Coronary artery disease generally presents in three ways - angina, a heart attack or sudden death - but the underlying problem is the same in each case: narrowings or blockages have developed in the arteries which take blood to the heart muscle. This can starve the heart muscle of oxygen.
When the presentation is angina enough blood gets through at rest but, when the heart is stressed and needs more oxygen, not enough can get past the narrowed areas and the heart muscle cries out for a rest. The angina pain that it produces is protective and generally makes a person stop so that the heart has a chance to catch up. It takes a significant amount of work to digest a meal so angina can sometimes be brought on more easily after eating as the heart has to work harder with the exercise.
The coronary arteries are not inert and their calibre can change during the day. This means that as well as fixed narrowings, any spasm or contraction of the artery can make the effect worse. Typically this is seen when someone goes out in cold weather or a wind and their exercise capacity before the angina comes on is lessened. It is also the reason people can develop anginal pains whilst tense or emotional.
Single, double and triple vessel coronary artery disease
Personally, I do not like the terms single, double and triple vessel coronary artery disease and find they can be confusing. It would perhaps be better to talk about single, double and triple system disease, because the terms do not refer to the number of individual vessels diseased but rather how many of the three systems are affected.
Single vessel disease
It is possible for a patient to have two narrowings in the left anterior descending artery (LAD) system and receive three heart bypasses to correct the situation and yet remarkably that patient would be defined as having single vessel disease! What is more, the definition single vessel disease would be correct.
The reason we use these terms with regards to patient's anatomy is that they have a predictive value in calculating what the future holds for an individual. Triple system disease carries a worse outlook than double system disease - which is usually worse than single system disease.
The number of bypasses a patient receives depends, not only on the number of narrowings present, but also where these obstructions lie. If there is a narrowing at an important branching in a vessel then it may be necessary to do two bypasses, one to each branch, to get blood past the obstruction.
If that same narrowing were slightly further upstream then it may well be that a single bypass would perfuse both branches. The number of bypasses, therefore, does not necessarily reflect the risk of the operation. The aim of surgery is to get blood flowing freely to all areas of the heart. If that is achieved then the heart will be happy whether it takes one or five bypasses.