Coronary heart disease (CHD) or coronary artery disease (CAD) usually refers to narrowing of coronary artery which supplies blood to the heart muscles. This narrowing results from deposition of lipid particles on the inner wall of the artery (for details atherosclerosis) which leads to decrease in blood flow to the heart muscles. During rest, it does not matter much but, when the heart starts working harder (as in case of exertion), its demand for oxygen and blood increases. The narrowed artery is not able to cope with the increased demand, which manifests in the form of chest pain. After rest the acute need for oxygen passes off and chest pain is relieved. This condition is known as angina pectoris. But when the previously narrowed coronary arteries are blocked by a blood clot, the portion of heart muscle supplied by these vessels become completely devoid of oxygen and gets damaged permanently. This condition is known as Myocardial infarction (heart attack).