With all due respect to the post above, because bad data could be the root cause, I would report to your doctor just in case. I’ve had two incidents, one about 3.5 years ago and another a couple months ago. In both cases I believe the HRM, as explained below. Reported both to my doctor, we did some tests after the first incident and nothing found.
First time was January 2018 and I was on 20+ minute descent before a 2.5 hour climb. HR and elevation on the chart below… hit 186bpm while not pedaling at about 9 minutes into the ride. My observed max on many rides, where I sprint after a long threshold effort, is 175bpm.
It happened at the beginning, and lasted about 6 minutes total. Had no idea at the time, only saw it after the ride. This was a hard 2.5 hour climb at upper tempo / lower sweet spot, the rest of the HR data looks normal.
Temps were dropping as we descended. It was 50F at the start and 36F at the bottom. I was a bit nervous about the road, a lot of potholes, switchbacks, steep drop offs, and some incredible views while descending toward the cloud deck at the bottom. This is a pic at that mid-descent uphill:
Definitely some anxiety and excitement and adrenaline. Because I didn’t notice high HR during the ride, I didn’t attempt to confirm the accuracy of the reading. It is possibly a bad HR reading from the Wahoo TickrX used on the ride. However on mid-winter days like that (cold and dry) I use electrode gel.
The other incident was this summer, following a hard 12.5 hour week - my usual max is ~10 hours - this was a Tuesday recovery ride at 40% FTP ramping up to 50% and then short 3x3-min tempo at 85% FTP:
HR was a little high at end of 1st 3-min interval, dropped quickly during 3-min recovery, jumped above max during 2nd interval, and HR returned to normal on the 3rd interval. About 5 minutes total of abnormally high HR.
When it happened, I cross-checked HR on my Apple Watch and it was showing the same HR readings. So in this case I’m forced to believe the Garmin dual HRM strap was accurate.
My post ride notes:
The strap is about 2 years old, no problems since then. A second note I added to that ride:
Certainly the heat was a factor, although my sweat rate was unusually high for the temp. And I was experimenting with preloading different concentrations of sodium, maybe I drank too much in combination with the heat.
My family doctor is a triathlete, he told me to chill on the sodium preloading and let him know if it happened again.
In both cases there was some anxiety and some adrenaline, otherwise not much else in common (winter vs summer, descent vs flat roads). Thankfully it has only happened twice.
When I see bad data on Wahoo Tickr or Garmin HRM, my experience is the opposite of mcalista and HR is artificially too low for first 5-10 minutes during dry/cool conditions. If I put electrode gel on before a ride/workout, and get a low HR reading, it is time to toss the strap. Once a strap is done, I usually see too low HR but as referenced in the post ride note above I did see it jump to ridiculous levels in 2019. But too high (above HRmax) is unusual, its almost always too low (stays at resting HR for 5+ minutes).
The Garmin straps usually last me about 3 years, and I do wash them after each use and toss into the washing machine after every 5-10 rides. I agree with the rest of what mcalista wrote above.