Heart surgery. Surgeon cutting open the pericardium prior to a coronary artery bypass graft (CABG). The pericardium is a double membranous sac that envelopes and protects the heart. Metal spreaders are being used to keep the rib cage open. During CABG a healthy artery is attached to the diseased coronary artery. This re-routes the blood flow, bypassing the blocked coronary artery. Blood vessels can become blocked by the build-up of fatty plaques, atherosclerosis. This reduces blood flow to the areas served by the vessel. Inadequate blood flow to the heart can lead to heart pain (angina) or a heart attack (myocardial infarction).