Hi josh
When ever you go to ER they put a little clamp on your finger but unless you ask they don't tell you what it is for. It is O2 sat. Oxygen saturation and should be between 95% and a 100% although by taking a few deep breaths you can raise it over this which usually gets me funny looks from the tech. If your heart is beating fast and you are not breathing then your O2 sat goes way down and you faint. In which case the involuntary side of your brain overrides the voluntary side and you will breath at a rate necessary to sustain sufficient O2 to your heart and brain so you don't have a heart attack. It is lack of O2 to your heart muscle that causes heart attacks, not the speed that you work it at.
Your breathing has two systems controlling it because it has too. There are times when you need to stop breathing for a short while. Like underwater. Or when you don't want to be noticed. (fight or flight) So here in lies the problem. Since you can control your breathing but only your heart rate to a small extent your heart rate has to speed up to compensate. So breath. Tense muscles use more oxygen to keep them there. (basket ball) This is why muscle relaxation and box breathing work. One reduces your O2 consumption and the other increases your O2 sat and reduces your CO2 levels.
So the question is not will it cause a heart attack but rather why is it too fast.
Stress, tension, and especially fight or flight. But some other things too. A big meal raises it. All your internal organs need O2 also. Water retention. To much water in your blood reduces your O2 carrying capacity. Low iron. But all these things show when your O2 sat is checked.
The other test you can have done is O2 transfer efficiency, but I wouldn't bother. If you can walk a block with out passing out you are fine.
So write it off to just part of the panic condition and know it will go away. Your heart is a very efficient muscle and use the relaxation tools to bring down the rate.
On the subject of Valium, It is a muscle relaxant as are all the benzo's, on top of their sedative effect. It will bring down your heart rate, but not right away, it needs 15 minutes about to work.
It is a safe benzo and relatively easy to get off if and when you no longer need it. I still use it periodically. (seldom) It does make a very good adjunct to the program.
Davit.