How to Assess Heart Size on a Dog’s Chest X-ray — Key Q&A on Reading VHS Scores

We’ve compiled the essential information every pet owner should know about how to assess heart size using a dog’s chest X-ray and what those findings mean.

What Is Heart Size Assessment on a Dog’s Chest X-ray?

Chest X-rays are used to assess heart size in dogs by quantifying the heart’s dimensions to help determine whether cardiomegaly is present. However, chest radiographs may not be sensitive enough to detect mild to moderate chamber enlargement, so early-stage changes can sometimes appear normal. Despite this limitation, chest X-rays remain a valuable tool for evaluating overall heart size and identifying signs of congestive heart failure, such as pulmonary edema or pleural effusion. In particular, they are one of the basic diagnostic tests veterinarians routinely perform when a dog is experiencing respiratory distress.

VHS Measurement: The vertebral heart score (VHS) is a numerical value obtained by measuring the heart’s length and width relative to the number of vertebrae. In many dogs, a VHS greater than 10.5 raises suspicion of cardiomegaly, though breed-specific variations exist.

Cardiomegaly: An enlarged heart can lead to impaired cardiac function.

Early Detection: Heart enlargement can occur without clinical signs and may not be clearly visible on X-rays in the early stages, making regular check-ups and follow-up imaging essential.

What Are the Key Criteria for Assessing Heart Size?

There are several key criteria used to assess heart size on a dog’s thoracic radiographs. These standards help veterinarians make objective evaluations of cardiac dimensions.

Vertebral Heart Score (VHS): This is calculated by measuring the length (from the carina to the cardiac apex) and width of the heart on a lateral view, then converting these measurements into the number of vertebral bodies starting from the fourth thoracic vertebra and summing them. In many dogs, a VHS exceeding 10.5 suggests cardiomegaly, though breed-specific variations exist.

Cardiothoracic Ratio: This method compares the maximum width of the heart to the maximum width of the thoracic cavity on a ventrodorsal or dorsoventral view. Cardiomegaly is suspected if the heart width exceeds 60% of the thoracic width or if the heart spans more than three intercostal spaces on a lateral view.

Lateral and Ventrodorsal/Dorsoventral Radiographs: Imaging from both projections helps evaluate the heart’s shape and position, aiding in the identification of specific patterns of cardiac change.

What Is the Difference Between Normal and Abnormal Heart Size?

Veterinarians use objective criteria to determine whether a dog’s heart size is normal or abnormal on a chest X-ray. This distinction serves as an important indicator for monitoring disease progression. However, because X-rays are not sensitive to mild-to-moderate enlargement, results must be interpreted comprehensively alongside other diagnostic tests.

Normal heart size: The Vertebral Heart Score (VHS) is generally 10.5 or less, and the heart occupies no more than 60% of the thoracic width. The average VHS for healthy dogs is reported to be approximately 9.7.

Suspected enlargement: A VHS exceeding 10.5 suggests possible cardiomegaly. However, due to significant breed variations, a definitive diagnosis cannot be made based on this single value alone.

Marked enlargement: Significantly elevated values accompanied by changes in heart shape indicate advanced enlargement, warranting a concurrent assessment of heart failure risk.

Changes in heart shape: Morphological changes, such as a distinct increase in the heart’s transverse diameter, may signal declining cardiac function.

What Causes an Enlarged Heart?

There are many reasons why a dog’s heart may enlarge. It can be due to simple aging or genetic factors, but it may also signal a serious underlying disease.

Valvular heart disease: Acquired valvular insufficiency, particularly mitral valve regurgitation, is a common cause. Left atrial enlargement is the earliest and most consistent change, and as the disease progresses, the heart grows larger.

Cardiomyopathy: In conditions such as dilated cardiomyopathy (DCM), the heart muscle weakens, causing the atria and ventricles to stretch or the muscle to thicken, leading to cardiac enlargement. DCM primarily affects large-breed dogs.

Heartworm infection: This can cause right ventricular enlargement, making the heart’s silhouette appear larger on imaging, but it may not be clearly visible on X-rays until the disease becomes severe.

Other cardiovascular factors: Other cardiovascular conditions can also increase the heart’s workload and affect its size, so a comprehensive evaluation is necessary.

VHS (Vertebral Heart Size) is a value that quantifies heart size based on the number of vertebral bodies starting from the 4th thoracic vertebra; in many dogs, exceeding 10.5 suggests enlargement, with variations by breed.

What Signs Should You Watch For After Heart Size Assessment?

An enlarged heart on a dog’s chest X-ray doesn’t necessarily mean there’s a serious condition. However, if you notice any of the following signs, be sure to consult your veterinarian.

Coughing: Especially frequent coughing at night or after exercise. This could be a sign of pulmonary edema.

Difficulty breathing: Rapid or labored breathing, with noticeable chest movement.

Fatigue: Less energy than usual, reluctance to run, or needing more rest.

Abdominal swelling: A bloated or distended abdomen. This can be a sign of heart failure.

How to Conduct Regular Checkups for Heart Size Assessment

It’s a good idea to include heart size assessment as part of your dog’s routine health checkups. This is especially important for senior dogs or breeds prone to heart disease.

Regular checkups: As dogs age, it’s recommended to shorten the interval between checkups. The specific schedule should be determined in consultation with your veterinarian based on your dog’s condition and risk factors.

Breed-specific care: Certain breeds are more susceptible to specific heart conditions, such as acquired valvular disease or dilated cardiomyopathy. Dogs of these breeds benefit from more frequent cardiac evaluations.

Home monitoring: Keeping track of your dog’s breathing, activity level, and any coughing can provide valuable information for discussions with your veterinarian.

Record keeping and tracking: Consistently documenting checkup results and X-rays makes it easier to monitor changes over time and assess disease progression.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Related Guides

Top Recommendations and Selection Criteria for Managing Feline Bradycardia and Atrioventricular Block

Top Prescription Heart Disease Diets for Dogs: Best Picks and Selection Criteria

Heartworm Preventative Comparison (Heartgard, Interceptor, Tri-Hart)

What Is a Dog Food Allergy Panel? — A Complete Guide to Cost, Accuracy, and Necessity

Is My Dog Anemic? — Checking Iron Levels with Ferritin and Transferrin

What is the Pancreatitis SNAP Test? How to Interpret Results and Accuracy

The Complete Guide to Bilirubin Tests and Jaundice Stages

Lymph Node Excision Surgery: When Is It Necessary and How Is It Performed?

What is ORIF for Dog Fractures? — From Surgery to Recovery

What Is Canine Disc Surgery? — From Hemilaminectomy Procedure to Recovery