Codes for the mediastinum and diaphragm are found in the 39000 – 39599 range in the Surgery section of the CPT manual.
The mediastinum is also known as the thoracic cavity. It is a collection of organs and structures held together by loose connective tissue, which includes the heart and its attendant vessels, lungs, esophagus, trachea, thymus, thoracic duct and other anatomical structures. The diaphragm is a sheet of skeletal muscle that separates the thoracic cavity from the abdominal cavity.
You should note, however, that none of the procedures in the mediastinum and diaphragm subsection pertain directly to these organs or structures. Each of the organs contained in the mediastinum is important enough to have its own specific section. Procedures of the heart, for example, would be found in cardiovascular, while procedures of the lungs would be found in respiratory.
Instead, the mediastinum and diaphragm subsection of Surgery consists of procedures operating on the thoracic cavity and diaphragm themselves. These include incision codes, excision codes, and various exploratory procedures (e.g. those using endoscopes). The diaphragm has a few codes for repair procedures, but procedures other than will be found elsewhere in the CPT manual.
Since the mediastinum and diaphragm subsection of Surgery is so brief, you should not worry too much about it. If a question does come up on the CPC exam, you’ll be able to recognize it fairly quickly by its use of ‘mediastinal’ or ‘diaphragmatic.’