Heart Disease in Dogs: Signs, Diagnosis and Treatment

At a glance

What it is	Conditions that stop the heart pumping blood efficiently, most often a leaking heart valve or weakened heart muscle
Who's at risk	Valve disease: mainly older small and medium breeds. DCM: mainly large and giant breeds
Main signs	Often none at first; later tiring easily, faster breathing at rest, coughing, fainting or a swollen belly
Contagious?	No
Can it be prevented?	Mostly not, but screening and early treatment can delay heart failure in some dogs

What it is

The heart has four chambers and four valves that keep blood flowing in the right direction. When the heart can't pump effectively, blood backs up and fluid can leak into the lungs or belly. This stage is called congestive heart failure.

Mitral valve disease is the most common heart disease in dogs. The mitral valve sits between the two chambers on the left side of the heart. With age, it thickens and becomes misshapen, so it no longer closes properly and some blood leaks backwards with every beat. The leak creates a sound your vet can hear through a stethoscope, called a heart murmur. Many dogs live with a murmur for years without problems. In others, the leak worsens over time, the heart enlarges and eventually heart failure can develop.

Dilated cardiomyopathy (DCM) is a disease of the heart muscle itself. The muscle becomes thin and weak, the chambers stretch, and the heart pumps less forcefully. DCM is often accompanied by abnormal heart rhythms, which can cause fainting or, sadly, sudden death. There can be a long hidden phase before any signs appear. DCM is often inherited. Some research has also explored a possible link between certain diets and DCM in breeds not usually affected; this is still being studied, so it is worth discussing your dog's diet with your vet.

Other heart problems exist, including conditions present from birth, abnormal rhythms and heart disease caused by parasites, but these two account for most cases in adult dogs.

Signs to watch for

In the early stages there are often no signs at all. As disease progresses you may notice:

• Faster or harder breathing when resting or asleep
• Tiring more quickly on walks, or lagging behind
• A cough, especially at night or after lying down
• Restlessness at night, or difficulty settling comfortably
• Fainting or collapsing, particularly during excitement or exercise
• A swollen tummy from fluid build-up
• Weight loss and muscle wasting, especially along the spine
• Reduced appetite

A cough has many causes, from kennel cough to lungworm. Our article on dog coughing explains more.

Counting your dog's resting breathing rate

One of the most useful things you can do at home, especially if your dog has a murmur or diagnosed heart disease, is to count their breathing rate while they are sound asleep. Breathing speeds up when fluid starts to collect in the lungs, often before other signs are obvious.

• Wait until your dog is deeply asleep in a cool, calm room — not dreaming, and not just after exercise.
• Count each rise and fall of the chest as one breath.
• Count for 30 seconds and double it, or count for a full minute.
• Write it down, and repeat every few days so you get to know your dog's normal.

Your vet can tell you what range is normal for your dog and at what number you should call them. A rate that is creeping up over several days, or that is consistently above the limit your vet has given you, is a reason to get in touch promptly. Our guide to knowing your pet's normal explains why a baseline matters.

When to see a vet

Book an appointment if your dog is tiring more easily, coughing, breathing faster at rest or seems generally "not right". If your vet has heard a murmur, follow their advice on further tests and check-ups.

Treat these as an emergency and contact your vet straight away:

• Struggling to breathe, breathing with an open mouth at rest, or standing with elbows out and neck stretched
• Blue, grey or very pale gums or tongue
• Collapse or fainting
• A rapidly swelling belly
• Coughing up frothy or pink-tinged fluid
• A sleeping breathing rate well above your dog's usual or above the limit your vet set, especially with restlessness

Keep your dog calm and cool while you arrange to travel.

How vets diagnose it

• Listening to the heart and lungs. Murmurs, abnormal rhythms and crackles in the lungs give important clues. Many heart conditions are first picked up at a routine vaccination check.
• Echocardiography. An ultrasound scan of the heart shows the size of each chamber, how well the muscle contracts and how the valves are working. It is the key test for both valve disease and DCM.
• Chest X-rays show the size of the heart and whether there is fluid in the lungs.
• ECG. A recording of the heart's electrical activity identifies rhythm problems. Some dogs wear a small monitor for 24 hours to catch abnormal beats that come and go.
• Blood tests, including a marker that rises when the heart is under strain, and checks of kidney function before and during treatment.
• Blood pressure measurement.

Treatment and management

Treatment depends on the type and stage of disease. Your vet, sometimes alongside a veterinary cardiologist, will tailor the plan.

• Monitoring only. Dogs with a mild murmur and a normal-sized heart usually need regular checks rather than medicine.
• Medicine to delay heart failure. In mitral valve disease, once the heart has enlarged past a certain point, starting medication can delay the onset of heart failure in many dogs. Similar medicine is often used in DCM.
• Treating heart failure. Diuretics (often called "water tablets") remove excess fluid, alongside medicines that help the heart pump more strongly and reduce its workload. Dogs often need several medicines given together, with doses adjusted over time.
• Managing rhythm problems with specific medicines.
• Hospital care for dogs in crisis, which may include oxygen and medicines given by injection.
• Valve repair surgery is available at a small number of specialist centres for some dogs with mitral valve disease.

Heart disease is usually progressive, but treatment can give many dogs months to years of good-quality life.

Living with it

• Keep counting resting breathing rate and keep a record to share with your vet.
• Give medication exactly as prescribed and don't run out.
• Let your dog set the pace on walks; avoid strenuous exercise and hot weather.
• Help them stay at a healthy weight, but tell your vet about muscle loss or poor appetite.
• Ask your vet about salt in treats and food — very salty human foods are best avoided.
• Attend regular rechecks, including blood tests, as heart medicines can affect the kidneys.

Prevention

Most heart disease in dogs is inherited or age-related, so it can't be prevented. However, some breed clubs run heart screening programmes using stethoscope checks and scans, and choosing a puppy from screened parents reduces risk. Regular vet checks catch murmurs early, and for DCM-prone breeds, screening before signs appear allows earlier treatment. In areas where heartworm exists, preventive parasite treatment protects the heart.

Breeds more at risk

Mitral valve disease is particularly common in Cavalier King Charles spaniels, often appearing at a younger age than in other breeds. It is also frequent in other small breeds such as the Chihuahua, dachshund, miniature poodle and miniature schnauzer. DCM is seen mainly in large and giant breeds, including the Dobermann, boxer, Great Dane and Irish wolfhound.