Just to be sure - you've cleared your bradycardia as healthy with a doctor, right? If you're not exceptionally fit as you say, that could also be a sign of something being wrong.
I go to get diagnosed, and they give you a nebulizer with something that triggers symptoms if you have it. I didn't visibly react, so the tech did it again, then measured the difference in my breathing. Whoops, the lack of visible response meant that I should have stopped after one dose.
So, I couldn't leave until after enough doses of an inhaler that it went back to normal. About 7 hits, I looked like I was sunburned for a while.