There is no way you are dehydrated.
That is a nonsense diagnosis.
You need a different evaluation for tachycardia.
See an internist.
Unless you have kidney disease, or some reason you are losing fluid.
Such as being on a diuretic.
If not, it is not dehydration.
You need a holter monitor, to watch the heart.
OK, so you might have more questions or want to give me more information: Please use reply to expert if you have further questions. When you are ready, please click a positive rating [hopefully excellent—that’s how we are paid, per rating]. If you forgot something, come back. I am here http://www.justanswer.com/medical/expert-dr-thomas/