Angina is chest pain that occurs because of inadequate delivery of oxygen to the heart muscle (often described as a heavy or squeezing pain in the midsternal area of the chest). Angina signals that a part of your heart muscle is not receiving an adequate supply of blood and oxygen. The heart requires a particularly rich blood supply because of its heavy workload, and receives this nourishment through the coronary arteries. When these vessels are narrowed or blocked, restricting blood flow, they fail to supply adequate oxygen.

The body has a response to pain and problems caused by narrowed arteries. The body often can increase the amount of blood flowing to the muscle by opening up tiny branches of nearby vessels. This network of tiny blood vessels - known as " collateral circulation " - makes it possible for blood to detour around blocked or narrowed arteries. The development of collateral circulation is particularly important in the heart muscle where it may be life-saving. However, the development of collateral circulation is a gradual process and not everyone has the same natural ability to develop these networks at the rate that will relieve angina.