A perfusionist is a highly specialized member of the surgical team in a cardiothoracic surgery department. 

The main duty is to operate the heart-lung machine, thus bypassing the patient's heart and lungs during surgery, allowing for the heart to be stopped and operated on. This procedure is also called "perfusion" or "ECC" (ExtraCorporeal Circulation), or more correctly: "CPB" (Cardio-Pulmonary Bypass).

The performance of CPB takes place in the operation room during open heart surgery. The heart-lung machine consists of a number of pumps and various monitor equipment. The blood from the patients great veins is lead through tubings to a reservoir, a pump and an artificial lunge (oxygenator). The oxygenated blood is returned to the patients aorta.

The perfusionist also have a number of other duties outside the OR, such as various forms of heart and lung assist (ECMO, CPS, VAD, IABP) and maintenance, logistics, development and science. 

See the PDF file for a deeper explanation of CPB.