Escalating and non-escalating energy in AEDs
Answered by Dave Bingham, director of AED and Emergency Services, Cintas First Aid & Safety, Cincinnati.
Let's start at the beginning. An AED with escalating energy means that after the first shock is delivered, each successive shock is delivered with higher energy. Non-escalating AEDs deliver the same level of energy on each shock. All AEDs must receive FDA approval before they can be sold, and FDA endorses both escalating and non-escalating energy devices. In reality, though, the biggest factor in saving a victim of sudden cardiac arrest is time to defibrillation, not the amount of energy delivered.
As it turns out, there was a very good reason why early defibrillators used escalating energy. Most defibrillators of the past employed a monophasic waveform designed in the early ’60s that made no compensation for patient impedance and which became degraded as impedance increased, so higher energy levels made sense.
Today's AEDs, however, use a biphasic truncated exponential waveform that achieves successful defibrillation at lower energies. Vastly superior to the outdated monophasic waveform, this BTE waveform compensates for each individual's impedance characteristics and does not degrade in the face of higher impedance. So energy current, not energy voltage, is the tool for effective defibrillation.
Out-of-hospital studies involving all AED models have shown that a high percentage of patients who have been saved have been successfully defibrillated by the first shock, which is always 200 Joules or less. This shows that even with those AEDs offering escalating energy, because the first shock is usually successful in defibrillating the victim, the unit's higher energy levels don't even come into play. Studies also have shown that when subsequent shocks are needed, higher energy shocks are no more effective than repeated low energy ones.
Finally, in its "Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiac Care" the American Heart Association has stated, "there is no clear relation between body size and energy requirements for defibrillation in adults."
While there still is no conclusive human study that proves that escalating energy is better than non-escalating energy, and vice versa, AHA has given low energy its highest endorsement (Class IIa).
What is not in dispute is that the true key to increasing the likelihood of saving a life is delivering the first shock as quickly as possible. This can be done by deploying an adequate number of units in readily accessible places, ensuring that everyone knows the location of units, and training an appropriate number of people on the unit and in CPR. Ultimately, the simpler the AED is to operate, the quicker it can be used to deliver the lifesaving shock to the patient.