Jack Crowley was 15 when a baseball hit him in the chest and stopped his heart. The Long Island teen survived thanks to a police officer who grabbed a defibrillator and shocked his heart back into rhythm. A blow to the chest — one that hits at just the wrong spot, at just the wrong time — can trigger deadly cardiac arrest. Fortunately it's rare. But most victims are otherwise healthy kid athletes. And survival hinges on fast use of those heart-zapping defibrillators that not every athletic league or school keeps near the playing fields.