythm by initiating the electrical activity leading to contraction -, inflammation or irritation of the heart muscle, medications, and toxic substances. Re-establishing the normal heart beat depends on determining the underlying cause and taking specific measures to reverse it. Clinical signs are usually associated with weakness, fainting episodes, extremely rapid heart beat and circulatory disturbances. The diagnostic procedures are the same as for other heart
conditions, with the electrocardiogram playing a particularly important role. Most rhythm abnormalities produce very specific patterns on the electrocardiogram
(EKG), which can often localize the lesion within heart, thus pointing to the diagnosis. For example, ventricular tachycardia characterized by an extremely rapid heart beat, is a very serious problem that is often missed with the stethoscope, but can be diagnosed with the EKG. Making the diagnosis is essential for, if left untreated, it can lead to ventricular fibrillation - a condition in which the contraction of the heart muscle is totally disorganized and chaotic, often resulting in sudden death. Treatment depends on the particular defect involved, and can usually be accomplished with appropriate medication. Most problems associated with the pacemaker, itself, are usually best treated with the surgical implantation of an artificial pacemaker.

THE HEART MUSCLE AND ITS LININGS

There are many diseases that can affect the heart muscle and its linings, including such conditions as myocarditis, cardiomyopathy, hypertrophy, dilatation,
endocarditis and pericarditis.

Myocarditis is an inflammation of the heart muscle, usually caused by either bacterial or virus infections, although parasites and other irritants can cause it, as
well. Depending on the extent and severity of the lesions, myocarditis can be a very serious and life threatening condition. Rapid and accurate diagnosis is essential so that proper treatment can be given.

Cardiomyopathy is a condition that has only been recognized and diagnosed in the past ten years or so. The term means, literally, sick heart muscle, and is seen in several forms. There is no specifically known underlying cause for most cases, and treatment is usually directed at relieving the clinical signs in affected animals.
Unfortunately, very little can be done to halt the progression of the condition in most cases, and many animals ultimately die of heart failure.

Hypertrophy is a thickening or enlargement of the heart muscles and is not uncommon in older animals. Any muscle, exercised or worked hard, will enlarge to compensate for the extra demands placed on it. The same is true of the heart. As a result of increased demands placed on the heart due to exercise, faulty heart
valves or any condition increasing blood pressure, the muscle fibers will enlarge to compensate for the extra load placed on them. Hypertrophy, itself, is not
necessarily serious, but when it is found on X-ray or on the ECG, it is important to investigate the cause so that, if disease does exist, steps can be taken to correct
it.

Cardiac dilatation is a much more serious condition, as a result of inability to compensate for increased load on the heart muscle. The muscle wall thins and balloons out, decreasing its ability to adequately pump blood. As the condition progresses, it can lead to heart failure and death. Endocarditis is an inflammation of the inner lining of the heart wall and, when due to bacteria (bacterial endocarditis), is extremely serious and life-threatening. Fortunately, it is not seen very often in the dog, as the diagnosis is not always easy to make. Yet, the patient's life depends on a quick and accurate diagnosis. Even when the treatment is successful and the patient recovers, he is often left with serious consequences, such as thickenings on the margins of the heart valves, which prevent them from closing properly.

The pericardium is the lining that surrounds the heart and forms a bag-like structure called the pericardial sac. The lining of the sac secretes a thin, lubricating fluid which prevents irritation of surrounding tissue due to friction from the beating heart. Inflammation or infection results in a condition called pericarditis, with an increased or abnormal secretion (effusion) of the pericardial fluid. If the amount of secretion becomes excessive, it can press on the heart and interfere with its ability contract properly and circulate blood.

Echocardiography, using ultrasound, is particularly useful in the diagnosis of pericardial effusion, and can be life-saving. It is important to both treat the underlying cause of the pericarditis and remove excess fluid. Many times this can only be accomplished by directing a needle into the sac and drawing off the fluid.
Treatment is usually curative and the condition does not tend to recur.

Obviously, it is impossible to discuss all of the diseases affecting the heart, while touching only the highlights of those I consider to be the most significant. To keep your pet heart healthy, have it receive a thorough examination each year, with an annual electrocardiogram, starting at about seven years. Annual chest X-rays are also desirable. One should also know the common signs of heart disease, and be on the lookout for easily provoked, or chronic cough, especially after exercising, weakness, easy fatiguability, listlessness, cyanosis (bluish discoloration of the membranes of the mouth and tongue) and unexplained swellings of the legs or abdomen.