Rapid Response Revival’s ground-breaking CellAED represents significant innovation in automated external defibrillator (AED) design. As we prepare to introduce the new paradigm in rapid defibrillation for out-of-hospital cardiac arrest patients, it’s important to recognise how far AEDs have come – and for that, a history of defibrillation is in order.
This article explains the history of defibrillation, how AEDs work, and why they remain the single most effective tool for treating out-of-hospital cardiac arrest.
Access to an automated external defibrillator (AED) can mean the difference between life and death. But how does defibrillation work?
Your Electric Heart
It’s not just a song by Anevo. Your heartbeat depends on electrical signals telling the cells to expand and contract – to pump blood in and out – in a coordinated way.
In ventricular fibrillation (VF) and ventricular tachycardia (VT), chaotic electrical activity in the heart stops blood from being pumped to the body and brain.
Defibrillation has been the only effective treatment for VT and VF for several decades. Successful defibrillation involves delivering sufficient current through the heart to depolarize a large number of myocardial cells, and thereby terminate the electrical disruption. Voltage stored by the defibrillator conducts electrical current (a shock) through the chest via electrodes or paddles placed on the chest.
By disrupting that chaotic activity, defibrillation gives the heart a chance to restore its regular pumping action.
A brief history of defibrillation
Scientists in Switzerland in 1899 demonstrated that electric shocks could both induce and reverse VF.
Cardiac defibrillation (direct to the heart muscle) was first attempted on a human during a surgical procedure in 1947, by American cardiac surgeon Claude Beck (whose at the time, cutting-edge device is pictured). External defibrillation (applied to the chest) was first used to treat ventricular fibrillation in 1956.
Early defibrillators were heavy, manually operated, and only available for use by trained medical professionals.
The first defibrillator to automatically analyse the heart rhythm, detect a shockable rhythm, and deliver a shock was tested in 1978. Subsequent technological and clinical advancements refined shock waveforms, energy storage, and shock delivery.
This led to the development of smaller, safer, and more effective automated external defibrillators (AEDs) that could be used by people with minimal training.
AEDs became available to the public for the first time in 2004, after Phillips Medical gained FDA approval for the over-the-counter sale of their home-use AED without a prescription.
The Modern AED
Modern AEDs are computerised devices. They contain a battery to power the device and provide the energy needed to deliver the shock to defibrillate, a capacitor that is charged to the high voltage needed to provide clinically effective current (the shock), a microcontroller to control the functions and user interface of the AED, and electronic circuitry.
Electrodes are attached to the AED and placed on the patient. The electrodes collect electrocardiogram data (ECG), which is analysed by the AED rhythm detection software. The AED informs the user whether defibrillation is needed, and either prompts the user to take action to deliver the shock, or delivers the shock automatically.
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How does the electrical current work?
A waveform describes how the magnitude of current changes over time during the defibrillation shock.
Defibrillators can deliver energy in a variety of waveforms. The most common types can be broadly characterised as monophasic or biphasic.
Early defibrillators used monophasic waveforms, where the current flows in one direction from one electrode to the other to depolarise the heart, giving it the chance to re-start on its own.
Monophasic waveforms have largely been replaced by biphasic waveforms. Here, the direction of current depolarises heart cells in the first phase of the shock, and then repolarises the cells in the second phase.
Biphasic waveforms require less energy to deliver a clinically effective shock than monophasic waveforms, and consequently are less harmful to myocardial (heart muscle) tissue, resulting in less post-shock injury than monophasic waveforms.
Biphasic waveforms have also enabled the development of smaller, lighter devices with extended battery life, making them more portable.
What is the difference between energy and current?
Good question. The distinction between energy and current is important in defibrillation.
Defibrillators use energy to deliver a shock, and are often described in terms of energy (measured in Joules). That said, it is actually the current (measured in amplitude, or amps) that provides the defibrillation therapy (shock), not the energy itself.
Current has actually been found to be a better predictor of success for both monophasic and biphasic defibrillation than energy.
The amount of electrical energy required depends upon the voltage applied, and the magnitude and duration of the flow of current. This means that the amount of energy delivered to the patient is determined by how long the current flows.
Different AED models deliver different amounts of energy, but no research has identified the optimal energy levels for either monophasic or biphasic defibrillation.
So, where to from here?
Most biphasic defibrillators have one capacitor, releasing part of the energy stored to polarise the heart cells in Phase-1, and using a switch to truncate energy to reverse the polarity in Phase-2.
A dual-capacitor biphasic waveform that changes capacitors between phases can reduce the defibrillation threshold (DFT), and reduce the energy (Joules) required for clinically effective defibrillation.
An AED using two smaller capacitors to generate the biphasic waveform, instead of one large capacitor and a switch, would allow for the current to be split equally between the two phases using the same voltage and full tilt discharge of each capacitor.
This means that a higher Phase-2 current delivery could fully reverse the charge deposited from Phase-1, and restore the heart to ‘zero potential’ to better support a return to normal sinus rhythm.
