It is very important to distinguish between two types of angina: stable angina and unstable angina. Both types result from problems within the coronary arteries.

• Stable angina results from a fixed obstruction of blood flow to the heart . It occurs when there is not enough blood for a fast-pumping heart, but sufficient blood can get through when the heart slows down and the individual is at rest. Stable angina typically is caused by widespread, irregular disease throughout the coronary arteries. The blockages that result may not seriously hinder the flow of blood, and they usually do not damage the heart unless a plaque (atheroma; fatty deposit within a blood vessel) suddenly ruptures.
  
• Unstable angina is due to a sudden interruption of blood flow to the heart due to a partial or complete blockage of the artery. Unstable angina comes on when a person is resting, asleep, or undergoes physical exertion (unlike stable angina, which usually comes on with a physical exertion). Symptoms of moderate or severe discomfort suddenly may develop in a person who has never experienced angina before, and attacks may become more frequent or increase in intensity.

Unstable angina can be dangerous, while stable angina generally is less serious.

• Stable angina usually occurs during physical exertion or emotional stress or excitement. Stable angina doesn't lead to a heart attack in most people.
  
• Unstable angina can occur during rest, can awaken a person from sleep, and can appear suddenly during physical exertion. Unstable angina may quickly progress to a heart attack.