7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Basic principles and technique of external electrical cardioversion and defibrillation : Bradley P Knight, MD, FACC : Richard L Page, MD : Nisha Parikh, MD, MPH All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jun 2023. This topic last updated: Jan 23, 2023. INTRODUCTION Electrical cardioversion and defibrillation have become routine procedures in the management of patients with cardiac arrhythmias. Cardioversion is the delivery of energy that is synchronized to the QRS complex, while defibrillation is the nonsynchronized delivery of a shock randomly during the cardiac cycle. In 1956, alternating current (AC) for transthoracic defibrillation was first used to treat ventricular fibrillation in humans [1]. Following this breakthrough, in 1962 direct current (DC) defibrillators were introduced into clinical practice [2]. Subsequent studies in the early 1960s demonstrated that electrical countershock or cardioversion across the closed chest could abolish other cardiac arrhythmias in addition to ventricular fibrillation [3-5]. This topic will review the basic principles and technique of electrical cardioversion and defibrillation. The clinical indications for these procedures, procedural sedation, potential side effects, and the use of the automated external defibrillator (AED) are discussed separately. (See "Cardioversion for specific arrhythmias" and "Procedural sedation in adults: General considerations, preparation, monitoring, and mitigating complications" and "Procedural sedation in children: Approach" and "Automated external defibrillators".) BACKGROUND INFORMATION ON DEFIBRILLATORS Most defibrillators are energy-based, meaning that the devices charge a capacitor to a selected voltage and then deliver a prespecified amount of energy in joules. The amount of energy that https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 1/25 7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate arrives at the myocardium is dependent upon the selected voltage and the transthoracic impedance (which varies by patient). Two other types of defibrillators are used less frequently in clinical practice: Impedance-based defibrillators allow selection of transthoracic current based upon the transthoracic impedance; the latter is assessed initially with a test pulse with the capacitor subsequently charged to the appropriate voltage [6]. Current-based defibrillation, in which a fixed dose of current is delivered, results in defibrillation thresholds that are independent of transthoracic impedance and are invariant for the individual [7,8]. Defibrillation success for a current-based method is independent of both transthoracic impedance and body weight [9]. Furthermore, this method achieves defibrillation with considerably less energy than does the conventional energy-based method. Defibrillators can also deliver energy in a variety of waveforms, broadly characterized as monophasic or biphasic ( figure 1). Initially defibrillators delivered only monophasic waveforms. Although monophasic defibrillation is highly effective, biphasic defibrillation terminates arrhythmias more consistently and at lower energy levels. Biphasic defibrillators have largely replaced monophasic defibrillators. (See 'Monophasic versus biphasic waveforms' below.) ELECTROPHYSIOLOGY OF DEFIBRILLATION AND CARDIOVERSION Cardioversion terminates arrhythmias by delivering a synchronized shock that depolarizes the tissue involved in a reentrant circuit ( figure 2 and figure 3). By depolarizing all excitable tissue of the circuit and making the tissue refractory, the circuit is no longer able to propagate or sustain reentry. As a result, cardioversion terminates those arrhythmias resulting from a single reentrant circuit, such as atrial flutter, atrioventricular nodal reentrant tachycardia, atrioventricular reentrant tachycardia, or monomorphic ventricular tachycardia. (See "Reentry and the development of cardiac arrhythmias".) Despite its widespread clinical use, controversy remains concerning the electrophysiologic mechanisms by which electrical cardioversion or defibrillation terminates atrial or ventricular fibrillation, arrhythmias which involve multiple microreentrant circuits. Fibrillation involves the entire atrial or ventricular myocardium and is considered to be a very persistent rhythm. (See "The electrocardiogram in atrial fibrillation" and "Pathophysiology and etiology of sudden cardiac arrest", section on 'Mechanism of ventricular fibrillation'.) https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 2/25 7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Most investigators agree that defibrillation occurs when a certain amount of current density reaches the myocardium. However, it is unclear what amount of current density is needed and what energy setting is necessary to achieve a homogeneous current density. At the cellular level, the delivered current flows around and through the myocardial cells, resulting in alteration of the transmembrane potentials [10]. At the organ level, the mechanisms responsible for termination of fibrillation are still controversial [11]. Two explanations have