4. a (call 911 immediately). Wait no more than 5 minutes — less is better — if you have signs of a heart attack. It doesn't matter if some symptoms go away, because they may return. If you call 911, emergency medical staff can start treating you when they arrive, and they're trained to revive someone whose heart has stopped. Clot-busting drugs only work if they're given soon after a heart attack (or stroke) begins