Hi David,
I had something very similar happen to me this year. It was not quite the same but close: I am 56. What I was doing was regularly this summer was riding with an average HR of around 165 for 3,4 hours (weekly) when training for my Century attempt. Sometimes, not all the time, my heart would suddenly jump up to 220, 230, and even 240. It was usually after 2-3 hours of pretty heavy exercise though. During this time I also would push my HR up near 190 at least once a week for power intervals It never happened on workouts less than 90 minutes no matter hard hard I exerted myself. If I did not have the display on my Garmin I would not have even known it was beating that fast. At first I thought it was just an error with my HR monitor, but then I also saw it on fit-bit watch so to me that confirmed it. I would usually just keep doing what I was doing and eventually it would go back to a normal range. I don't think it ever lasted for more than 5 minutes at a time.
I went to at least different Cardiologists. I did an Echo stress test (could not reproduce it during the test), and finally worn a "holter" type monitor for about 2 weeks - ending when I road in the UMCA 100 mile National Championship race. This device caught it happening several times. I finally was referred to a specialist and he said that probably had 1 of 3 "things" happening, but he would have to evasive testing to really find out. (I'm sorry but I could not tell you what those "3" different things were, but they all seemed to be related to a sort of "short circuit" that my heart electrical signals would default to when I was tired or maybe even low of some chemical or nutrient.) He said that that testing would have about 1-2% chance of damaging my heart. Then based on what they found, then would probably want to do an ablation to the error prone area to make it stop doing what it was doing. That also carried another 1-2% chance of damaging my heart. The doctor said if he was me he would not do anything. He said that since my fitness level was so high (with a probable maximum heart rate of close to 200) that in his opinion my heart was healthy enough to beat at 240 and not have any problems. He said if I saw it near 300, then it might be a cause for alarm.
Anyway, I suspect you might have something similar, so I would go and ask for an echo stress test, and also one of those fancy monitors that you wear for a couple of weeks so you can catch it happening.
Also, here is another (rather frightening article) on something also similar:
http://velonews.competitor.com/cycling-extremes
My doctor assured me that I did not have this. I also have not be an ultra endurance rider my whole life that the people in the articles. I have only been doing it for one year, but it does make you think more about how hard to go and for how long.
Good luck, and I'll be interested in hearing what you find out.
Larry
