Dr. John D. Bonagura, D.V.M., M.S., Dr. Kate Meurs, D.V.M.,
Ph. D., Dr. Virginia Luis Fuentes, D.V. M.
Diseases of the heart can be subdivided into cardiac malformations (birth defects of the heart or major blood vessels)
and acquired diseases of the heart. Malformations – also called congenital heart defects - are present at birth. Most congenital defects are believed to be genetically predisposed. Acquired heart disease refers to a problem that is
not present at birth, but is "acquired" during life. Many acquired diseases have a genetic basis, but the actual condition is not evident until the dog grows or is fully mature.
Doberman pinschers are particularly prone to an acquired heart disease that affects the heart muscle. This condition is properly called dilated cardiomyopathy, and is often referred to by breeders by the nonspecific abbreviation "cardio."
Normal Heart Function
The normal canine heart consists of four chambers covered by a thin membrane (the pericardium). Blood returns to the heart via veins, emptying into blood reservoirs (the left atrium and right atrium). Two muscular chambers pump blood to the body (left ventricle) and to the lungs (the right ventricle). The filling and pumping of the heart are
coordinated by an electrical system that begins in the cardiac pacemaker (the sinus node) located in the right atrium. Current spreads systematically across the atrial chambers, is delayed slightly in the atrioventricular node, and then rapidly spreads downward across the muscle of the ventricles. Electrical activation of heart muscle cells
stimulates contraction of the cell and the entire heart chamber. Coordinated contraction and relaxation of the heart is responsible for the development of blood pressure in the arteries and the circulation of blood throughout the body. Blood is kept moving in one direction within the heart chambers by a series of heart valves. There are four valves.
One is located between the left atrium and left ventricle (mitral valve) and another is between the right atrium and right ventricle (tricuspid valve). There are two additional valves located at the outlets of the two ventricles. These are named for the great vessel connected at that site, the aortic valve and the pulmonic valve. Heart disease involves
one or more of these structural components: the pericardium (pericardial diseases), the heart muscle (myocardial diseases), the heart valves (valvular disease), the electrical system (arrhythmias), or the blood vessels (vascular diseases).
Congenital Heart Disease
Congenital heart disease (CHD) includes a number of specific malformations of the heart and of the great vessels exiting the heart (aorta, pulmonary artery). Though congenital heart defects are not common in the Doberman pinscher when compared to other breeds, a number of malformations have been observed by veterinary cardiologists over
the years. Examples of congenital heart defects include the following:
_ Malformation (dysplasia) of the atrioventricular valves (mitral valve, tricuspid valve)
_ Malformations of ventricular outflow tracts leading to obstruction of blood flow (aortic
stenosis, pulmonic stenosis)
_ Defects of the cardiac septa (atrial septal defect, ventricular septal defect)
_ Patent ductus arteriosus (PDA)
_ Abnormal development of the great vessels or other vascular structures (such as
persistent right aortic arch)
_ Complex, multiple, or other congenital disorders of the heart, pericardium, or blood
vessels (such as the tetralogy of Fallot)
Detection - Virtually all congenital heart defects can be identified by veterinarians through careful use of the stethoscope (cardiac auscultation). This is very important, especially during the veterinary examinations conducted between seven and 16 weeks of age. The hallmark feature of CHD is a heart murmur. It is rare for a CHD to be present without a murmur. Infrequently, a murmur is present at birth but escapes detection during the initial examinations. The greatest difficulty arises in distinguishing a soft innocent (or puppy) murmur from that caused by CHD. Most innocent heart murmurs become softer and are no longer evident by the time the pup is 4 months of age. Should a heart murmur persist, a veterinary cardiologist should be consulted if possible.
Hereditary basis - There is no doubt than many, if not most cases of CHD have a genetic basis. The work of Dr. D. Patterson in the 1960’s and 1970’s established clearly the genetic basis of CHD in a number of breeds (the Doberman pinscher has never been systematically studied for CHD). The mode of inheritance is rarely straightforward, and it may be difficult to determine if the sire, dam, or both are at fault. Currently there are no blood or tissue tests for determining carrier states. If CHD is detected in a dog, that dog should never be bred, even if the defect can be corrected surgically or by a cardiac catheterization. A veterinary cardiologist or specialist in congenital heart diseases should be consulted if possible.
