Physical principles of defibrillators

Abstract 

Defibrillation is the only effective, and therefore life-saving, treatment for ventricular fibrillation. Defibrillators are devices that store and then discharge pre-programmed quantities of electrical energy through the heart to synchronously depolarize myocytes and allow return of sinus rhythm. Energy is supplied from batteries and is stored in a capacitor before being discharged through leads to electrodes and then across the heart either directly or via the chest. The ratio of charge stored to potential difference is known as capacitance and will determine the energy discharged by the defibrillator. Capacitance is equal to charge/voltage, and energy is equal to charge × voltage. Inductors and other electronic components are used to ensure that the waveform of the discharge is of optimal shape and duration. Most modern defibrillators use a biphasic waveform, which has a positive and a negative component and reduces the energy required for defibrillation. Patient factors, including thoracic and cardiac impedance, will also determine the energy delivered. Defibrillators can be external (transthoracic) or implantable. Owing to the energy used in defibrillators, care must be taken to prevent electrocution of medical staff and burns to the patient.

Keywords: biphasic waveform, capacitance, defibrillation, defibrillator, thoracic impedance, ventricular fibrillation