In the case of unconsciousness due to tachyarrhythmia, countershock 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 in order to decrease the risk of aspiration. Constant heart rhythm monitoring should be used throughout the procedure and a 12-lead electrocardiogram should be obtained before and after the countershock.

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 frequent 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 conscious sedation should be followed, including frequent assessment of blood pressure and pulse oximetry. Supplemental oxygen is delivered via nasal cannula, face mask, or—in the case of heavier sedation—AmbuBag. The goal of sedation should be minimal or no response to verbal stimulus. In such a state, a patient may cry out during the actual cardioversion but will nonetheless usually have no recollection of the procedure after recovery.

Defibrillators that employ biphasic waveforms have largely replaced those utilizing monophasic waveforms. The chief advantage of biphasic waveforms is a lower defibrillation threshold, meaning shocks using biphasic waveforms require less energy to achieve defibrillation. It is thought that the first phase of the biphasic waveform stimulates the myocardium to defibrillate while the second phase lowers the defibrillation threshold, although the mechanism is not well understood [8]. Biphasic truncated exponential waveform and biphasic rectilinear waveform are both commercially available, with the former being more common. Randomized trials comparing the two types of waveforms in the cardioversion of atrial fibrillation have failed to show any significant difference in efficacy [9, 10 and 11].

The efficacy of biphasic shocks in the termination of VF has been well established [12, 13]. Furthermore, clinical studies of elective cardioversion have established the superiority of biphasic over monophasic waveform shocks [14, 15]. For instance, one study demonstrated the equivalent efficacy of a 120 J to 200 J biphasic sequence with a 200 J 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 coated with conductive gel were the sole type of electrode used to deliver countershock. Self-adhesive pads have become more common in the past few years, although paddles are still used, especially in emergent cases. 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], and by delivering the shock during expiration [20].

The optimal anatomic placement of pads and paddles is controversial. Anterior-lateral and anterior-posterior placements are both acceptable (Fig. 6-1). The anterior paddle is placed on the right infraclavicular chest [21]. 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 [22]. 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 [23]. It is thought that anterior-posterior positioning directs more of the delivered energy to the atria than anterior-lateral placement. Since it has been shown that only 4% of the current flow from shock reaches the myocardium with the anterior-lateral position [24], any method that directs more energy to the atria should be beneficial. However, a study using defibrillators employing a biphasic waveform suggested pad position was not associated with cardioversion success [25].