A heart attack can occur as a result of the following: * The slow buildup of plaque may almost block one of your coronary arteries. A heart attack may occur if not enough oxygen-containing blood can flow through this blockage. This is more likely to happen when you are exercising. * The plaque itself develops cracks (fissures) or tears. Blood platelets stick to these tears and form a blood clot (thrombus). A heart attack can occur if this blood clot completely blocks the passage of oxygen-rich blood to the heart.