Coronary Sinus Reducer Objective Impact on SymptoMs, Ischaemia and miCrovascular Resistance in Non-obstructive Hypertrophic CardioMyopathy

Hypertrophic cardiomyopathy (HCM) is a common heart condition in which the heart muscle becomes abnormally thick. In non-obstructive HCM, blood flow out of the heart is not blocked, but many patients still experience chest pain, breathlessness and limitation of daily activities. This may be because the small blood vessels within the thickened heart muscle do not supply enough blood and oxygen, particularly during exercise or stress. Current treatment options for symptomatic non-obstructive HCM are limited. This study will test a small device called the coronary sinus reducer (CSR), which is placed into a vein of the heart using a catheter. The CSR is designed to improve blood flow to areas of heart muscle that are short of blood supply. Fifty adults with non-obstructive HCM, ongoing symptoms and reduced blood supply on a heart MRI scan will take part. Participants will be randomly assigned to receive either CSR implantation or a placebo procedure. Neither participants nor the follow-up research team will know which procedure was performed until the 6-month follow-up assessments are complete. The investigators will use detailed heart MRI scans to measure blood flow, a smartphone app to record daily symptoms, coronary sinus blood tests to look for biochemical evidence of reduced oxygen supply, and invasive coronary physiology measurements to assess the small blood vessels of the heart. The main aim is to find out whether the CSR improves blood flow and reduces symptoms. The results may identify a new treatment option for patients with non-obstructive HCM and improve understanding of the disease.

Inclusion Criteria: * Non-obstructive hypertrophic cardiomyopathy (LVOT gradient ≤30 mmHg at rest on echocardiography) * Ongoing symptoms despite current medical therapy * Significant perfusion defects (≥ 2 myocardial segments) on stress perfusion CMR Exclusion Criteria: * Age \< 18 years * Left ventricular impairment LVEF \