A feature of many AED models is that they have escalating energy.
Escalating energy means that each subsequent shock given by the AED will increase in energy. Usually, there are three levels: low, medium, and high.
The first time the casualty requires to be shocked, the AED will use low dose of therapeutic energy. If another shock is required, the AED will increase the level of energy to the medium setting. If a third shock is required, the AED will use the high energy setting. The fourth shock (and any other subsequent shocks) will be at the high level as well.
The rationale for increasing the energy level is that if the low energy setting did not send a strong enough current to re-start the heart, a higher level of energy may.
The reason why the initial shock is not set at the high energy level is because it takes a slightly longer time to charge the capacitor of the AED with energy to send to the casualty. Time is of the essence, so the faster to give the initial shock the better. A lower energy setting will also reduce the risk to damage to the heart tissue. Not everyone requires the highest level of energy, therefore the AED will escalate the amount of energy energy to shock the casualty.
Fixed energy is the opposite of escalating energy. Fixed energy means that only level of energy is given to the casualty regardless of the number of shocks.
The Cardiac Science, Physio-Control, and Zoll AED models all have escalating energy.
The Philips AED models have fixed energy.