Myxomatous Mitral Valve Disease (MMVD)
A degenerative cardiac disease where the mitral valve leaflets thicken and leak, allowing blood to back up into the left atrium.
💬 In Plain English
The heart has four chambers separated by one-way valves. In MMVD (the most common heart disease in dogs), the mitral valve on the left side becomes knobby, floppy, and thickened with age. When the heart pumps, the valve no longer seals tightly, allowing blood to leak backward (regurgitation). This creates a heart murmur. Over time, the backward pressure causes the left heart chambers to enlarge and can eventually lead to fluid pooling in the lungs (congestive heart failure).
🔬 Clinical & Technical Depth
For Vets & TechniciansFormal Clinical Definition
Age-related progressive glycosaminoglycan expansion and collagen degeneration of the atrioventricular valve leaflets and chordae tendineae, leading to mitral regurgitation, left atrial and ventricular eccentric hypertrophy, and pulmonary venous hypertension.
Why Veterinarians Use It
Veterinarians use this term to classify the specific type of systolic murmur heard over the left heart apex and assign an ACVIM stage (Stage A, B1, B2, C, or D).
Clinical Context Encountered
Extremely prevalent in older, small-to-medium breed dogs—nearly 100% of Cavalier King Charles Spaniels develop changes by senior years, along with Poodles, Chihuahuas, and Shih Tzus.
Clinical Reference Guide (Medical Condition)
- •A sleeping respiratory rate consistently exceeding 30 breaths per minute while resting or asleep indicates early pulmonary edema (fluid in the lungs) requiring immediate veterinary attention.
❓ Questions to Ask Your Veterinarian
Take these actionable questions with you to your next clinic visit or appointment:
- 1What ACVIM stage (B1, B2, C, or D) is my dog’s heart disease?
- 2Has my dog reached Stage B2 criteria (heart enlargement on X-rays and echocardiogram) that warrants starting pimobendan (Vetmedin)?
- 3What is my pet’s baseline sleeping respiratory rate (breaths per minute while asleep)?
- 4What signs of congestive heart failure (such as nighttime coughing or rapid breathing) should trigger an immediate visit?
Academic & Clinical References
- ACVIM Consensus Guidelines for the Diagnosis and Treatment of Myxomatous Mitral Valve Disease in DogsOrganization: American College of Veterinary Internal Medicine (ACVIM) • Published: 2019-04-01 • Accessed: 2026-01-10https://www.acvim.org