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.

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.

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.

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].