For me, panic attacks were scariest when I had no idea what they were or while I was (or am) still uncertain that they're panic attacks. The fear that they're something worse drove the feedback loop.

Being able to identify them as anxiety (focusing quite acutely on constellations of familiar symptoms and reasoning about how they might or might not be anxiety-related) made me ride all subsequent ones out successfully, and they also grew milder and more short-lived.

If a mobile device acting as crappy medical equipment told me I was having heart problems, though, my anxiety would definitely increase.

Explored rather amusingly in The Pale King

https://books.google.com/books?id=gW0K6X-ThtAC&lpg=PA12&dq=p...

Exactly. I was in traffic jam, checked my pulse routinely and it was slightly more than usual: 80 BPM. I started to think that something wrong. now it became 90BPM, and here closed feedback loop caused almost 1800BPM. I have rushed to the ER and good doctors (thank god we have cheap medicine) helped me to go through this. Panic attacks continued for almost half year.

“The study used an older version of the Apple Watch, not the latest one launched in September, which had added a built-in electrocardiogram to its pulse sensor.”

Presumably the new version is more accurate