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Enlarged Heart in Dogs: What You Need to Know | BetterVet

Our dog’s hearts are vital for their good health, so finding out that your dog may have an enlarged heart can be very worrying. There are several different health conditions that can cause an enlarged heart in dogs, and it’s important to diagnose the exact problem so that your pet can get the treatment they need. 

What Causes an Enlarged Heart in Dogs?

There are two common causes of an enlarged heart in adult dogs: mitral valve disease, and dilated cardiomyopathy. These illnesses are rare in puppies, but there are other congenital conditions (ones that a puppy is born with) that can cause an enlarged heart at a young age. 
 

Mitral Valve Disease

The mitral valve is one of the valves that keep blood flowing in the right direction through the heart. It is normally thin and flexible, but in some dogs, it becomes thickened and stiff as they age. This is called “myxomatous mitral valve disease,” or MMVD. Over time, this thickened valve will start to leak, and the dog’s heart cannot pump blood around the body as effectively. This stretches the heart over time, causing it to become enlarged. 

Dilated Cardiomyopathy (DCM)

DCM directly affects the muscles of the heart, causing them to become thinner and weaker. This means the heart cannot pump blood around the body as effectively. As the muscles of the heart become weaker, more and more blood is left behind when the heart tries to pump it out, causing the heart muscle to be stretched and the heart to enlarge. 

Congenital Heart Conditions 

There are many different heart conditions that a puppy can be born with. These are all rare, but most of them will lead to an enlarged heart over time if they are severe. Some of these conditions can cause heart enlargement as a puppy, or become an issue later into adulthood.

Examples include:

  • Patent Ductus Arteriosus (PDA)
  • Ventricular or Atrial Septal Defect (a hole in the heart)
  • Aortic or Pulmonic Stenosis (narrowing of valves)
  • Mitral or Tricuspid Dysplasia (valves that have not properly formed)

Which Dogs Are At Risk of an Enlarged Heart?

Many illnesses can cause an enlarged heart in dogs, and they can happen in any age, breed, or size. However, certain illnesses are more common in certain kinds of dogs.

Mitral Valve Disease

Smaller breeds and cross-breed dogs under 44 pounds are more likely to develop Mitral Valve Disease. Older dogs are at higher risk of developing this illness, but middle-aged dogs can be affected in at-risk breeds. 

Specific breeds at higher risk of Mitral Valve Disease include:

  • Cavalier King Charles Spaniels
  • Dachshunds
  • Miniature and Toy Poodles

Dilated Cardiomyopathy (DCM)

DCM mostly affects large pure-breed dogs, and the risk is thought to be genetic. This means that if a dog has a sibling or parent who develops DCM at some point in their life, then they are also at a high risk of developing this illness.

Breeds at high risk of DCM include:

  • Great Danes
  • Dobermanns
  • Boxers
  • Newfoundlands
  • Irish Wolfhounds
  • Deerhounds

A different form of DCM, linked with diet, has also been recognized more recently. This is linked to feeding diets high in legumes and peas, which includes some grain-free diets. This can affect dogs of any size or breed, including crossbreeds.

Symptoms of an Enlarged Heart in Dogs

In the early stages, there are often no obvious symptoms that a dog has an enlarged heart. Your vet might hear a heart murmur or an irregular heartbeat, but in some cases (particularly with DCM) the dog’s heart may sound normal. 

In the later stages, when the heart is having difficulty coping, there may be more obvious symptoms. These include:

  • Being reluctant to exercise
  • Breathing more quickly, even at rest
  • A soft cough, particularly after lying down
  • Weight loss
  • Lack of interest in food
  • A swollen belly

Diagnosing an Enlarged Heart in Dogs

Your vet may suspect that your dog has an enlarged heart when they listen with a stethoscope during a routine wellness checkup. However, it is not possible to diagnose the problem just by listening. Tests that your vet may recommend running include:

Blood tests

Your vet may recommend running general blood tests to check for any other cause of your dog’s symptoms. Some specific blood tests can help to measure heart health, including Pro-BNP and Cardiac Troponins. However, these tests are usually not enough by themselves to diagnose a problem. 

X-Rays

The outline of the heart can usually be seen easily on x-rays, and your vet can measure this to look for signs that a dog’s heart is enlarged. They can also look for other signs that the dog’s heart is struggling, such as enlarged blood vessels or fluid in the lungs. 

Echocardiography 

This more advanced diagnostic allows a vet to scan a dog’s heart as it beats using an ultrasound scan. They can measure the size of the chambers of the heart to look for enlargement and examine the structure of the inside of the heart to determine the exact cause of the enlargement. Most heart conditions can only be definitively diagnosed on a heart scan. 

Your vet may be able to do a basic heart scan in the clinic, but many dogs will need to see a veterinary cardiologist to get a full scan. 

Electrocardiography (ECG)

An ECG measures the electrical activity in the heart and is important for dogs who have an irregular heartbeat. It can give clues as to what is causing this irregular beat and allow the vet to determine the best treatment. It is also used to monitor dogs’ response to treatment. 

Some shorter ECG measurements can be done at home or in the clinic using clip-on electrical leads. For certain problems – including irregular beats that come and go – dogs may need to be monitored for a few days using a Holter monitor. This is a type of ECG that can be worn on the body at all times. 

Treating an Enlarged Heart in Dogs

The exact treatment needed for an enlarged heart will depend on what is causing it.

Medication

Once dogs are showing symptoms from an enlarged heart, then they will need medication to relieve this and help keep them comfortable. Sadly, most heart conditions will get progressively worse over time, so your dog will need regular checkups so that their medication can be adjusted to help them maintain a good quality of life. 

Some medications have also been shown to help dogs before they are showing symptoms. Pimobendan, a drug that helps the heart beat more strongly, has been shown to delay heart failure in dogs with Mitral Valve Disease or DCM. 

Surgery

Some heart conditions can be treated with surgery. For example, Patent Ductus Arteriosus (PDA) is a congenital heart disease caused by a blood vessel that remains open after birth where it should close. It can sometimes be treated by surgically closing the vessel, but only if the condition is not too advanced.

Mitral Valve Disease in humans is treated by surgically replacing the leaky valve. This procedure has started to be performed in dogs, too, but is a very advanced procedure and is only done in a few places. 

Some minimally invasive techniques can directly alter the heart without needing surgery. These can help dogs with PDAs or pulmonic stenosis. 


Dogs of any size and age can develop an enlarged heart. However, in the early stages, there are very few signs of this from the outside. If your vet hears a murmur or suspects that your dog may have a problem with their heart, it’s important to run some tests and find out exactly what treatment will help your dog.