The purpose of this study is to see if an experimental device, called the Shockwave Medical Inc. Reducer System (Reducer) is safe and useful in treating refractory angina compared to standard treatment.

Experimental means that the device is not approved by the U.S. Food and Drug Administration (FDA).

The Reducer is permanently implanted in the main vein of the heart that drains the blood from the heart muscle (the coronary sinus) and creates a narrowing in the vein. The narrowing creates a slight increase in blood pressure within the veins of the heart which can lead to a change in blood flow to areas of the heart muscle that need it. This movement of blood may reduce the symptoms of refractory angina and improve quality of life and the ability to exercise.

The implantation of the Reducer is performed through a balloon catheter(tube) that is inserted into a vein in the neck (jugular vein) and is advanced into the main vein of the heart (the coronary sinus). The balloon is then used to expand the Reducer to the correct size and shape at the proper location.

Participants will be randomized into one of two study groups. Randomization is like flipping a coin and there is an equal chance of being assigned to either group. You will not know what group you are assigned to.

Group 1 will be implanted with the Reducer device and group 2 will receive no device, known as the control group. Participants in group 2 may be able to receive the Reducer after completing the 1-year follow-up visit.

All participants will undergo various tests and exams as part of the study. Total study participation may last about five (5) years.