MODEL RELEASED. Heart bypass surgery. Surgeon using diathermy to cauterise blood vessels in an incision made in a patient's leg during a coronary artery bypass operation. Diathermy is the use of heat to seal blood vessels and stop bleeding. During this stage of the operation, a vein from the leg is removed. The harvested vein will then be attached to arteries on the heart to re-route blood around a blockage. Blocked coronary arteries can cause angina pectoris (heart pain) and a heart attack, and are a leading cause of death.