Many heart attacks happen because a short length of the artery that brings blood and oxygen to the heart muscle is narrowed. Clots form at the site of the narrowing, and they eventually block the artery completely. The area of heart muscle beyond the block dies away, and that leads to the heart being weakened. This is the heart muscle protesting at having too little oxygen to cope with its needs. The pain is similar to a stitch in the side when running.
Coronary angiograms show exactly where coronary arteries are narrowed.
From them the specialist heart team determine how to proceed. Bypass, balloon or stent surgery is now routine in every developed country, and the European complaint about Britain is that we are lagging behind the rest in offering these services. People with milder angina, who get some chest pain when, say, walking up hills, that goes away when they stop, also need angiograms. It is far better to sort out angina before a heart attack than to let people develop one. So, basically, everyone with angina and certainly anyone who has had a heart attack, should be offered an angiogram.