Beta blockers: How do they affect exercise?

My doctor recently prescribed a beta blocker to lower my blood pressure. Now, when I exercise, I have trouble getting my heart rate higher than 135. Is this normal?

- No name / Connecticut

Mayo Clinic hypertension specialist Sheldon Sheps, M.D., and colleagues answer select questions from readers.

Answer

Beta blockers slow your heart rate, which can prevent the increase in heart rate that typically occurs with exercise. As a result, the usual age-adjusted target heart rate doesn't work for measuring exercise intensity. No matter how vigorously you exercise when taking a beta blocker, you may never achieve your "target."

There's no precise method for predicting the effect of beta blockers on your heart rate. An exercise stress test can evaluate your exercise capacity on beta blockers and provide information on which to base an adjusted target heart rate. You can also try lowering your target heart rate by the amount that your resting heart rate has been lowered by the beta blocker. For example, if your resting heart rate has decreased from 70 to 50, then try working at a target heart rate 20 beats per minute lower than what you used to do. Again, this is not a precise method, and sometimes the peak exercise heart rate is affected much more than is the resting heart rate. An exercise stress test is the best way to establish a new target heart rate on beta blockers.

If you haven't had an exercise stress test, you can use the perceived exertion scale, which relies on your own judgment of how hard you're working based on effort, breathlessness and fatigue. The scale ranges from 6 (at rest) to 20 (maximal effort). For most workouts, your best bet is to aim for moderate intensity, or a rating between 12 and 14.