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Enlarged Heart in Dogs: Causes, Signs and Life Expectancy

By the Dog Health Handbook team · Updated · Fact-checked against current veterinary references (see sources)

Key takeaways

  • An enlarged heart is a sign of heart disease, most often mitral valve disease or dilated cardiomyopathy.
  • Many dogs have no symptoms at first; later signs include coughing, fast breathing at rest and tiring easily.
  • Vets measure heart size on X-rays (vertebral heart score) and, best of all, with an echocardiogram.
  • Normal heart size varies by breed; Bulldogs, Boston Terriers and Boxers normally measure larger.
  • Treatment can shrink the heart a little and delay heart failure; some diet-related DCM partly reverses.
  • Life expectancy ranges from years in mild valve disease to months once heart failure develops.

Table of Contents

Research Causes Signs How it's measured Treatment Can it shrink? Life expectancy FAQ Sources

Heart and Blood QuickLinks

Heart Disease Congestive Heart Failure Heart Murmurs Heart Rate High Blood Pressure Heartworm Overview of Dog Blood Blood Disorders

An enlarged heart (cardiomegaly) isn't a disease on its own. It's a sign that something is making the heart work harder or weakening its muscle, most often mitral valve disease in small dogs or dilated cardiomyopathy (DCM) in large dogs.1,2 Many dogs with a mildly enlarged heart have no symptoms for months or years, but more enlargement raises the risk of heart failure.1,3 This guide explains the causes, symptoms, how vets measure heart size, treatment, whether the heart can shrink back, and life expectancy.

Go to a vet or emergency clinic now if a dog with an enlarged heart:

  • Breathes with obvious effort at rest, or can't settle or lie down (call your vet promptly if the sleeping breathing rate is consistently above 30)4,5
  • Faints or collapses6,7
  • Has blue, gray or very pale gums or tongue4,8
  • Has a quickly swelling belly9,10

What the Research Shows

  • Normal is about 9.7 vertebrae. In the original study of 100 normal dogs, heart size on X-rays averaged 9.7 vertebral lengths, and 98% measured under 10.5.11
  • "Normal" depends on breed. Average heart scores are about 12.7 in Bulldogs, 11.7 in Boston Terriers, 11.6 in Boxers and 10.6 in Cavaliers.11
  • Treatment can shrink the heart slightly. In the EPIC trial, pimobendan reduced left ventricle size within 35 days, and dogs whose hearts shrank more stayed out of heart failure longer.12
  • Size predicts risk. In dogs with valve disease and an enlarged heart, a left atrium more than 2.1 times the size of the aorta tripled the risk of heart failure or cardiac death.3

What Causes an Enlarged Heart in Dogs?

  • Mitral valve disease: by far the most common cause. A leaky valve lets blood flow backward, so the left atrium and ventricle stretch. It's most common in older small breeds such as Cavaliers, Chihuahuas and Poodles.1,13
  • Dilated cardiomyopathy (DCM): the heart muscle weakens and the chambers balloon. It mainly affects large breeds such as Dobermans, Boxers and Great Danes, and some cases are linked to diet.2,14
  • Congenital heart defects such as patent ductus arteriosus (PDA) or stenosis of a valve.15
  • Heartworm disease, which enlarges the right side of the heart.16
  • Fluid around the heart (pericardial effusion), which makes the heart look large and round on X-rays even though the muscle itself isn't enlarged.17,10
  • Anemia: long-standing, moderate to severe anemia makes the heart work harder and can enlarge it.17

See heart disease in dogs for more on each type.

Symptoms of an Enlarged Heart in Dogs

Early on there are often no signs; the first clue is usually a murmur or an abnormal rhythm at a checkup.1,9 As enlargement worsens or heart failure develops, you may notice:7,8,18

  • Fast or labored breathing while resting or asleep
  • Coughing, especially at night
  • Tiring easily or not wanting to walk or play
  • Restlessness at night
  • Fainting
  • A swollen belly, weight loss or a poor appetite

See dog heart disease symptoms.

An enlarged dog heart specimen with a rounded, stretched shape
An enlarged dog heart, examined after death. In DCM the chambers stretch and the heart becomes rounder. Photo © Washington State University.

