Flecainide & Marathon Training

[RunnerM]

I am a 60 year old male and a distance runner.  I experienced an episode of Atrial Fibrillation about 4 years ago but spontaneously converted to normal sinus rhythm as I waited in the ER.  I had no further episodes until last spring when I was went through electro conversion in the ER.  I had another episode a month later and was put on "pill in a pocket" protocol after being converted with flecainide.  I had 3 more episodes with the last being over 5 months ago -- the last after being put on a daily maintenance dose of Flecainide (250mg a day - 100--mg AM, 150 PM).  I have experienced a very frustrating condition -- while my generally low resting heart rate (45-50bpm) does not appear to be affected, under exercise I now am unable to get my heart rate above about 70-75% of tested max.  This has had a profound effect on the quality of my training and racing.  As an example, a hill workout I have done for years during which I would regularly achieve a heart rate of 90%, I am now unable to get above 70%.  This change coincided with my maintenance dosage of flecainide.  Despite my cardiologist insists that "...the medication shouldn't have any effect on your heart rate or training," whether or not it should, it definitely has.  Some research I have done on the internet indicates that it is advised that trough plasma levels between 0.2 and 1.0 µg/mL are desired but my cardiologist has never ordered any monitoring of plasma levels or made any attempt to determine the lowest dosage that will prevent episodes.

I have been unable to find any research done on the effects of this or other antiarrhythmics on endurance athletics.  I clearly don't fit the normal profile of my doctor's patient base nor does he seem to have any particular insights regarding my situation.  For me running and marathoning in not a hobby, it is a lifestyle and I would like to not only be able to continue to train and race but also do it at a level I find satisfying.

Please advise what course I might/should pursue with my cardiologist.

[IowaRunner]

I noticed that same thing (61 year old male, runner all my life).  Was having a hard time getting my pace down after significant training and couldn't figure it out.  Last night I forgot to take the 150 mg tablet and went out this morning and easily ran 30-45 sec/mile faster with same/less effort.   Trying to figure out how to handle this (it sure felt good to maintain a decent pace.  I was getting a bit embarrassed about how slow I was going)

[tsco]

I started Flecainide about 6 weeks ago to stop PACs and SVT.  It has greatly reduced my PACs however it seems to be effecting another problem that recently showed it's ugly head, Left Bundle Branch Block.  After some research the description on action of flecainide specifically states it is less active at lower heart rate and increases it's effectiveness as heart rate increases.  I am not a performance athelete but I have noticed that I can not get my heart rate near as high on my treadmill exercising.  I do the same program I did before the flecainide.  Flecainide delays conduction in the heart so it makes sense to me that it is retarding the lower part of my heart.   This is why I believe my LBBB is now acting up.