01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Free gas may enter the peritoneal cavity after gastroduodenal, small-bowel or colonic perforation, following surgery or endoscopy, through a drain or occasionally from thoracic or gynaecological sources. The same sign therefore has very different meaning according to timing, symptoms and procedure history. In an unwell patient with acute pain, treat it as perforation until rapidly clarified.
Erect radiography can reveal crescentic subdiaphragmatic gas, usually most clearly beneath the right hemidiaphragm. Supine gas may collect anteriorly and outline both sides of bowel wall, the falciform ligament or other peritoneal reflections. Small, sealed, retroperitoneal or early perforations can remain occult.
CT depicts tiny extraluminal gas bubbles, fluid, focal bowel-wall discontinuity, inflammatory change, foreign body, abscess and distant complications. Distribution may suggest the source but is not perfectly specific. The report should state the likely compartment and site, degree of contamination and signs of ischaemia, then be communicated immediately.
Key points
- Pneumoperitoneum is a radiological sign; perforation remains a clinical diagnosis when imaging is negative or equivocal.
- CT is preferred when promptly available because it detects small gas volumes, maps fluid and inflammation, and may suggest the perforation site.
- Erect chest or abdominal radiography is useful when CT is not promptly available, but negative plain imaging cannot exclude perforation.
- On supine radiography seek Rigler sign, falciform-ligament outlining and abnormal gas over the liver, while recognising limited sensitivity.
- Distinguish postoperative or post-procedural gas by timing, volume trend, symptoms and operative context rather than appearance alone.
- Resuscitation, broad-spectrum antimicrobial treatment and urgent surgical decision-making proceed alongside imaging when sepsis or peritonitis is present.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
A crescent of lucency beneath a hemidiaphragm on an adequately positioned erect film supports free intraperitoneal gas.
Gas outlines both luminal and peritoneal surfaces of bowel wall on a supine film, usually requiring a substantial gas volume.
CT gas adjacent to inflamed or discontinuous bowel can help localise a contained perforation even without diffuse pneumoperitoneum.
Sealed, small or retroperitoneal leaks may show only focal fluid, fat stranding or subtle gas; negative plain films are insufficient.
Recent laparoscopy, laparotomy, endoscopy or drain manipulation can explain free gas, but increasing volume or clinical deterioration remains concerning.
Pneumatosis with portal venous gas, reduced enhancement, vascular occlusion or shock raises concern for necrosis and urgent source control.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Contrast CT abdomen and pelvisFirst step - Why
- Detect extraluminal gas, localise source and assess contamination, abscess, obstruction and ischaemia.
- Interpretation and limitations
- Absence of a visible wall defect does not exclude a sealed leak; integrate focal gas, fluid and inflammatory change.
- 02
Erect chest radiograph - Why
- Detect larger subdiaphragmatic free-gas volumes when CT is not promptly available.
- Interpretation and limitations
- A negative image cannot exclude perforation and must not override peritonism or shock.
- 03
Supine abdominal radiograph - Why
- Look for indirect free-gas signs and coexisting obstruction when obtained for a selected indication.
- Interpretation and limitations
- Sensitivity is poor; apparent double-wall signs can be mimicked by adjacent gas-filled loops.
- 04
Water-soluble contrast study - Why
- Test for a sealed gastroduodenal leak in an exceptionally selected non-operative pathway.
- Interpretation and limitations
- This is specialist use after stability and source assessment, not a routine substitute for CT.
- 05
Blood gas and lactate - Why
- Assess perfusion and severity during simultaneous resuscitation.
- Interpretation and limitations
- Rising values increase concern, but normal early results do not exclude perforation or bowel ischaemia.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked caseSudden pain with guardingFirst stepA stable adult develops sudden severe epigastric pain and generalised peritonism.+
- 1Context: identify shock, sepsis, ulcer risk, recent procedures, medication, previous surgery and the time symptoms began.
- 2Reasoning: arrange urgent CT if immediately available because it can detect and localise perforation better than plain radiography.
- 3Outcome: start resuscitation and antimicrobial treatment, obtain surgical review and use CT distribution to plan source control.
- 4Verification: document direct communication, repeat physiology and ensure operative or closely supervised exceptional management is enacted.
02Emergency routeTreat suspected perforationPeritonism, sepsis or shock makes a hollow-viscus perforation clinically likely.+
- 1Give oxygen when indicated, obtain vascular access, resuscitate, provide analgesia and start appropriate broad-spectrum antimicrobials.
- 2Contact the surgical team immediately and state physiology, examination and any imaging already available.
- 3Perform urgent CT if it will guide intervention without delaying necessary source control; use plain radiography only when CT is not prompt.
- 4Communicate the likely site, contamination, obstruction and ischaemia signs, and verify acknowledgment.
03Negative radiographDo not close the pathwayPlain imaging shows no free gas but clinical suspicion remains substantial.+
- 1Reassess haemodynamics, guarding, pain trajectory and sepsis rather than lowering urgency automatically.
- 2Check whether positioning, exposure and anatomy made the film adequate and remember contained or retroperitoneal leaks.
- 3Proceed to CT and surgical review, or directly to source control when instability makes further imaging unsafe.
- 4Maintain observation and clear ownership until perforation is excluded by the total clinical and imaging assessment.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Repeat observations, abdominal examination, urine output, acid-base status and lactate during resuscitation.
- Review postoperative free gas against procedure type, elapsed time and symptom trend rather than applying a universal time cutoff.
- Track whether critical imaging findings were acknowledged by the operating or acute surgical team.
- After intervention, monitor for persistent leak, collection, ileus, sepsis and organ dysfunction using clinical change to guide re-imaging.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Air volume does not grade illness
A tiny leak can cause severe sepsis, while a large postoperative gas volume may be benign; physiology and source matter.
CT localises probabilistically
Gas concentration, wall abnormality and adjacent fluid suggest a source, but contamination can spread far from the perforation.
Retroperitoneum differs
Duodenal, ascending or descending colonic perforation may create retroperitoneal gas rather than classic intraperitoneal crescents.
Rigler sign needs context
Apparent bilateral bowel-wall outlining can result from adjacent distended loops, so review the entire pattern and CT when needed.
Procedure history is essential
Recent operation, endoscopy, peritoneal dialysis or drain manipulation changes the differential but does not neutralise new peritonism.
Contained is not harmless
A localised leak may form an abscess or deteriorate; management depends on source, stability, drainage feasibility and specialist review.
07Common pitfallsFrequent interpretation and management errors.
- 01
Treating a negative erect chest radiograph as excluding perforation.
- 02
Delaying resuscitation or surgical contact until CT reporting is complete.
- 03
Assuming all postoperative free gas is benign without clinical correlation.
- 04
Reporting free gas without suggesting compartment, likely source or associated complication.
- 05
Using oral or barium contrast routinely before deciding whether urgent CT or surgery is needed.