Day after Rivington Pike many moons ago, I started feeling strange in the middle of the night, similar to butterflies in my stomach.
Every 5 to 10 minutes I had to take a deep breath.
When I checked it with my Garmin chest strap it read 35bpm.
Phoned 111, fast response arrived and wired me up. 33bpm he said.
Ambulance arrived 31bpm they said.
Dropped to 28bpm while travelling.
Then after 3 days in hospital went back to normal with not a pill or injection administered.
After a year of monitoring I asked for a referral. Got sent to LGI and the fell walking Doctor said 3 out comes. Pacemaker now, pacemaker in future or obituary.

Easy decision. He showed me his Statistics and he had 100% success rate.

They did an interesting test on a treadmill. Every minute it got slightly fast and inclined.
Max was 20minutes. The technician pressed the emergency stop at 19.5mins. I was fully wired and my HR was 176, I did tell him I could max at 184 but he wouldn't risk it.

When the consultant got the results and heard I had complained about not being able to max he laughed and said are there many like you. I said no, I usually finish in the bottom half of the field the others are better than me.

He concluded that because we train on hills and work in oxygen debt we don't show the symptoms of normal people.

I should have felt light headed or dizzy similar to being drunk but I felt perfectly normal all the time. (Well as normal as a fat welder can be)