Atenolol lowers blood presssure. the way it achieves this is by lowering the heart rate. although 60 to 100 is considered normal, your numberes may be acceptable as long as long as you're not experiencing any symptoms. how is your blood pressure? If it is okay and you have no syptoms do not worry about numbers.
My average is 46 beats per minute. I have heart failure which is very treatable. I know there are other causes of low pulse rate, however. You will have to talk to your doctor about it. I would keep my appointment if I were you.