been proposed, which are not necessarily mutually exclusive: the critical mass hypothesis and the upper limit of vulnerability. Critical mass hypothesis There is general agreement that total elimination of fibrillatory activity can be achieved with relatively high defibrillation energy levels [12,13]. According to the critical mass hypothesis, a certain amount of myocardium must be available to sustain atrial or ventricular fibrillation and the entire myocardium must be uniformly depolarized in order to terminate the arrhythmia [14]. Electrophysiological evidence to support this theory has been obtained using a computerized mapping system that recorded simultaneous electrograms from 120 sites [12]. Defibrillation was successful only when fibrillatory activity was annihilated at all sites. Upper limit of vulnerability hypothesis According to the upper limit of vulnerability hypothesis, low-energy shocks can induce fibrillation up to a limit where further increases in energy do not induce fibrillation, and the largest shocks that do not induce fibrillation are slightly weaker than necessary for defibrillation [15]. Although shocks abolish small areas of localized reentry during ventricular fibrillation, shocks at strengths below the upper limit of vulnerability stimulate other regions of myocardium during their vulnerable period, giving rise to new areas of local reentry that reinitiate ventricular fibrillation [16-18]. To successfully defibrillate, therefore, the shock strength must be greater than the largest shock that reinitiates fibrillation (the upper limit of vulnerability). This hypothesis is supported by the observation that almost identical changes in the upper limit of vulnerability and the defibrillation threshold occur with changes in electrode polarity and waveform duration [19]. Transcardiac shocks of up to 30 joules were delivered to canine myocardial tissue at different times of the cardiac cycle [20]. A shock applied 10 milliseconds before the end of the refractory period extended the refractory period by 63 milliseconds, whereas the same shock given 40 milliseconds earlier produced only a 25-millisecond increase in the refractory period. In contrast, a five-joule shock applied at the same times during the cardiac cycle produced only a 36- and 10- millisecond increase in the refractory period, respectively. https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 3/25 7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Because the higher intensity shock selectively extends the refractory period of the myocardium, it may establish a zone of tissue in which repolarization is delayed. This zone is always immediately in front of the newly depolarized tissue, and the inability of the depolarizing impulse to propagate across these refractory zones may be the mechanism responsible for terminating fibrillation. The size of these zones and the duration of increased refractoriness both increase with increasing shock intensity; this may account for the relatively high shock energies required for defibrillation relative to the intensities required to directly depolarize the tissue. An electrical shock will also induce different degrees of action potential duration prolongation and dispersion of ventricular repolarization, depending upon shock strength and timing [21]. One study, performed during the post-implantation testing of implantable cardioverter- defibrillators, found that shocks delivered on the T wave generated variable degrees of action potential prolongation, depending upon myocardial repolarization [22]. The creation of high dispersion of repolarization facilitated reentry by creating functional blocks and subsequently favored the induction of ventricular fibrillation and the prevention of its termination by a shock. Defibrillation and cardioversion do not appear to cause significant myocardial necrosis [23]. (See "Cardioversion for specific arrhythmias", section on 'Myocardial necrosis'.) FACTORS AFFECTING DEFIBRILLATION AND CARDIOVERSION SUCCESS A variety of device-related and patient-related factors will influence the chances of successful cardioversion and/or defibrillation. The device-related variables include factors related to the electrodes (ie, position, size, hand-held versus adhesive patch) and factors related to the energy delivered (ie, number of joules, type of waveform), while the patient-related variables include the transthoracic impedance through which the energy much travel as well as the type and duration of arrhythmia. Device-related variables Electrodes A number of electrode characteristics can affect the outcome of cardioversion. These include electrode position, pad size, and hand-held versus patch electrodes. Electrode position The placement of defibrillation electrodes on the thorax determines the transthoracic current pathway for external defibrillation. There are two conventional positions for electrode placement ( figure 4): Anterolateral orientation Anteroposterior orientation https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 4/25 7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate In some patients, one (but not the other position) may be effective. As a