Canine Heart Diseases: Myxomatous Valve Disease and Dilated Cardiomyopathy
An overview of two common canine heart conditions: Myxomatous Valve Disease (MVV) and Dilated Cardiomyopathy (DCM). This content details their pathophysiology, clinical signs, diagnostic methods, staging, and treatment options, including management of congestive heart failure (CHF).
In veterinary cardiology, distinguishing between valvular and myocardial diseases is fundamental to effective patient management. While both conditions can ultimately lead to congestive heart failure (CHF), they arise from different pathological mechanisms and affect different anatomical structures of the heart. This article explores the two most common primary cardiac pathologies in dogs: Myxomatous Valve Disease (MVV) and Dilated Cardiomyopathy (DCM).
Myxomatous Valve Disease (MVV)
Myxomatous Valve Disease, also referred to as degenerative mitral valve disease or mitral endocardiosis, is the most common cardiovascular lesion in dogs. It is a progressive, degenerative condition that primarily affects the mitral valve, which is located between the left atrium and the left ventricle.
Pathophysiology and Clinical Presentation
The disease is characterized by the nodular thickening of the valve leaflets. This structural change leads to valvular insufficiency, where the valve fails to close properly during systole. This results in mitral regurgitation, the backward flow of blood from the left ventricle into the left atrium.
As blood is regurgitated into the atrium, the increased volume causes left atrial enlargement. Over time, this volume overload can lead to pulmonary congestion and eventually congestive heart failure (CHF).
Signalment: Most common in small-to-medium sized breeds (e.g., Cavalier King Charles Spaniels, Chihuahuas, and Terriers) and typically affects middle-aged to older males.
Clinical Signs: The hallmark sign is a left-sided, systolic heart murmur heard at the left apex. As the disease progresses, patients may present with a cough (often due to atrial enlargement), exercise intolerance, or dyspnea (difficulty breathing) if CHF develops.
Diagnosis and Staging
Diagnosis relies on a combination of clinical examination and imaging:
Auscultation: The loudness of the murmur is a good predictor of disease severity; a murmur grade <3/6 is unlikely to have associated atrial enlargement.
Echocardiogram: This is essential to visualize thickened mitral valves, turbulent flow via Doppler, and left atrial enlargement.
Radiography: Used to identify pulmonary edema, tracheal displacement, and left-sided cardiac enlargement (where the heart appears "taller").
Staging is the most critical step in determining whether to initiate medical intervention.
This diagram illustrates the stages of heart disease in dogs, from high risk to advanced heart failure, including diagnostic indicators like murmurs and x-ray findings.
Exam Tip: Staging Differentiation
The most important clinical distinction is between Stage B1 and Stage B2. While both stages are largely asymptomatic, Stage B1 dogs require no treatment, whereas Stage B2 dogs show evidence of cardiac remodeling (atrial enlargement) and require pimobendan to delay the onset of CHF.
Treatment Options
Stage B1: No treatment indicated.
Stage B2: Pimobendan is indicated to delay the onset of CHF.
Stages C & D (CHF): Management shifts to controlling congestive heart failure.
Dilated Cardiomyopathy (DCM)
Unlike MVV, which is a disease of the valves, Dilated Cardiomyopathy (DCM) is a disease of the heart muscle (myocardium) itself. It is the most common form of myocardial disease in dogs.
Pathophysiology and Phases
DCM is characterized by impaired myocardial contractility and the subsequent dilation of the ventricles (often affecting both the left and right sides). This reduction in contractility leads to both forward failure (reduced cardiac output) and backward failure (congestion).
The disease typically progresses through two distinct phases:
Occult Phase: The patient is asymptomatic and may have no detectable murmur. These patients are difficult to identify without screening tools like Holter monitoring.
Symptomatic Phase: Clinical signs become apparent, and the prognosis is generally poor by this stage.
Clinical Presentation and Signalment
Signalment: Primarily affects large breed dogs, such as Dobermans, Great Danes, and Irish Wolfhounds.
Clinical Signs:
Arrhythmias: Patients frequently experience ventricular tachyarrhythmias and atrial fibrillation.
CHF: Signs include dyspnea, tachycardia, ascites (abdominal fluid), and pleural effusion.
Murmur: A soft left apical murmur may be heard; this is often secondary to the ventricle dilating and pulling the mitral valve leaflets apart.
Other: Exercise intolerance, lethargy, weight loss, and sudden death.
Diagnosis and Management
Diagnosis: An echocardiogram is vital to demonstrate a large, poorly contracting heart. An ECG is used to identify dysrhythmias, and a 24-hour Holter monitor is the gold standard for detecting arrhythmias during the occult phase.
Treatment:
Occult Phase: Pimobendan may be used, though identification is challenging.
Symptomatic Phase: Focuses on managing CHF and arrhythmias.
Cardiogenic Shock: If the patient shows signs of poor cardiac output (e.g., cold extremities, pale mucous membranes), treatment aims to increase contractility using pimobendan or a dobutamine CRI.
Summary of Congestive Heart Failure (CHF) Management
Both MVV and DCM can culminate in CHF. Regardless of the cause, the pathophysiology involves a drop in cardiac output, which triggers the Renin-Angiotensin-Aldosterone System (RAAS), leading to fluid retention and increased cardiac workload.
Clinical Management of CHF
When a patient enters CHF, treatment is divided into acute and chronic phases.
Acute Management (Stabilization):
The priority is diuresis to relieve pulmonary edema.
Furosemide (IV/IM): Typically administered at 1−2 mg/kg and repeated every 30–60 minutes until dyspnea resolves.
Chronic Management (Maintenance):
A "four-pillar" approach is often used to manage the disease long-term:
Loop Diuretics (e.g., Furosemide): To manage fluid overload.
Pimobendan: To increase contractility and reduce afterload via vasodilation.
ACE-Inhibitors (e.g., Benazepril): To antagonize the RAAS.
Aldosterone Antagonists (e.g., Spironolactone): A potassium-sparing diuretic that works synergistically with furosemide.
Clinical Pearl: Monitoring at Home
Owners should be taught to monitor their dog's Resting Respiratory Rate (RRR) while sleeping. An increase in RRR is often the first indicator of an impending CHF episode, allowing for early veterinary intervention.
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