Maximum Heart Rate

Your maximum heart rate is the number of beats per minute that your heart should not surpass. This figure varies by person and is dependent on your age. To determine your maximum heart rate, subtract your age from 220. Using this calculation, a 40-year-old person's maximum heart rate is 180 beats per minute. For this person, a heart rate of 200 is unhealthy. For a 20-year-old person, a heart rate of 200 would equal the person's maximum heart rate -- however, this person should not exercise in the maximum heart rate zone.

Measuring Heart Rate

For a quick method of measuring your heart rate, place your index and middle finger on the inside of the opposite wrist, directly below the thumb. When you detect a pulse, look at a clock and count the beats for 15 seconds, and then multiply the number of beats by four. If you count 50 beats in 15 seconds, your heart rate would be 200 beats per minute. If using a cardio machine such as a treadmill or stationary bike, place your hands on the designated handles and hold them in place until the machine displays your heart rate.

Target Heart Rate

Even if your maximum heart rate is 200 beats per minute, avoid elevating your heart rate to this level during cardio exercise. During exercise, your heart rate should be in the target zone, which is between 50 and 85 percent of your maximum heart rate, according to the American Heart Association. For a 40-year-old person with a maximum heart rate of 180, the target heart rate is between 90 and 153 beats per minute. If you're 20 and have a maximum heart rate of 200, keep your heart rate between 100 and 170 beats per minute during exercise.

Lowering Your Heart Rate

If you measure your heart rate at 200 beats per minute, take immediate steps to lower the rate. Reduce the intensity of your cardio exercise immediately; for example, lower the speed of your treadmill or reduce the pace of your swim stroke. Deep breathing can start to lower your heart rate, but when exercising, the most effective step to take is to reduce your intensity. Always visit the washroom before exercising, as a full bladder can boost your heart rate by up to nine beats per minute, according to a study quoted on NBC News.