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INTRODUCTION

Key Clinical Questions

  • image What are the indications for cardioversion?

  • image What is the difference between cardioversion and defibrillation?

  • image What is the difference between monophasic and biphasic cardioverter defibrillators?

  • image What are the indications for anticoagulation prior to cardioversion?

Electrical cardioversion is an effective, rapid, and safe technique that has become a routine procedure in the management of patients with cardiac arrhythmias. Cardioversion disrupts the abnormal electrical circuit(s) in the heart and restores a normal heartbeat. The shock causes a critical mass of cardiac myocytes to depolarize simultaneously, thereby interrupting and terminating the abnormal electrical rhythm. This split second interruption of the abnormal beat allows the heart’s electrical system to regain control and restore a normal rhythm. The delivery of energy during the shock is synchronized to the QRS complex, which represents cardiac depolarization, in order to terminate the arrhythmia safely and allow sinus rhythm to resume.

Defibrillation, on the other hand, delivers nonsynchronized energy to treat life-threatening arrhythmias such as pulseless VT or VF. Defibrillation is the treatment of choice for the arrhythmias most commonly associated with sudden cardiac arrest. The electrical shock delivered during cardioversion or defibrillation can be delivered externally to the heart using electrodes (external pads or paddles) placed on the chest (most common); directly to the heart using internal paddles during an open chest surgery; or through the electrodes of a permanently implanted cardioverter defibrillator (ICD).

Pharmacologic or chemical cardioversion uses antiarrhythmic medication instead of an electrical shock to restore the heart’s normal rhythm.

MECHANISM OF CARDIOVERSION

Electrical cardioversion appears most effective in terminating arrhythmias that arise from a single reentrant circuit such as AFL, atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reentrant tachycardia (AVRT), and monomorphic VT. The high-energy shock depolarizes the myocardium and the conduction tissue involved in the reentry circuit of the arrhythmia simultaneously. The depolarization of the myocardium is followed by a period of refractoriness that has the effect of interrupting the reentrant circuit, thereby breaking the repeating cycle and terminating the arrhythmia. When the reentrant circuit is broken and the arrhythmia stops, the sinus node begins to fire again and a normal heart rhythm is restored. This mechanism of action does not explain how electrical cardioversion terminates AF, which typically arises from multiple reentrant circuits. As a result of the multiple circuits, successful cardioversion of AF often requires higher energy (eg, 120 J) for termination. Other arrhythmias, such as junctional tachycardia and multifocal atrial tachycardia, originate from ectopic sites (ie, nonpacemaker) in the heart. Cardioversion is not effective for these types of arrhythmias.

TYPES OF DEFIBRILLATORS

Cardioverter defibrillators deliver energy in a variety of waveforms, broadly characterized as monophasic or biphasic. Monophasic devices provide a unidirectional pulse of energy, meaning that electrons flow in a single direction through the heart. Monophasic energy is highly effective, and monophasic cardioverter defibrillators remain widely used. Alternatively, ...

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