This study is a multicenter, multi-country three-arm prospective and retrospective investigation in up to 400 subjects conducted at a maximum of 40 investigational centers. Arm 1 will include eligible prospective subjects. Arm 2 will include subjects who were previously enrolled and treated with the Reducer during the COSIRA (Coronary Sinus Reducer for Treatment of Refractory Angina) study. Arm 3 will include subjects who received a Reducer under CE Mark (unrelated to the COSIRA study).