Some tachycardia is normal and may be caused by anxiety, some tachycardia is not normal but may be benign, and other tachycardia is a serious problem.
Learning which can be frustrating and take years. It is common for people with abnormal tachycardia to have either nothing turn up on the monitor or almost as often, something turns up but it gets misread. Having someone inappropriately suggest valium is common as well.
Sedatives shouldn't be used to treat any of these things. Either you have an anxiety disorder and it needs more specific medication, or you have a heart problem and psychiatric meds are completely inappropriate - though they could be appropriate for your psychiatric problem.
Stress can exacerbate any arrhythmia but is not the cause, usually isn't the only trigger, and an actual arrythmia needs the arrythmia treated.
If you have panic attacks, your heart speeds up somewhat and takes a few beats to get there. If you have, for instance, supraventricular tachycardia, your heart speeds up to well over 160, often over 200, and takes a beat or two to get there. Sometimes it may do a few rapid beats and settle back down, or alternate between a normal rhythm and a very rapid rhythm. None of this is something a panic disorder can do.
If you have supraventricular tachycardia, your heart beat is regular but far faster than it could get on its own. This seems to be the hardest beat for doctors to spot on a monitor.
This discussion is related to trachycardia due anxiety / panic attack - risk of stroke or heart attack