Had that happen to me on an AA flight DFW-DCA a few years back. FO had the attack as soon as we hit level flight and we dropped like a stone to get into Kansas City. Capt said FO was able to man comms until the final approach. Made for a real sporty ride in an MD-80 dropping from 32k' to WOW in less than 15 minutes tops!
We were in first row that biz class that flight so I got a chance to chat with Capt as we waited on the deck for the relief crew to land and take us on to DCA. He said the saving grace was that this was this guys 3rd or 4th attack so he knew the early signs and allowed them to declare emergency and begin decent before it got too far. (So my next question is why a guy with prev heart attacks was behind the sticks and not manning the sims or the flight manual desk!)