Postoperative free intraperitoneal gas

Last revised by Henry Knipe on 17 Nov 2021

Postoperative free intraperitoneal gas refers to the presence of gas in the peritoneal cavity following a surgical procedure and may result from open or laparoscopic surgical techniques.

Postoperative free intraperitoneal gas is also referred to as postoperative pneumoperitoneum 1.

Free intraperitoneal gas after surgery occurs in up to 60% of laparotomies (air) and 25% of laparoscopic procedures (carbon dioxide) 2. Larger volumes of gas are associated with a smaller body mass index 3.

A tympanic percussion sound on percussion may be elicited on abdominal examination. Post laparoscopic shoulder pain is a common complication following laparoscopic surgery and is caused by irritation of the phrenic nerve by the retained gas used during laparoscopy 4

In open surgical procedures, direct incision of the skin and then closure of the incision trap gas within the peritoneal cavity. In laparoscopic procedures, insufflation of the peritoneum introduces gas (usually carbon dioxide) which is then retained. The gas is gradually reabsorbed by surrounding tissue and blood vessels and is then expired via the lungs.

In one study the volume of postoperative intraperitoneal gas in a group of patients who underwent laparoscopic gynecological surgery was an average of 17 mL but measured as much as 130 mL in some patients 4.

Signs of pneumoperitoneum such as subdiaphragmatic free air and the cupola sign may be visible on chest radiographs. The volume of gas on serial radiographs should decrease. If the volume of gas increases then it is concerning for another cause of pneumoperitoneum following surgery such as bowel perforation or anastomotic leak.

On abdominal radiographs, signs of pneumoperitoneum may be visible (eg. Rigler sign).

Post-operative free gas is non-specific and can be a normal postoperative finding or indicate serious pathology such as a bowel perforation. The differentiation between these two entities is primarily clinical and the job of the radiologist is to help put the gas into context for the clinical team:

  • trending the volume of gas over time is important
    • gas that does not resolve or increases, is a sign concerning for perforation and/or infection 9
    • the point at which all postoperative gas will resolve varies, but most will resolve by 5 days and almost always within 14 days 7-9
  • ​CT is more sensitive than radiographs for detection of postoperative free gas; non-significant postoperative free gas has been detected rarely on CT up to postoperative day 18 8
  • perforation is associated with larger (>10 mL) volumes of free gas 9
  • for bowel surgery, postoperative gas that is clustered around an anastomosis is not definitely a perforation, but is certainly suspicious and follow up is often warranted, especially if the patient's clinical status is not improving
    • follow up with water-soluble contrast may be useful to confirm a leak
  • postoperative gas confined to a loculated collection may indicate infected fluid

If there are no features suggestive of bowel perforation or other intra-abdominal pathology, management is non-interventional and the gas will gradually be reabsorbed 5,6. The volume of postoperative free gas is positively associated with the intensity of postoperative pain 4.

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