There are both breed and organizational systems of registry for CHD in dogs. Some breed organizations informally register dogs by requiring a letter describing the results of a cardiologist’s physical +/- Doppler echocardiographic examination. In the OFA system,dogs can be provisionally approved as puppies, but must be 12 months of age or older to receive final certification as "free from congenital heart disease." Specific forms are used by some registry organizations. Examinations can vary from a simple auscultation of the heart with a stethoscope to more detailed studies such as echocardiography or ambulatory electrocardiography (Holter ECG).
Congenital vs. genetic? It is crucial to distinguish a congenital heart defect – a disorder present at birth – with an acquired heart disease. While an acquired heart disease condition, such as dilated cardiomyopathy, can most certainly have an underlying genetic basis, the disease is not considered a congenital heart defect. For this reason, registry organizations, such as the OFA, do not list cardiomyopathy under congenital heart diseases. Some organizations establish separate registries for congenital and for acquired heart diseases.
Acquired Heart Diseases – General Considerations
Acquired heart diseases include a variety of disorders. These can be summarized as diseases of the:
_ Pericardium (the lining around the exterior of the heart)
_ Heart valves (degeneration of the valves, infection of the valves or endocarditis)
_ Myocardium (disease of the heart muscle)
_ Impulse forming and conduction system of the heart (abnormal electrical
activity of the heart, also called cardiac arrhythmias)
_ Blood vessels (for example, heartworm infection injures the heart by
damaging the blood vessels in the pulmonary arteries)
Heart disease can also develop secondary to a problem elsewhere in the body, for example:
_ Moderate to severe anemia can lead to heart enlargement and can precipitate heart failure in a dog with underlying heart disease (such as cardiomyopathy)
_ Systemic hypertension (high blood pressure) from chronic kidney disease can cause the heart muscle to thicken (hypertrophy)
_ Severe hypothyroidism (emphasis: very severe, not the usual forms) can decrease heart muscle function. However, there is no evidence that cardiomyopathy is caused by hypothyroidism.
_ Excessive administration of thyroid hormone can increase the demand for heart work, enlarge the heart, and cause fast or irregular heart rhythms.
Dogs at greatest risk are those taking relatively high doses (doses exceeding 0.75 mg twice daily). This condition can be detected by having the blood concentration of thyroid checked periodically (once a stable dose of thyroid supplementation has been initiated). Many Doberman pinschers receive thyroid supplementation, and this condition may be more common than realized.
_ Tumors of the chest can press on the heat and impair heart function.
The clinical signs of the above conditions are quite variable and a complete review of each disorder is beyond the scope of this discussion. Dogs with mild disease may appear completely normal. Common signs of heart disease include tiring or exercise intolerance, difficulty breathing or respiratory distress, inability to be comfortable when lying down, coughing, fainting, or collapse. NONE of these symptoms is specific for heart disease, but cardiac conditions must be considered. Diagnosis of the above conditions can be made by a combination of physical examination, x-rays, ultrasound examination of the heart (echocardiogram), electrocardiogram, and laboratory tests (usually blood tests). The acquired disease of primary importance to the Doberman pinscher breed is dilated cardiomyopathy, which is addressed below.
Dilated Cardiomyopathy in Doberman Pinchers
Definition - Cardiomyopathy in the Doberman pinscher is a genetically predisposed disease of the heart muscle. This heart muscle disease is "primary" and cannot be explained by another medical or cardiac problem. For example, cardiomyopathy in the Doberman pinscher is not caused by an endocrine (hormone) disorder, by diet, by narrowing of the coronary arteries, or by a heart valve problem. Many breeders refer to this heart condition by the abbreviation "cardio." Though this "word" is well entrenched among breeders and dog owners, it is neither an accepted name nor abbreviation for any disease (it is a prefix meaning "heart"). "Cardiomyopathy" (meaning heart muscle disease) is less ambiguous and is a preferable word. Dilated cardiomyopathy is the most precise term for advanced cardiomyopathy associated with a failing heart muscle.
