As described in an earlier post, your maximum heart-rate (HR max) is important to know, when you start exercising based on heart-rate. Usually your HR max will depend on your age, but research has shown that several types of medications, including some medications to lower blood pressure, can lower your maximum heart rate. This means your target heart rate zone will be lowered too. 

Ask your doctor if you need to use a lower target heart rate zone because of any of your medications or medical conditions.