Electricity Generated by Automated External Defibrillators
Modern Automated External Defibrillators (AEDs) typically deliver between 150-360 joules of energy, with biphasic waveform devices generally using lower energy levels (150-200J) than older monophasic devices (200-360J). This energy translates to approximately 30-40 amperes of current when delivered through the chest wall to the heart.
Energy Levels in AEDs
Biphasic vs. Monophasic Waveforms
Biphasic waveforms (most common in modern AEDs):
- Deliver current in two directions
- More efficient at lower energy levels (150-200J)
- Higher first-shock success rate (approximately 90%) 1
- Require less energy for effective defibrillation
Monophasic waveforms (older technology):
- Deliver current in one direction
- Typically use higher energies (200-360J)
- Less efficient, requiring escalating energy protocols
Energy Protocols
AEDs typically follow one of two energy delivery approaches:
Fixed energy protocol: All shocks delivered at the same energy level (typically 150-200J for biphasic devices)
Escalating energy protocol: Increasing energy with subsequent shocks
- Traditional monophasic protocol: 200J → 200J → 360J 1
- Some biphasic devices: 150J → 150J → 200J
Current Delivery Considerations
The actual current delivered to the heart (rather than the energy setting) is what terminates ventricular fibrillation. The optimal current for ventricular defibrillation appears to be 30-40 amperes with monophasic damped sinusoidal waveforms 1.
Factors affecting current delivery include:
Transthoracic impedance: Higher impedance reduces current flow
- Adult average: 70-80 ohms
- Affected by: pad size, placement, skin contact, and patient characteristics
Electrode placement: Anterolateral placement is the standard default position 1
- Proper pad placement ensures optimal current flow through the heart
- Poor placement can reduce effectiveness
Special Considerations
Pediatric Applications
- Adult AEDs deliver 150-360J, which can be excessive for children
- For children 1-8 years old, ideally use pediatric dose-attenuating systems 1
- Without pediatric systems, standard AEDs may deliver 6-15J/kg to an 8-year-old child (25kg) 1
- The recommended pediatric dose is 2J/kg 1
AED Effectiveness
- AEDs have nearly 100% accuracy in detecting ventricular fibrillation 2
- First-shock success rates with biphasic AEDs approach 90% 1
- Survival rates of 40% have been reported for in-flight cardiac arrests treated with AEDs 3
Technical Evolution
Modern AEDs are moving toward current-based defibrillation rather than energy-based approaches, as current is the physiologically relevant parameter that terminates arrhythmias 1. This transition allows for more precise therapy delivery based on individual patient characteristics.
The key to AED effectiveness is minimizing the delay between cardiac arrest and shock delivery, as the chances of successful defibrillation decline substantially with each passing minute 1.