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?
Answer from Sheldon G. Sheps, M.D.
Beta blockers slow your heart rate, which can prevent the increase in heart rate that typically occurs with exercise. This means that it might not be possible for you to reach your target heart rate — the number of heartbeats per minute you should have to ensure you’re exercising at the proper intensity level. No matter how hard you exercise when taking a beta blocker, you may never achieve your target heart rate.
There’s no precise way to predict the effect of beta blockers on your heart rate. An exercise stress test, which checks blood flow through your heart while you exercise, can measure how hard your heart pumps while you’re taking beta blockers. Your doctor can use information to adjust the target heart rate you should work to.
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. This way of calculating your adjusted target heart rate isn’t precise, 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 a perceived exertion scale, such as the Borg scale, which relies on your own judgment of how hard you’re working based on effort, breathlessness and fatigue. Ask your doctor for help finding and using an exertion scale. For most workouts, your best bet is to aim for moderate intensity.