Concept of Invasive Coronary Physiology: Focus on FFR

The coronary artery system has three components with different functions: conductive epicardial coronary arteries, arterioles, and capillaries although the borders of each compartment cannot be clearly defined anatomically (Fig. 20.1) [1]. The proximal compartment, conductive epicardial coronary arteries, has a capacitance function and possesses little resistance to coronary blood flow, and its diameter ranges from approximately 500 μg to 4 mm. The intermediate compartment is represented by prearterioles with measurable pressure drop along its length with diameter ranging from 100 to 500 μg. The most distal compartment is represented by intramural arterioles, which are characterized by a considerable pressure drop along its length. The arteriolar segment possesses diameter less than 100 μg. Prearterioles and arterioles cannot be clearly delineated by coronary angiography. These microvessels take most of the coronary vascular resistance and are called as resistance vessels. They can modulate the vascular tone and resistance under various physiologic and pharmacological conditions to control the myocardial blood flow. Prearteriolar vessels are responsive to flow and pressure changes, and their function is to maintain the pressure in a narrow range at the origin of arterioles when coronary perfusion flow or pressure changes. Intramural arterioles are the main part of the metabolic regulation of coronary blood flow. When oxygen consumption increases, arterioles are dilated and vascular resistance is reduced in response to myocardial metabolites. This induces the dilatation of other vessels by the increase in flow and shear stress (Fig. 20.2) [1, 2].