Canine Valvular Heart Disease – NAVLE Study Guide

Overview and Clinical Importance

Valvular heart disease represents the most common category of acquired cardiovascular disease in dogs, accounting for approximately 75% of all canine heart disease cases. The most prevalent form is myxomatous mitral valve disease (MMVD), also known as degenerative valve disease, endocardiosis, or chronic valvular heart disease. Understanding the pathophysiology, diagnosis, staging, and treatment is essential for NAVLE success.

Figure 1 - Canine heart anatomy diagram showing all four cardiac valves - mitral, tricuspid, aortic, and pulmonic - with labeled chambers.

Myxomatous Mitral Valve Disease (MMVD)

Epidemiology and Breed Predisposition

MMVD is predominantly a disease of small to medium-sized dogs, with prevalence increasing markedly with age. Up to 85% of dogs in predisposed breeds show evidence of valve lesions by 13 years of age. The disease affects the mitral valve alone in approximately 60% of cases, both mitral and tricuspid in 30%, and tricuspid alone in less than 10%.

Pathophysiology

The pathophysiology involves progressive myxomatous degeneration characterized by: expansion of extracellular matrix with glycosaminoglycans; valvular interstitial cell transformation to myofibroblast phenotype; attenuation of the collagen-laden fibrosa layer; and chordae tendineae changes with potential rupture.

Figure 2 - Gross pathology of myxomatous mitral valve showing thickened, nodular valve leaflets

Reference: Menciotti G, Borgarelli M. Review of Diagnostic and Therapeutic Approach to Canine Myxomatous Mitral Valve Disease. Vet Sci. 2017 Sep 26;4(4):47. doi: 10.3390/vetsci4040047. PMID: 29056705; PMCID: PMC5753627.

Hemodynamic Cascade

• Systolic regurgitation from LV into LA creates volume overload
• Compensatory LA and LV dilation (eccentric hypertrophy)
• Increased LA pressure transmitted to pulmonary veins
• Pulmonary venous congestion and pulmonary edema (left-sided CHF)
• Secondary pulmonary hypertension may develop

Clinical Presentation

• Murmur: Left apical systolic (holosystolic); intensity correlates with severity
• PMI: Left 5th-6th intercostal space
• CHF signs: Cough (nocturnal), tachypnea, dyspnea, exercise intolerance, syncope

ACVIM Staging System for MMVD

The ACVIM consensus staging system (2019 update) guides diagnosis and treatment. This is essential knowledge for the NAVLE.

Diagnostic Evaluation

Key Parameters

Figure 3 - Echocardiographic LA:Ao measurement in right parasternal short-axis view. The method for measurement of left atrial to aortic root ratio (LA/Ao). (A) A LA/Ao obtained from a right parasternal short axis (LA/AoSx) by method 1 (see text). (B) A LA/Ao obtained from a right parasternal short axis (LA/AoSx) by method 2 (see text). (C and D) A LA/Ao obtained from a right parasternal long axis (LA/AoLx). For LA diameter (C), the measurement was made at end-systole 1 to 2 frames before the opening of the mitral valve leaflets. The measurement bisects the atrium extending from the mid-atrial septum in the near field to the bright pericardial echo of the LA lateral wall in the far field and is roughly parallel to the mitral annulus. For Ao diameter (D), the measurement of the aortic valve was made between the opened aortic valve leaflets in an early systolic frame when the Ao diameter is the greatest.

Reference: https://www.intechopen.com/chapters/71522 DOI: 10.5772/intechopen.91819

Treatment of MMVD

Pimobendan (Vetmedin)

An inodilator combining positive inotropic effects with vasodilation via calcium sensitization and PDE-III inhibition. Dose: 0.25-0.3 mg/kg PO every 12 hours. Give 1 hour before or 2 hours after food. Start at Stage B2 or C/D.

Stage C Triple Therapy

• Furosemide: Loop diuretic; 2-4 mg/kg PO BID-TID maintenance
• Pimobendan: 0.25-0.3 mg/kg PO q12h
• ACE inhibitor: Enalapril/benazepril 0.5 mg/kg PO q12-24h
• Spironolactone: 2 mg/kg PO q12-24h (quadruple therapy)

Prognosis

• Stage B1: Excellent; many never progress
• Stage B2 with pimobendan: ~3.5 years to CHF
• Stage C: Median 9-15 months survival
• Stage D: 3-6 months survival

Congenital Valvular Diseases

Tricuspid Valve Dysplasia (TVD)

Congenital malformation causing tricuspid regurgitation and right heart volume overload. Labrador Retrievers have highest predisposition (heritable - chromosome 9). Presents with ascites, jugular distension, right-sided murmur. Treatment: medical management of right-sided CHF.

Pulmonic Stenosis (PS)

Third most common congenital defect. Breeds: English/French Bulldogs, Boston Terriers, Samoyeds, Labs. Type A (dome-shaped, fused leaflets) vs Type B (hypoplastic annulus in brachycephalics). Severity by gradient: less than 50 mmHg (mild), 50-80 mmHg (moderate), greater than 80 mmHg (severe - balloon valvuloplasty indicated).

Infective Endocarditis

Bacterial infection of heart valves (usually aortic/mitral). Predisposing factors: subaortic stenosis (most important), immunosuppression, chronic infections. Common organisms: Staph, Strep, E. coli, Bartonella. Clinical signs: fever, shifting leg lameness, new murmur, weight loss. Echocardiography shows vegetations. Treatment: prolonged IV antibiotics (6-8 weeks).