Tachycardia, in simplistic terms, is an abnormally fast heart rate. As a child, I suffered from bouts of tachycardia that were triggered by ephedrine or variants of ephedrine, most commonly found in decongestants. I seemed to have outgrown this 'allergy' as I have been able to take decongestants with ephedrine or pseudo-ephedrine and not have a tachycardia attack.
As a child, my resting heartbeat was normally around 100-120 with it getting into the 200's when the tachycardia set in. The worst for me was the pounding chest pains; it would be so bad that I literally had to lay still and take long, long, deep breaths, which would force the heart to slow down. I have a few relatives who suffered from the exact same problem, so I assumed it was genetic, although I seemed to have outgrown it, and others in my family have not.
Here is a link to a website that discusses what Tachycardia is. Do you know of anyone who has or had this problem?
https://www.studio-delos.com/tachy/tachy2.htm
Thanks for the information and link Malexander. I was born with Tachycardia, I do not know if the reason for it, was that my mother smoke through my pregnancy. There are times I feel it very strong, specially if I am under a lot of stress, I feel my heart beating very fast and I have chest pains. My grandmother also, who now was diagnosed with an arrhythmia in her heart, is also suffering from tachycardia.