Heart rhythm problems

I try to keep fit.

Years ago I did a lot of running but had to give that up due to knee damage - more on that later - so in the last few years I've taken up cycling.  Mostly road with a bit of mountain biking.

A peak distance of 7500 miles in one year, more than 3000 in each of the last 5 years.

I use a Garmin computer with a heart rate strap and can track a range of statistics including HR.

When training for the bike leg of a 2 person duathlon, I notice that I get some periods where I can feel my heart fluttering, with a high HR being reported by Garmin; this is mostly under certain specific conditions - for example, when I reach the top of a hard hill and ease off both in effort and breathing intensity. There is no pain, discomfort or breathlessness, just a mild fluttering sensation.

I print out a few example Garmin traces and go to the GP. Sensible recommendation - wear a medical-quality monitor for 24 hours. So I'm referred to the local hospital, the monitor is fitted. I can replicate these events at will and do so, and can pinpoint the time by cross reference to the Garmin trace.

A month later I have an appointment with a consultant to discuss the results. Apart form a fairly crude line of questioning to gauge if I'm really thick or not, I get a very clear explanation of how my heart's electrical circuits operate and what may be happening. An investigative operation is recommended, and I agree.

This takes place, after the necessary preparations. General anaesthetic, drugs administered in an attempt to provoke the problem, sensors posted through an artery in my groin to see what's going on.

No fault found!

Follow-up with the consultant. I explained that I am able to manage the problem, mostly, by careful breath control, and I'm returned to my GP as 'no further action needed'.

I still get spikes but with care these do not get into dangerous territory. But I can't attempt to get to higher HR zones without seeing higher spike levels. Therefore, more slow and steady and less high intensity training.

Update April 2016 - after 6 weeks of winter sun in NZ, and lots of walking and clean air, I found that the spiking had pretty much disappeared when I restarted cycling on my return to the UK mid-March.     I can still provoke spikes still by silly effort levels with limited breathing, but even then there is an improvement. I wonder why?