Subdiaphragmatic free gas

Last revised by Daniel J Bell on 25 Oct 2018

Subdiaphragmatic free gas is one of the ways of detecting presence of free intraperitoneal gas (i.e. pneumoperitoneum). It is the presence of free, extraluminal gas in the anterior subhepatic space. 

Subdiaphragmatic free gas is well appreciated as the gas under the diaphragm and air in the lungs outline the diaphragmatic contour well.

As little as 1 mL of free gas can be detected but the patient may be needed to be kept in upright position for about 10 minutes for the gas to rise.

Erect chest x-rays covering the upper abdomen are often considered the best for detecting free subdiaphragmatic gas but an erect abdominal x-ray covering the lung bases will be just as sensitive.

When there is a large amount of free subphrenic gas the continuous diaphragm sign may be present.

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