Exercise induced very high heart rate

I am a 43 year old male, with BMI of 26.  I've been reasonably physically active all my life.  I recently started training for long runs and decided to pace myself by monitoring my heart rate when I am running.  What I've noticed is that my heart rate >150 even when I am running at a low pace of 5 miles/hour, and goes well over 185 when I am running normally at about 7 miles/hour, and if I have to traverse a hill, it climbs into the 200s. These rates remain high over the duration of my long runs -- typically an hour to hour-and-half. My resting heart rate is in the 50s.

Is this something I should be concerned about?  I have spoken to my primary physician who seemed to think it'll come down with more conditioning.  I have been told by others that this may be a result of atrial fibrillation.  Any advise is appreciated.

S.

[Michael J. McWilliams, M.D.]

It is true that your heart rate should come down with conditioning but it does not in some people.  As long as you doctors think it is ok for you to exercise, I would not focus that much on heart rate.  If you feel ok with you exercise, it should be ok to continue.  your body will tell you when to stop.

You would need  a monitor to confirm the atrial fibrillation.  It is much more likely that your monitor is picking up artifact (like mine often does) than that you have atrial fib.