
If you're taking a beta-blocker for a heart condition, and you'd like to work out at high intensity, it's imperative that you talk to your doctor about a safe heart rate. Even a trainer - unless he or she is part of a cardiac rehabilitation program - can't prescribe a heart rate for you.
Your doctor may suggest that you use a subjective exercise intensity scale to rate exertion, instead of relying on your heart rate. You could show him or her the scale on page 155 of Strong Women Stay Slim and ask if it's suitable.