Dilated Cardiomyopathy & Arrythmogenic Right Ventricular Cardiomyopathy in the Dog

Michael Luethy, DVM
Chicago Veterinary Emergency & Specialty Center
Chicago, IL, USA

The term cardiomyopathy denotes a disease process in which the primary feature involves structural abnormality and functional impairment of the heart muscle. Primary cardiomyopathies result from abnormalities of the myocardium itself and not from other systemic diseases or other congenital or acquired heart disease. Secondary cardiomyopathies result from other well defined disease processes and may be broken into a number of subcategories such as metabolic, infiltrative, fibroplastic, genetic, neuromuscular, and physical agents.

Cardiomyopathies may also be classified by other factor such as morphologic phenotype, effect on myocardial function or pathophysiology. Unlike feline cardiomyopathies in which hypertrophic cardiomyopathy represents the predominant functional and structural phenotype, canine cardiomyopathy is phenotypically dominated by the dilated class. Canine dilated cardiomyopathy (DCM) has established genetic etiologies in the Doberman pinscher, Irish wolfhound, New Foundland, Portuguese water dog, and Labrador retriever. Genetic etiologies have also been suggested in the American cocker spaniels, Great Dane, and German shepherd. Hypothyroidism, tachyarrhythmias, Chagas disease, and anthracycline toxicity have also been associated with DCM in the dog. Idiopathic DCM is a diagnosis of exclusion and still accounts for the majority of canine cases.

Successful management and treatment of the patient requires logical and consistent treatment decisions that take into account the severity of the disease and likely treatment benefit(s) to be obtained. Treatment and management decisions in veterinary medicine are limited by a lack of prospective studies with enough statistical power to provide useful information on response to various therapeutic options. The ABCD system of heart disease staging allows DCM cases to be placed in a consistent staging system and facilitates reasonable and consistent decision making with the best data available.

Class Description
Stage A Patient at high risk for developing heart disease but that currently does not have evidence of structural disease
Stage BstrongPatient with structural heart disease but that has never had clinical signs of heart failure
B1: Asymptomatic patient with no radiographic or echocardiographic evidence of cardiac enlargement or remodeling
B2: Asymptomatic patient with radiographic or echocardiographic evidence of cardiac enlargement or remodeling
Stage C Patient with past or current clinical signs of heart failure associated with structural heart disease
Stage D Patients with advanced heart failure having clinical signs that are refractory to standard therapy

Stage A DCM patients require no medical therapy or intervention. The emphasis for at risk patients includes owner education and screening programs for at risk breeds or patients that will be receiving anthracycline therapy (echocardiography or Holter monitoring).

Stage B patients may be identified because of the presence of a soft murmur or gallop or the presence of ventricular arrhythmias noted on a routine physical examination, but the majority of these patients will go undetected or be identified through a screening program. A careful patient history, cardiac and pulmonary auscultation, and thoracic radiographs are indicated in all stage B patients. Blood pressure measurement, ECG rhythm strip, 24-hour ambulatory ECG recording, event monitoring, echocardiography, NT proBNP, and other clinical laboratory tests may be beneficial in many patients.

Optimal therapy for stage B1 patients remains uncertain. Extrapolations from human medicine and theoretical considerations suggest that beta blockade or antiarrhythmic therapy may be beneficial. Consultation with a veterinary cardiologist is recommended prior to initiating therapy.

Patients confirmed to be in stage B2 should be started on an ACE inhibitor (ACEi). A recent study (PROTECT) suggests that pimobendan delays the onset of congestive heart failure at this stage. Consult a veterinary cardiologist about the use of a beta blocker or antiarrhythmic therapy.

Stage C patients may present with an owner complaint of decreased energy, and coughing is a frequent complaint in large breed dogs. Increased resting respiratory rate or frank dyspnea may also be noted. Diagnostic recommendations are similar to those for class B patients.

All stage C dogs with DCM should be started on standard “triple therapy” with an ACEi, furosemide, and pimobendan. Spironolactone is often added to maintenance therapy. Digoxin and/or diltiazem are indicated to maintain average ventricular rate at < 150 bpm if atrial fibrillation is present, and other antiarrhythmics may be needed for ventricular arrhythmias. Excessive dietary sodium should be avoided, and adequate protein intake should be maintained. Council the owner on maintaining a reasonable level of activity but avoiding prolonged strenuous physical activity.

Stage D patients have advanced, end stage heart failure and may present with ascites, severe cardiac cachexia, frequent syncope, or refractory pulmonary edema. Diagnostic recommendations are similar to those for previous stages with an emphasis placed on evaluating the patient’s overall condition and locating factors that may have “changed” and be preventing a satisfactory response to medical therapy. Honestly discuss the patient’s condition with the owners and place an emphasis on overall quality of life.

Standard therapy for stage D patients includes ACEi, furosemide, pimobendan, and spironolactone. Digoxin therapy is recommended in all patients that tolerate the drug, and diltiazem may be needed for additional heart rate control. Antiarrhythmic therapy may be required for ventricular arrhythmias. Rescue diuretics, additional vasodilators, or catecholamine therapy may be needed. Nutritional intake should be optimized and owners will need careful guidelines for activity. A veterinary cardiologist should be consulted about ideal management of a stage D patient.

ARRHYTHMOGENIC RIGHT VENTRICULAR CARDIOMYOPATHY (ARVC)
ARVC or boxer cardiomyopathy is an adult onset muscle disease that may lead to sudden death due to ventricular arrhythmias or to CHF secondary to deterioration in muscle function. While the disease is adult onset, it is known to be an inherited abnormality in the boxer breed. Recent work has indicated that a genetic deletion in the gene coding for the striatin protein is responsible for the development of the disease in some boxers. Other genetic mutations are also likely present in some affected individuals of the breed. The average age of onset for ventricular arrhythmias in affected dogs is 6 years, but this is highly variable among individuals. Screening boxers with 24-hour ambulatory ECG recordings annually beginning at 3 years of age is recommended. Individuals with ventricular ectopic pairs, runs of ventricular tachycardia, or R on T are considered candidates for antiarrhythmic therapy. Sotalol (1 to 2 mg/kg BID) is typically the first line drug for treating arrhythmias associated with ARVC with mexiletine (4 to 8 mg/kg TID) plus metoprolol (0.5 to 1 mg/kg BID) a good second option. Amiodarone may be effective in controlling resistant ventricular arrhythmias, but has too high an incidence of complications to consider it a first line drug.

A small percentage of boxers will develop myocardial rather than, or in addition to, ventricular arrhythmias. These patients will present with CHF signs typical of other breeds with dilated cardiomyopathy and treatment recommendations are consistent with the ABCD staging recommendations for DCM.

Genetic testing is available for the striatin deletion with current recommendations being that heterozygously affected, asymptomatic individuals be bred with unaffected individuals and homozygously affected individual be eliminated from a breeding program. A disease that is clinically similar to ARVC is also seen in the English bulldog.

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