Unfortunately, there aren't any tests that could be run at the wrist that would be even remotely specific for an MI.
Really the only thing I can think of is some sort of transcutaneous monitoring of troponin levels in the blood, but that's hard to do (without needles) and a late indicator... troponin levels don't start rising for a few hours after the damage has started.
http://en.m.wikipedia.org/wiki/Straw_man
The important idea is that I'm not asking the wearable to tell someone that they're having a heart attack but simply indicate some one "test" is not within range.