In rehab they take 220 and subtract your age. This is the max heart rate you are allowed to achieve. If you take beta blockers, they subtract a further 30.
E.G. 60 year old not on beta blockers is allowed (220-60) = 160 beats per minute.
60 year old on beta blockers is allowed (220-90) = 130 beats per minute.
I had to raise concerns at rehab though asking what they would do if a 10 year old child
attended lol. That equates to an allowed heart rate of 210, I think not.
Is there a cardiac rehabilitation center in your area? If so, I strongly advise your husband to participate. Exercise is done while wired up and one gains confidence as success is achieved. That being said, is he taking beta blockers? If so, it may be almost impossible for his heart rate to exceed 100 beats per minute, and the rules for aerobic exercise and heart beat are thrown out the window... the rehab folks teach "perceived exercise thresholds" instead.