Hi there, my name is Luke from Detroit Michigan and have a question about heart rate. I know heart arrhythmias have been discussed before on here and my question involves my experience. I get high HR during lower intensity workouts. Zone 1-2. It will stay relatively normal for about 30 minutes to an hour but then will jump up to 160bpm sometimes higher. I’ve experienced high rates going up to 217+ on rides that aren’t very strenuous. I’m wondering if my heart is just over stressed? Electrolyte imbalance? Overtrained? Sometimes I can go months without having an issue then out of nowhere hr will be very sporadic. I am on thyroid medication (under active) but recently have been taking a higher dose. Possibly raising my heart rate during the day and during activity. Sometimes I feel like my stress response is active during many hours of the day. I have trouble sleeping for periods of time as well. I also have experienced disordered eating with training… any help would be greatly appreciated
The first thing I would make sure is that your HR monitor isnt the problem. Low batteries or a strap issue have given me some erratic heart rates similar to what you are describing. Are you noticing your heart beating out of your chest when the HR is 217? If not that might imply it is the HR monitor is the issue. Assuming that it is real you will need to see a cardiologist to get a work up. There is really no way to say what is going on without having a ECG to look at what is happening during the periods of high heart rates.
I would contact your PCP for a workup - they could start the ekg/labs/exam and get you connected with a cardiologist if needed for a holter monitor, stress test or other additional work up.
Recently wore a holter monitor for about 12 days. We’ll see what it shows.
I was able to exercise with it and noticed a few things. HR was pretty normal during intervals (sweet spot/threshold) reading 155-175 depending on wattage/percent of FTP. The moment the intervals were over my heart rate would spike up to 190-200 bmp when my HR should have been declining. Happened more than once during different workouts that I did during these 12 days so at least they have something. We’ll see what my doctor says and if he recommends seeing a cardiologist
FWIW, in my experience over many years, HR does not decay in a rapid fashion. For me as well, it always is never a direct reflection of the actual rate, whatever that is. What has most concerned me is an HR that rises w/ effort, and then when the effort stops, HR rapidly increases as can be seen in paroxysmal supra-ventricular tachycardia. Did a C2 rowing workout many years ago, Tabata 10" on 20" off and after several reps it rose to 165, and at the next rest interval it continued to rise to 220bpm. PSVT it was, acc to my Doc, a Sports Cardiologist who himself was a Master’s sculler - and he looked it too.
Beware that if you are an accomplished athlete w/ a history of endurance training especially and over many years, you will likely have forgotten more than your MD knows, especially when you take a look at him or her and they are over-fat and out of shape. 
Find a sports cardiology doc who might actually know something about athletic metrics.
I have found it most useful in measuring ECG via www.movesense.com, the company’s MD version of it. Pricey and it takes 10 days to come from Europe and will likely get flagged at CBP as a medical device. DHL will then notify you of that and require that you give your SS # or TIN# for it to clear. Advise the company to put it in a plain wrapper.
Alternatively, you could take a look at this:
HTH, Anthony