Enlarged Heart in Dogs: What It Means and What Comes Next

Go to an emergency veterinary hospital now if your dog is struggling to breathe, breathing with an open mouth at rest, has blue or gray gums, or has collapsed or fainted. These can be signs of heart failure or a serious heart rhythm problem.

An enlarged heart, called cardiomegaly, isn't a disease by itself. It's a sign that something is making one or more of the heart's chambers stretch or work harder. In dogs, the most common cause is degenerative mitral valve disease, which accounts for about 75% of heart disease in dogs (Kittleson, 2025). Another major cause is dilated cardiomyopathy (DCM), a weakening of the heart muscle that mostly affects large breeds (Kittleson, 2025).

Many dogs with an enlarged heart feel completely normal at first. When signs do appear, they can include faster breathing during sleep, coughing, tiring easily, fainting, and a swollen belly (Keene et al., 2019; Kittleson, 2025). An enlarged heart can be seen on chest X-rays and confirmed with an echocardiogram (Keene et al., 2019).

Finding heart enlargement early can matter a great deal. In dogs with mitral valve disease and an enlarged heart but no signs of failure yet, treatment delayed the onset of heart failure or cardiac death by about 15 months in a large clinical trial (Boswood et al., 2016).

Key points

• An enlarged heart is a sign of heart disease, not a diagnosis on its own.
• Degenerative valve disease accounts for about 75% of heart disease in dogs (Kittleson, 2025).
• Dilated cardiomyopathy mostly affects large breeds, including Doberman Pinschers, Boxers, Great Danes, German Shepherds, and Labrador Retrievers (Kittleson, 2025).
• Treating qualifying dogs before signs appear delayed heart failure or cardiac death by a median of about 15 months (Boswood et al., 2016).
• Fluid around the heart, sometimes caused by a tumor, can make the heart look enlarged on X-rays (Kittleson, 2025).
• Small, flat-faced, and short-legged breeds often have a larger-looking heart on X-rays, so breed-specific ranges matter (MedVet, n.d.).

Why a dog's heart gets bigger

Several different diseases can enlarge a dog's heart, and which chambers are affected depends on the cause.

Degenerative mitral valve disease. This is the most common heart disease in dogs (Kittleson, 2025). The mitral valve, which separates the left upper and lower chambers, thickens and leaks. Blood flows backward with each beat, increasing the volume and size of the left atrium (Kittleson, 2025). It's far more common in dogs under about 20 kg (44 pounds), becomes more frequent with age, and develops earlier in Cavalier King Charles Spaniels (Keene et al., 2019).

Dilated cardiomyopathy (DCM). In DCM, the heart muscle weakens and the chambers stretch. It most commonly affects large breeds, including Doberman Pinschers, Boxers, Great Danes, German Shepherds, and Labrador Retrievers, as well as some spaniels. Our breed articles on Great Danes and German Shepherds cover those breeds' cancer risks. Portuguese Water Dogs can develop it young (Kittleson, 2025). DCM is inherited in Doberman Pinschers, with a reported prevalence of 58%, and yearly screening is recommended in the breed from age three (Wess et al., 2017). Collapse and sudden death occur in as many as 20% of affected Dobermans (Kittleson, 2025).

In some breeds, including American Cocker Spaniels, Golden Retrievers, and Dalmatians, heart failure can be caused by taurine deficiency, which blood tests can confirm (Kittleson, 2025).

Fluid around the heart. When fluid collects in the sac around the heart (pericardial effusion), X-rays may show an enlarged or abnormally shaped heart even though the heart itself isn't enlarged (Kittleson, 2025). Cardiac tumors, especially hemangiosarcoma and heart base tumors, are among the most common causes in dogs, and a swollen belly is by far the most common sign (Kittleson, 2024).

What about grain-free diets?

The U.S. Food and Drug Administration investigated reports of DCM in dogs eating certain diets. In December 2022, the FDA said it had received 1,382 DCM reports from 2014 through early November 2022, but that the reports alone "do not supply sufficient data to establish a causal relationship" with specific products. It said it wouldn't release further updates until it had meaningful new scientific information (Wall, 2022). If your dog has DCM or is a breed at risk, talk with your veterinarian or a cardiologist about diet.

