I'm a 64 year old male with low body fat. I've exercised 6 days a week for many years. My routine has been to, in addition to my moderate sessions, twice a week push myself to the limit for about a minute at the end of my exercise session. I usually can get my heart rate up to 160 to 164.
A couple of weeks ago right at the beginning of an exercise, when I hadn't even worked up a sweat, my heart rate monitor showed my pluse rate in the upper 170's. I stopped and it took about a half hour for it to work its way back to under 90 and another half hour to get back to 70. I did some interenet research and the issue of dehydration caught my eye. I started drinking more and my next several exercise sessions, including those that I pushed mysefl, were normal - except for one a week later where it elevated abnormally to 160 at the start of a session for just a few minutes and then came back down and I continued exercising with no problem. Again, since then I've had many normal exercise sessions. During none of these episodes did I feel any different. Had I not been wearing a HRM, I wouldn't even have known that my pulse rate was elevated. And BTW, the monitor on my exercise machine read the same as my wrist monitor.
This morning it happened again, and I'm pretty sure, although not positive since I don't know how accurate my scale that reads hydration is, that I'm not dehydrated. At the start of my exercise my pulse rate jumped to the upper 170's and stayed there for about 5 to 10 minutes. When I tried my exercise bike again after it came down, it went back up to the 150's with very little exertion. I stopped and have been wearing my HRM for the past hour to see what happens. For a brief period it went down to 56 even though I wasn't totally resting and then leveled off at 70 to 80 except for normal increases to 120 or so when I run up the stairs. I feel fine, but wondering why this has started.