Feasibility and Efficacy of Coronary Sinus Narrowing in Patients With Coronary Microvascular Dysfunction

Inclusion Criteria * Age \>18 * Able to provide written informed consent and willing to participate in all required study follow-up assessments * Symptomatic CAD with refractory angina defined as CCS class II to IV, despite optimal tolerated medical therapy * Abnormal coronary microvascular function indices: CFR≤2.5 and/or IMR≥25 Exclusion Criteria * Recent (within 3 months) acute coronary syndrome * Patients with prior coronary artery bypass surgery * Unstable angina (recent onset angina, crescendo angina, or rest angina with ECG changes) during the last 30 days * Subjects in cardiogenic shock (systolic pressure \< 80mm/Hg, on vasopressors or intraaortic counter pulsation) at the time of consenting. Subjects who recover from cardiogenic shock by the time of consenting are eligible. * Obstructive CAD on coronary angiography (\>70% stenosis or 50-70% stenosis with iFR\<0.89 or FFR\<0.8 in epicardial artery) * Inability to perform invasive coronary flow evaluation and/or measure CFR and IMR in the LAD * Severe valvular heart disease * LVEF\<30% * Decompensated congestive heart failure (CHF) or hospitalization due to CHF during the last 3 months * Patient with a pacemaker electrode in the CS * Mean right atrial pressure \>15 mmHg * Anomalous or abnormal CS anatomy (e.g., tortuosity, aberrant branch, persistent left superior vena cava (SVC) as demonstrated on angiogram * CS diameter at the site of planned implantation greater than 13mm or less than 9.5 mm as measured by angiogram * Severe chronic obstructive pulmonary disease (COPD) indicated by a forced expiratory volume in one second that is less than 55 percent of the predicted value * Tricuspid valve replacement or repair (tissue or mechanical) * Chronic renal failure (serum creatinine \>2mg/dL), and or on chronic hemodialysis * Moribund, or with comorbidities limiting life expectancy to less than one year * Known severe reaction to required procedural medication * Known allergy to stainless steel or nickel * Magnetic Resonance Imaging (MRI) within 8 weeks of Reducer implantation * Participation in another ongoing investigational trial * Additional factors deemed unsuitable for trial enrollment per discretion of principal investigator * Inmates