I've had a problem with skipped/extra heart beats, which I talked about in this thread:
https://www.bikeforums.net/general-cycling-discussion/125068-palpitations-while-cycling.html

I dug my 10-year-old bottom of the line Polar HRM out of the closet to see if I could notice the irregular beats on a ride. But it only gives a reading every couple of seconds or so, which isn't enough to tell. Can any recommend a HRM that takes frequent enough readings to notice this, as well as be able to download to a computer in case I need to do a printout for the doc?

A skipped beat may still be read by the HRM. Skipped beats, like premature ventricular contractions, are not truly skipped. The heart is still triggered to beat and will give off some electrical signals that may be picked up by the monitor.

Polar makes one model (the 810?) that will measure beat-to-beat rates but I doubt that it will store very much data so you would have to know when the condition occurs. However, if your doctor wanted more info, he would probably have you wear a Holting monitor, which is a battery operated EKG that will store about 24 hours of data.