Defibrillation has been used to treat disturbances of the cardiac rhythm since the late 1800s.^1w1 In order to appreciate the mechanisms of defibrillation, one must also understand the basic electrophysiological principles of fibrillation.

ELECTROPHYSIOLOGICAL BASIS OF FIBRILLATION

Atrial and ventricular fibrillation are both based upon a re-entrant mechanism, a phenomenon that was first recognised almost 100 years ago and initially described by Mines and Garrey.^w1 Mines also suggested that conduction block was necessary to facilitate re-entry.^w1 It was not until the 1950s that Moe and colleagues advanced the circus movement theory of re-entry when they suggested the multiple wavelet hypothesis of atrial fibrillation (AF).² This proposed that AF occurred in the presence of inhomogeneous atrial tissue.² It was postulated that multiple wavelets propagated randomly through the atria. These waves of excitation were presumed capable of spreading through the atrial myocardium at a time when some of its components had recovered while others remained partially or fully refractory as a result of a preceding activation.² Thus AF could be maintained as a turbulent arrhythmia in a stable state for long periods of time. By 1985, after the development of high resolution electrode mapping systems, Allessie and colleagues provided evidence of multiple propagating wavelets that created turbulent atrial activity in the canine heart.² Subsequent experiments have proven Moe’s idea that multiple wavelets distributed randomly throughout the atria gave rise to the chaotic activation patterns observed on the ECG.²

More recent studies have further expanded on some of the experimental findings of Allessie and colleagues, suggesting that some forms of AF may actually be the result of high frequency activation by a single re-entrant source in some patients.² The drivers of AF may be relatively stationary “rotors” of electrical activity that are anchored to an unexcited (anatomical) …