Another pneumoperitoneum on chest X-ray.
Pneumoperitoneum seen on X-ray with the patient lying on his left side.
Pneumoperitoneum is the presence of air or gas in the abdominal (peritoneal) cavity. It is often seen on X-ray, but small amounts are often missed, and CT is nowadays regarded as a criterion standard in the assessment of a pneumoperitoneum. CT can visualize quantities as small as 5 cm³ of air or gas. The most common cause is a perforated abdominal viscus, generally a perforated peptic ulcer, although any part of the bowel may perforate from a benign ulcer, tumor or abdominal trauma. A perforated appendix seldom causes a pneumoperitoneum.
A pneumoperitoneum is deliberately created by the surgical team in order to perform laparoscopic surgery. This is achieved by insufflating the abdomen with carbon dioxide.
Differential diagnosis 
Subphrenic abscess, bowel interposed between diaphragm and liver (Chilaiditi syndrome), and linear atelectasis at the base of the lungs can simulate free air under the diaphragm on a chest X-ray.
See also 
- ^ Ali Nawaz Khan. "eMedicine.com: Pneumoperitoneum".
- ^ Necrotizing Enterocolitis Bugs, Drugs and Things That Go Bump in the Night