Canine myxomatous mitral valve disease (MMVD)

Symptoms an owner can spot at home

• Soft cough, often worse at night or after exercise
• Reduced exercise tolerance—stopping earlier on walks, panting more
• Increased resting respiratory rate (a normal sleeping dog breathes under ~30 breaths/min)
• Fainting (syncope) episodes
• Abdominal distention or laboured breathing in advanced disease

When to see a vet

• Any new cough lasting more than a few days in an older small-breed dog
• Resting (sleeping) respiratory rate above ~35 breaths/min on repeated counts
• Fainting, collapse, or sudden weakness
• Refusing food, severe lethargy, or open-mouth breathing—emergency

What it is

MMVD is a chronic degeneration of the mitral valve leaflets. The valve thickens and stops sealing cleanly, so blood leaks backward (regurgitation) each beat. The leak shows up first as a heart murmur on physical exam, often years before any clinical sign. Over time, chronic volume overload enlarges the left atrium and ventricle; eventually pulmonary pressures rise and congestive heart failure (CHF) develops.

It is overwhelmingly a small-breed disease—Cavalier King Charles Spaniels are the textbook example, but Dachshunds, Chihuahuas, small terriers, and Miniature Poodles are all over-represented. The ACVIM stages it A through D: A (at-risk breed, no murmur), B1 (murmur, normal heart size), B2 (murmur with enlargement), C (current or past CHF), D (refractory CHF).

How vets diagnose it

Diagnosis starts with auscultation (the murmur) and progresses to thoracic radiographs and echocardiography to stage the disease. Echocardiogram is the standard for moving from B1 to B2—that boundary changes the treatment plan. A home sleeping respiratory rate diary is one of the most useful owner contributions to monitoring; sudden rises predict CHF onset.

Treatment overview

Editorial overview only—your cardiologist or vet builds the plan based on ACVIM stage. Stage B1 is monitored. Stage B2 is treated with pimobendan, which the landmark EPIC trial showed extends the symptom-free interval by a median of ~15 months. Stage C adds diuretics and other classes. Dietary management focuses on modest sodium control, maintaining muscle (cardiac cachexia is real), and ensuring adequate taurine and omega-3 EPA/DHA per cardiologist guidance.

What buyers can do

• Track sleeping respiratory rate weekly with a phone timer—share the trend with your vet. Sudden rise above 35 bpm is an early CHF warning.
• Avoid high-sodium treats—no cheese, deli meat, jerky-style treats. Single-ingredient freeze-dried protein or vet-approved cardiac treats are safer.
• Keep the dog at lean body weight; obesity worsens cardiac workload, cachexia worsens prognosis—both extremes are bad.
• Discuss an omega-3 EPA/DHA fish-oil supplement only with your vet; dose matters and product quality varies.
• If you breed Cavaliers or other predisposed breeds, follow MVD breeding scheme protocols (MRI/echo screening) before mating.