Overview
Delivery of direct current (DC) shocks to the heart has long been used successfully to convert abnormal heart rhythms back to normal sinus rhythm. In 1775, Abildgaard reported using electricity to both induce and revive a hen from lifelessness.
Beck was the first physician to use DC defibrillation on a human to treat ventricular fibrillation (VF, vfib) in a 14-year-old during cardiac surgery in 1947. Fifteen years later, Lown applied synchronized DC shocks to the heart to convert atrial fibrillation (AF, afib) and ventricular tachycardia (VT, vtach) to normal sinus rhythm.
Cardioversion is defined as a “synchronized DC discharge, and … does not apply to ventricular defibrillation or to the pharmacologic reversion of arrhythmias.”
It is typically used to terminate life-threatening or unstable tachycardic arrhythmia (unstable ventricular and supraventricular rhythms) in patients who still have a pulse but who are hemodynamically unstable.
The DC electrical discharge is synchronized with the R or S wave of the QRS complex. Synchronization in the early part of the QRS complex avoids energy delivery near the apex of the T wave in the surface electrocardiogram (ECG), which coincides with a vulnerable period for induction of ventricular fibrillation. The peak of the T wave represents the terminal portion of the refractory state when adjacent heart fibers are in differing states of repolarization.
Defibrillation refers to an unsynchronized discharge of energy and is only recommended for ventricular fibrillation. It is used in patients who are in cardiac arrest.
Basic principles
Transient delivery of an electrical current causes a momentary depolarization of most cardiac cells, thereby allowing the sinus node to resume normal pacemaker activity. In the presence of a reentrant-induced arrhythmia, such as paroxysmal supraventricular tachycardia (PSVT) and ventricular tachycardia (VT), electrical cardioversion interrupts the self-perpetuating circuit and restores a sinus rhythm. Electrical cardioversion is much less effective in treating arrhythmia caused by increased automaticity (eg, digitalis-induced tachycardia, catecholamine-induced arrhythmia) because the mechanism of the arrhythmia remains after the arrhythmia is terminated and therefore is likely to recur.
At present, two types of defibrillators are in use for external cardioversion and defibrillation: a monophasic sinusoidal waveform (positive sine wave) and a biphasic truncated waveform. In 1997, a low-energy, impedance-compensating biphasic waveform was evaluated for atrial and ventricular arrhythmia management. This defibrillator automatically adjusted to the patient’s transthoracic impedance, which is a feature that the monophasic models lack. Currently, there are several biphasic devices with similar efficacy available on the market.
The more recent use of biphasic cardioversion has shown that less energy is required to convert an arrhythmia to a normal sinus rhythm than with monophasic cardioversion. Consequently, use of biphasic energy results in fewer delivered shocks to the patient and less cumulative energy delivered. Potential benefits include fewer burn wounds, less tissue damage, and reduced damage to the heart muscle than is found with higher voltage shocks.