Angina is usually due to obstruction or spasm of the arteries that supply blood to the heart muscle. Other causes include anemia, abnormal heart rhythms and heart failure. The main mechanism of coronary artery obstruction is atherosclerosis as part of coronary artery disease .There is a weak relationship between severity of pain and degree of oxygen deprivation in the heart muscle (i.e., there can be severe pain with little or no risk of a myocardial infarction (heart attack) and a heart attack can occur without pain). In some cases, angina can be quite severe, and in the early 20th century this was a known sign of impending death. However, given current medical therapies, the outlook has improved substantially. People with an average age of 62 years, who have moderate to severe degrees of angina (grading by classes II, III, and IV) have a 5-year survival rate of approximately 92%.Worsening angina attacks, sudden-onset angina at rest, and angina lasting more than 15 minutes are symptoms of unstable angina (usually grouped with similar conditions as the acute coronary syndrome). As these may precede a heart attack, they require urgent medical attention and are, in general, treated in similar fashion to myocardial infarction.