Free air or pneumoperitoneum is air that is in the peritoneal cavity of the abdomen. This is abnormal because the only air seen should be within bowel. The peritoneal cavity contains the intestines, the liver, gallbladder, pancreas and spleen. Any air that is contained in the abdominal peritoneal cavity outside of bowel is therefore abnormal.
Air that is outside the bowel in the abdominal peritoneal cavity can be from a bowel perforation. The bowel wall is torn and air escapes along with contents, such as stool. The abdominal peritoneal cavity is normally sterile, meaning there are no organisms growing. Spillage of bowel contents into the cavity will result in infection and peritonitis. This can be deadly.
When we see free air on CT, we are most concerned about bowel perforation. Most commonly, I see this from a perforated ulcer of the stomach. A perforated ulcer will penetrate the wall of the stomach and let air and other contents spill into the peritoneal cavity. It is difficult to see the ulcer on CT, however, majority of the free air is located by the stomach.
Another common cause of bowel perforation is diverticulitis. This occurs when a small outpuching of the colon or a diverticulum becomes inflamed and than perforated. This lets air and contents from the colon escape into the peritoneal cavity. This can lead to peritonitis and other complications.
While there are other causes of bowel perforation that leads to free air on CT, there are some mimics that are not life threatening. It is common to see free air after patients have surgery. Surgery often introduces air into the abdominal peritoneal cavity. It can be difficult to tell if the air is from surgery or bowel perforation.
Air can dissect down from the chest when patients have pneumothorax or air in the mediastinum (pneumomediastinum). Pneumoperitoneum can also result from peritoneal dialysis. This is introduced through a catheter patients have to get the dialysis. Air can also be introduced through the vagina from a variety of causes.
Free air on CT or Pneumoperitoneum often indicates a life threatening perforation of bowel. There are many causes but a few of the most common are stomach ulcers and diverticulitis. Others can be from medical procedures like colonoscopy where the bowel wall is perforated. This often requires surgery. There are benign causes as well. These are most common from peritoneal dialysis, extension from processes in the chest and air that is introduced into the vagina. These patients will often not have abdominal symptoms like those with a perforated bowel.