Pre-Owned Philips HeartStart FRx Infant/Child Key. The FRx uses an energy reducing "key" which, when inserted into the device, converts the FRx into a pediatric defibrillator. The key can be reused with no need to replace it. Compared to the cost of replacing pediatric electrode pads on other AEDs and you can see the savings potential. HeartStart FRx Smart Pads II are used for all emergencies, adult and pediatric.