Angina is a pain or discomfort in your chest caused by plaque that narrows or blocks the coronary arteries and reduces the amount of oxygen-rich blood that reaches the heart muscle. Stable (reversible) angina occurs when the heart works harder and needs more oxygen. Progressive (unstable) angina occurs when a plaque in one or more of your coronary arteries ruptures (bursts). Blood clots form in the area and may cause partial or complete blockage of the artery. Reversible angina goes away when heart demand is decreased. Progressive angina may become worse and cause permanent damage to heart muscle tissue.