no one can hear you scream "I'm having a heart attack!" Right now, you'd probably die if it was severe enough. Honestly, I think the higher risk would be from a stroke; zero-g might cause random blood clots to dislodge. The article doesn't mention this device actually getting deployed yet; their still testing it on the Vomit Comet. After what our guys did during Apollo 18 though, I'll put money down on them fabricating some type of defibrillator from the ISS itself. Since your in free-fall zero G, you might get lucky and have a longer time period since your heart doesn't need to pump nearly as hard; or that might screw up your internals even more! As Dennis Leary said "we just don't know". We probably should be inducing heart attacks in mice up there to see what happens.
I kinda find it hard to believe that NO ONE has sent a single defibrillator up there by now, just from a risk-assessment standpoint. Even though we send all our astronauts through rigorous health checks pre-flight, some of our astronauts are pushing 65-70. We've sent up student projects, a small defib doesn't weigh much. I have a feeling that a heart attack victim would fare well in a Soyuz emergency landing, if they even could in a rapid enough time frame.