What an odd thing to witness live, or at least televised... it really was about 40 minutes before they really knew what had happened as they had thought he got hit by a puck but then on replay the puck wasn't near the bench, then they thought from looking at the replays again (though they didn't have any good angles since cameras don't show the benches much) that maybe someone coming off had been hit from behind and caught him as he was sitting down with their stick in the neck...
As they say now, they had to use a defibrulator on him and it was clear from when they were showing the benches live before they really knew what was going on that someone was over there doing CPR chest compressions on him as well. That was the really scary part

so I'm not surprised it was a heart related matter. Best wishes to Jiri for a speedy recovery and hopefully he'll be able to come back and do what he loves to do which is play hockey. Thankfully his fiancee was in the stadium and they escorted her over to him at the bench so he had someone special to be with him in the ambulance and later in the hospital.
From NHL.com:
http://www.nhl.com/news/2005/11/244372.html