QUOTE:
"why should nt i think that the heart is trying to offset the effects of atenolol 50mg which i am taking for past three years?"
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It is possible after 3 years your system has changed and the same dose of atenolol is now an overdose and expectation of an overdose may include uneven heartbeats, shortness of breath, dizziness, weakness, fainting, etc.
An aerobic exercise for the heart should be limited to a heart rate of 220 minus your age less 10% that calculates to 149 bpm. If a person is vulnerable for arrhythmia, it is possible to have an event when there is an increase of heart rate. The condition of arrhythmia usually happens at the beginning of the exercise if it is going to happen.
If you are exercising with 149 bpm as your target rate, and if you are not experiencing ill effects, then you should maintain that level for optimum effect.