Cardioversion and Defibrillation
Mark S. Link
Naomi F. Botkin
The use of electric shock to terminate arrhythmia is one of the critical findings of the last century and underlies much of the modern treatment of arrhythmias. Thanks to the pioneering work of Zoll et al. [1] and Lown et al. [2] in the second half of the twentieth century, the use of electric shock gained widespread acceptance. Although incorporating the same mechanism and physics, Cardioversion refers to the use of direct-current electric shock to terminate arrhythmias other than ventricular fibrillation, while Defibrillation refers to the termination of ventricular fibrillation. Cardioversion shocks are synchronized to the QRS to avoid the initiation of ventricular fibrillation which may result from shocks on the T-wave while defibrillation occurs with unsynchronized shocks.
Physiology of Arrhythmia and Shock
Arrhythmias may be due to reentry, increased automaticity, or triggered activity. Reentry refers to the phenomenon in which a wave of excitation travels repeatedly over a closed pathway or circuit of conduction tissue. Reentry requires slow conduction in a portion of myocardium so that by the time the impulse exits the slowly conducting portion the remaining myocardium has repolarized and is hence able to be depolarized again.
Many of the commonly encountered arrhythmias are due to a fixed reentrant mechanism, including atrial flutter, atrioventricular (AV) nodal reentrant tachycardia (AVNRT), AV reentrant tachycardia (AVRT), and most ventricular tachycardias. Atrial fibrillation, once thought exclusively reentrant, has been shown to be caused by foci in the pulmonary veins in many individuals [3]. Atrial fibrillation may also be secondary to functional reentry. Ventricular fibrillation is also due to functional reentry. Cardioversion and defibrillation terminate these arrhythmias by simultaneously depolarizing all excitable tissue, disrupting the process of reentry.
Arrhythmias may also be due to disorders of impulse formation (increased automaticity or triggered activity). These include sinus tachycardia, focal atrial tachycardia, and idiopathic ventricular tachycardias. Sinus tachycardia is a physiologic response and not a pathologic tachycardia; thus, sinus tachycardia will not respond to cardioversion, but atrial tachycardias and ventricular tachycardias generally will terminate.
Insight into the effect of shock on fibrillating myocardial cells has grown in the past few decades. Although it was initially thought that all activation fronts had to be terminated simultaneously to stop atrial and ventricular fibrillation [4], it is now believed that if the vast majority of myocardium is silenced, the remaining mass is insufficient to perpetuate the arrhythmia [5]. The effect of shock on fibrillating myocardium is complex and is dependent on multiple factors including energy, waveform, and myocardial refractory state [6]. Electric shocks at low energy levels may fail to terminate atrial and ventricular fibrillation [7]. Atrial and ventricular arrhythmias may also be terminated by the shock and then reinitiated shortly thereafter. And finally, ventricular fibrillation can be triggered in patients not already in this rhythm if shock occurs on the vulnerable portion of the T wave. Thus, synchronization of shocks with the R wave will minimize the risk.
Indications and Contraindications
Cardioversion and defibrillation are performed for a variety of reasons in the intensive care setting. In the case of hemodynamic instability due to tachyarrhythmia of nearly any type, the urgent use of shock is strongly indicated. One must be careful,
however, not to shock sinus tachycardia, which is commonly present in patients who are hypotensive for noncardiac reasons. Acute congestive heart failure and angina that are secondary to an acute tachyarrhythmia are also indications for urgent cardioversion; however, there is usually sufficient time to provide some sedation. Care must be taken not to shock tachycardias that are secondary to the heart failure or chest pain. In the absence of hemodynamic instability or significant symptoms, cardioversion is usually considered elective and the risks and benefits of the procedure must be carefully weighed.
however, not to shock sinus tachycardia, which is commonly present in patients who are hypotensive for noncardiac reasons. Acute congestive heart failure and angina that are secondary to an acute tachyarrhythmia are also indications for urgent cardioversion; however, there is usually sufficient time to provide some sedation. Care must be taken not to shock tachycardias that are secondary to the heart failure or chest pain. In the absence of hemodynamic instability or significant symptoms, cardioversion is usually considered elective and the risks and benefits of the procedure must be carefully weighed.
Extreme caution should be exercised in patients with digitalis toxicity or electrolyte imbalance because of the increased risk of ventricular tachycardia or fibrillation after being shocked. Patients with severe sinus node disease may exhibit significant bradyarrhythmia after cardioversion from atrial fibrillation. In addition, patients who have been in atrial fibrillation for greater than 48 hours are at risk for thromboembolism after cardioversion; appropriate measures should be taken to minimize this risk (see later).