result, if initial attempts are unsuccessful in terminating the arrhythmia, the electrodes should be relocated to the other position and attempts at defibrillation or cardioversion repeated. Several studies have suggested that less energy is required and the success rate is higher with the anteroposterior electrode position in patients cardioverted for atrial fibrillation [24-27]. As an example, in one study of 301 patients, sinus rhythm was restored in 87 percent using an anteroposterior position compared with 76 percent with an anterolateral orientation [25]. In the same study, the mean energy required for successful cardioversion was lower in patients with an anteroposterior electrode position (237 versus 287 joules) [25]. There is also evidence that anteroposterior pad position may be effective in treating refractory ventricular fibrillation [28]. The supporting study is described in detail separately. (See "Adult basic life support (BLS) for health care providers", section on 'Defibrillation'.) However, other reports have failed to confirm these findings and suggest that there is no clear advantage to either electrode configuration [29-31]. In a contemporary randomized trial of patients presenting to the emergency department with acute atrial fibrillation who underwent cardioversion, rates of successful cardioversion to sinus rhythm for at least 30 minutes were similar regardless of electrode position (94 percent with anterolateral placement versus 92 percent with anteroposterior placement) [32]. Pad size Electrode pad size is an important determinant of transthoracic current flow during external countershock [29,33-35]. One study analyzed the outcome of cardiac arrests in 105 patients using self-adhesive defibrillator pads of different sizes [35]. A single shock of 200 joules was successful in 31 percent of cardiac arrests using two small pads (each 8 cm in diameter), in 63 percent with one small and one large pad (8 and 12 cm in diameter), and in 82 percent when two large pads (each 12 cm in diameter) were used. A larger pad or paddle surface is associated with a decrease in resistance and increase in current and may cause less myocardial necrosis [36-38]. However, there appears to be an optimal electrode size (approximately 12.8 cm) above which any further increase in electrode area causes a decline in current density [39]. Hand-held versus patch The use of hand-held paddle electrodes may be more effective than self-adhesive patch electrodes. This was illustrated in a randomized trial of 201 patients referred for cardioversion of persistent atrial fibrillation [40]. Success rates were slightly higher for patients assigned to paddle electrodes (96 versus 88 percent with patch electrodes). Improved electrode-to-skin contact and reduced transthoracic impedance with hand-held electrodes may explain the benefit. https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 5/25 7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate However, there are no published data comparing hand-held paddle electrodes with self-adhesive patch electrodes for other arrhythmias requiring cardioversion (eg, atrioventricular nodal reentrant tachycardia, atrial flutter) or defibrillation (eg, ventricular fibrillation). Therefore, the decision to use hand-held or self-adhesive electrodes should be made based on the available equipment and the opinion of the operator regarding which electrodes are more likely to be effective for the patient at hand. Monophasic versus biphasic waveforms Defibrillators can deliver energy in a variety of waveforms that are broadly characterized as monophasic or biphasic. Defibrillators developed prior to 2000 deliver a monophasic wave of direct electrical current. Since then, "biphasic" devices, which reverse current polarity 5 to 10 milliseconds after discharge begins, have been developed ( figure 1). Biphasic waveforms defibrillate more effectively and at lower energies than monophasic waveforms. This benefit has been demonstrated in both animals and humans, and with both ventricular and atrial fibrillation [41-50]. However, monophasic defibrillation is still highly effective in most situations, and it is not clear that the superior efficacy of biphasic defibrillation results in important clinical advantages [51]. Ventricular fibrillation Several randomized trials have compared monophasic and biphasic waveforms in the treatment of ventricular fibrillation. In the Optimized Response to Cardiac Arrest (ORCA) trial, 115 patients with an out-of- hospital cardiac arrest due to ventricular fibrillation were randomly assigned to defibrillation using a 150 joule biphasic shock or traditional high-energy (200 to 360 joules) monophasic shocks [46]. Successful defibrillation was significantly more likely with biphasic waveforms compared with monophasic waveforms after one shock and total treatment under Emergency Medical Services care (96 versus 59, and 100 versus 84 percent respectively). In addition, the rate of return of spontaneous circulation was higher with biphasic shock therapy (76 versus 54 percent). However, there was no difference in the rate of survival to hospital discharge between the two therapies. Among