According to the pre-existing condition you note in your question(heart attack), you are taking "Warfarin" as a preventive medication for further vascular thrombosis that can cause another heart attack. Warfarin reduces the blood’s coagulation ability by stopping the synthesis of some coagulant factors in the liver. At the beginning, the medicine is taken in larger doses and then the dose is decreased. The important parameters for monitoring the Warfarin’s efficiency is not its blood level, but the prothrombin time. It should be 3-4,or 5 times longer than in normal persons.