- Clinical signs
- Radiographic findings
- Clinicopathological findings
- Breed variations in clinical presentation
- Treatment of dilated cardiomyopathy
Dilated cardiomyopathy (DCM), next to valvular endocardiosis, is the most commonly diagnosed cardiac disorder in dogs. In most cases the cause is not apparent; the most popular concept is that the aetiology for dilated cardiomyopathy in dogs is multifactorial. Cases of dilated cardiomyopathy in association with a deficiency of myocardial L-carnitine have been reported in one family of boxers suggesting that nutritional factors may be involved in the pathogenesis. L-Carnitine is necessary for the transport of long chain fatty acids into the mitochondria of cardiac muscle cells and a deficiency results in impaired mitochondrial energy production. More recently, it has been shown that some dogs with dilated cardiomyopathy have reduced plasma taurine levels. In humans, cardiomyopathy is associated with depression of the cellular Na+, K+-ATPase pump and a reduction (down regulation) of myocardial beta-adrenergic receptors. Whether a similar situation exists in dogs with dilated cardiomyopathy is not clear. A recent study reported no significant difference in the beta receptor density in four dogs with dilated cardiomyopathy compared to normal dogs. Immune-mediated disease with the production of antibodies against altered myocardial proteins, for example following viral infection, is a known cause of cardiomyopathy in humans but has not been recognized in dogs.
The high incidence in certain breeds of dog suggests that genetic influences may also be important. Dilated cardiomyopathy is more common in medium-sized and large, giant breeds of dog with the highest incidence occurring in English cocker spaniels, old English sheepdogs, boxers, Dobermanns, Irish wolfhounds, great Danes and Saint Bernards. There is an increased incidence in males and the average age of onset is 4-6 years of age (occasionally earlier especially in certain breeds, for example, cocker spaniels).
Dilated cardiomyopathy results in impairment of systolic function. Progressive dilatation of the atria and ventricles causes reduced myocardial contractility and increased end-diastolic pressures. As the chambers of the heart dilate so too does the fibrous atrioventricular annulus supporting the atrioventricular valve cusps. Affected animals may, therefore, be expected to show signs of low output failure due to myocardial dysfunction and also signs of congestive heart failure associated with regurgitation of blood into the atria.
Dilated cardiomyopathy is characterized by lethargy, inappetence or anorexia, generalized weakness, exercise intolerance and weight loss; the latter may be rapidly progressive once cardiac function becomes severely compromised. Animals which develop cardiac dysrhythmias, the most common of which is atrial fibrillation, may experience acute syneopal episodes.
The clinical signs of dilated cardiomyopathy are those of left and / or right-sided congestive heart failure. In most animals there are signs consistent with both low output failure and congestive heart failure.
Severely affected animals may present with acute dyspnoea due to pulmonary oedema, pleural effusion or both. Increased bronchovesicular lung sounds with pulmonary crackles and pallor of the mucous membranes with prolongation of the capillary refill time may be evident. In some cases right-sided signs may predominate with evidence of jugular distension, hepatosptenomegaly and ascites. The heart sounds may be muffled due to the presence of pleural fluid. A systolic murmur associated with atrioventricular regurgitation and / or a gallop rhythm due to accentuation of the third (S3) heart sound may be present.
The electrocardiography findings are variable. Changes consistent with severe left atrial and left ventricular enlargement (wide, sometimes notched P waves, wide QRS complexes, tall R waves and ST slurring) are often present. The most common arrhythmia associated with dilated cardiomyopathy is atrial fibrillation which is characterized by an irregular tachycardia with variable R-R intervals and an absence of P waves. In certain breeds ventricular arrhythmias are more common and occasionally atrial fibrillation may co-exist with ventricular premature complexes.
