Pre industrial terms for "heart attacks"

Seizure?

"You almost scared me to death"?

If it's just for that particular scene, perhaps instead of looking for a literal substitute for "heart attack" you could just go for a similar phrase, e.g. "You almost scared the life out of me."

"I could feel my heart almost stop!"

ETA - Or, "My heart just skipped a beat!"

Acute Exacerbated Angina could work...A lot of heart attacks are describes as "just like my Angina, only worse."

Informally perhaps: chest spell

"Acute indigestion" is something my grandparents used often.

-cb

If I had a character with a heart attack in a medieval setting, I'd probably just say, "His heart stopped."

As for your needs, I've never been fond of, "You nearly gave me a heart attack." I prefer more accurate phrases, like, "You scared the pee out of me," Or, "You scared the crap out of me." It makes sense, as people sometimes lose bodily functions when they're terrified, whereas actually having a heart attack is more rare (if it's ever happened at all). Also, "You nearly gave me a heart attack," is very cliched.

Personally, I would go for a phrase that means the same basic thing (being scared to death), but that's unique to your world. Do they have a deity they worship? You could say, "You nearly sent me to St. Peter's gates." Or "to the shores of the River Styx," etc. How does your character react to being startled? If he strikes out automatically, like a soldier might, he could say, "What were you thinking? I could have stabbed you/zapped you out of existence/turned you into a frog."

Or you could show the character's reaction, instead. Have him startle awake, then make a big deal out of putting a hand to his heart and widening his eyes, as he lets out a cuss word, perhaps.

Just a few ideas.

I had an ancestor die of a heart attack; he was a Civil War veteran. (American Civil War, 1860's.) His death certificate read, 'Flabby heart.'

We're pretty certain it was a heart attack as he was 60, and many of his male descendants, down to the present time, have also died in their late 50's or early 60's of a flabby heart, or heart attack.

Of course, it doesn't sound that scientific, and I don't know how prevalent the phrase was. Maybe the doctor who signed the certificate was a quack.