I think it’s actually quite common. So people just tend to have higher heart rate than others. And I’m myself among those with a “wild” heart. Throughout the year though, I have figured out a way to deal with this. It is very simple: whenever your heart rate goes off the chart, just sit down and relax, and exhale all the air out of your lungs, then hold your breath for a little while, then gradually start to inhale again. Repeat this process several times, and you will find that your heart rate is going down. This technique works for me perfectly, and I have since then recommended to many people who have the same condition, and it turned out to be effective for most of them.
The rationale behind it is not well-known to me, I have to say. But I can tell you my hypothesis. I think it has something to do with the body’s contingency mechanism for the lack of oxygen: since the body is getting less air, most body functions start to compensate for it, and as a result the heartbeat goes down.
Now, this is only a guess. I don’t really know the mechanism, but what I do know is: this method does work. Try it and judge for yourself.