Lack of blood supply is called ischemia. Angina pectoris can occur when blood circulation to the heart is sufficient for normal needs but inadequate when the heart's needs increase, such as during physical exertion or emotional excitement. Running to catch a bus, for example, could trigger an attack of angina while walking to a bus stop might not. Some people, such as those with a coronary artery spasm, may have angina when they're resting. Angina can be a warning sign that someone is at risk of heart attack.
Some people have ischemia without pain.
As many as three to four million Americans may have ischemic episodes without knowing it. These people have silent ischemia. They may have a heart attack with no prior warning. In addition, people with angina also may have undiagnosed episodes of silent ischemia. Collateral circulation involves small arteries that connect two larger coronary arteries or different segments of the same artery.