Free gas under the diaphragm

Free gas under the right hemidiaphragm on an erect chest radiograph is, in a patient with acute abdominal pain and peritonism, a perforated abdominal viscus until proven otherwise. In a prospective cohort of 83 perforated-viscus patients, the height of the air column under the diaphragm correlated with the size of the perforation, the volume of peritoneal soiling and mortality — more free air means worse soiling and higher mortality.^[1] On a supine film (the trauma or the breathless patient), free gas is sought by the Rigler double-wall sign (gas outlining both sides of a bowel loop), the football sign (a large ovoid lucency over the abdomen), the cupola sign (gas trapped under the central diaphragm), and gas outlining the falciform ligament. A normal erect film does not exclude perforation; CT localises the source when the patient is stable enough. The image triggers the resuscitation-and-referral pathway: large-bore intravenous access, intravenous fluid resuscitation, a broad-spectrum intravenous antibiotic, analgesia (titrated opioid), a nasogastric tube to decompress the stomach, an urgent surgical review, and a CT abdomen and pelvis to localise the source if the patient is stable enough. The unstable patient with frank peritonitis goes to theatre without further imaging.