By the person suffering the attack or by people who witness it?

I've had anxiety and full blown panic attacks in the past, and they are not always exactly the same. Some times I just feel shaky, sometimes I have an extremely rapid heart beat, sometimes I might start hyperventilating. If I DON'T recognize it as an onset of an attack as soon as it starts, it gets worse because of the anxiety of not know what's going on. IF I do recognize it as being just another anxiety or panic attack I can control it's severity to some extent.

I would say in regards to the person suffering the attack, that don't recognize it are thinking what may be happening is:

Heart Attack
Stroke or similar brain related problem
They've been poisoned by something they've recently eaten or drank
Or simply it's their time to die

So the biggest thing is the person suffering the attack BELIEVES they are about to die, and there's nothing they can do to stop it. That causes even more panic and what leads to full blown panic attacks.

As far as people witnessing it, I'd say probably most of the time it's mistaken for a heart attack if they talk to the person suffering the attack.

In people who have never had panic attacks and have never had a heart attack, the two are very similar by the symptoms alone. The rapid beating heart, difficulty breathing and pressure in the chest are all symptoms of both a panic attack and a heart attack.

A panic attack in an older person under stress is likely to be suspected as a heart attack until proven otherwise as well, just to be on the safe side

It could also be mistaken for a stroke, depending on how it presents and because many people have no actual understanding of what stroke symptoms are. The symptoms are actually quite different, but during a panic attack a person will often fear the worst case scenario and a stroke is pretty bad especially if the panic attack is not accompanied by tightness in the chest and other heart attack-like symptoms.

I used to work in a doctor's office, and I can't count the number of times someone would come rushing into the office, screaming, that they were having a heart attack. People often think that they are having a heart attack when, actually, what they are experiencing is a panic attack. This is usually the case with first-time panic attack sufferers, which, of course, makes the panic attack all the more worse.

Frequently people who are experiencing anxiety attacks come in believing that they are about to die. I have also had people tell me they are suffering from asthma attacks, that they can't breather, or that they are having heart attacks.

A quick exam and a couple of questions usually point to the real culprit, hyperventilation from an anxiety attack, pretty quickly. In my experience, once I explained to the person what wast going on, I could usually help them stop hyperventilating and then their symptoms would resolve very quickly.

The symptoms from an anxiety attack are caused because your body needs a certain amount of carbon dioxide. If you hyperventilate (breathe fast) then you blow off too much carbon dioxide. This can leave you feeling breathless, cause chest pain, cause tingling in your extremities, and cause cramping - particularly of the fingers.

All of this information is not intended to keep people away from the ER who may in fact be having a heart attack. Whenever you are concerned about your health, see a doctor.