Dilated cardiomyopathy is usually characterized by generalized cardiomegaly although in some breeds (for example Dobermanns and boxers) thoracic radiographs often appear remarkably normal or show signs only of left atrial and mild left ventricular enlargement. There may be evidence of pulmonary venous congestion with an interstitial or mixed interstitial / alveolar lung pattern typical of pulmonary oedema involving the perihilar and dorsocaudal regions of the lungs.
The echocardiographic features of dilated cardiomyopathy include :
- An increased left ventricular internal dimension (LVID) during systole and diastole;
- Reduced fractional shortening (FS) of the left ventricle during systole indicating decreased myocardial contractility;
- Increased mitral valve E-point septal separation (EPSS);
- Decreased thickness of the interventrieular septum during systole and diastole;
- Evidence of pericardial fluid indicative of right-sided failure (usually the volume of fluid is minimal).
Dogs with reduced renal perfusion may become azotaemic. Passive venous congestion of the liver causes mild increases in liver enzymes. The total plasma protein concentration is frequently reduced especially in dogs which are ascitic.
A few cases of dilated cardiomyopathy, particularly in Dobermanns and great Danes, have been associated with hypothyroidism. Total and free T4 levels should he checked in breeds which are susceptible to hypothyroidism although it should be noted that T4 levels may be depressed in dogs with dilated cardiomyopathy because of a *sick* euthyroid syndrome.
The prognosis for dilated cardiomyopathy is invariably extremely guarded since many dogs fail to survive 6 months. Dogs which survive longer than seven months have a good probability of being long-term survivors (up to 40 months after diagnosis in some cases). The presence of pleural effusion or pulmonary oedema appear to be the poorest independent prognostic indicators in dogs with dilated cardiomyopathy.
Breed variations in clinical presentation
There is a higher incidence of dilated cardiomyopathy in male boxers, the average age of onset being approximately 8.0-8.2 years. During the early stages of the disease animals may show no clinical signs or may present with signs of weakness or syncope.
The clinical features may be divided into three categories :
1. asymptomatic with arrhythmias;
2. syncope or episodic weakness with signs of congestive heart failure;
3. signs of left sided or biventricular congestive heart failure with arrhythmias.
Approximately 50% of cases may have a murmur due to mitral insufficiency. Ventricular arrhythmias (ventricular premature contractions or paroxysmal ventricular tachycardia) are common. The ventricular extrasystoles are typically of a left bundle branch block type.
Asymptomatic cases and dogs which have syn-copal episodes but which are not showing signs of congestive heart failure may show few significant radiographic abnormalities. Cardiomegaly and pulmonary oedema occur only with advanced cardiac failure; pleural effusions are uncommon. The prognosis for long-term survival is poor especially in dogs with congestive heart failure and / or arrhythmias. Many dogs fail to survive 6 months and sudden death is common.
Cardiomyopathy of Dobermann pinschers
Dilated cardiomyopathy in Dobermanns may resemble classic dilated cardiomyopathy seen in large giant breeds of dog (generalized cardiomegaly with signs of biventricular congestive heart failure). Affected Dobermanns often present with severe dyspnoea and acute signs of low output failure. Usually there is a marked reduction in left ventricular contractility and increased left ventricular end-systolic dimensions (fractional shortening of the left ventricle during systole may be less than 10%). There is a subset of dogs, however, which present with radiographie signs of left atrial enlargement (rather than generalized eardiomegaly) and engorgement of the cranial lobar pulmonary veins. Pulmonary oedema if present tends to be diffuse rather than perihilar in distribution. Ventricular arrhythmias are common and tend to persist throughout the course of therapy.
The prognosis is usually extremely poor with many dogs dying within 3-6 weeks of diagnosis. A few dogs may appear clinically normal but nevertheless have radiographic evidence of mild left atrial enlargement and, echocardiograpliically, a reduction in left ventricular contractility. Most of these dogs may be expected to develop signs of acute left-sided congestive heart failure within 12-15 months. Sudden death is common.
