I am not sure I know what 'panic attacks' really are. Whenever I or a family member has had an attack of panic then we were preoccupied with what we were panicking about, not heart rate. But a sudden increase in heart rate is a common reason for panicking. So I would interpret the problem as some sort of autonomic discharge that leads to anxiety rather than the reverse.
I actually went through a week of something a bit like this on a ski-ing holiday. I say that just to suggest that all of us may go through periods of autonomic instability at times. I suspect for PWME that will be much more likely.
Is it a 'crash'. I am not sure that there is an answer to that but it does not sound like what people typically describe.
In general terms I would say that if the high heat rate is not associated with any heart abnormality and ECG shows sinus tachycardia (rapid but normal electrical rhythm) then there is no immediate cause for concern. As to what is actually going on I suspect that may as hard to answer as it is for ME itself. But episodes like this tend to fade away. You need to follow medical advice but my only other thought would be to 'hang in there' a bit - the best advice anybody ever gave me when I was as low as it gets.
Thanks for your thoughtful reply Jonathan!
Interesting to hear u have experienced something similar.
I don`t know how to explain what it is either, funny u mention it. I would say that what the literature describes as an "impending sense of doom" fits quite nicely, which comes when my heart starts pounding fast, and then my mind starts to race frantically to understand what is going on, i get shaky, and i
feel that I am struggling to stay conscious. Since I have ME as well, u feel so miserable that Ive just assumed "this is it" the three times its been really bad

Obviously if u assume that, it gets way worse, so that`s something to work on.
I`m not concerned about heart issues at all actually, cause ive had echocardiography and the rest earlier in my illness, so im just trying to understand at what angle I should approach this. Although they did find i have irregular heartbeat (supraventricular tachycardia) when jogging, I assume that's more related to whatever is awry ME-wise. I havent felt any irregular heartbeat during the attacks, and i clearly felt them when jogging.
If these episodes happens due to the strength training i was doing the last month and therefore is ME-related, then the best thing is to do not much at all for a while probably. If there is some psychological issue that has happened, then I`m thinking i should try and just keep going to store etc so I don`t develop any phobia. I mean I have to anyway cause i live alone and need to buy food haha. I get scared about having these attacks, cause if its so bad I go to the ground it is extremely embarrassing. A 28 year old man spazzing out on the floor is a ridiculous sight indeed.
At the moment I`m just doing what you have suggested, hanging in there a bit, and tracking when it happens. So far i think its a mix of ME and psychological, cause when i go to the store first thing in the morning, ive managed to do the shopping without it getting to bad. This kinda tells me my functioning level is relevant for its occurrence.