I have had the same question for my Dr.'s. At one point I had a low PBM of 41, it was 44 in the Dr.'s Office. Each of us have a peculiar set of circumstances, and I am not a Dr., however I was told as long as you are not exhibiting other symptoms of some type of heart concern then the low rate is OK. I am getting of Digoxin that I had taken for almost 20 years. I can't help but feel that my heart has been trained by the chemicals to beat slowly. I've been off Digoxin for almost three months and my average BPM in the morning is 56. So, unless you have some type of heart problem other than a slow beat, it may not be much of a problem. Hope you get a medical professionals answer. Good luck.
The key about slow heart rates is does it cause symptoms. In general, if a slow heart rate does not cause symptoms it is ok. If your heart rate is slow (usually 30's or lower) and you are extremely fatigued, passing out, etc, it is probably too slow. It also depends on whether your heart rate increases with exercise appropriately. Most people with heart rates in the 40-60 range tolerate it pretty well. Of course this varies depending on if you are 18 or 80 (an 18 year old should be able to get their heart rate much higher than 60's). If there are any questions, an exercise stress will help determine if your heart rate increases appropriately with exertion.
I hope this helps.