Suffer anxiety (panic attacks), could a fast heart beat lead to sudden death?

Not usually. Folks suffering from panic attacks may often think that they are having a heart attack and call 911 and go to the emergency department where they are told their heart is fine. There are good treatments available for panic disorder. In my practice, i try and control a patient's panic/anxiety symptoms without the use of benzodiazapines like xanax, (alprazolam) as they can be habit forming.
No risk of death. Heart beating fast is quite safe. Think if it as exercise for your heart, very much like when you run or play tennis. During panic attack, it is common for sufferers to feel as if they are going to die. They don't actually die.