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Free Intraperitoneal Air




  • Etiology
    • Disruption of wall of hollow viscus
      • Blunt or penetrating trauma
      • Perforating foreign body (eg, thermometer injury to rectum)
      • Iatrogenic perforation
        • Laparoscopy / laparotomy (58%)
        • Absorbed in 1-24 days depending on initial amount of air introduced and body habitus (80% in asthenic, 25% in obese patients)
        • Leaking surgical anastomosis
        • Endoscopic perforation
        • Enema tip injury
        • Diagnostic pneumoperitoneum
      • Diseases of GI tract
        • Perforated gastric / duodenal ulcer
        • Perforated appendix
        • Ingested foreign-body perforation
        • Diverticulitis (ruptured Meckel diverticulum / sigmoid diverticulum, jejunal diverticulosis)
        • Necrotizing enterocolitis with perforation
        • Inflammatory bowel disease (eg, toxic megacolon)
        • Obstruction* (gas traversing intact mucosa): neoplasm, imperforate anus, Hirschsprung disease, meconium ileus
        • Ruptured pneumatosis cystoides intestinalis
        • Idiopathic gastric perforation = spontaneous perforation in premature infants (congenital gastric muscular wall defect)
    • Through peritoneal surface
      • Transperitoneal manipulation
      • Abdominal needle biopsy / catheter placement
      • Mistaken thoracentesis / chest tube placement
      • Endoscopic biopsy
    • Extension from chest
      • Dissection from pneumomediastinum (positive pressure breathing, rupture of bulla / bleb, chest surgery)
      • Bronchopleural fistula
      • Rupture of urinary bladder
      • Penetrating abdominal injury
    • Through female genital tract
      • Iatrogenic
      • Perforation of uterus / vagina
      • Culdocentesis
      • Rubin test = tubal patency test
      • Pelvic examination
      • Spontaneous
      • Intercourse, orogenital insufflation
        • Douching
      • Knee-chest exercise, water skiing, horseback riding
    • Intraperitoneal
      • Gas forming peritonitis
      • Rupture of abscess
      • Air in lesser peritoneal sac gas in scrotum (through open processus vaginalis)
  • Imaging findings
    • Large collection of gas
    • Abdominal distension, no gastric air-fluid level
    • "Football sign" = large pneumoperitoneum outlining entire abdominal cavity
    • "Double wall sign" = "Rigler sign" = air on both sides of bowel as intraluminal gas and free air outside (usually requires >1,000 mL of free intraperitoneal gas + intraperitoneal fluid)
    • "Telltale triangle sign" = triangular air pocket between 3 loops of bowel
    • Depiction of diaphragmatic muscle slips = two or three 6-13 cm long and 8-10 mm wide arcuate soft-tissue bands directed vertically inferiorly + arching parallel to diaphragmatic dome superiorly outline of ligaments of anterior inferior abdominal wall:
    • "Inverted V sign" = outline of both lateral umbilical ligaments (containing inferior epigastric vessels)
    • Outline of medial umbilical ligaments (obliterated umbilical arteries)
    • "Urachus sign" = outline of middle umbilical ligament

Blue arrows point to falciform ligament, made visible by a large amount of free air in the peritoneal cavity.
The red arrows demonstrate both sides of the wall of the stomach (Rigler's sign), a sign of free air. The yellow arrow points to a skin fold.


  • RUQ gas (best place to look for small collections)
    • Single large area of hyperlucency over the liver
    • Oblique linear area of hyperlucency outlining the posteroinferior margin of liver
    • Doge's cap sign = triangular collection of gas in Morison pouch (posterior hepatorenal space)
    • Outline of falciform ligament = long vertical line to the right of midline extending from ligamentum teres notch to umbilicus; most common structure outlined
    • Lligamentum teres notch = inverted V-shaped area of hyperlucency along undersurface of liver
    • Ligamentum teres sign = air outlining fissure of ligamentum teres hepatis (= posterior free edge of falciform ligament) seen as vertically oriented sharply defined slitlike / oval area of hyperlucency between 10th and 12th rib within 2.5-4.0 cm of right vertebral border 2-7 mm wide and 6-20 mm long
    • "Saddlebag / mustache / cupola sign" = gas trapped below central tendon of diaphragm
    • Parahepatic air = gas bubble lateral to right edge of liver



Dahnert 5th edition

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