How Vets Measure Heart Size

  • Vertebral heart score (VHS): on a chest X-ray, the heart's length and width are compared with the bones of the spine. The average in normal dogs is about 9.7 vertebrae, and 10.5 is a common upper limit for most breeds.11,1 Some breeds normally measure higher, so vets use breed-specific values.11,17
  • Echocardiogram (heart ultrasound): the most accurate test. It measures the left atrium against the aorta (LA:Ao) and the size of the left ventricle, and shows the cause.1

For mitral valve disease, ACVIM specialists define Stage B2 as a moderate or loud murmur plus an echocardiogram showing a left atrium at least 1.6 times the aorta and an enlarged left ventricle, with a breed-adjusted VHS above 10.5; without an echo, a VHS above 11.5 suggests B2.1 X-rays alone have limits: in one study, a VHS above 11.7 was very reliable for true enlargement but missed most enlarged hearts, so many dogs still need an echocardiogram.19

A rounder heart outline, being overweight, which side it lies on for the X-ray, and spine abnormalities in breeds such as Bulldogs can all affect the score.17

Treatment for an Enlarged Heart in Dogs

  • Mitral valve disease with enlargement but no symptoms: pimobendan delays heart failure. In the EPIC trial, it extended the median time to heart failure or cardiac death by about 15 months.1,20
  • Heart failure: a diuretic such as furosemide, pimobendan, an ACE inhibitor and often spironolactone.1
  • DCM: pimobendan and other heart drugs, plus a diet review. Dobermans with early DCM benefit from starting pimobendan before symptoms.21,2
  • Congenital defects: some can be corrected by surgery or catheter procedures.15
  • Diet and exercise: avoid salty treats, keep your dog lean and keep exercise gentle once heart failure develops.22,23

See dog heart medication.

Can an Enlarged Heart in Dogs Go Back to Normal?

Usually not completely, but it can improve. In the EPIC trial, pimobendan produced a small reduction in heart size within about a month, and dogs with bigger reductions stayed out of heart failure longer.12 Some dogs with diet-associated DCM improve after a diet change: in one study, dogs switched from grain-free diets showed more improvement in heart size and needed lower medication doses.24,25 Open-heart mitral valve repair is offered at a few specialist centers; in one UK program, 81% of 132 dogs survived to leave the hospital.26 Heart enlargement from fluid around the heart can resolve once the fluid is drained and the cause treated.10

Life Expectancy for Dogs With an Enlarged Heart

It depends on the cause and how far the disease has progressed. In a study of 558 dogs with mitral valve disease, median survival from any cause was about 19.5 months; mild disease was relatively benign, and a left atrium more than 1.7 times the aorta was the only independent predictor of cardiac death.27 Dogs with enlargement but no symptoms often live for years, especially with pimobendan.20,1 Once heart failure develops, survival is usually months to a year or two.28,29 DCM generally has a poorer outlook than valve disease.2

Count your dog's sleeping breathing rate regularly; a rise above 30 a minute is often the first sign that heart failure is developing.5,1

Frequently Asked Questions

How long can a dog live with an enlarged heart?

It varies widely. Dogs with mild valve disease often live for years, and pimobendan can extend that. Once heart failure develops, many dogs live months to a year or two with treatment.

Can an enlarged heart in a dog go back to normal?

Usually not completely, but treatment can shrink it a little, and some diet-related DCM and heart enlargement from fluid around the heart can improve substantially.

Is a dog in pain with an enlarged heart?

An enlarged heart isn't usually painful. Breathlessness, coughing and weakness from heart failure are what cause distress, and treatment can ease them.

Is it okay to walk a dog with an enlarged heart?

Usually yes. Gentle walks are fine for most dogs. Once heart failure develops, keep walks short, avoid heat and stop if your dog breathes hard or lags.

What is a normal vertebral heart score in dogs?

About 9.7 on average, with 10.5 a common upper limit. Some breeds, such as Bulldogs, Boston Terriers and Boxers, normally measure higher.

What is the most common cause of an enlarged heart in dogs?

Mitral valve disease in small and medium breeds, and dilated cardiomyopathy in large breeds.

What are the final stages of an enlarged heart in dogs?

Heart failure that's hard to control: fast or labored breathing despite medication, coughing, poor appetite, fainting and a swollen belly. Your vet can help with comfort and quality-of-life decisions.

Can diet cause an enlarged heart in dogs?

The FDA has investigated a possible link between grain-free, pea- or lentil-heavy diets and DCM but hasn't proven a cause. Ask your vet whether your dog's diet is a concern.

