Hello Stuart,
A pulse rate of 50 is somewhat low, especially if you are older. A normal heart rate is 60-100 beats per minute.
There are many causes of a low heart rate. In older patients (and by the number of medications you are on, I'm guess you are older), heart disease and a previous history of heart attacks can cause a low heart rate. Both of these can interfere with the conduction system of the heart which keeps your heart rate regular and at a normal rate. Also, you are on a beta-blocker (the atenolol), a medication which can lower blood pressure and slow the heart rate.
For your piece of mind, you should discuss your concerns about your low heart rate with your physicians!
To your health,
Neil MD