I've recently been diagnosed with PSVT. Checked graphs for outdoor and indoor training rides last couple of years and found HR spikes every few rides. They might occur 1-3x per ride when they occurred. Interestingly, it was usually shortly after a hill or increased speed/cadence when HR should be lowering.
Had an accelerated cardio stress test on treadmill and it showed up shortly after recovery phase began. I never felt any physical symptoms when my HR moved up. I'm 57 and HR would go from 145-160 range up to 190-210 for about 30 seconds. 3 options explained - do nothing, medication, and ablation. I just started on a calcium channel blocker as I also have slightly high blood pressure. Waiting to see a cardiac specialist as well.
Apparently one way to stop this spike is to cough vigorously or strain.
Will be doing a 160 km ride followed by a 100km next day this weekend so we'll see what effect this has. Guess I'll focus more on my HR during recovery portions of my rides.