There are two types of AED: those offering escalating shocks (with increasing intensity) and those that offer non-escalating (or fixed) energy shocks. An escalating defibrillator may deliver a shock at 200 Joules, and then a second at 300 Joules, and a third at 360 Joules. Many fixed energy defibrillators shock at 150 Joules, but vary the wavelength based on patient impedance. Total joules delivered, the waveform of the energy, and the impedance of the patient all play important roles in the effectiveness of the shock. While the debate surrounding which shock delivery method is most effective has continued for decades, the important take-away is that all FDA approved AEDs have been shown to be effective in delivering life-saving defibrillation in sudden cardiac arrest.