A myocardial bridge (MB) is a congenital heart defect in which one of the coronary arteries tunnels through the heart muscle itself (myocardium). In normal patients, the coronary arteries rest on top of the heart muscle and feed blood down into smaller vessels (ex. septal arteries) which then take blood into the heart muscle itself (i.e. populate throughout the myocardium). However, if a band of muscle forms around one of the coronary arteries during the fetal stage of development, then a myocardial bridge is formed - a "bridge" of heart muscle over the artery. Each time the heart squeezes to pump blood, the band of muscle exerts pressure and constricts the artery, reducing blood flow to the heart. This defect is present from birth. It is important to note that even a very thin ex. < 1 mm