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The heart is the engine that powers life, and the coronary circulation represents the fuel pipes, providing blood with oxygen and other nutrients to keep the heart beating. Under normal physiologic circumstances, there will always be an equilibrium between oxygen demand of the myocardium and oxygen supply provided by the blood flow in the coronary arteries. There is an ingenious regulation system in the coronary circulation to maintain this equilibrium, called autoregulation. Due to the enormous reserve of the coronary circulation to provide blood to the myocardium, early stages of coronary atherosclerosis and narrowing in the coronary arteries will hardly be noticed, and if the coronary arteries become more severely narrowed, complaints will only occur in situations where the oxygen demand is increased, such as physical exercise or stress. Under those circumstances, myocardial ischemia will present itself by a characteristic pain or unpleasant sensation in the chest, arms, neck, or back, called angina pectoris. However, under resting circumstances, blood flow in the coronary circulation can be kept sufficient for a long time despite the presence of important narrowing.1

For those reasons, it is important to realize that in assessing the coronary circulation, it is not the coronary blood flow at rest that should be studied as a measure for the severity of coronary artery disease, but the maximal achievable blood flow as can be provoked by maximum exercise or vasodilatory stimuli like adenosine. Stated in another way, the degree to which maximum blood flow in the coronary circulation is decreased determines the exercise level at which angina pectoris occurs.

Also, in case of acute coronary syndromes, fundamentally the same principles hold true. However, in those situations, a rapid and sharp decrease of coronary blood flow usually occurs due to plaque rupture and/or thrombosis, often superimposed upon preexisting lesions.

It should be realized that the severity of disease on the coronary angiogram has only a poor correlation with the degree to which coronary blood flow is decreased. In other words, it is difficult to assess the physiologic significance of a coronary artery stenosis from the coronary angiogram.2,3 Nevertheless, the angiogram is important because in performing coronary interventions, it is a road map for the interventionalist to manipulate with wires and other equipment and to place the stent(s). In this chapter, the structure of the coronary circulation is discussed, followed by the regulation of coronary blood flow. There is a brief discussion on the development of atherosclerosis and the physiologic methods to detect this in early and later stages. Some words are also spent on coronary stenoses and myocardial ischemia and the consequences of ischemia for the heart. This is followed by the paramount question why functional testing is important, leading to the concept of fractional flow reserve. Some physiologic aspects on background and features of fractional flow reserve are briefly discussed. The clinical use of fractional flow reserve is ...

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