@d102 So yeah, I’ve dealt with bradycardia as an athlete my whole life - enlarged heart. RHR usually in the low 40’s, though when in great shape with lots of aerobic conditioning it dropped into the mid-30’s. (Side note: Miguel Indurain had a RHR of 28bpm!).
Obv. I’m no cardiologist, with the usual caveats. But in personal experience, I can’t say that I’ve ever had issue with bradycardia impacting my top-end MHR - I think in my 20’s my MHR was up around 210ish. Once the gets up to speed, I’ve not had problems.
Now that said, what bradycardia actually is: an abnormally low heart rate - for whatever reason. The risk is that your heart can’t deliver enough oxygen for the amount of effort required, which IRL manifests as dizziness, lightheadedness, up to syncope/fainting. The biggest issue I’ve always had is that bradycardia tends to cause (or occur in parallel with) hypotension/low blood pressure. Slower HR = slower blood flow = less pressure on the blood vessels. Have you ever stood up too quickly and gotten suddenly lightheaded? That’s what that is. Ironically that can be exacerbated by improved fitness, which lowers BP, increasing the likelihood of hypotensive issues.
With cycling, and now including SUFF, the biggest trigger for me has always been a too-rapid easing off the accelerator after a hard effort, like suddenly dropping off from max HR. With bradycardia and hypotension, this causes a rapid drop off in oxygen and I get dizzy. In my younger days I learned on hard training rides that I couldn’t suddenly back off after a hard climb in the mountains, or I’d wake up laying on the side of the road a few seconds later.
TL;DR: yes, bradycardia can absolutely affect your body’s ability to produce sufficient oxygen during high-demand efforts, causing your cardiovascular system to “bonk” maybe sooner than your respiratory and muscular systems would need to. Training, health, and fitness will improve things, but it’s just a condition you need to understand, manage, and learn to work around.
Final caveat - it sounds like you’re making guesses based on family history and what you’re feeling on the bike. As @KevinD stated, there are MANY other issues and situations that could be impacting your workouts - including fitness, nutrition, hydration, and experience on the bike. The only way to know would be to see a cardiologist and get stress-tested, EKG, the whole shebang. If you’re concerned (and if you really are bradycardic), I strongly recommend you find a cardiologist.
And welcome to Sufferlandria.