Signs of an enlarged heart

Heart enlargement often causes no signs until the disease is advanced. Signs to watch for include:

• Faster breathing while asleep or at rest (Keene et al., 2019)
• Coughing, restlessness, or labored breathing, which can mean fluid in the lungs (Keene et al., 2019)
• Tiring easily or unwillingness to move (Kittleson, 2025)
• Weakness, collapse, or fainting (Kittleson, 2025)
• A swollen belly from fluid buildup (Kittleson, 2025)

Healthy adult dogs rarely breathe more than 30 times a minute during sleep (Rishniw et al., 2012), and a rise above your dog's own baseline is one of the best early warnings of heart failure (Keene et al., 2019). Our article on early signs of CHF in dogs shows you how to count, and the Dog Cancer Tracker can store the numbers. If your dog is coughing or panting, our articles on coughing and panting in dogs with cancer cover the overlap with other causes.

How an enlarged heart is diagnosed

Chest X-rays. X-rays can show enlargement of the heart (Kittleson, 2025). Veterinarians often use the vertebral heart score, which compares the size of the heart to the length of the spine bones. The original normal range was 8.7 to 10.7 vertebrae (Buchanan & Bucheler, 1995, as cited in MedVet, n.d.). Small, flat-faced, and short-legged breeds often have scores well above that range, so applying it to them could wrongly suggest an enlarged heart. Breed-specific ranges help, and enlargement of the left atrium is more meaningful than the overall score for judging left-sided heart disease (MedVet, n.d.). Our article on why dogs need X-rays explains chest imaging more broadly.

Echocardiogram. An ultrasound of the heart shows each chamber and valve directly. Guidelines use echocardiography or X-rays to decide whether a dog with mitral valve disease has enough heart enlargement to benefit from early treatment (Keene et al., 2019).

Other tests. An electrocardiogram can find abnormal heart rhythms, which are especially important in Doberman Pinschers and Boxers with DCM (Kittleson, 2025). A blood test called NT-proBNP can help sort out whether a cough or breathing problem is coming from the heart (Keene et al., 2019).

Stages and treatment

Veterinary cardiologists stage mitral valve disease from A to D. Stage B dogs have a heart problem but no signs. Stage B2 dogs also have heart enlargement, and stage C dogs have current or past signs of heart failure (Keene et al., 2019).

Stage B2 is where early treatment can make a real difference. In the EPIC trial, dogs with mitral valve disease and an enlarged heart who received a heart medication went a median of 1,228 days before developing heart failure or dying of cardiac causes, compared with 766 days for dogs given a placebo (Boswood et al., 2016). Current guidelines recommend treatment for dogs that meet these criteria (Keene et al., 2019).

For DCM, treatment depends on the breed and findings. Rhythm medications are often needed, especially in Doberman Pinschers and Boxers with severe arrhythmias, and dogs with taurine deficiency can respond dramatically to supplementation over 2 to 4 months (Kittleson, 2025). Treatment decisions belong to your veterinarian or a veterinary cardiologist.

Enlarged hearts and cancer

Two connections matter for families in our community. First, a tumor can cause fluid around the heart that makes it look enlarged on X-rays (Kittleson, 2024, 2025). If your dog has a swollen belly, pale gums, or sudden weakness, it needs prompt evaluation. Our article on hemangiosarcoma life expectancy explains that cancer's outlook.

Second, some chemotherapy can affect the heart. In 494 dogs treated with doxorubicin, 4% developed clinical heart problems overall, but the rate was 15.4% in Boxers and other breeds prone to DCM (Hallman et al., 2019). If your dog has an enlarged heart and needs chemotherapy, make sure your oncologist knows. Our overview of chemotherapy for dogs explains how drugs are chosen.

