LD stands for Lactic Acid Dehydrogenase, which is an enzyme found in many tissues of the body (kidney, heart, muscle, brain, liver). Since it is widely distributed, this is a non-specific test used to determine if a heart attack may have happened. SInce your levels are low, this would mean that there is no heart damage. So, the chances that you had a heart attack are low.
Bottom line: The low serum result is a good result, and not something you want to raise!