How well does it work compared to just stopping and not taking the pill. I ask because when considering paroxysmal AFib there are a couple of potential endogenous triggers. It tends to occur because of either vagal stimulation or adrenergic stimulation. In the event where the trigger is hard efforts, in races in particular, the adrenergic stimulation seems the most likely cause and this generally does not respond well to flecainide. Generally in this case a Beta Blocker works best. No reason you couldn't do a pill in pocket type strategy with a short acting Beta Blocker.
In regards to performance effects of flecainide or Beta Blockers the news isn't great. I would not be as worried about dropping your heart rate with flecainide as the fact that it decreases contractility, stroke volume and ejection fraction of the heart. Essentially when taking the med the heart does not pump as much blood with each stroke, as you can imagine this is not good for a cyclist. Beta Blockers generally are the opposite they are bad for rate and response of the heart rate to stressors so it doesn't rise as much, again not good for cyclists.
I am assuming your cardiologist ruled out structural issues in the heart before starting as this is a dangerous contraindication. I would also be concerned about daily flecainide here as low potassium increases risk of arrhythmias and I do always worry about medications effected by hypokalemia when riding aggressively in hot weather.
My other question is do we know this is a fib versus PSVT or just sinus tachycardia? How was this captured during exercise, Holter, Loop monitor?
I am not a cardiologist but take interest in cyclists. If it was me I would ask the cardiologist about seeing the EP to determine if radiofrequency ablation is a possibility as in the case of a competing cyclist a drug free solution would seem best.
I have copied an excellent though fairly in depth and jargon rich link about Paroxysmal afib: http://qjmed.oxfordjournals.org/content/94/12/665.full
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