Free gas under the diaphragm – Xray

Free gas under the diaphragm – Xray
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  • Free gas under diaphragm (also known as air under the diaphragm) is a finding in the chest Xray seen in case of perforation of hollow viscus.
  • When there is perforation, gas within the hollow viscus escapes into the peritoneal cavity along with other contents
  • When an chest xray is taken in the upright position, gas being lighter rises up and settles under the diaphragm and is seen in the xray as a radioluscent (dark) area
  • If the patient is supine when the xray is taken, the gas will settle at the region of the umbilicus and hence such a film is not useful in diagnosing hollow viscus perforation

Why is it called free gas?

  • It implies that the gas is localised within the peritoneal cavity and changes position with the posture of the individual
  • In other cases, as in case of gas within the retroperitoneum, the location of the gas is fixed and does not change with posture

When to suspect hollow viscus perforation:

  • Patients usually presents as a case of acute abdomen
  • The abdomen is tense and tender
  • Board like rigidity of the abdomen may be present

Differential diagnosis for free gas under diaphragm:

  • Most common cause is hollow viscus perforation. Site of perforation – ordered proximal to distal – with causes:
    • Oesophagus -rupture (Boerhaave syndrome) – rare
    • Stomach and duodenum
      • Peptic ulcer perforation – commonest
      • Malignancy eroding the wall
    • Small bowel
      • Inflammatory bowel disease (eg: Chron’s disease)
      • Primary cancers of small bowel – extremely rare
      • Tumors encroaching upon bowel from adjacent structures (eg: mesenteric tumours)
    • Large bowel
      • Diverticulitis (more seen in left side, especially sigmoid colon)
      • Malignancies
    • Injury to vagnina or anus
  • Abdominal trauma as in stabs, gun shots, road traffic accidents
  • After laproscopic surgery


  • The diagnosis of free gas under the diaphragm necessitates emergency management.
  • Along with air, the gastric/intestinal contents also leak into the peritoneal cavity and the effects can be catastrophic.
  • Laparotomy has to be done immediately to localise site of leak.
  • Tissue should be taken for histopathology to rule out malignancy.
  • The perforation is to be repaired.
  • Peritoneal lavage can be done to remove as much of the spilled contents
  • There is high fatality rate for hollow viscus perforation as it may be due to a malignant process and associated complications like infections

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