English cocker spaniel cardiomyopathy
Dilated cardiomyopathy has been reported in young to middle-aged cocker spaniels. Affected animals vary in age from 2 to 9 years and an increased incidence in males has been reported. A familial predisposition has been suggested. Clinical signs include a cough, exercise intolerance and dyspnoea; sudden death may occur. Unlike other breeds the clinical signs may be preceded by a long asymptomatic period. During this time ECG abnormalities consistent with left ventricular enlargement (tall R waves. ST slurring) may be noted. Deep Q waves and supraventricular arrhythmias (atrial premature contractions) are relatively common. Radiographs may show biventricular enlargement and evidence of pulmonary oedema. Echocardiographic findings are more variable; left ventricular end-systolic dimensions may be increased but myocardial function may be less suppressed than in other breeds with dilated cardiomyopathy. Progressive systolic dysfunction usually results in congestive heart failure.
Treatment of dilated cardiomyopathy
Dogs which show signs of acute decompensation should be given oxygen and confined to a kennel. Intravenous frusemide (2-4 mg kg-1 body weight) and nitroglycerine, a venodilator, can be administered to alleviate the pulmonary oedema and reduce cardiac preload (approximately 1 cm of nitroglycerine ointment 2% can he applied cutaneously, usually on the medial surface of the pinna, eveny 6-8 h). Morphine (0.2-0.3 mg kg-1 body weight) can be given subcutaneosly or intramuscularly.
The use of cardiac glycosides such as digoxin as positive inotropic agents is controversial. The myocardium of dogs with dilated cardiomyopathy generally has little or no contractile reserve capacity and any improvement in myocardial contractility is minimal especially in dogs which are still in sinus rhythm. A more significant clinical response may be observed in dogs which are in atrial fibrillation but this is not due to improved myocardial contractility but to the negative chronotropic effect of cardiac glycosides. This results in a reduction in the ventricular response rate which in turn leads to improved diastolic filling and increased cardiac output.
Large dogs may develop signs of digitalis toxicity at relatively low doses, thus it is important to calculate the dose of digoxin on a body surface area basis (see section on cardiovascular therapeutics). Renal function should be evaluated beforehand; digoxin is excreted intact by the kidneys and compromised renal function may result in digitalis toxicity. Slow oral ‘digitalization’ is preferred to fast intravenous digkalization. Digoxin is contraindicated if multiple ventricular premature complexes are present on the ECG since even therapeutic levels of digoxin may aggravate ventricular arrhthmias during the first week of therapy.
Sympathomimetic drugs such as dopamine and dobutamine have yet to be fully evaluated in the treatment of dilated cardiomyopathy. Although both drugs have to be administered by slow intravenous infusion they may have a role to play in the treatment of animals which are showing signs of acute cardiogenic shock.
Angiotensin convening enzyme (ACE) inhibitors such as captopril, enalapril and benazapril have been shown to significantly improve exercise intolerance and short-term survival times in dogs with dilated cardiomyopathy. The reduction in ejection impedance leads to increased cardiac output, a reduction in left ventricular end-diastolic pressure, increased myocardial perfusion and a decrease in myocardial oxygen demand. Enalapril (0.5 mg kg-1 body weight once or twice daily) has been shown to significantly reduce heart rate, blood pressure and mean capillary wedge pressure (PCWP). The reduction in PCWP is reflected clinical I v by decreased oedema formation in dogs with dilated cardiomyopathy.
If digoxin fails to significantly reduce the ventricular response rate the use of beta adrenergic blocking drugs such as propranolol, or calcium blockers such as verapamil or diltiazem, should be considered. Both types of drug are negative inotropes and should be given in combination with digoxin. In humans, betablockers have been shown to increase myocardial responsiveness to inotropic stimulation by up-regulating beta-adrenergic receptor function. Calcium blockers may potentate digitalis toxicity and the dose of digitalis should be adjusted accordingly.