Mitral Valve Disease in Small Senior Dogs: From a New Murmur to Counting Breaths at Home

The checkup was supposed to be routine. Then the vet moved the stethoscope across the left side of the chest, listened a little longer than usual, and said the dog has a heart murmur. For owners of older Cavaliers, Dachshunds, small Poodles and other small breeds, this is a very common moment, and it raises the same questions every time: is the heart failing, does the dog need medication, and what should be watched at home?

In a small senior dog, a new murmur most often points to myxomatous mitral valve disease (MMVD), a slow degeneration of the valve between the two chambers on the left side of the heart. Many dogs live with it for years without symptoms. What changes the outcome is knowing which stage the dog is in and noticing the shift toward heart failure early. One of the most useful tools for that costs nothing: counting breaths while the dog sleeps.

A leaking valve, in plain terms

The mitral valve closes with each heartbeat so blood moves forward into the body instead of back toward the lungs. In MMVD the valve leaflets thicken and stop sealing properly, so some blood leaks backward with every beat. That leak is what the vet hears as a murmur. Over time, the extra volume can enlarge the left side of the heart, and eventually pressure backs up into the lungs. Fluid in the lungs is what vets mean by left-sided congestive heart failure.

The 2019 ACVIM consensus guidelines on MMVD in dogs describe it as the most common heart disease of dogs in many parts of the world, accounting for about 75% of the heart disease cases seen in practices in North America. Prevalence rises sharply with age in small breed dogs, with up to 85% showing the valve lesion by 13 years of age. The guidelines describe the disease in small breeds as generally slow but at times unpredictably progressive. VCA Animal Hospitals adds that it is more common in males, that a murmur can appear as early as four to six years of age, and that a murmur does not mean heart failure is imminent.

The stages, and what each one asks of the owner

The ACVIM guidelines sort dogs into stages. Knowing your dog's stage tells you what to expect from the vet and what to do at home:

• Stage A, at risk with no murmur yet. This includes every Cavalier King Charles Spaniel or other predisposed breed without a murmur. The guidelines recommend that small breed dogs, including Cavaliers, Dachshunds and Miniature and Toy Poodles, have their heart listened to by the family vet every year. No medication is recommended.
• Stage B1, murmur without significant heart enlargement. The dog has no symptoms and the heart is normal or only mildly enlarged on X-rays or ultrasound. The guidelines do not recommend treatment, because there is no evidence that medication helps at this stage. The owner's job is regular rechecks and, once a baseline exists, the home breath count.
• Stage B2, murmur with clear enlargement. Still no symptoms, but imaging shows enlargement that meets clinical trial criteria, typically with a murmur of grade 3 out of 6 or louder. This is where treatment before symptoms begins. In the EPIC study, a randomized trial in dogs at this stage, pimobendan prolonged the period before heart failure or cardiac death by about 15 months compared with placebo, and the dogs on pimobendan also lived longer.
• Stage C, current or past heart failure. The dog has had signs of heart failure and needs ongoing medication, decided and adjusted by the vet. The guidelines describe the typical history as fast breathing, restlessness, breathing difficulty or cough.
• Stage D, heart failure that no longer responds to standard treatment. Management becomes more intensive and is usually guided closely by the vet or a cardiologist.

Telling B1 from B2 takes imaging, usually chest X-rays and an echocardiogram. If your dog has a murmur and the stage has never been worked out, ask the vet whether imaging is needed now. The answer decides whether medication can buy time before symptoms start.

Why the sleeping breath count matters so much

When fluid starts to build in the lungs, breathing gets faster before it gets visibly labored. A dog asleep on the sofa breathing a little quicker than usual is easy to miss. The ACVIM guidelines state that, among the things monitored in these dogs, an increase in resting respiratory rate above the dog's normal baseline has the best predictive value for impending heart failure. VCA describes the same logic for owners: the rate should stay steady while the condition is stable, an increasing rate indicates the condition is worsening, and a decreasing rate after treatment suggests the dog is responding.

A baseline taken while the dog is well is what makes the count useful. The number on its own means less than the change from the dog's own normal.

