Deborah M. Fine, DVM, MS, DACVIM (Cardiology)
Dilated cardiomyopathy (DCM) is the most common form of cardiomyopathy in dogs. It is the second most common heart disease in dogs, after degenerative valve disease (endocardiosis). Other less common forms of cardiomyopathy that will be reviewed include arrhythmogenic right ventricular cardiomyopathy (also known as "Boxer" cardiomyopathy), and tachycardia induced cardiomyopathy.
DCM is characterized by enlarged, poorly contracting ventricles. It usually involves both the right and left ventricles, though in some instances one side may be more severely affected then the other. The atria enlarge secondary to the failing ventricles as preload (blood returning to the heart, and increased fluid retention by the kidneys) increases to compensate for the poor cardiac output. The papillary muscles often appear flattened, and the mitral and tricuspid valve annuli become stretched, resulting in secondary valve regurgitation. The valve leaflets themselves are generally normal.
On gross examination, the heart muscle is pale and flabby, and the chambers themselves are markedly dilated. Although it is commonly believed that the chamber dilation is secondary to the heart muscle stretching like an over-inflated balloon, the actual cause is eccentric hypertrophy. Eccentric hypertrophy results in an increase in cardiac muscle mass by adding sarcomeres end-to-end within the myocardial cells. The histologic abnormalities are relatively mild relative to the degree of systolic dysfunction. There are small, scattered areas of myocyte atrophy, myocardial degeneration with necrosis, and fibrosis. Small collections of cellular infiltrates are sometimes seen (usually lymphocytes, plasma cells, and macrophages), and are usually confined to focal areas of active myofiber necrosis. The myocytes themselves take on a characteristic wavy appearance.
The specific etiology of DCM is unknown. Some proposed etiologies include viral infection, microvascular hyperreactivity, nutritional deficiency, immune-mediated, myocardial toxins, and a variety of genetic disorders.1 In many breeds, DCM has been found to have genetic basis, though the specific defects remain elusive.2,3 It is probable that more than one primary disease process can induce the same functional and morphologic abnormalities that are clinically recognized as DCM. In most cases of DCM there are no widespread inflammatory changes or extensive myocardial damage on histopathology. This suggests that one or more sub-cellular abnormalities are responsible for the development of myocardial failure.
DCM is almost exclusively a disease of large and giant breed dogs, especially Doberman pinschers, Irish wolfhounds, great Danes, Boxers, and Newfoundlands. The exception to the size rule is seen in English and American cocker spaniels who are also predisposed to DCM. The majority of dogs presented for evaluation and treatment of DCM are between 4 and 10 years, and the prevalence of DCM increases with age. DCM occurs about twice as frequently in male dogs than in females.
DCM has a prolonged asymptomatic phase known as occult cardiomyopathy. In this phase, there is systolic dysfunction observed on echocardiogram and possibly a soft murmur on auscultation, but there are no overt clinical signs. The clinical signs of symptomatic DCM include weakness, lethargy, syncope, and respiratory distress. On auscultation, a soft systolic left apical murmur of mitral regurgitation is commonly heard, usually a 3/6 or softer in intensity. Arrhythmias are also common in symptomatic DCM, and may include atrial and ventricular premature contractions, and ventricular tachycardia which may be sustained or non-sustained. The most common arrhythmia associated with symptomatic DCM is atrial fibrillation. This causes the heart sounds to be markedly irregular (like "tennis shoes in a dryer") and very fast, with rates often exceeding 200 bpm. Arrhythmias are usually accompanied by abnormalities in pulse quality. These include weak or hypokinetic pulses, pulses varying in strength from beat to beat, and pulse deficits. If the patient is in congestive heart failure then crackles and wheezes are frequent findings on auscultation. The presence of pleural effusion is indicated by the finding of muffled heart and lungs sounds.
The diagnosis of DCM is typically made using a combination of radiographs, ECG, and echocardiogram. Radiographically, the heart will appear enlarged, particularly the left atrium and ventricle. In some instances the heart will take on a globoid appearance. If heart failure is present this will be identified by the presence of an interstitial infiltrate that is most severe in the perihilar region and the caudal and dorsal lung fields. In severe cases an alveolar pattern will be present. The pulmonary veins will be distended due to the increase in preload, and the pulmonary arteries may also enlarge if pulmonary hypertension has developed.