Dilated cardiomyopathy (DCM) occurs in many different species of animals as well as in people. The essential abnormality of DCM is inability of the heart muscle cell to contract normally. Furthermore, the heart chambers dilate increasing heart size. The best way to identify this reduced muscle contractility is with an echocardiogram (ultrasound examination of the heart). The precise trigger for this failure of muscle contraction is unknown, but is most certainly related to the presence of certain genes (alleles). Other factors (for example, sex, age, or specific breed line) probably influence the onset, development, or severity of DCM. These factors, while not completely understood, may explain some of the variation observed in this disease. The mode of inheritance has not been conclusively determined, nor is there currently a blood or tissue test available to identify affected individuals or carriers. The entire situation is very frustrating to breeders and veterinarians alike. The disease can develop at almost any age (from less than 6 months to greater than 10 years). There is no simple method to identify mildly affected dogs. Many dogs with otherwise excellent traits go on to develop this condition (often after successfully breeding for many years).
There are two major clinical features of cardiomyopathy in Doberman pinschers. The first is reduced heart muscle contraction that can lead to heart failure (classic DCM). The second is electrical instability of the heart (arrhythmia) that leads to a heart rhythm that is too fast, too slow, or too erratic. These electrical disturbances often begin in the lower chambers of the heart, the ventricles. In their most malignant form, these arrhythmias can cause fainting or sudden cardiac death. Electrical disturbances can develop as an isolated problem (sometimes-called arrhythmogenic cardiomyopathy), or in association with obvious DCM. C. Calvert and M. O’Grady, two veterinary cardiologists who have worked extensively with this disease, have both suggested that isolated ventricular rhythm disturbances are quite common as an early sign of cardiomyopathy that will eventually progress to DCM.
Occult Dilated Cardiomyopathy - Very subtle decreases in heart muscle contraction are not likely to be detectable by any clinical method, and these dogs represent "carriers" for the disease of DCM. Mild heart muscle failure may be detectable by sophisticated examinations, such as echocardiography, but may not be evident by other routine methods (stethoscope examination of the heart, x-rays, routine electrocardiogram). Both of these situations represent what has been called occult DCM (occult = something that is obscure or hidden from view). Occult cardiomyopathy usually refers to the following situation: an apparently healthy Doberman pinscher with echocardiographic evidence of reduced heart muscle contraction.
The practice of screening for occult DCM with echocardiography is popular, but has caused some concern among veterinary cardiologists and breeders alike. There is no doubt that moderate to advanced DCM can reliably be detected by echocardiography. However, the situation is more difficult in "occult" disease. Most echo studies of the left ventricle depend on only two measurements to calculate the heart function (these are the left ventricular minor dimensions in diastole – the filling phase of the ventricle – and the ventricular dimension at end systole – after contraction of the ventricle). Dogs of most other breeds have a shortening fraction (value in diastole, minus the value in systole, divided by the value in diastole) of >25%. For example, if the diastolic dimension is 40 mm and the systolic dimension is 30 mm, the shortening fraction is 10/40 or 25%. Values of less than 25% are often considered abnormal. In fact, some laboratories use values as high as 28% to 30% as lower limits of normal. This has caused great consternation because hundreds (if not thousands) of long lived Doberman pinschers have a left ventricular shortening fraction of <25%. In fact, in a recent UDC screening (Luis Fuentes, Bonagura, Meurs, Hitchcock; unpublished data of May 1997), the average shortening fraction of dogs examined was approximately 26% using the short axis imaging approach, and only 22.5% using the long axis approach. Many of these apparently healthy dogs had a SF of <25%. These echocardiographic data have been interpreted in two ways. To some, it indicates that a large percentage of healthy Doberman pinschers have occult DCM. To others, it suggests that the normal values used for other breeds may not be applicable to the Doberman pinscher. This situation is also observed in some other larger canine breeds wherein normal values for shortening fraction often average 22 – 25% in healthy dogs.
