/sci/ - Science & Math

If anyone could give me an advice regarding my heart rate I would greatly appreciate it.

Every time I exercise my heart rate goes stupidly high and I don't know what to do. Today I went for a run and few times I measured it it went slightly above 200 (I'm 21) after 1km. Pace was around 6 min/km. After 250m of walking the heart rate went to around 160-170. After another 1k heart rate again raised to above 200. Then after I finished running it stayed 150-160 for probably more than 20 mins. Sometimes my HR is high when I stand up, even when I'm not doing anything, could be around 120-130. It only takes for me to bend and heart rate drops to normal 70-80.
Probably worth mentioning that even though my HR goes above 200, I can feel no symptoms at all. No pain in chest, no SOB etc.

I've seen cardiologists, electrophysiologist. They said it's "arrhythmia caused by sensitive autonomous regulation" (sensitive sympathetic nervous system to be more precise). Respiratory arrhythmia is very noticeable. I failed veloergometry (exercise stress test) 2 times (because I get scared really quickly, heart rate was simply too high (at around 150) to begin the test). One time I passed veloergometry test, though physician said HR ramped up a bit too quickly, but apart from that, nothing unusual was seen.

Basically everything I got tested for was good except for vitamin D (53nmol/l). Could vitamin D be the cause of this?

To cut a long story short, physicians said I'm fine and there's generally nothing to worry about. Though I really want to believe them and even though they are very good and competent physicians, is it even safe to exercise at HR above 200? Anyone has any advice on how to approach this problem and how should i try to solve it?