Management of acquired canine heart disease: part 1 & 2 (Proceedings)


Management of acquired canine heart disease: part 1 & 2 (Proceedings)

Acquired heart diseases of dogs include chronic degenerative valvular diseases (endocardiosis), pericardial diseases, cardiac neoplasia, dilated cardiomyopathy (DCM), arrhythmogenic right ventricular cardiomyopathy (ARVC), pulmonary hypertension (PH), infective endocarditis, and heart rhythm disturbances, some of which represent primary electrical disorders and others that develop secondary to cardiac remodeling. These conditions can lead to clinical signs of limited exercise capacity, heart failure, weakness/collapse, or sudden cardiac death. Effective management of cardiac diseases in dogs requires an appreciation of these different disorders, understanding of diagnostic criteria for these diseases, and delivery of appropriate patient monitoring and interventional strategies.

Congestive heart failure (CHF) is a clinical syndrome characterized by a cardiac lesion that limits cardiac output, causing arterial under-filling and evoking maladaptive compensations to restore blood pressure (BP). Most compensatory responses triggered in heart failure, including activation of the sympathetic nervous system, renin-angiotensin-aldosterone system, and proinflammatory cytokines, ultimately injure heart muscle and blood vessels. Furthermore, CHF is characterized by renal sodium retention that promotes elevated venous pressures behind the failing side(s) of the heart. Effective management controls these compensations with multifaceted medical therapy.

Causes of heart disease

The most common heart diseases leading to CHF in dogs are valvular endocardiosis, dilated cardiomyopathy, pulmonary hypertension, and pericardial effusion. Various congenital malformations (including patent ductus arteriosus, pulmonic stenosis, subaortic stenosis, atrioventricular valve dysplasia) are important causes of heart disease and heart failure young animals.

Valvular endocardiosis is characterized by progressive mitral/tricuspid valvular degeneration and apical systolic murmurs typical of mitral regurgitation (MR) and tricuspid regurgitation (TR). Atrial arrhythmias, left mainstem bronchus compression, PH, and rarely atrial tearing may complicate the clinical picture. Systemic hypertension from renal or Cushing's disease increases the regurgitant fraction and represents a comorbid condition. In contrast to endocardiosis, infective endocarditis is a multisystemic inflammatory disorder originating from a cardiac infection and is a relatively rare cause of CHF in dogs. The conditions should not be confused.

Dilated cardiomyopathy (DCM) is a primary myocardial disorder caused by an inexplicable loss of myocardial contractility. This idiopathic/genetic disease is often associated with cardiac arrhythmias, such as atrial fibrillation (AF) and ventricular tachycardia (VT). Occult or preclinical DCM refers to the echocardiographic finding of reduced left ventricular (LV) ejection fraction in the absence of CHF. Left- and right-sided CHF as well as sudden cardiac death are common outcomes of DCM. In some breeds such as Doberman pinschers, development of ventricular or atrial arrhythmias may predate the development of DCM. Right ventricular arrhythmogenic cardiomyopathy (ARVC), is especially common in boxers and English bulldogs.

Pulmonary hypertension (PH) stems most often from three disorders: chronic left sided heart failure; dirofilariasis; and severe interstitial lung disease. This disorder also can be idiopathic (primary) in dogs. PH is very common in dogs with chronic mitral regurgitation (MR) and typically leads to a progressively louder murmur of tricuspid regurgitation, signs of low cardiac output, right sided failure (including ascites and exertional syncope). With the exception of heartworm disease, PH due to primary lung disease infrequently leads to heart failure.

Pericardial effusion is a frequent cause of heart failure in dogs but often is misdiagnosed. Acute effusions can provoke collapse related to hypotension. Right-sided CHF, including pleural effusions, can develop in chronic cardiac tamponade. In younger dogs (and some older ones) idiopathic pericardial hemorrhage is the underlying cause and carries a very good prognosis with proper management. In dogs >7 years of age there is often a cardiac-related neoplasia involved with the effusion (hemangiosarcoma, chemodectoma, mesothelioma, ectopic thyroid neoplasia). Treatment of pericardial disorders does not involve drugs, but instead, pericardiocentesis often followed by form of surgical or endoscopic procedure.

