Disclaimer: I have been diagnosed with severe ME/CFS in the last few years, retired on ill health and now housebound. Only 40!
Now ignore that bit.
My main symptoms (apart from pain) are debilitating vertigo, orthostatic intolerance and tachycardia. I only realised I was tachycardic when I bought a POLAR monitor last year - identical to the ones that are used -but broken- in the ME/CFS service. I showed my HR's to the ME/CFS Service and they were not concerned that it was reaching 150 on climbing stairs or on my 5 minute physio walk.
Now I have noticed that my chest strap monitor keeps recording spikes of over 200 bpm and often dips very low. I thought it was faulty (& changed the battery) but have tried it on DH and it displays a very regular healthy HR.
Right from the flu virus (that triggered ME?) I have been unable to pick things off the floor, go from lying to standing, or be active, without severe disequilibrium. Recently have been very breathless too.
So neither my GP or the ME/CFS clinic seem concerned about this and have not been interested in screening for POTS. I have had a rough winter and keep dragging myself up from my bed to wander about a bit in the home to see if there's any improvement.
Last week I managed to get downstairs (going up is the problem) and sit outside the back door on a recliner in the sun. Yay! Only I realised I felt quite dizzy and that I had tachycardia of mainly 120 - 239 bpm....whilst sitting resting happily in the sun 
. I expected it to drop (am happiest in the garden) but it didn't seem to be slowing at all.
I tried slow walking and even went inside and meditated calmly on the sofa to see if I could bring it down. Nothing. I then went upstairs to lie in bed to see if this helped. Still ridiculously high and had been for nearly two and a half hours. I was very close to calling DH and thinking about A&E, when I googled it and some advice said to hold your breath for a minute. I did and it began to drop and gradually returned to normal. Well, normal for me 
Now, what happened here?!! Anything over 200 bpm is normally just brief spikes for me, here it was mainly between 140-230 for two hours, 30 mins. I was not anxious (have been tachycardic for ages now) and had had quite a good day.
What I am anxious about is seeing my GP & ME/CFS bod and asking again about POTS, tachycardia and a cardiologist referral! Do they think this is anxiety? Any top tips to actually see a cardiologist this time? I would really like to avoid any eye-rolling when I pop up again with HR readings 
Surely this is something that should be investigated? Or am I always to languish in the deep bucket of an ME/CFS diagnosis 
No improvement at all in my recovery, so I'm looking for clues as to why not.