Free intraperitoneal air (pneumoperitoneum)
- Finding
Free air is gas that has escaped from the gut into the belly cavity. Outside recent surgery it usually means a hole in the bowel and needs a surgeon.
What it is
Pneumoperitoneum is gas in the peritoneal cavity outside the bowel. In a patient who has not had recent surgery or a procedure, it almost always means a perforated hollow organ: most often a perforated peptic ulcer, then diverticulitis, trauma, tumour, ischaemia or an endoscopic injury. Gas rises to the highest point, so on an erect chest X-ray it collects as a thin dark crescent under the diaphragm; the patient should sit upright for several minutes first. On a supine abdominal film the signs are subtler: gas on both sides of the bowel wall (Rigler sign), a large oval lucency over the abdomen outlining the falciform ligament (the football sign, mainly in infants) or small triangles of gas between loops. CT is the most sensitive test, finds small amounts, and often shows the site of the perforation.
What it looks like
- Erect chest X-ray: a thin dark crescent between the hemidiaphragm and the liver or stomach.
- Rigler (double-wall) sign: both sides of the bowel wall are visible on a supine film.
- Football sign: a large oval lucency over the abdomen, often with the falciform ligament as a white line (infants).
- Triangle sign: small triangular gas pockets between adjacent loops.
- Left lateral decubitus film: gas collects over the liver, along the right flank.
- CT, best viewed on lung window: small bubbles anterior to the liver and near the leak.
Why it looks that way
- Free gas moves to the highest part of the cavity: under the diaphragm when upright, the anterior abdomen when supine.
- Gas against the outer bowel wall plus gas in the lumen outlines the wall from both sides (Rigler sign).
- Sitting upright for a few minutes gives the gas time to rise, which is why technique matters.
Differential and pitfalls
- Commonest causes: perforated peptic ulcer, diverticulitis, trauma, tumour, ischaemia, iatrogenic injury.
- After abdominal surgery or laparoscopy free air is expected for several days; growing amounts suggest a leak.
- Mimic: colon between the liver and diaphragm (Chilaiditi sign) shows haustra crossing the lucency.
- Mimics: gas in the gastric fundus under the left diaphragm, subphrenic abscess, basal lung lucency.
- Pneumatosis intestinalis (gas in the bowel wall) can rupture into the peritoneum; benign forms can give free air without peritonitis.
- A normal X-ray does not exclude perforation: CT is the test when suspicion stays high.
Next step
- Free air with peritonitis or sepsis: call the surgical team now; resuscitate while waiting.
- Unclear X-ray but clinical suspicion: CT abdomen and pelvis with IV contrast to confirm and locate the leak.
- Suspected leak where oral contrast may help (oesophagus, duodenum, anastomosis): water-soluble contrast, never barium.
- Free air after recent surgery: compare with earlier imaging and discuss with the operating team and the radiologist.
- Pneumatosis with portal venous gas or a sick patient: treat as ischaemia until proven otherwise and call surgery.
Test yourself
Think of the answer, then tap the question to see it.
Why should a patient sit upright for several minutes before an erect chest X-ray for free air?
Free gas needs time to rise to the space under the diaphragm, where it shows as a dark crescent. Taken too soon, a small amount can be missed.
What is the commonest cause of pneumoperitoneum without recent surgery?
A perforated peptic ulcer, usually duodenal or gastric.
How do you tell Chilaiditi sign from true free air?
In Chilaiditi sign the lucency under the right diaphragm is colon, so haustral folds cross it; true free air is a smooth crescent without folds.
Which contrast is used when a gastrointestinal leak is suspected, and why?
Water-soluble iodinated contrast, because barium leaking into the peritoneum causes a severe inflammatory peritonitis.
Elsewhere in DurgiPedia
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Bowel Perforation (StatPearls (NCBI Bookshelf), 2025); Pneumatosis Intestinalis (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.