By Dr Duncan Houston
A heart murmur does not automatically mean heart failure. Many dogs with mitral valve disease have no outward symptoms, while others need treatment to control fluid build-up and breathing problems.
The useful next step is establishing the stage and knowing what to monitor at home.
Quick Answer
Myxomatous mitral valve disease causes a heart valve to leak and is especially common in older small dogs. Treatment depends on heart enlargement and whether congestive heart failure has occurred, not simply the presence of a murmur. Monitor sleeping breathing, give medicines exactly as prescribed and seek emergency care for laboured breathing or collapse.
What changes in the heart?
The mitral valve sits between the left atrium and left ventricle. When it no longer seals properly, some blood moves backwards during contraction. Over time, the heart may enlarge; in some dogs pressure eventually causes fluid to accumulate in the lungs.
An echocardiogram evaluates valve leakage and chamber size. Chest imaging and other tests help assess breathing signs, heart failure and alternative causes. A cough can also come from airway disease, so it does not confirm fluid in the lungs.
What do the stages mean?
| Stage | Broad meaning | Usual focus |
|---|---|---|
| A | At risk, without identified disease | Routine examination |
| B1 | Valve disease without qualifying enlargement or heart-failure signs | Monitoring; routine cardiac drugs are generally not indicated |
| B2 | Defined heart enlargement, without prior heart failure | Veterinary assessment for pimobendan and monitoring |
| C | Current or previous congestive heart failure | Medication to control failure and ongoing reassessment |
| D | Failure difficult to control with standard treatment | Individual specialist management |
The ACVIM consensus guidelines define these stages and treatment recommendations. B2 requires specific evidence of enlargement; a murmur alone is insufficient.
Why prescribed treatment matters
Pimobendan benefits appropriately selected B2 dogs and is also used in heart-failure care. Dogs with congestion usually need diuretics and may require other medicines according to the case. Doses depend on response, kidney function, electrolytes and other health factors.
Do not start, stop or increase a diuretic without the treating team's plan. Keep water freely available unless your vet gives a specific instruction. Increased thirst and urination may occur with treatment and should be accommodated with toilet access.
Monitor breathing while asleep
- Count when your dog is asleep, comfortable and not panting or actively dreaming.
- Count one chest rise and fall as one breath for a full minute.
- Record the result regularly and learn your dog's usual pattern.
- Ask your vet for the specific rate or change that should trigger a call.
A sustained rise from baseline matters, even before obvious distress. A repeatedly elevated sleeping rate should prompt advice. Tufts' breathing guidance explains home counting. Breathing effort is urgent regardless of the number.
When is this an emergency?
Go immediately for laboured or open-mouth breathing at rest, blue or pale gums, collapse, or inability to lie down comfortably. Keep your dog calm, call ahead and let them choose an easier breathing position. Do not force food, water or tablets during respiratory distress.
Food, exercise and supplements
Maintain appropriate body weight and muscle with a complete diet. Sodium advice should match the stage and total diet, including treats and medication foods. Use comfortable activity and avoid pushing a breathless or exhausted dog.
Taurine deficiency is not the usual cause of mitral valve degeneration. Hawthorn, CoQ10 and other supplements should not be presented as proven valve treatments. Discuss every product and avoid scented oils or other unreviewed remedies.
Common mistakes
- Assuming every cough means heart failure.
- Restricting water because a diuretic causes urination.
- Changing medication based on a single internet threshold.
- Replacing proven treatment with natural heart-support products.
Frequently asked questions
Can my dog live well with a murmur?
Many do, but the outlook depends on the cause, stage and progression.
Does every dog need medicine immediately?
No. Stage and diagnostic findings guide treatment.
Should exercise stop completely?
Not automatically. Ask for an activity plan suited to the stage and your dog's tolerance.
Know the stage, learn your dog's breathing baseline and keep the treatment plan current. These steps are more useful than a generic heart supplement.
For practical explanations of heart-health care, explore ASK A VET™.
This article is general veterinary education and does not diagnose or treat an individual animal. Always confirm medication names, doses and interactions with the veterinarian who knows your pet.
If your animal is struggling to breathe, collapsed, bleeding heavily, seizing, or has eaten something toxic, contact your vet or the nearest emergency clinic now.
Animal Poisons Helpline (AU) 1300 869 738 · Animal Poisons Helpline (NZ) 0800 869 738 · ASPCA Animal Poison Control (US) (888) 426-4435
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Ask A Vet articles are written by Dr Duncan Houston BVSc, a practising veterinarian, unless another author is named in the byline.
