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.