10 Chapter 1 GENERAL INTRODUCTION Ischemic Heart Disease (IHD) is the leading cause of morbidity and mortality worldwide, affecting both men and women. Chronic Coronary Syndrome (CCS) represents a stable yet progressive form of IHD.1 CCS encompasses multiple underlying pathophysiological mechanisms. For decades, clinical cardiology has focused on obstructed coronary artery disease (CAD) as a probable cause of ischemic complaints. However, nearly half of the patients undergoing invasive coronary angiography for suspected CCS do not have obstructed CAD. This condition is called Angina with Non-Obstructed Coronary Arteries (ANOCA). In these cases, angina can arise from changes in vasomotor responses of the coronary circulation including coronary vasospasm and/or alterations in the microcirculation.2 The coronary vascular bed originates at the root of the aorta, where two primary coronary arteries emerge: the right coronary artery (RCA) and the left main coronary artery. The left main coronary artery bifurcates into two major coronary arteries, the left anterior descending (LAD) artery and the left circumflex (LCx) coronary artery. These three large coronary arteries run across the epicardial surface of the heart, branching into various smaller branches, from pre-arterioles, to arterioles and capillaries.3 These small branches (diameter <200 µm) constitute the coronary microcirculation (Figure 1). In a healthy heart, the microcirculation plays an important role in regulating coronary blood flow, supplying oxygen, nutrients and metabolites to the myocardium. Conventional imaging techniques cannot provide adequate visualization and evaluation of the coronary microcirculation. Techniques exist that are capable of for detailed assessment of the coronary microcirculation, however these are not commonly used in clinical practice. Figure 1. Microcirculation Ischemic symptoms occur when oxygen delivery is insufficient to meet oxygen demand in the myocardium. In the microcirculation, oxygen delivery is determined by two parameters,
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