Your situation is amazingly similar to my own. I also have been competitively cycling and running since 1975 and now have been diagnosed with paroxymal atrial fibrillation. Resting pulse also 45. I have been on a low dose of Toprol XL-25mg which seems to help somewhat but I do notice that my heart rate is slow to come up while running and I still get some episodes of Afib which last about 24 hours. I tried a 50 mg dose of immediate acting metoprolol, the generic version of Toprol, and found running to be almost impossible. The cardio prescribed the anti arhythmmic amiodarone at the low dose of 100 mg although he has some reservations about possible side effects of this powerful drug.
My episode of fib seem to be brought on by sudden exertion on the bike when "jamming" up a hill which suddenly takes my pulse over 160. Intense emotions are also triggers. A recent event monitor showed many PAC's while I was running hard although they don't seem to progress into fib.
My 6 year younger brother had similar problems and ended up with an ablation due to his strong ventricular response leading to ventricular rates approaching 300. I'm hoping the amiodarone helps me avoid an ablation although I am not overly optimistic. I have read theories that Afib in heavy exercisers may be a result of some hypertrophy of the atria. The doc cautioned me not to expect results for at least two weeks due to the time it takes for the drug to reach full concentration in the body.
I'm very interested in the doctor's answer to your question.
First, I'm not a competitive athlete, nor am I of an age that I might be. I was a jogger, however.
I've taken both Rythmol and Metoprolol, but at much higher dose levels. I'm now off of Rythmol as it didn't do anything to reduce AFib.
I know Toprol does lower the HR, one of its prime purposes when subscribed, the other to lower blood pressure. Clearly, you didn't need anything to lower your HR. Still, at the low level dose you are on, I am surprised it has any noticeable affect on your HR.
I strongly recommend not changing your dose before discussing with your doctor, or the associated nurse service.
I don't think its the Toprol that is causing your decreased heart rate. I feel it reduces mine by maybe 10 bpm at the most at the 25 mg dosage. The prescribing info claims 14% reduction for a 50 mg dose, the lowest tested. Its commonly prescribed as high as 200 mg, so our 25 mg is very low.
For what its worth, the effect of beta blockers seems to wear off with me as I run further. When I tried a 50 direct acting metoprolol tablet prior to running, I couldn't get pulse above 120 for the first 2 miles. By mile 6, I was as 145 or so, although I still didn't feel very good. With my normal 25mg dose of the XL, I often run at 160 bpm. Beta blockers also reduce the heart's stroke volume, so they hit you with a double whammy.
Hum, I knew/experienced both lower HR and BP due to Toprol... the lower HR was all I was seeking. I didn't know it also reduced the heart's stroke volume. I have enough trouble with that due to AFib.. That may be a contributor to the dizziness I sometimes get when I stand up too fast. Low BP added to reduced volume should be enough to make a sudden change in physical activity a kick in the head.