A variety of ECG abnormalities may be identified with DCM. The R wave height of the QRS complex may increase secondary to left ventricular enlargement, and the P wave duration may be prolonged secondary to left atrial enlargement. Atrial premature complexes are identified by a normal QRS complex that appears earlier in the cycle then expected. It is usually associated with an abnormal appearing or absent P waves. Ventricular premature complexes are identified as wide bizarre biphasic complexes that are premature relative to the expected sinus beat. Atrial fibrillation is characterized by four criteria: 1) normal appearing QRS complexes, 2) markedly irregular R to R intervals, 3) tachycardia, and 4) no P waves. The diagnosis of DCM is strongly supported by abnormalities noted on radiographs and ECG, and in particular when found in a susceptible breed.
To definitively make the diagnosis of DCM requires an echocardiogram. The echocardiographic appearance of DCM is almost always unmistakable. The left ventricle takes on a very rounded appearance (normal is oval-shaped) and the contractility is very severely depressed. A commonly used echocardiographic index of systolic function is fractional shortening. This is obtained by subtracting the minimal chamber size in systole from the maximal dimension in diastole, and then dividing this by the maximal diastolic dimension to achieve a shortening ratio. The normal fractional in dogs is >27%; in dogs with DCM this ratio is often < 10%. Other abnormalities seen on echocardiogram include marked enlargement of the left atrium, and moderate right atrial and ventricular enlargement. Mild pleural or slight pericardial effusion is not uncommon.
When DCM is seen in cocker spaniels or in atypical breeds, a blood taurine level should be obtained to test for deficiency. Taurine deficiency has been associated with a form of cardiomyopathy that has been found to be reversible in some patients, and therefore carries a potentially good prognosis.
The mainstay of treatment for all dogs with congestive heart failure is diuretics (furosemide) and ACE inhibitors (enalapril). Dogs with DCM will also need drugs to help improve contractility. Until recently the only option was digoxin. Digoxin has a unique combination of actions: it increases force of contraction while decreasing heart rate. This combination is particularly useful for patients who are also in atrial fibrillation. In patients with atrial fibrillation it is our goal to decrease the heart rate to below 160 bpm in the hospital. For patients that are not in atrial fibrillation, another option has recently become available. Pimobendan (Vetmedin®) is a newly approved drug that increases the force of contraction, but has little affect on heart rate. These two drugs can be given safely in combination if one of them alone is not adequate to improve the patient's clinical signs.
If a patient is having frequent ventricular arrhythmias then antiarrhythmic therapy is probably indicated. One of the most common oral drugs for treatment of ventricular arrhythmias in DCM is mexiletine. Mexiletine is the oral equivalent of lidocaine, which can only be administered intravenously. Mexiletine is sometimes combined with sotalol for additional antiarrhythmic effects. Sotalol should be used with caution in patients with DCM as the beta blockade effects will decrease the force of ventricular contraction.
All cocker spaniels and atypical breeds with DCM should be treated with taurine pending the results of blood testing. Dogs that are not taurine deficient are unlikely to gain any benefit to supplementation, but there is also few to no side-effects from the therapy. There are a variety of other potential therapies for DCM, including various homeopathic preparations. At this time, the only alternative therapy that I routinely recommend is omega-3 fatty acid supplementation with fish oil. There is fairly good evidence to suggest that it may help improve clinical signs and decrease the severity of weight loss in these patients. There has also been shown an antiarrhythmic effect in humans and dogs. It is unlikely that severely sodium restricted diets are of benefit in symptomatic patients. Furosemide therapy causes sodium loss and it is important for the patients not to become hyponatremic. Further, these patients are often anorexic and low sodium diets are sometimes unpalatable. Although, the patients should not be fed an excessive sodium diet, the most important thing is to keep the patient eating.
Dogs with DCM in the occult stage may remain asymptomatic for years. However, once symptoms develop the long-term prognosis is poor. The average survival for a dog with DCM and heart failure is 6 to 12 months. The average survival for a Doberman with DCM is 4 to 6 months.4 Sudden death is common with DCM, particularly with Dobermans. The mechanism of sudden death is most likely ventricular fibrillation.
