I know asking LetsRun for medical insight is somewhere between crazy and desperate, but here goes ...
Any thoughts about what might cause a drastic drop in MAX heart rate while running? General running HR also much lower in comparison to pace and effort. Add in breathing noticeably harder than pace and HR would suggest.
Here are a few running details, but I'm leaving medical stuff out for now because I don't want to bias any possible responses. It's been a very frustrating period. I'm almost 50 and male and as recently as 4-5 years ago I know my max could still get up into the low 170's. Earlier this year I started to notice I was breathing harder and harder at even easy paces. I was also noticing weakness in my leg muscles - decreased strength up hills was what I noticed first (even felt weakness walking up stairs), but speed also evaporated. I started paying attention to heart rate more and realized it was unusually low - 7:30-7:45 pace used to be trivial, but I was breathing hard and feeling tired at 7:45-8 even though my HR would be in the low 130's at best.
About a month and a half ago I did a running max HR test and could only get my HR to 145. Yes, I know those tests aren't "exact", and yes I did push to the point my legs were failing, so I don't think it was an effort issue.
Over the past couple of weeks it's gotten even lower. The last week or so, runs have averaged 122-125 with max rates of 129-132 during the run. That includes hills and miles as fast as very low 7's
Yeah, I'd love to think the low averages mean I'm in fantastic shape, but since I don't have any ability to elevate it or push hard, I don't think that's the case.