Cardiac arrhythmias often complicate the atrial and ventricular remodeling observed in structural heart diseases. Heart rhythm disturbances can precede the development of heart failure in some disorders, especially in forms of cardiomyopathy. Tachyarrhythmias, if relentless (as with atrial flutter, orthodromic, reentrant supraventricular tachycardia, or sustained ventricular tachycardia) induce a potentially-reversible decrease in ventricular function. This impairment of cardiac output is additive to any preexisting structural heart disease. Bradyarrhythmias such as sinus arrest and atrioventricular blocks are more often related to primary disease (degeneration) of the conduction system in dogs and can lead to collapse or syncope. Management approaches for arrhythmias may involve directed follow-ups (with no therapy), antiarrhythmic drugs, cardiac pacing, or catheter based interventions. (Diagnosis and Management of Cardiac arrhythmias are considered elsewhere).

The diagnosis of heart disease and the recognition of CHF require a careful clinical examination. No historical findings are specifically diagnostic of heart disease or CHF. Exercise intolerance can be identified and respiratory signs are common in patients with failure. Auscultation may indicate a heart murmur, arrhythmia, or gallop sound. The lungs may be abnormal to auscultation if there is pulmonary edema. Blood pressure may be normal (from compensations), low in profound CHF (cardiogenic shock), or surprisingly high, indicating a complicating condition of systemic hypertension. Echocardiography is the noninvasive gold standard for diagnosis of heart disease and is helpful in confirming the cause in cases of suspected CHF. Thoracic radiography is useful for evaluating heart size and following the progression of cardiomegaly. Radiographs are also essential in the differential diagnosis of respiratory signs, as many dogs with compensated heart disease are symptomatic because of a primary respiratory, pleural, or thoracic disorder. The electrocardiogram (EKG) in advanced heart disease may delineate cardiac-enlargement patterns (wide or tall P-waves or QRS complexes), conduction disturbances, or arrhythmias. Unfortunately the 6-lead ECG is too often within normal limits and cannot be relied upon for establishing a diagnosis of heart disease (low diagnostic sensitivity). The EKG is the test of choice for delineating heart rhythm disturbances.

Confirmation of the diagnosis of left-sided CHF requires integration of history, physical examination, and radiography; echocardiography can also be instructive when performed by an experienced examiner. Key radiographic findings of left sided heart failure include left atrial and ventricular enlargement; pulmonary venous congestion or distension (this is variable); and pulmonary infiltrates compatible with cardiogenic edema. Interstitial and alveolar infiltrates due to severe heart failure should improve within 24 to 48 hours of diuretic therapy. Pleural effusions also may be evident in biventricular CHF. The diagnosis of right-sided CHF is usually suspected from physical examination (jugular venous distention, abdominal distension from hepatomegaly and ascites, and abnormal auscultation). Confirmation requires identification of cardiomegaly or pericardial effusion by radiography and often with echocardiography (to establish the exact type of heart disease).

Clinical laboratory tests may be contributory in canine patients with heart disease. Elevated blood troponin (cTnI) indicates heart muscle injury. High circulating NT pro-BNP (brain natriuretic peptide) suggests structural heart disease and heart failure. There are emerging data regarding the use of this biomarker, but the test should not be assessed in isolation (as it may be high in dogs with respiratory disease or from other noncardiac conditions). Serum biochemistries, especially renal function tests and electrolytes, should be evaluated in CHF patients. These can be abnormal owing to pre-existing disease or drug therapy. Anemia and hyperthyroidism (from excess or inappropriate supplementation) increase demands for cardiac output and should also be ruled out in cardiac patients.

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