Outlook

The outlook depends on the cause and stage. In the EPIC trial, even dogs given a placebo went a median of 766 days before heart failure or cardiac death, and treated dogs went longer (Boswood et al., 2016). For Doberman Pinschers with DCM, the outlook is poor in most dogs (Kittleson, 2025). Regular rechecks, home breathing counts, and prompt attention to new signs give your dog the best chance.

When to contact a veterinarian

Go to an emergency hospital now if your dog:

• Is struggling to breathe or breathing with an open mouth at rest
• Has blue, gray, or very pale gums
• Collapses or faints, even briefly
• Has a rapidly swelling belly with weakness

Call your veterinarian the same day if:

• Your dog's sleeping breathing rate is repeatedly above 30 per minute or clearly above its usual rate
• A cough is new or getting worse
• Your dog is tiring much sooner on walks or seems restless at night

Book an appointment soon if your veterinarian has mentioned a heart murmur and you haven't discussed staging, or if your dog is a breed at risk and hasn't been screened.

Questions to ask your veterinarian

• What's causing my dog's heart to enlarge?
• Which chambers are enlarged, and how much?
• Should we do an echocardiogram, and should we see a veterinary cardiologist?
• What stage is my dog's heart disease, and would early treatment help?
• What sleeping breathing rate should prompt me to call you?
• Is my dog's diet appropriate for its heart?
• My dog has cancer or may need chemotherapy. How does the heart change the plan?

Sources

Boswood, A., Häggström, J., Gordon, S. G., Wess, G., Stepien, R. L., Oyama, M. A., Keene, B. W., Bonagura, J., MacDonald, K. A., Patteson, M., Smith, S., Fox, P. R., Sanderson, K., Woolley, R., Szatmári, V., Menaut, P., Church, W. M., O'Sullivan, M. L., Jaudon, J.-P., . . . Watson, P. (2016). Effect of pimobendan in dogs with preclinical myxomatous mitral valve disease and cardiomegaly: The EPIC study. A randomized clinical trial. Journal of Veterinary Internal Medicine, 30(6), 1765-1779. 

Hallman, B. E., Hauck, M. L., Williams, L. E., Hess, P. R., & Suter, S. E. (2019). Incidence and risk factors associated with development of clinical cardiotoxicity in dogs receiving doxorubicin. Journal of Veterinary Internal Medicine, 33(2), 783-791. 

Keene, B. W., Atkins, C. E., Bonagura, J. D., Fox, P. R., Häggström, J., Fuentes, V. L., Oyama, M. A., Rush, J. E., Stepien, R., & Uechi, M. (2019). ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. Journal of Veterinary Internal Medicine, 33(3), 1127-1140. 

Kittleson, M. D. (2024). Pericardial disease in dogs and cats. Merck Veterinary Manual. Retrieved November 10, 2026, from https://www.merckvetmanual.com/circulatory-system/various-heart-diseases-in-dogs-and-cats/pericardial-disease-in-dogs-and-cats

Kittleson, M. D. (2025). Acquired heart and blood vessel disorders in dogs. Merck Veterinary Manual, Dog Owners Version. Retrieved November 10, 2026, from https://www.merckvetmanual.com/dog-owners/heart-and-blood-vessel-disorders-of-dogs/acquired-heart-and-blood-vessel-disorders-in-dogs

MedVet. (n.d.). Veterinary cardiology tip: Vertebral heart score. Retrieved November 10, 2026, from https://archive.medvet.com/?p=2030

Rishniw, M., Ljungvall, I., Porciello, F., Häggström, J., & Ohad, D. G. (2012). Sleeping respiratory rates in apparently healthy adult dogs. Research in Veterinary Science, 93(2), 965-969. 

Wall, T. (2022, December 26). FDA ends DCM updates; no causality data with dog foods. Petfood Industry. https://www.petfoodindustry.com/news-newsletters/pet-food-news/article/15469411/fda-ends-dcm-updates-no-causality-data-with-dog-foods

Wess, G., Domenech, O., Dukes-McEwan, J., Häggström, J., & Gordon, S. (2017). European Society of Veterinary Cardiology screening guidelines for dilated cardiomyopathy in Doberman Pinschers. Journal of Veterinary Cardiology, 19(5), 405-415.