Pneumoperitoneum (Free Air)
- Pneumoperitoneum is free air loose in the belly — outside the bowel, where air should never be. It usually means a hollow organ has sprung a leak.
- The classic cause is a perforated viscus (a hole in stomach or bowel). It's a "page someone now" finding, not a "mention it at the end of the report" finding.
- On an upright chest X-ray, hunt for a thin sliver of black hugging the underside of the diaphragm — free air rising to the top.
- CT is far more sensitive: even a few tiny bubbles of gas where they don't belong count.
- A little free air is normal for days after abdominal surgery. Context is everything.
Your bowel is basically a long, tidy tube that keeps its gas politely inside. Pneumoperitoneum is what happens when that tube springs a leak and the air escapes into the open space around the organs. Air, being air, immediately floats to the highest point it can find — and that simple bit of physics is what saves the patient.
The word looks scary: pneumo- (air) peritoneum (the slick lining bag that holds your abdominal organs). Put together: free air in the belly. Radiologists also just call it "free air," because we are nothing if not literal.
Why this is a drop-everything finding
In most cases, free air means something hollow has perforated — a stomach ulcer eroded through the wall, a diverticulum burst, the colon ruptured, or a loop of obstructed bowel blew out under pressure. Bowel contents are now spilling where they shouldn't, and the body responds with peritonitis — the kind of belly that's rigid as a board and makes patients lie very, very still.
Free air from a perforated viscus is a surgical emergency. The longer the spillage continues, the worse the contamination and sepsis. If you spot it, you don't sit on it — you call the surgeons.
This sits squarely in the world of the acute abdomen, and it's one of the findings that turns an ordinary day into a fast one.
Finding it on plain film
The trick rests on a foggy-windshield principle: air is the blackest thing on an X-ray (it lets nearly all the beam through), so even a thin ribbon of it stands out against the gray of soft tissue. If you'd like the why behind that, it's worth a detour through the four radiographic densities.
The single best plain-film view is an upright chest X-ray. Stand the patient up, give the air a minute to float, and it collects under the dome of the diaphragm. You're looking for a crescent of black tucked between the diaphragm and the liver on the right (the left is muddier, because the stomach's gas bubble already lives up there and likes to impersonate the real thing).
On a supine abdominal radiograph, the air can't rise to a tidy crescent because the patient is lying flat — so it pools differently and gets sneaky. The classic sign here is being able to see both the inside and outside walls of a loop of bowel, because there's now air on each side of it making both edges visible. That outlined-wall look is the giveaway.
If the patient genuinely can't sit up, a left lateral decubitus film (lying on the left side) works too — free air rises over the liver, away from the stomach bubble, where it's easy to spot.
CT: the sensitive detective
Plain films are good for big leaks, but CT is the real workhorse — it'll catch a handful of tiny gas bubbles that an X-ray would shrug off. On abdominal CT, free air shows up as pockets of pure black gas sitting outside the bowel: drifting along the front of the abdomen under the wall, tracking around the liver, or bubbling near the leak itself. CT often points right at the culprit — a focal wall defect, a cluster of bubbles, and surrounding inflammatory haziness like a smudge marking the scene.
The traps that fool people
Not all free air is an emergency. After abdominal surgery — open or laparoscopic — air is deliberately introduced and can linger in the belly for days, sometimes longer. Recent surgery in the history changes the whole story, so always check it before you panic.
A few other mimics worth knowing:
| Looks like free air | What it actually is | The tell |
|---|---|---|
| Crescent under right diaphragm | Free air (real) | Pure black, conforms to the diaphragm's curve |
| Bubble under left diaphragm | Stomach gas bubble | Has a curved wall below it; air-fluid level inside |
| Gas between liver and diaphragm | Bowel looped up there (a known variant) | You can trace bowel folds within the gas |
That last one — a loop of colon riding up between liver and diaphragm — is the famous impostor. The fix is simple: real free air is featureless black, while a bowel loop has its own internal folds. When in doubt, look for those folds, and lean on CT.
The one thing to remember
Air belongs inside the tube. When you see it loose in the abdomen — floating under the diaphragm on an upright film, or bubbling outside the bowel on CT — assume a perforation until proven otherwise, check whether the patient just had surgery, and pick up the phone.