Clinical Competence
A clinical competence statement by the American College of Cardiology and American Heart Association outlines the cognitive and technical skills required for the successful and safe performance of elective external cardioversion (Table 6.1). A minimum of eight cardioversions should be supervised before a physician is considered competent to perform the procedure independently. In addition, a minimum of four procedures should be performed annually to maintain competence [8].
Methods
Patient Preparation
In the case of unconsciousness due to tachyarrhythmia, the shock must be performed urgently. In more elective settings, patient safety and comfort become paramount. As with any procedure, informed consent should be obtained. Patients should refrain from eating and drinking for several hours to decrease the risk of regurgitation and aspiration. Constant heart rhythm monitoring should be used throughout the procedure and a 12-lead electrocardiogram should be obtained before and after the shock.
Table 6.1 Cognitive and Technical Skills Necessary for Performing External Cardioversion | ||
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Medications with rapid onset and short half-life are favored for achieving analgesia, sedation, and amnesia. The combination of a benzodiazepine, such as midazolam, and a narcotic, such as fentanyl, is a common choice in the absence of anesthesiology assistance. Propofol is often used when an anesthesiologist is present to assist with airway management and sedation. Existing hospital policies for monitoring during moderate sedation should be followed, including frequent assessment of blood pressure and pulse oximetry. Supplemental oxygen is delivered via nasal cannula or face mask.
Shock Waveforms
Defibrillators that employ biphasic waveforms have largely replaced those using monophasic waveforms. Advantages of biphasic waveforms are lower defibrillation thresholds, meaning shocks using biphasic waveforms require less energy to achieve defibrillation [6], and they are less likely to cause skin burns and myocardial damage. Both biphasic truncated exponential waveform and biphasic rectilinear waveform are commercially available, with the former being more common. Randomized trials comparing the two types of biphasic waveforms in the cardioversion of atrial fibrillation have failed to show any significant difference in efficacy [9,10,11].
The efficacy of biphasic shocks in the termination of ventricular fibrillation has been well established [12,13]. Furthermore, clinical studies of atrial fibrillation cardioversion have established the superiority of biphasic over monophasic waveform shocks [14,15]. For instance, one study demonstrated the equivalent efficacy of a 120 to 200 J biphasic sequence with a 200 to 360 J monophasic sequence [15]. Biphasic waveforms allow fewer shocks to be given and a lower total energy delivery
[14]. Whether or not this translates into a significant clinical advantage remains to be demonstrated. However, there is evidence that biphasic shocks result in less dermal injury [14]. Although an animal model suggested better maintenance of cardiac function after biphasic shocks [16], human data on myocardial function are unavailable.
[14]. Whether or not this translates into a significant clinical advantage remains to be demonstrated. However, there is evidence that biphasic shocks result in less dermal injury [14]. Although an animal model suggested better maintenance of cardiac function after biphasic shocks [16], human data on myocardial function are unavailable.
 Figure 6.1. A: Self-adhesive defibrillator pads in the anterior and lateral positions. B: Self-adhesive defibrillator pad in the posterior position. When posterior positioning is used, the second pad is placed anteriorly. |
Electrodes
Until recently, hand-held paddles were the only available means of cardioversion or defibrillation. Self-adhesive pads have become more common in the past few years, although paddles may still be used. Limited data are available comparing the two modalities, but one study suggested the superiority of paddles over pads in cardioverting atrial fibrillation [17]). This phenomenon might be explained by the lower transthoracic impedance achieved with paddles [18]. Whichever modality is used, impedance can be minimized by avoiding positioning over breast tissue, by clipping body hair when it is excessive [19], by delivering the shock during expiration, and by firm pressure on the pads or paddles.
The optimal anatomic placement of pads and paddles is controversial; however, the general principal holds that the heart must lie between the two electrodes [6]. Both anterior–lateral and anterior–posterior placements are acceptable (Fig. 6.1). The anterior paddle is placed on the right infraclavicular chest. In anterior–lateral placement, the lateral paddle should be located lateral to the left breast and should have a longitudinal orientation, since this results in a lower transthoracic impedance than horizontal orientation [20]. When anterior–posterior positioning is used, the posterior pad is commonly located to the left of the spine at the level of the lower scapula, although some physicians favor placement to the right of, or directly over, the spine. There are data to suggest that anterior–posterior placement is more successful in the cardioversion of atrial fibrillation than anterior–lateral positioning when monophasic waveforms are used [21]. It is thought that anterior–posterior positioning directs more of the delivered energy to the atria than anterior–lateral placement. However, a study employing biphasic waveforms failed to show any difference of success with anterior–lateral compared with anterior–posterior pad positions [22].
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