A monophasic defibrillator is a type of defibrillator that delivers a high-energy electrical shock in a single direction through the heart, from one electrode pad to the other. The term ‘monophasic’ (meaning ‘one phase’ or ‘one direction’) distinguishes this older technology from biphasic defibrillators, which deliver a current that first flows in one direction and then reverses, flowing back in the opposite direction.
Monophasic defibrillators were the standard technology for decades and were widely used in hospitals and by emergency services from the 1960s onwards. They are effective at terminating ventricular fibrillation and other shockable arrhythmias, but require higher energy levels (typically 360 joules) compared to biphasic devices to achieve the same efficacy.
Biphasic defibrillators, which deliver the same or better defibrillation success at lower energy levels (typically 150 to 200 joules), have largely replaced monophasic technology in both hospital and community settings. The lower energy requirements of biphasic devices reduce myocardial injury from the shock, improve first-shock success rates, and allow for smaller, lighter portable defibrillators and AEDs.
Monophasic defibrillators may still be encountered in some older hospital settings and are referenced in older resuscitation guidelines and clinical literature. In current UK and European Resuscitation Council guidance, biphasic waveform defibrillation is the standard recommended approach.
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