patients who survived to discharge, those treated with a biphasic shock were more likely to have good cerebral performance (87 versus 53 percent). In the Out-of-hospital cardiac arrest rectilinear biphasic to monophasic damped sine defibrillation waveforms with advanced life support intervention trial (ORBIT) of 169 patient with out-of-hospital cardiac arrest, biphasic shocks (escalating 120, 150, or 200 joules) were more effective than monophasic shocks (escalating 200, 300, and 360 joules) as defined by conversion at 5 sec to an organized rhythm (52 versus 34 percent) [52]. However, there https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 6/25 7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate were no significant differences in return of spontaneous circulation (47 percent in both groups) or survival to discharge from hospital (9 versus 7 percent). In the Transthoracic Incremental Monophasic versus Biphasic defibrillation by Emergency Responders (TIMBER) trial, 168 patients with an out-of-hospital cardiac arrest due to ventricular fibrillation were randomly assigned to treatment with either monophasic or biphasic defibrillation [53]. Defibrillation was initially performed by Emergency Medical Service with an automated external defibrillator (AED), and if necessary, with a manual defibrillator by paramedics who arrived later. Patients were included only if all shocks were delivered with the same waveform. In contrast to the ORCA trial, in TIMBER the defibrillation energies were the same for both waveforms (200 followed by 200 and then 360 joules). There were no statistically significant differences between the treatment arms with regard to the success of initial shocks, the rate of survival, or neurologic outcomes. However, biphasic defibrillation did result in nonsignificant trends towards earlier return of spontaneous circulation and increased overall survival (41 versus 34 percent, compared with monophasic defibrillation). There is no clear explanation for the discrepant findings in these three trials. Due to the complexities and urgencies associated with resuscitation, in combination with the small sample sizes available for controlled studies, it is difficult to demonstrate or exclude important clinical benefits. Based upon the greater efficacy of biphasic defibrillation demonstrated in other settings, the lack of evidence of harm from biphasic defibrillation, and the trends towards outcome benefits suggested by clinical trials, we support the use of biphasic defibrillation for the treatment of ventricular arrhythmias. Atrial fibrillation The benefit of the biphasic waveform for the treatment of atrial fibrillation has been illustrated in two randomized trials: In one trial, 174 patients with AF were randomly assigned to cardioversion with a monophasic waveform, using sequential shocks of 100 to 360 joules, or biphasic waveform, with energies of 70 to 170 joules [48]. First shock efficacy was greater with a biphasic waveform (68 versus 21 percent), delivered energy was 50 percent less, and the overall cardioversion rate was higher (94 versus 79 percent). Similar benefits were noted in a comparable randomized trial of 210 patients [49]. Biphasic waveforms were associated with the following significant benefits: greater first shock efficacy (60 versus 22 percent); fewer total shocks (1.7 versus 2.8); less energy delivered (217 versus 548 joules), and a lower frequency of dermal injury (17 versus 41 percent). https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 7/25 7/6/23, 2:48 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate In a larger nonrandomized experience, 2546 patients with AF were cardioverted using a monophasic (1996 to 1999) or biphasic (1999 to 2001) defibrillator [50]. With the biphasic waveform, the overall success of cardioversion was greater (99.1 versus 92.4 percent) and the energy level required for cardioversion was lower (median 100 versus 200 joules). Similar findings have been reported for patients with atrial flutter, in whom cardioversion was successful more frequently and at lower energy levels when using biphasic waveforms [54,55]. Optimal biphasic defibrillation energies Because biphasic shocks are more effective at lower energies than monophasic shocks, initial protocols often suggested using lower-energy biphasic shocks during resuscitation (eg, 150 joules). The value of higher energy biphasic shocks was demonstrated in the BIPHASIC trial [56]. A series of 221 patients with out-of-hospital cardiac arrest were randomly assigned to either fixed lower energy defibrillation (150 joules, followed by up to two additional shocks at the same energy as necessary), or to escalating higher-energy shocks (200 joules, followed by 300 and 360 joule shocks as necessary). Shocks were administered by an AED. The following results were noted: The rate of successful defibrillation with the first shock was the same in both groups (37 versus 38 percent in the escalating higher and fixed lower treatment groups, respectively). Among patients requiring multiple shocks (n = 106), patients assigned to the escalating higher-energy protocol were significantly more likely to be successfully