Canine Cardiology Quiz

Canine Cardiomyopathy

• Myocardial disease in dogs, particularly dilated cardiomyopathy (DCM)
• Learning objectives include describing DCM pathophysiology, diagnostic testing, etiologies, clinical signs, treatment for congestive heart failure (CHF), and breed predisposition.
• Samantha, a 6-year-old Doberman Pinscher, presented with a 2-week history of progressive coughing, lethargy, and exercise intolerance, along with 24-hour history of difficulty breathing, inappetence, and syncope.

Samantha - Physical Exam

• Respiratory rate (RR): 70 breaths/minute
• Heart rate (HR): 300 beats per minute
• Temperature: 99.5°F
• Physical exam findings include: pale pink, dry mucous membranes; distended jugular veins; bilateral pulmonary crackles; a Grade III/VI left apical systolic murmur; and weak femoral pulses.

Case Study Questions

• What causes Samantha's arrhythmia? Is it an emergency?
• What are the signs of congestive heart failure?
• What are the differential diagnoses for left-sided heart failure in dogs?
• What are the radiographic and echocardiographic abnormalities?

Canine Cardiomyopathy

• Primary:
  • Dilated cardiomyopathy (DCM): A disorder of the cardiac muscle, characterized by heart enlargement
  • Arrhythmogenic right ventricular cardiomyopathy (ARVC): another primary cardiomyopathy affecting the right ventricle
• Secondary:
  • Nutritional deficiencies
  • Infectious myocarditis
  • Drug-induced/toxic cardiomyopathies
  • Tachycardia-induced cardiomyopathy
• Differential diagnoses for left heart failure:
  • Likely a primary cardiomyopathy like DCM in Samantha
• Additional considerations: Congenital heart defects

Dilated Cardiomyopathy (DCM) - Pathology

• Four-chamber enlargement, with the left side typically more affected than the right.
• Walls are thinned to normal.
• Valves are normal.
• Myocardial fibrosis is present.

Pathophysiology

• Systolic dysfunction leads to decreased stroke volume.
• Activation of the renin-angiotensin-aldosterone system (RAAS) due to hypotension.
• Secondary (functional) valvular regurgitation due to annular stretch.
• Cardiac chamber dilation (profound left atrial and ventricular dilation).
• Increased atrial pressure leading to pulmonary edema/left-sided congestive heart failure (or ascites if the right side is involved as well).
• Arrhythmias can be due to atrial or ventricular ischemia.

Clinical Findings - DCM

• Compensated:
  • Murmur (systolic, apical, left or right).
  • Gallop (S3) sound due to ventricular dilation.
  • Weak femoral pulses
  • Arrhythmias with pulse deficits
• Decompensated (CHF):
  • Tachypnea
  • Cough
  • Tachycardia
  • Crackles
  • Right-sided CHF signs include ascites, distended jugular veins, muffled heart sounds, and tachycardia

Radiographic Findings

• Generalized cardiomegaly
• Prominent left atrial and left ventricular enlargement.
• Left-sided congestive heart failure (L CHF):
  • Distended pulmonary veins
  • Pulmonary interstitial/alveolar infiltrates
• Right-sided congestive heart failure (R CHF):
  • Distended caudal vena cava
  • Pleural effusion
  • Ascites

Electrocardiography

• Tall R waves indicate left ventricular enlargement.
• Tall or wide P waves may suggest right or left atrial enlargement.
• Atrial fibrillation is common.
• Ventricular arrhythmias (VPCs or VT) may occur.
• Tachycardias can contribute to CHF.

Breed Predisposition - DCM

• More common in large breed dogs:
  • Doberman Pinscher
  • Great Dane
  • Irish Wolfhound
  • American Cocker Spaniel
  • Boxer
  • Juvenile Portuguese Water Dog

Echocardiography

• Chamber dilation
• Systolic dysfunction
• Systolic mitral regurgitation (MR)
• Typically a central jet.
• Left atrial (LA) enlargement.
• Left ventricular (LV) eccentric hypertrophy
• Measure fractional shortening (FS).
  • Decreased (typically less than 25%).

Degenerative Valve vs. DCM

• Differentiators based on physical exam and echocardiography

Acute Therapy - Congestive Heart Failure (CHF)

• Oxygen supplementation
• Intravenous (IV) furosemide (decrease preload).
• Pimobendan (PO), dobutamine (IV) (increase contractility).
• Sedation +/-
• Afterload reducer (nitroprusside) +/-

Chronic CHF Therapy

• Furosemide (loop diuretic)
• Enalapril or benazepril (ACE inhibitor).
• Pimobendan (inodilator).
• Spironolactone (aldosterone antagonist)

Follow-up

• Recheck visit 5–7 days post initial treatment.
• Thoracic radiographs are taken.
• Renal panel is assessed.
• Blood pressure is measured.
• Electrocardiogram (ECG) might be used.

Additional Diagnostic Testing

• 24-hour ambulatory Holter monitoring
• Ambulatory event monitor
• Serum biochemistries, PCV/TS (or CBC), +/- urinalysis (U/A).

Antiarrhythmic Therapy - Ventricular Tachycardia

• Acute Therapy:

  • Lidocaine (first choice for dogs)
  • IV bolus (2 mg/kg).
  • Short-lived effect, follow with continuous IV infusion (CRI) (50-75 mcg/kg/min).
  • Monitor serum electrolytes.

• Other acute options

  • Procainamide IV
  • Magnesium sulfate or Magnesium chloride
  • Esmolol
  • Amiodarone

• Chronic Therapy

  • Sotalol (with caution in heart failure)
  • Mexiletine
  • Sotalol + Mexiletine combination
  • Amiodarone

Lidocaine Toxicity

• Do not exceed 8 mg/kg IV over 30 minutes
• Gastrointestinal upset
• Tremors
• Seizures can occur.

Therapy - Atrial Fibrillation (AF)

• AF is rarely reversible; the goal is to slow the transmission of impulses to the ventricle, bringing the ventricular contraction rate to a safe range.
• Medications including diltiazem +/-digoxin.
• Beta Blocker

Therapy - Right-Sided CHF(RCHF)

Specific Cases - Dilated Cardiomyopathy (DCM)

• Doberman Pinscher DCM

  • Breed screening for occult disease (young dogs 3-4 years of age).
  • 24-hour Holter monitor (and/or recheck after a period of time).
  • Echocardiogram
  • Genetic testing (e.g., pyruvate dehydrogenase 4 deficiency).

• American Cocker Spaniel DCM

  • Potential association with taurine deficiency (common in the late 20th century).
  • Measure blood taurine levels (normal 200-350 nmol/L)
  • Can result in heart failure.
  • Supplementation with taurine.

Diet-Associated DCM

• Nutritionally based cardiomyopathy may occur in some dogs
• Diet changes and echo improvement (taurine supplementation) but blood tests normal in some situations.

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC)

• "Boxer Cardiomyopathy"

• Primary disease of the heart muscle

• Arrhythmias, syncope, sudden cardiac death

• Inherited disease, adult onset (typically after 6 years old)

• Forms:

  • Asymptomatic ventricular arrhythmias
  • Symptomatic ventricular arrhythmias (with syncope or sudden death).
  • Ventricular arrhythmias with congestive heart failure
  • Echocardiogram findings may be normal.
  • 24-hour Holter monitor to diagnose (looking for criteria: ≥100 ventricular premature complexes/couplets in 24 hours, R-on-T phenomenon).