Bowel gas is usually confined to the lumen of the gut, but owing to a variety of pathological processes, gas may escape into the peritoneal cavity. It is important to recognise the radiographic signs of pneumoperitoneum because bowel perforation usually requires surgical intervention. Patient morbidity and mortality is often significantly increased if this is delayed.

Sub-diaphragmatic air is usually easy to identify on an erect chest radiograph (CXR) and appears as a dark crescent of gas under each hemidiaphragm, provided the patient has remained erect long enough for free air to rise, and there is a sufficient volume of air to allow identification (fig 1A⇓). Free gas is best seen on the right side above the distinct dome shaped outline of the liver (fig 1B⇓). In sick patients, particularly those on ventilatory support, it is not possible to obtain an erect CXR when bowel perforation is suspected. Free intraperitoneal air can also be identified on abdominal radiographs (AXR), the majority of which are taken supine in both adults and children. A careful inspection is required in order to identify the signs associated with free intraperitoneal air, particularly if there is clinical suspicion of perforation or the patient has risk factors such as necrotising enterocolitis (NEC) or steroid treatment. Remember that no free gas may be seen if a bowel perforation has been sealed off by the omentum.