Another concern about current practices is the use of a single dimension to characterize the contraction of a three-dimensional structure. When other echo models of left ventricular function are used to characterize heart contraction, some dogs with "low" shortening fractions become normal. These issues have sparked some debate among cardiologists, and there is not a definitive study published in the scientific literature to answer this question. Thus, in the "healthy" dog with a low shortening fraction (say 18%) and relatively frequent ventricular arrhythmias, most cardiologists would have little reluctance indicating that the dog has occult disease. However, in the dog with a normal heart rhythm, normal chamber dimensions, and a ventricular shortening fraction in the 20 – 25% range, one may wish to exert some caution before labeling the dog occult DCM. This is especially true if follow up examinations (6 to 12 months later) show no obvious progression of any disease.
The issue of the heart rhythm and the electrocardiogram is also important. There is evidence that ventricular arrhythmias may be the earliest sign of cardiomyopathy in some dogs. It is likely that many cases of sudden cardiac death in apparently healthy Doberman pinschers are due to asystole or ventricular fibrillation, the two lethal heart arrhythmias often associated with cardiomyopathy. In other words, the dog with ventricular arrhythmias may have occult cardiomyopathy despite a normal echocardiogram. These dogs may appear completely normal with the exception of an irregular heart rhythm. It may be difficult to detect sporadic arrhythmias, and for that reason, a routine electrocardiogram (EKG) is not likely to be an effective screening tool for occult cardiomyopathy. Despite suggestions that an EKG is a good screening test, there is no reported evidence that the typical 30 – 60 second EKG is useful for this purpose. Prolonged auscultation of the heart using a stethoscope (4 or 5 minutes) may be helpful, but even normal dogs can have some irregularity of the rhythm. Because of these limitations, the 24-hour, ambulatory ECG (Holter ECG) has been recommended to screen for cardiac arrhythmias and "arrhythmogenic" cardiomyopathy. These studies are best interpreted by a specialist in cardiac care. Unfortunately, such a screen is neither simple nor inexpensive. Moreover, normal results do not guarantee that DCM will not develop in the future.
If occult DCM is diagnosed, then what is the best course of action? Firstly, one must consider the degree of confidence in the diagnosis. If relatively certain, then any future breeding is not advised. Initiation of treatment in otherwise healthy dogs also requires careful consideration. The issue of "prophylactic" therapy with an angiotensin converting enzyme inhibitors (e.g., Enacard brand of enalapril; benazepril; lisinopril) i unresolved. These drugs can potentially slow the progression of heart muscle disease.
There appears to be sufficient laboratory research to recommend such therapy in clear -cut cases of occult DCM. The use of medication to control irregular heart rhythms is much more complicated and controversial and is best determined after consultation with a veterinary specialist. There are no studies reporting the long-term benefits of therapy in Doberman pinschers with occult cardiomyopathy.
Congestive Heart Failure - With moderate to severe heart muscle disease, the ability of the heart to pump can be compromised sufficiently to cause "symptoms" of heart failure.
These signs can include exercise intolerance or development of edema fluid in the lungs (pulmonary edema). Fluid accumulation in the lungs or in the abdomen (ascites) are clinical signs of congestive heart failure, a most serious condition. Cardiac arrhythmias may be present as well, including ventricular arrhythmias or atrial fibrillation. Diagnosis is usually straightforward, although the signs may resemble those caused by pneumonia or other respiratory diseases. However, a thorough clinical examination, chest x-rays,and an echocardiogram will permit an accurate diagnosis.
Treatment of congestive heart failure typically includes diuretics (furosemide),angiotensin converting enzyme inhibitors (e.g., enalapril), and often digoxin. If cardiac arrhythmias are present, other treatments may be prescribed. Dietary restriction of sodium and supplementation with nutriceuticals (e.g., L-carnitine) or omega fatty acids is sometimes recommended; however, these treatments are more controversial and should be discussed with a veterinarian. The development of congestive heart failure in a Doberman pinscher is a very poor prognostic sign, and the likelihood of survival beyond 6 months, even with good medical therapy and home care, is probably < 25%. The arrhythmia, atrial fibrillation, is a particularly ominous finding in this breed. Overall , congestive heart failure in the Doberman pinscher dog is a discouraging condition. Often this problem ends in sudden death or in euthanasia to prevent suffering from intractable heart failure.
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