Cardiothoracic ratio
Read Your Scan Editorial Team·Last updated
Written from published radiology references and reviewed for plain language. Not written or signed by a physician, and not a diagnosis. How we write these pages →
In plain language
A comparison of the heart’s width to the chest’s width, used to gauge whether the heart looks enlarged.
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Where you'll see this
This wording turns up on Chest X-ray reports — X-ray imaging of the chest. A chest X-ray provides a broad picture of the lungs, the heart’s size and outline, the major airways, the bones of the chest, and the diaphragm. Doctors order it for cough, fever, shortness of breath, chest pain, before surgery, or as a routine check. It is a good first look, and if something needs closer inspection, a CT or other test may follow. The report describes what the picture shows; your doctor decides what it means.
Questions worth asking your doctor
- What was the reason for this X-ray, and did it answer that question?
- Does anything on the report need a closer look with another test?
- Do the findings fit my symptoms, or were they unexpected?
- Do the results change my current treatment in any way?
- Should I have a follow-up X-ray, and if so, when?
How common is this?
The cardiothoracic ratio appears on a great many chest X-ray reports and is one of the oldest measurements in radiology. It compares the widest part of the heart shadow with the widest part of the chest, and a ratio above about half is conventionally described as an enlarged heart shadow. It is quick, needs no extra equipment, and is calculated from the film itself.
When should I worry?
The measurement is far less reliable than its precision suggests, and that is the important thing to know. It is affected by how deep a breath was taken, by whether the film was taken from front to back rather than back to front — as it is for anyone scanned lying down or in a bed — and by body shape. A raised ratio on a portable film taken in a hospital bed frequently means nothing about the heart at all. Even when genuine, it describes a shadow rather than a diagnosis: an enlarged shadow can come from the heart muscle, from a chamber, or from fluid around the heart, and an X-ray cannot distinguish them.
What happens next
A raised ratio in someone with symptoms usually leads to an echocardiogram, which is the test that actually assesses the heart — chamber sizes, valve function and the ejection fraction. On a technically limited film in someone with no symptoms, the usual response is to repeat the X-ray properly or to disregard it. Nothing is treated on the basis of the ratio alone.
Frequently asked questions
- What is a normal cardiothoracic ratio?
- Conventionally up to about half on a standard back-to-front chest X-ray taken with a full breath. Above that the heart shadow is called enlarged — though whether that reflects the heart itself depends heavily on how the film was taken.
- Does an enlarged heart shadow mean heart failure?
- No. It describes a shadow on an X-ray, not a diagnosis. It can reflect the heart muscle, an enlarged chamber, fluid around the heart, or simply how the film was taken. An echocardiogram is what distinguishes them.
- Why does the type of X-ray matter?
- Because a film taken from front to back, as portable bedside films are, magnifies the heart shadow. So does a shallow breath. Both make the ratio look higher without anything having changed, which is why radiologists note the technique alongside the measurement.
Other words on the same report
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