cardioverted (37 versus 25 percent, compared with patients assigned to the fixed lower-energy protocol). There were no significant differences between regimens in adverse events including cardiac enzyme elevation and left ventricular systolic dysfunction. There were no differences between regimens in survival outcomes, although this could be due in part to the relatively small sample size. Based upon the absence of adverse events and the greater efficacy in patients requiring multiple shocks, we agree with the use of escalating higher-energy shocks with biphasic defibrillators in the treatment of cardiac arrest due to a ventricular tachyarrhythmia that does not respond to an initial lower energy shock. Double sequential external defibrillation Double sequential external defibrillation (DSED) consists of rapid sequential shocks from two defibrillators; this has been shown to be effective at terminating refractory ventricular fibrillation [28]. The supporting study is described in detail separately. (See "Adult basic life support (BLS) for health care providers", section on 'Defibrillation'.) https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 8/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate If more studies are supportive of DSED for patients with refractory ventricular fibrillation, this could be a strategy employed in a hospital setting. Patient-related variables Transthoracic impedance To compensate for transthoracic impedance during transthoracic defibrillation, a considerably larger current must be delivered to the thorax than is required for internal defibrillation. Impedance results in the dissipation of energy due to shunting to the lungs, the thoracic cage, and other elements of the chest. In an animal study, 82 percent of the transthoracic current was shunted to the thoracic cage, 14 percent to the lungs, and only 4 percent passed through to the heart [57]. Transthoracic impedance is determined by multiple factors including: Energy level Electrode-to-skin interface Interelectrode distance Electrode pressure (with hand-held electrodes) Phase of ventilation Myocardial tissue and blood conductive properties In animals, repetitive shocks delivered at three minute intervals decrease the transthoracic impedance to a greater extent than repetitive shocks delivered at 15 or 60 second intervals [58,59]. Similar trends have been reported in humans, but the changes in impedance were not found to be clinically relevant [59,60]. With repeated shocks, impedance decreased by as much as 8 percent and the peak current increased by 4 percent [61]. Therefore, despite only a minimally larger amount of energy being delivered to the heart, an unsuccessful shock should be followed promptly by a higher energy shock. An animal study examined the mechanisms responsible for the decline in transthoracic impedance after direct current (DC) shocks [62]. After three 100-joule shocks, there was an 11 percent decline in transthoracic impedance which correlated with a tenfold increase in skeletal muscle blood flow. This observation suggests that tissue edema contributes to the DC shock- induced decline in transthoracic impedance. A second report evaluated the effect of prior sternotomy on transthoracic impedance [63]. Transthoracic impedance decreased after sternotomy and remained below preoperative measurements even after wound healing was complete, suggesting that the hyperemia, inflammation, tissue edema, and pleural effusion associated with sternotomy were the major contributors to the reduction in impedance. https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 9/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate The phase of ventilation is another factor that alters transthoracic impedance. Inspiration (and the increased volume of air within the lungs) is associated with a 13 percent higher transthoracic impedance than expiration [64]. The composition of the gel used during cardioversion also affects the transthoracic impedance. In a study comparing a non-salt-containing gel with a salt-containing gel, transthoracic impedance was 20 percent higher with the non-salt-containing gel [65]. Type of arrhythmia The type of arrhythmia and the patient's clinical condition are important determinants of defibrillation success [66-68]. As an example, patients with ventricular fibrillation as the primary event are easier to defibrillate than patients with secondary ventricular fibrillation resulting from uncompensated congestive heart failure and hypotension. The different energy requirements between organized and non-organized arrhythmias may be related to the electrophysiologic characteristics of the arrhythmia. Organized arrhythmias, such as sustained monomorphic ventricular tachycardia, arise from a discrete reentrant circuit which is easily depolarized by smaller amounts of current [69,70]. In contrast, in non-organized rhythms such as polymorphic ventricular tachycardia and ventricular fibrillation, the wavefronts are multiple and involve more myocardial mass, thereby requiring more energy for termination [18]. The electrical current and energy required to terminate ventricular tachyarrhythmias vary by arrhythmia. Ventricular