Nobody who actually knows anything is going to diagnose this over the interweb. But since I don't actually know anything, I'll give it a try.

I've had almost exactly this experience and my HR graph looked almost exactly like yours. I would be pounding away near threshold power/HR and my HR would suddenly jump from ~155 to ~205 (my normal max is 175). As long as I continued to exert, it would remain up there. One time I completed the last 30 minutes of a 40K with a HR above 200. It didn't feel terrible, but didn't feel so great either. This was diagnosed as re-entrant tachycardia. It is treatable.

If your bottom bracket is doing weird shit and creaking, you go to your bikeshop. Why not go to the cardiologist if your heart is doing weird shit?

---

My latest book: "Out of the Melting Pot, Into the Fire" is on sale on Amazon and at other online and local booksellers

I see that kind of thing when my battery needs to be replaced in my Garmin HRMs. They either don't work at all or some very odd HR stuff... super high, low, big jumps... replace your battery.

Hey, many thanks!
Yeah it's not "super" weird:
My RHR is about 47, my HRmax is around 207 and my LHTR is 187 and i am well able to run at 187 for more than an hour without feeling completely shattered afterwards. It seems like this is a thing I only experience with running as I have no problem cycling between 140-180 but with running I have to go very slow (6-7min per km) to keep my HR below 165. I would say I am "more or less" fit - doing aroung 10h of sports a week.
But sounds like i should get a checkup just to be on the safe side :)

That was my first guess as well but I just replaced the battery 2 days ago...
I also tought it could be something with the "contact" i.e. the water I apply on the sensors before running does not do its job and the strap only starts picking up a proper signal after i start to sweat or so...

When I've had chest-strap problems due to wind/static/battery, it tended to more erratic, with spikes or dropouts as opposed to what you're seeing. That could easily be some kind of heart arrythmia, I suggest getting checked out.

If you’re using a 735, don’t use the HR strap for a run or two and use the HR on the watch. Eliminate the strap as the cause of the issue first.

Good idea! :)

It is your strap.
Artifact 1 - Before the jump, the measurements are flatlined (with only a brief hop) for about a minute. This shows that the strap is struggling.
Artifact 2 - The jump looks like it happens instantly...hard to tell without looking at the raw file. Your heart cannot do that.

Either -

1. the battery is failing - not likely, but CR2032s are cheap
2. shirt static - use the watch's HR monitor
3. bad/dry contact - use the watch's HR monitor or electrode gel (spit isn't good enough)

Also, you can start your run and manually check your pulse and compare it to your strap's number.

That's what I was thinking as well, as I was running at a constant pace on flat ground and such an increase in HR would be super random...

Thank you!

I've been dealing with afib for the last year, and prior to having an ablation a few months ago I would see similar readings, where my heart rate would suddenly jump 50-60 bpm. I would also feel like I had the wind knocked out of me when this happened. After the ablation, I would still sometimes see similar results but I wouldn't feel anything different. Taking my pulse at my wrist would show a drastically lower HR than what my watch was telling me. I've been using a 920 with the tri strap, and a few weeks ago I thought about this and tried using an older Timex strap instead. Since then I haven't seen the sudden jumps in HR...

---

"I'm thinking of a number between 1 and 10, and I don't know why!"

seems like old is gold :P