Free air under the diaphragm means that there is air under the diaphragm either on an upright chest X-ray or abdominal X-ray. This most frequently indicates free air in the peritoneal or abdominal cavity and can signify a life threatening condition. This can mean disruption of the wall of the bowel. The air then escapes the bowel and rises underneath the diaphragm on an X-ray. It has to be upright because air rises.
There are many causes of air under the diaphragms, many of which can be life threatening and some which are benign. Some of the most common causes I see in practice include diverticulitis which is an inflamed outpouching of the colon with perforation. Peptic ulcer can cause a perforation of the stomach and free air under the diaphragm. Sometimes patients after a colonoscopy who have had a perforation can have this finding.
More benign cause is common in post operative patients. This is air that is introduced into the peritoneal cavity during surgery rather then from a perforation of the bowel. Air introduced from a dialysis catheter in the abdomen can also cause free air. Sometimes air can dissect from the chest when a patient has abnormal air collections such as pneumothorax or pneumomediastinum.
Often, a patient with free air under the diaphragm and perforation will have abdominal pain and will be sick. When these patients present to the emergency room, an X-ray may be ordered which can show free air. A CT will often follow to identify the source of the free air and any associated conditions or complications. Sometimes even with CT it is not possible to identify the source of the free air under the diaphragm.
A surgeon will often be consulted who may choose to take the patient to the operating room. The surgeon will identify the cause of the perforation and surgically fix it or resect the diseased bowel. It is important to promptly identify free air under the diaphragm as this can be a life threatening condition.