Why is the term free air in Radiology a misnomer?
Gas in the stomach is initially similar in composition to atmospheric air swallowed by the patient. When various digestive processes occur, the gas in the small or large bowel no longer has the same composition as atmospheric air. Carbon dioxide is generated when acid secreted by the stomach combines with bicarbonate secreted by the pancreas. Oxygen and carbon dioxide are resorbed in the small bowel. Nitrogen is not absorbed. Colonic gas is also composed of gases such as methane and hydrogen sulfide, produced by bacterial fermentation.
Gas in the peritoneal cavity ascends to a nondependent position under the anterior abdominal wall with the patient in a supine position and underneath the diaphragm with the patient in an erect position. Intraperitoneal gas is also trapped in various crevices of the organs and mesenteries. A patient should generally be positioned in an erect or lateral decubitus position for about 5 minutes before a radiograph is obtained to look for “free intraperitoneal gas.”