How to count, step by step

The cardiology service of the University of Missouri Veterinary Health Center gives owners this method:

• Pick the moment: the best time is while the dog is sleeping, or resting comfortably in a room that is neither too hot nor too cold.
• Count full breaths: one breath is the chest rising and falling once. Count for 15 seconds.
• Multiply by four: that gives breaths per minute.

A few practical habits make the numbers more reliable. Count at roughly the same time each day, for example in the evening nap after dinner. Skip counts taken while the dog is dreaming, panting, or just after play. Write each count down with the date and time, in a notebook kept near the dog's bed or in a notes app, so the trend is visible at a glance. Over the first week or two a stable dog will show its own typical range. Bring that log to every recheck.

Reading the log: when to call and when to go

The Missouri cardiology service states that the normal sleeping rate should stay below 30 to 35 breaths per minute, with some variation between pets. It asks owners to contact the cardiology team if the rate rises 20 to 30 percent or more above the dog's normal over three consecutive days, or if it is consistently above 35 breaths per minute. Your own vet may give a different threshold for your dog; use theirs when they do.

Effort matters as much as the count. The same source lists these signs of increased breathing effort:

• The chest wall and ribs moving farther out with each breath.
• The belly muscles working hard on each breath in and out.
• Open-mouth breathing, a stretched-out neck, or the front legs held wide apart.
• A dog that suddenly prefers to sleep lying on its chest instead of on its side or back.

Visible breathing effort, gums that look pale or bluish, collapse, or a dog that cannot settle and keeps standing to breathe are emergencies. Go to the nearest emergency clinic and call on the way; do not wait for the three-day trend. The general rules for that decision are in the guide to what counts as an emergency in a senior pet.

The cough is not always the heart

Many owners assume that a coughing dog with a murmur is in heart failure. The ACVIM guidelines caution against that shortcut. Small older dogs, the same population most affected by MMVD, also commonly have chronic disease of the trachea and bronchi, so a cough in a dog with a typical mitral murmur does not necessarily mean heart failure. An enlarged heart can also press on the airways and cause a cough without fluid in the lungs. For that reason the guidelines value having baseline chest X-rays taken while the dog has no symptoms, so a later cough can be compared against them. A cough with a normal sleeping breath count and no breathing effort still deserves a vet visit, usually at a regular appointment.

Day-to-day life with a heart dog

Once a dog is diagnosed, several routine decisions change:

• Medication on schedule. In stages B2 and C the pills only help if they are given as prescribed. Ask the vet what to do if a dose is missed, before it happens, and never stop or change a heart medication on your own.
• Weight and appetite. The ACVIM guidelines encourage home-based programs that track body weight, appetite, breathing and heart rate. Weigh the dog regularly and note any loss of appetite. Keeping a lean body condition is covered in the article on weight management in senior dogs.
• Other checks. Because most dogs with symptomatic MMVD are middle-aged or older, the guidelines advise completing the workup with a blood pressure check, blood tests and a urine test, especially when heart failure treatment is expected.
• Procedures and anesthesia. Tell any clinic about the murmur and the stage before dental cleaning or surgery. How heart disease changes anesthetic risk is explained in the article on anesthesia risk in senior pets.

Small dogs, slow valves and a notebook by the bed

Mitral valve disease is so common in older small dogs that a murmur is best treated as the start of a monitoring plan. The steps that change the course are concrete: have the heart listened to every year, get the stage defined with imaging once a murmur appears, start treatment at stage B2 if the vet recommends it, and keep a written log of sleeping breath counts so the move toward heart failure is spotted in days rather than at the emergency clinic. A dog whose sleeping rate creeps up for three days in a row needs a call to the vet; a dog that is struggling to breathe needs to be seen now.

Sources and further reading

• ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs, Journal of Veterinary Internal Medicine, 2019 (PubMed Central)
• Effect of Pimobendan in Dogs with Preclinical Myxomatous Mitral Valve Disease and Cardiomegaly: The EPIC Study, Journal of Veterinary Internal Medicine, 2016 (PubMed Central)
• Mitral Valve Disease in Dogs, VCA Animal Hospitals
• At-Home Monitoring of Pets with Heart Disease, University of Missouri Veterinary Health Center