Thank you for writing in tonight.
Based on your past history, you are familiar with Coronary Artery Disease (CAD). CAD is the big one; the one that caused the bypasses. Your small vessel disease likely developed due to the same factors as your CAD. Small vessel disease is roughly the same thing, but on a significantly smaller scale; therefore, in comparison, small vessel disease is better than coronary artery disease. However, small vessel disease can cause problems none the less.
Small vessel disease just means that there is blockage of the smaller arteries throughout the heart. Therefore, unlike the large coronary arteries, the smaller arteries only perfuse a small amount of the heart muscle. Therefore, blockage of the small artery, will cause pain, heart muscle hypoxia, and possible muscle death; however, it is not of the same intensity if compared to a blocked coronary artery, which perfuses a large amount of heart muscle.
I suspect you were diagnosed with this after having chest pain, and the doctor did not find any blockage along the larger coronary arteries.
I hope this helps explain. I tried to explain this in patient friendly terms; however, if you need clarification or have additional questions, please let me know.