Heart bypass grafts. Coloured computed tomography (CT) scan of the chest of a 35 year old after heart bypass surgery. The front of the body is at right. The left and right internal thoracic arteries (indicated by arrows) have been grafted onto the surface of the heart, after coronary arteries that supplied the heart muscle with oxygen became blocked. Lack of oxygen can cause angina (heart pain) or myocardial infarction (heart attack). The most common cause of blockage is atherosclerosis, a build-up of fatty plaques on the inner lining of the arteries.