Why the insane Heart Rate suddenly?

In the last couple of weeks I've found that my heart rate has gone absolutely sky high in the first 10 minutes of a 5km run. I mean, really sky high. I'll start jogging and will feel absolutely fine and  breathing no harder than if I were sat on the sofa, but my HR has hit 170+ BPM, the kind of rate that should mean I'm sprinting, squat-jumping, or absolutely hammering it up a hill on my bike.

This is not normal. At Frimley Lodge parkrun last weekend I hit 190 BPM which is utterly insane given my peak HR is considered to be 192 BPM - I should be at near-maximum output to warrant that, not jogging along looking in alarm at my watch!

Saturday 1st December, Frimley Lodge parkrun

After 10 minutes it drifts down a bit, but is still very high. For jogging round a parkrun in 28 minutes I'd expect to be 125-135 BPM for most of the time.

Initially I suspected this was as a result of the Bulletproof Coffee I'd been having for breakfast - however there is another occurrence, this time in the evening. On Thursday I went for a run when I got home from work, unusually I hadn't cycle commuted so started the run from cold.

A 5km run after work on my local route

Similarly, I set off very slowly and suffered a huge spike in heart rate before it settled somewhat. Still higher than I would normally expect but closer.

This morning Mrs and I took a drive to Brighton & Hove parkrun. I had decided to warmup slowly, and then run at whatever pace was required to keep my heart rate in HR Zone 1, under 152 BPM. During the warmup I walked briskly with a heart rate of around 100 BPM as expected, but as soon as I took the first few slow jogging steps it rocket straight up to over 170 BPM, literally (and I mean literally) within 5-10 seconds. I've never seen anything like it. I stopped jogging after a few steps, and it came down just as quickly. I carried on in this fashion for 15 minutes, gradually able to jog very slowly for slightly longer each time.

Warmup, featuring ridiculous heart rate responses

For the run I decided there was no way I could stay in Zone 1, I'd have to walk round the course! Instead I set off very very slowly, and held a pace keeping my heart rate under observation.

Brighton & Hove parkrun

So it did come down, eventually! A big step down after 10 minutes, before finally getting more normal after 20 minutes or so. Note that my speed was relatively constant throughout this variation. Is it all due to not warming up slowly enough?

After a very slow first km my pace was relatively constant

There's an interesting data point from earlier this week. I went for a 5km on my local route earlier in the week, but this time I had cycled home first. My commute home starts in the West End of London, and is therefore very slow for the first 15 minutes, and not much better after that being punctuated as it is by hundreds of sets of traffic lights.

This heart rate plot looks totally normal for a cycle commute, well within HR Zone 1 (under 152 BPM)

I took this as a brick session, meaning I got in, got changed as fast as I could, and ran out immediately. I was still thoroughly warmed up form the bike ride. I wanted to blow out some cobwebs, so this run was harder than many recent runs, and I completed the 5km in a little over 24 minutes. Still a couple of minutes outside my personal best, but fast enough to have to work hard - the upward drifting heart rate during the run validates that I was certainly working hard to maintain pace.

A faster 5km brick run on my local route

This shows that following a 45 minute low to medium intensity bike ride I don't experience the same spike. This graph is exactly how a hard 5km run always used to look, and just what I'd expect to see. No starting spike whatsoever.

So it looks like there are a few options:

1. Warm up really very very slowly (so slow I can barely even jog!)
2. Warm up with a bike ride every time I run (potentially impractical), or find some other way to warm up (drills? some squat jumps?)
3. Warm up with a 15 minute jog ahead of the start of a run and just let any spike happen and pass - or if there's no time to warm up just accept that I have a huge heart rate spike at the outset of a run and jog through it, saving any harder effort for after 8-10 minutes

Maybe this is a side effect of a low carbohydrate and high fat diet? Why is it happening? My suspicion is that as my metabolism and fuelling has changed, there's some trigger which is not yet attuned to this physiological state.

My body goes "Right, get ready to exercise!" and everything plays according to the rules apart from my cardio-vascular system which for some reason thinks it needs to get a record amount of oxygen to my muscles, or the signal to say that it was already delivering enough is suppressed (fat metabolism takes more oxygen, but I wouldn't have thought that alone explains this).

So, why the insane heart rate suddenly?