Heart Disease in Dogs: Understanding the Diagnosis
Claire Greenway
BVM&S MRCVS
Your vet listened to your dog's chest and said they heard a murmur. Or maybe an echocardiogram has come back with terms you've never seen before. Either way, you're now trying to understand what heart disease means for your dog.
The good news is that heart disease in dogs is often very manageable. Many dogs live comfortably for years after diagnosis. But the terminology can be bewildering, so let's break it down.
What Is a Heart Murmur?
A heart murmur is a sound, not a disease. It's the noise made by turbulent blood flow within the heart. Your vet can hear it through a stethoscope. Murmurs are graded on a scale from 1 (barely audible) to 6 (audible without a stethoscope).
Not all murmurs indicate disease. Puppies sometimes have "innocent" murmurs that resolve as they grow. But in adult dogs, especially older small breeds, a murmur usually indicates a structural problem.
The Most Common Cause: Mitral Valve Disease
Mitral valve disease (MVD) accounts for roughly 75 percent of heart disease in dogs. The mitral valve sits between the left atrium and left ventricle. Over time, the valve thickens and doesn't close properly, allowing blood to leak backwards. This is the murmur your vet hears.
MVD is progressive but often slow. A dog can have a murmur for years before it causes any problems. The key is knowing what stage your dog is at.
The ACVIM Staging System
Cardiologists stage heart disease in dogs using this system:
Stage A: Dogs at risk but with no murmur yet. Breeds like Cavalier King Charles Spaniels fall into this category.
Stage B1: A murmur is present but there are no structural changes visible on echocardiography. No treatment needed, just monitoring.
Stage B2: A murmur with evidence of heart enlargement on echocardiography, but no symptoms. This is when pimobendan (Vetmedin) is typically started, based on the EPIC trial which showed it delays progression to heart failure.
Stage C: The dog has developed or is currently showing signs of congestive heart failure. Symptoms include coughing, increased breathing rate, exercise intolerance, and reduced appetite. Multiple medications are needed.
Stage D: End-stage heart failure that is no longer responding adequately to standard treatment.
Understanding Your Echo Report
An echocardiogram measures several things. The ones that matter most for MVD are:
LA:Ao ratio: This compares the size of the left atrium to the aorta. A normal ratio is under 1.6. Above this suggests the left atrium is enlarging because it's handling the backflow of blood.
LVIDDn (normalised left ventricular internal diameter in diastole): This tells you if the left ventricle is dilated. Values above 1.7 are considered enlarged.
If both measurements are above the thresholds, your dog is in Stage B2 and should start pimobendan.
Medications and What They Do
Pimobendan (Vetmedin)
Started at Stage B2. It strengthens the heart's contraction and opens blood vessels, making the heart's job easier. Given twice daily, 1 hour before food. This is genuinely one of the most impactful cardiac drugs in veterinary medicine.
Furosemide (Frusemide)
A diuretic, started when heart failure develops (Stage C). It removes excess fluid from the lungs and body. The side effect is that your dog will drink and urinate much more. This is the medication working, not a problem to solve.
ACE Inhibitors (Benazepril, Enalapril)
Often added at Stage C. They reduce the workload on the heart by relaxing blood vessels.
Spironolactone
Another diuretic with additional heart-protective effects. Often used alongside furosemide.
The Single Most Important Thing You Can Do
Monitor resting respiratory rate. Count how many breaths your dog takes in 60 seconds while they're sleeping or resting calmly. Normal is under 30. A sustained increase above 30 to 40 is often the earliest sign that fluid is building up in the lungs.
Do this daily. Write it down. It's more useful than any amount of worrying about what the echo showed. A rising respiratory rate tells you something is changing and your vet needs to know.
Living Well
Dogs with heart disease can and do live well. Small breed dogs with MVD often have a slowly progressive course. With appropriate medication, many dogs in Stage C live comfortably for 12 months or more, and some for considerably longer.
Keep exercise moderate but don't eliminate it. Heat and humidity are harder on heart dogs, so adjust accordingly in summer. Watch the weight, as extra weight means extra work for the heart.
And remember: your dog doesn't know they have heart disease. They know whether they feel well or not. With proper management, most heart dogs feel well for a long time.
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