Heart surgery. Image 4 of 24. Surgeon using a cautery knife to cut open a patient's pericardium, the sac that encloses the heart, during heart surgery. heart surgery. The breastbone (sternum) has already been cut and is being held open by metal retractors. A cautery knife is used because it burns (cauterises) the flesh to prevent bleeding. This chest is being opened to perform heart valve replacement surgery. For a sequence showing the operation, see images M560/494 to M560/517.