Arrhythmogenic Right Ventricular Cardiomyopathy
In addition to the classic form of DCM, boxer dogs also present with a cardiomyopathy that is known as arrhythmogenic right ventricular cardiomyopathy (ARVC). This is a disease primarily of dysrhythmogenisis (abnormal heart rhythm), rather than systolic dysfunction.5 ARVC is seen in breeds other than boxers, but they are most commonly affected. ARVC is caused by replacement of the normal myocytes of the right ventricle by fibrofatty cellular infiltrates. These abnormal cells are susceptible to arrhythmia formation.
There are unique ECG abnormalities that are very characteristic of ARVC. They manifest as ventricular premature complexes or ventricular tachycardia that is predominantly positively deflected in lead II. A positively deflected ventricular premature contraction indicates that the abnormal beat originated in the right ventricle and spread toward the left. The wave of electrical depolarization spreading toward the positive pole of the lead II electrode results in a positive deflection on the ECG. This is in contrast to most ventricular ectopy we see that originates from the left ventricle and propagates toward to right ventricle which results in a predominantly negative deflection in lead II. The following gives an example of a ventricular premature contraction typical of ARVC, and another that would be seen commonly with idiopathic DCM.
Click on an image to see a larger view.
Echocardiographically, ARVC often demonstrates a fairly normal appearing heart, although the right ventricle may appear dilated and the right atrium enlarged. In humans, ARVC is usually diagnosed using MRI, which can detect the fibro-fatty infiltrates characteristic of this disease. If echocardiogram demonstrates a dilated left atrium and ventricle, the diagnosis is more likely to be idiopathic DCM rather than ARVC.
ARVC is primarily diagnosed using an ambulatory ECG (Holter monitor), which records ECG data for 24 hours. A normal dog will usually have less than 25 ventricular premature contractions in a 24 hour period. Dogs are considered affected with ARVC if they have greater than 100 VPCs/24 hours. Although there are no studies that have definitively shown an affect of antiarrhythmic therapy on survival, most veterinary cardiologists consider treatment when >1,000 VPCs/24 hours are recorded. Response to therapy must be assessed with a repeat Holter examination. A 5 minute in-hospital ECG is NOT adequate. There must be a > 80% reduction in ventricular ectopy to document a treatment affect. Less than 80% reduction in ventricular ectopy may simply be the result of normal day to day variation in number of ectopic beats. Boxers presenting with syncope or weakness secondary to ARVC have a high risk of sudden death. The current recommended treatments for ARVC include Sotalol, mexiletine, and atenolol, or some combination of these drugs.
Tachycardia Induced Cardiomyopathy
Tachycardia Induced Cardiomyopathy (TICM) is a rare disease in which a persistently high heart rate causes ventricular systolic dysfunction. Echocardiographically this looks identical to classic idiopathic DCM. However, the signalment is not typical for DCM in that the patient is often a young dog, and frequently is a breed that is atypical for DCM. TICM is usually caused by an abnormal electrical connection between the atria and the ventricles known as a bypass tract. This allows conduction of a fast, often incessant tachycardia which eventually beats the heart into failure.
The ECG of TICM often demonstrates a short P-R interval, indicative of a bypass tract causing early activation of the ventricle. The diagnosis must ultimately be confirmed with an invasive electrophysiologic study. If a bypass tract is found, radiofrequency ablation can be used to destroy the tract, and return the rhythm to normal.6 In these instances, the prognosis is excellent for return to normal function and life expectancy. Medical management of these cases with antiarrhythmic therapy is difficult as they are often refractory to rate and rhythm control.
1. Tidholm A, et al. Vet J. 2001 Sep;162(2):92-107.
2. Meurs KM, et al. J Vet Intern Med. 2007 Sep-Oct;21(5):1016-20.
3. Wiersma AC, et al. J Hered. 2005;96(7):739-44. Epub 2005 Jul 13.
4. Fuentes VL, et al. J Vet Intern Med. 2002 May-Jun;16(3):255-61.
5. Meurs KM. Vet Clin North Am Small Anim Pract. 2004 Sep;34(5):1235-44.
6. Wright KN, et al. J Vet Intern Med. 1999 Jul-Aug;13(4):361-71.