I've had two recent experiences, quite similar to one another, with very high HR reading on my Garmin (Premium softstrap HRM, paired with an Edge 500) that I suspect are glitches, but.....
Both have occurred very early in my ride - in the first mile - with minimal exertion. In both cases, I looked down to see the HRM recording above my MHR (180 BPM). Today it was reading 182, the other time a couple of weeks ago, 189. Based on my perceived effort, I would have expected 110-130 BPM.
In both cases, I stopped as soon as practicable and took my pulse with my wristwatch and wrist pulse. These recorded something like 110 BPM (it's hard to be accurate in this way), but by the time I was able to do this, the Garmin too was reading low, reasonable values.
In both cases, I felt absolutely fine - no racing heart, no symptoms of exertion, nothing. Had I not been wearing my HRM, I would have had no inkling of a problem. But I have read that tachycardia can be otherwise asymptomatic.
In both cases, with cooler weather, I suspect that the HRM could have been malfunctioning (static electricity from my jersey? imperfect connection because I wasn't yet perspiring?
In one case, once I started up again, the HR climbed within less than a minute to about 160 BPM, which made little sense based on exertion. I stopped, the HR went down, my manual pulse seemed normal, and the rest of the ride I had normal reading. In the second case, I had normal readings the rest of the ride without the second coda.
But the readings weren't randomly wacky - again in both cases, when the HRM first coupled to the Edge, the HRs were normal, but as I pedaled they just went up really fast. And then when I stopped, they went down again, quite quickly. If it was just a HRM malfunction, would it behave in that particular way (e.g., mimicking my exertion, but with greatly exaggerated values)? And why were the spurious readings coincidentally be just above my normal actual MHR? The anecdotes I've heard about spurious HR readings have tended to be of the sort where the monitor went to 250 BPM for somebody who had a normal MHR (160, 180, etc.)
Anybody out there with similar experience with their HRMs (or with actual experience with tachycardia)?