The increasing number of coronary revascularization procedures, coupled with improvements in drug therapy, has significantly extended the lifespan of patients with coronary artery disease (CAD). However, there remains a significant number of CAD patients who experience disability due to chronic refractory angina pectoris. These patients typically have severe diffuse CAD and are not candidates for further revascularization involving surgical coronary artery bypass grafting (CABG) or percutaneous coronary intervention (PCI). The installation of a coronary sinus reducer (CSR) represents a new option for percutaneous treatment of patients with refractory angina pectoris who are not suitable for surgical or percutaneous revascularization. The CSR device is designed as an hourglass-shaped stent that is positioned transcatheterally in the distal part of the coronary sinus. This increases intramyocardial venous pressure, which is believed to lead to a more favorable perfusion ratio between the ischemic subendocardial and non-ischemic subepicardial myocardium. Previous research has demonstrated that the implantation of CSR is a safe and relatively straightforward procedure. However, broader implementation and better patient selection are still limited by the fact that the exact mechanism of action remains controversial. It has not been determined why some patients have better outcomes compared to others with seemingly similar coronary artery disease. It is known that patients with atherosclerotic changes in the epicardial coronary arteries also have a certain degree of coronary microcirculation disease (the coronary vascular bed encompassing vessels with a diameter \< 200 μm), which cannot be assessed through standard coronary angiography. This study aims to assess changes in coronary microcirculation after the implantation of CSR by measuring coronary flow reserve (CFR) and index of microcirculatory resistance (IMR) before and 6 months after the procedure. Furthermore, our goal is to associate these changes with clinical symptoms and myocardial ischemia.