40 BPM heart rate spike - why?

Yesterday I raced the Tour de Sportsolid (D category). About 3 minutes in, when my heart rate had just started to stabilize at 140, my heart rate spiked to 181 within 7 seconds. In the next 7 seconds it returned to 142. (This 14 second interval would account for about 32 beats if the spike hadn’t occurred.) The spike increases smoothly to the peak and likewise decreases smoothly (no jittering).

No other such spikes were observed for the rest of the race (or ever on Zwift during 2.5 years of monitoring). I didn’t feel anything at the time and only noticed the spike later on the two monitoring apps (Zwift Companion and WhatsHeart).

Although this spike might be an accurate measurement, I wonder if there are any other possible explanations. (If the strap had lost contact with my skin then I would have expected a drop in heart rate, not a spike.)

Polar H10 with generic strap.

EDIT (12/1/2021) –
During hard efforts my max heart rate is usually upper 150s, occasionally exceeding 160. Over the past 2.5 years on Zwift my absolute max heart rate has been 164. During a supervised stress test in 2016 my max heart rate was 166. Thus, my race spike of 181 seems very abnormal.

Attached is a handmade graph that closely approximates the spike.

Don C.

Could be equipment related but also may indicate a cloaked medical issue that is inducable in certain conditions.
A visit to your doctor to discuss your general health and risks may be in order.
That said, I bought a new heart rate monitor and my HR readings are much more stable.
I’ve started disconnecting the monitor from the strap and wipe it dry after each use.
Basically, we are soaking this peice of equipment in a brine bath each time we use it.
I have popped the back off after a ride and there was moisture on the battery.

I’ve experience in this field of both medical and technology.

Tell me about your caffeine habits. How long before the race did you have any?
Do you normally train after caffeine? Etc…

none of this is medical advice:

i’ve experienced this, too, with a wahoo tickr (chest strap). sometimes, it reports “way too high” for about 10 seconds and then creeps back down to “normal”.

i was super worried about it the first time, so i stopped pedalling and took my pulse manually. my HR was normal, and in fact the HRM reported it was still increasing to zwift while i was off the bike (it was in reality decreasing).

at this point, i am chalking it up to some sort of malfunction in the HRM itself. possibly, it’s a bug in zwift, but it’s really intermittent if so – i have no idea how to replicate it. i am not worried about my heart when it happens now.

if it happens again, might be worth trying what i did to reassure yourself it’s not your actual heart!

Flat batteries and metal nipple ring have caused spikes for me in the past.

Stuart – I don’t use any caffeine or stimulants of any kind. I normally warm up about 10 minutes before a ride or race.

Dan –

This has only happened once (briefly), which I didn’t notice until after the race (I’m usually concentrating on the race, not my HRM). The BPM spiked quickly (7 secs) and then quickly returned to normal (see the graph in my edited post). If this happens again as quickly then I wouldn’t have time to manually check may heart rate.

Since this spike was reported both in Zwift and the WhatsHeart app, it couldn’t be attributed to Zwift.

Tim –

I checked the battery compartment and there was no evidence of moisture. I disconnect one terminal of the monitor after every use. The battery is a CR2032 rated at 3 volts. My measurement showed 3.02 volts so it should still be good. My records (if up to date) indicate that the battery is somewhat over a year old. I’ll try replacing it but I don’t that this is the problem.

Don, it’s almost definitely a technology issue rather than medical based on the further evidence you suggest.

Arrhythmia caused by stimulants can be a root cause of this but not in your case.

My Garmin Fenix 6 wrist based watch will sometimes do almost exactly the same as you have experienced. Set off and its fine but a few minutes in I get a spike. Often reporting above the max HR I can attain. Couple of mins later it’ll drop.

I get this with chest straps too. It’s down to positioning and moisture.
Make sure you moisten the contacts before wearing it. Your warm up should have achieved this naturally but depends on how vigorous the warm up is.
Do some research on chest strap positioning as most of us get it wrong by placing the strap almost over the heart thinking that’ll give us the best contact.

It isn’t.

Stuart –

I found an article by DC Rainmaker (search " Troubleshooting your heart rate monitor/strap HR spikes"). He says that HRM spikes are not uncommon. As you have suggested, he primarily suggests wetting the electrodes (which I seldom do); also replacing the battery. I’ll try both. (He also mentions possible clothing issues but I always ride bare-chested on Zwift.)

It’s good to know that this is unlikely a medical issue.