For me the HRM is the most important piece of equipment I have. I do not ride without it. I have had 5 heart surgeries in the last 2 years including a 4x bypass, a pacemaker, and a stent.
I go into heart block, I would be at 140 to 150 BPM and then I would go to 70. One night I went actually went to zero on the trainer. That is when I got the pacemaker, I told the doc and within a couple of days he did the implant.
Now when I get to a sustained HR rate of 140 or so going uphill (my heart rate is lower due to meds), I need to stop and rest. This is about when the pacemaker will kick in. The Cardio doc said the pacemaker is set to have a max HRM of 130 so continuing riding uphill is not something I can do, but within a minute or so I am good to go , my HR drops, the pacemaker is no longer firing, I am back to a normal sinus rhythm and I can continuing climbing.