cat scan to check for free air in abdomen

Hi, my husband had the MIE last Friday. He is having some complications. A-fib, which they say is possible after cardiac-thoracic surgery and hopefully will convert to normal sinus rhythm with the meds. The one thing I don't understand is that they are doing a cat scan on his abdomen to check for "free air". Does anyone have any experience with that one? Waiting on results. Just wondering. Thanks.