If in doubt, just toddle over to wiki.
I had tachycardia
(due to a panic attack) for 15 hours once, I survived, its actually quite common. I'm personally thinking your heart rate
is fast because your body is fighting off some chest infection or something.
Try some deep breathing
(lame I know, but it will be essential in lowering your heart rate and keeping blood oxygen
levels up) for around 10 minutes, breathing slowly through your nose, not mouth.
Also, are you stressed out at the moment? Worried?