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Sources and further reading

This article was rewritten and fact-checked on October 15, 2026 against current veterinary references and specialist guidelines:

  1. Keene BW, Atkins CE, Bonagura JD, et al. ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. Journal of Veterinary Internal Medicine, 2019. Also in the IVIS ACVIM consensus statements collection.
  2. Kittleson MD. Dilated cardiomyopathy in dogs and cats. Merck Veterinary Manual, September 2024.
  3. Borgarelli M, et al. Predictive value of clinical, radiographic, echocardiographic variables and cardiac biomarkers for heart failure or cardiac death in dogs with preclinical myxomatous mitral valve disease (the DELAY study). Journal of Veterinary Cardiology, 2021.
  4. Cummings School of Veterinary Medicine at Tufts University. Difficulty breathing (dyspnea). HeartSmart.
  5. Porciello F, Rishniw M, Ljungvall I, et al. Sleeping and resting respiratory rates in dogs and cats with medically-controlled left-sided congestive heart failure. The Veterinary Journal, 2016.
  6. Cummings School of Veterinary Medicine at Tufts University. Fainting. HeartSmart.
  7. Kittleson MD. Heart failure in dogs. Merck Veterinary Manual (pet owner version), updated September 2024.
  8. Hunter T, Weir M, Ward E. Congestive heart failure in dogs. VCA Animal Hospitals, November 2023.
  9. Cummings School of Veterinary Medicine at Tufts University. Abdominal swelling. HeartSmart.
  10. Kittleson MD. Pericardial disease in dogs and cats. Merck Veterinary Manual, September 2024.
  11. Kallassy A, Calendrier E, Bouhsina N, Fusellier M. Vertebral heart scale for the Brittany Spaniel: breed-specific range and its correlation with heart disease assessed by clinical and echocardiographic findings. Veterinary Sciences, 2021.
  12. Boswood A, Gordon SG, Häggström J, et al. Longitudinal analysis of quality of life, clinical, radiographic, echocardiographic, and laboratory variables in dogs with preclinical myxomatous mitral valve disease receiving pimobendan or placebo: the EPIC study. Journal of Veterinary Internal Medicine, 2018.
  13. Mattin MJ, Boswood A, Church DB, et al. Prevalence of and risk factors for degenerative mitral valve disease in dogs attending primary-care veterinary practices in England. Journal of Veterinary Internal Medicine, 2015.
  14. U.S. Food and Drug Administration. Questions & answers: FDA's work on potential causes of non-hereditary DCM in dogs. Current as of August 2024.
  15. Tou SP. Congenital and inherited disorders of the cardiovascular system in dogs. Merck Veterinary Manual (pet owner version), updated September 2024.
  16. American Heartworm Society. Heartworm basics.
  17. Pasieka G. The vertebral heart scale. CVCA Cardiac Care for Pets, February 2018.
  18. Cummings School of Veterinary Medicine at Tufts University. Coughing. HeartSmart.
  19. Poad MH, Manzi TJ, Oyama MA, Gelzer AR. Utility of radiographic measurements to predict echocardiographic left heart enlargement in dogs with preclinical myxomatous mitral valve disease. Journal of Veterinary Internal Medicine, 2020.
  20. Boswood A, Häggström J, Gordon SG, et al. 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, 2016.
  21. Summerfield NJ, Boswood A, O'Grady MR, et al. Efficacy of pimobendan in the prevention of congestive heart failure or sudden death in Doberman Pinschers with preclinical dilated cardiomyopathy (the PROTECT study). Journal of Veterinary Internal Medicine, 2012.
  22. Cummings School of Veterinary Medicine at Tufts University. Heart disease: nutrition. HeartSmart.
  23. Cummings School of Veterinary Medicine at Tufts University. Reduced ability to exercise. HeartSmart.
  24. Walker AL, DeFrancesco TC, Bonagura JD, et al. Association of diet with clinical outcomes in dogs with dilated cardiomyopathy and congestive heart failure. Journal of Veterinary Cardiology, 2022.
  25. Freeman L, Rush J, Adin D, et al. Prospective study of dilated cardiomyopathy in dogs eating nontraditional or traditional diets and in dogs with subclinical cardiac abnormalities. Journal of Veterinary Internal Medicine, 2022.
  26. Brockman DJ, Greensmith TD, et al. Improvement in short-term outcome over time in a single center embarking on a canine mitral valve repair program. Veterinary Surgery, 2025.
  27. Borgarelli M, Savarino P, Crosara S, et al. Survival characteristics and prognostic variables of dogs with mitral regurgitation attributable to myxomatous valve disease. Journal of Veterinary Internal Medicine, 2008.
  28. Häggström J, Boswood A, O'Grady M, et al. Effect of pimobendan or benazepril hydrochloride on survival times in dogs with congestive heart failure caused by naturally occurring myxomatous mitral valve disease: the QUEST study. Journal of Veterinary Internal Medicine, 2008.
  29. Maharaj N. Congestive heart failure in dogs: what you need to know. American Kennel Club, August 2026.

About this article

Last updated : fully rewritten using the ACVIM consensus guidelines, the Merck Veterinary Manual, VCA and published studies on heart size, treatment and survival; added causes, signs, how heart size is measured, breed differences, treatment, whether enlargement can reverse, life expectancy, a key takeaways summary and FAQ.

This page is for general information and is not a substitute for an exam by your veterinarian.