I've been talking to Joe who got home from the hospital a bit ago. Seems they did a whole series of tests and none of them point to heart trouble. I think this is the best possible answer, although Joe feels a bit dumb for wasting everyone's time. I'm just glad he's okay.
It seems that what the first dr. saw in the EKG as a heart attack indicator was perceived by the other doctors at the hospital as a "maybe" but nothing definite. I guess reading EKGs is more of an art than a science. They did say that anxiety attacks can wake you up out of sleep, although that's an indicator that they take seriously. So, for now, it seems that all is well.
Thanks again to everyone for all the well-wishes.
(Oh, and if I were a doctor, I might phrase this sort of thing as "we just want to rule out anything serious" as opposed to "the EKG indicates you might have has a heart attack".