For suspected free intraperitoneal air, which radiographic view is most sensitive?

• A. Upright chest radiograph
• B. Supine abdominal radiograph
• C. Left lateral decubitus abdomen
• D. AP abdomen

Answer: (A)

Air that has leaked into the peritoneal cavity will rise to the highest point in the body. When a patient is upright, free air collects beneath the diaphragms and appears as a distinct radiolucent area just under the diaphragms on a chest radiograph. This makes even small amounts of pneumoperitoneum detectable because gravity helps separate the air from surrounding tissues and outlines the diaphragms clearly.

Supine abdominal films spread the air over the posterior abdomen and can be masked by bowel gas, so they’re less reliable for small amounts. The left lateral decubitus view can reveal free air by layering it along the lateral abdominal wall, but it’s generally less sensitive than an upright chest radiograph. An AP abdominal view doesn't take advantage of the diaphragmatic silhouette or the gravity-driven subdiaphragmatic collection of air as effectively.

So the upright chest radiograph is the most sensitive for detecting suspected free intraperitoneal air.