Heart bypass surgery. Image 3 of 7. Tubes from a heart-lung machine attached to a heart during coronary artery bypass graft (CABG) surgery. A heart-lung machine is used to maintain the body's vital functions while the heart is stopped temporarily during surgery. The metal bars either side of the heart are parts of a retractor, used to keep the chest open. CABG is used to treat stenosis (narrowing) of coronary arteries. An artery or vein, taken from the chest or leg is grafted onto the blocked artery, restoring blood flow to the heart. Blocked arteries can cause heart pain (angina) or a myocardial infarction (heart attack). See images M560/451 to M560/457.