Coronary Sinus Reducer Objective Impact on Symptoms, MRI Ischaemia and Microvascular Resistance

Inclusion Criteria: * Stable coronary artery disease (CAD) not eligible for percutaneous coronary intervention (PCI) or coronary artery bypass graft surgery (CABG) * Evidence of ischaemia on stress perfusion CMR * Angina - Canadian Cardiovascular Society Class II-VI on maximal medical therapy Exclusion Criteria: * Age\<18 years * Pregnancy * Inability to consent * Recent acute coronary syndrome (3 months) * Recent revascularisation (6 months) * Permanent pacemaker or defibrillator leads in the right heart * Severe left ventricular impairment (\<25%) * Indication for cardiac resynchronisation therapy (CRT) * Right atrial pressure ≥15mmHg * Life expectancy \<1 year * Severe renal impairment (eGFR\<15) * Contraindication to CMR * Contraindication to adenosine * Ischaemia isolated to inferior wall * Ongoing participation in a separate interventional study