External direct current (DC) cardioversion refers to the application of a synchronized electrical shock across a patient’s chest using a defibrillator. The aim being to convert abnormal tachyarrhythmias back into sinus rhythm. Anaesthetists are often requested to provide anaesthesia for cardioversion because it is an extremely stimulating and painful procedure, which requires an appropriate depth of anaesthesia to be tolerated without adverse psychological consequences.1 This may involve emergency scenarios in unfamiliar locations such as Coronary Care Units (CCUs) and Emergency Departments, as well as the more commonly performed elective case. Anaesthetists may also need to perform cardioversion themselves in theatre or Critical Care settings. An understanding of both the theoretical and practical aspects of this procedure are essential to perform cardioversion safely and hence form the basis of this review. The early 20th century saw a rapid expansion of the electrical power industry and accidents associated with electrocution. It soon became apparent that most deaths were due to ventricular fibrillation (VF). Ironically research at the time suggested that alternating current (AC) was more effective at terminating VF than DC. As a result the earliest defibrillators in the USA utilized AC shocks. In the USSR defibrillator development followed a different path, largely due to the work of Naum Gurvich.2 In 1939, Gurvich and Yuniev proposed using a single capacitor discharge to defibrillate VF. By 1952 Gurvich had designed the first commercially available transthoracic DC defibrillator that became rapidly utilized throughout the Soviet Union. It was only 10 yr later in 1962 that Lown reported the successful use of DC cardioversion to treat cardiac arrhythmias which revolutionized practice in the USA and elsewhere. ### Simple defibrillator function Modern defibrillators have become far more sophisticated than simple circuit diagrams suggest. They still rely on a power source which charges a capacitor, generating a potential difference …
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