MODEL RELEASED. Heart surgery. Image 5 of 24. Surgeon using a cautery knife to cut open a patient's pericardium, the sac that encloses the heart, during 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. The surgeon is wearing magnifying lenses over his glasses. This chest is being opened to perform heart valve replacement surgery. For a sequence showing the operation, see images M560/494 to M560/517.