Pneumoperitoneum is free air within the peritoneal cavity. It has various etiologies and is easily confused with several other causes of acute abdominal pain. Etiologies can broadly be classified as traumatic or non-traumatic. Traumatic causes of pneumoperitoneum include iatrogenic injuries or stab wounds. Non-traumatic causes are numerous, including perforated ulcer, infection, erosion, and ischemia.
The most common cause of pneumoperitoneum is the perforation of a hollow viscus, such as in a perforated stomach ulcer (Tanner).
On abdominal ultrasound, the most common finding for pneumoperitoneum is the Enhanced Peritoneal Stripe Sign (EPSS). This is when air within the peritoneal space rises and causes an “echoing” of the usually single, hyperechoic peritoneal stripe that separates the abdominal wall from underlying peritoneal fluid and fluid-filled organs (Indiran).
If there is a large amount of pneumoperitoneum, your image of abdominal organs will be obscured by gas wherever you place your probe.