tachycardia generally requires less energy than ventricular fibrillation. In one study of 203 patients who received 569 shocks for ventricular tachycardia or fibrillation, the heart rate and electrocardiographic degree of organization of the tachycardia were important determinants of transthoracic energy and the current requirements for cardioversion or defibrillation [71]. Transthoracic termination of monomorphic ventricular tachycardia required relatively low energy (70 to 100 joules) while polymorphic ventricular tachycardia required more energy (150 to 200 joules). The varying energy requirements for cardioversion for ventricular tachyarrhythmias are analogous to those of atrial tachyarrhythmias. Atrial flutter, a more organized rhythm than atrial fibrillation, generally terminates with lower electrical doses. Duration of arrhythmia An additional factor in the likelihood of a successful cardioversion or defibrillation, for both ventricular and atrial arrhythmias, is the amount of time an arrhythmia has been present. In ventricular fibrillation the duration of the arrhythmia is a determinant of the degree of organization of the electrical impulse. Even when using a biphasic waveform, the https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 10/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate effectiveness of defibrillation is reduced when the arrhythmia is of longer duration [72]. The more recent the onset of ventricular fibrillation, the coarser are the fibrillatory waves and the greater the success with defibrillation. As the arrhythmia persists (ie, more than 10 to 30 seconds), the fibrillatory waves become finer and the likelihood of successful termination decreases [73-75]. This relationship has also been demonstrated in a study of 22 patients with an implantable cardioverter-defibrillator [74]. Defibrillation was effective in 82 percent of patients when delivered after five seconds compared with 45 percent when delivered after 15 seconds. (See "Prognosis and outcomes following sudden cardiac arrest in adults".) In atrial fibrillation there is a similar relationship between duration of atrial fibrillation and the success of cardioversion, although longer time periods are usually involved. One study of 198 patients found that sinus rhythm was restored in 98 percent of patients when the duration of atrial fibrillation was less than 24 hours compared with a success rate of 62 percent when the duration was more than 24 hours [76]. The overall success rate for sinus rhythm restoration is approximately 90 percent when atrial fibrillation is of less than one year's duration compared with 50 percent when atrial fibrillation has been present for more than five years [77]. Use of antiarrhythmic drugs Antiarrhythmic drugs can increase or decrease the defibrillation energy requirements for ventricular fibrillation and the cardioversion energy requirements for atrial fibrillation. In general, sodium channel blockers increase the energy required for defibrillation, while potassium channel blockers and catecholamines decrease the energy needed. For example, lidocaine increases defibrillation energy requirements, and sotalol and ibutilide decrease the energy needed [78]. Epinephrine's effect on cycle length, synchronization, and dispersion of repolarization of fibrillatory waves may be the mechanisms by which it facilitates defibrillation [79]. These observations are relevant to external defibrillation success in the setting of a cardiac arrest, the ability to successfully defibrillate a patient with an implantable defibrillator during defibrillation threshold testing, and the use of ibutilide to facilitate restoration of sinus rhythm in patients undergoing elective cardioversion for atrial fibrillation [80]. Patients with an underlying cardiac implantable electronic device Patients with an underlying cardiac implantable electronic device (CIED) such as a permanent pacemaker or an implantable cardioverter-defibrillator require special attention to electrode placement. In such patients, we recommend placing the external electrode pads in the anteroposterior position and avoiding any contact with the skin overlying the CIED. Positioning of the electrode pads to avoid making contact with the skin overlying the CIED is critical to maximizing the efficacy of the https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 11/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate externally delivered shock and minimizing the likelihood of damage to the CIED from the externally delivered shock. ENERGY SELECTION FOR CARDIOVERSION AND DEFIBRILLATION The amount of energy selected for initial attempts of defibrillation has been controversial. The energy selected should be sufficient to accomplish prompt defibrillation because repeated failures expose the heart to damage from prolonged ischemia and multiple shocks. On the other hand, excessive energy should be avoided, since myocardial damage from high-energy shocks has been demonstrated in experimental studies, although the frequency with which this occurs in humans is not known. The choice of energy for initial attempts at cardioversion or defibrillation is discussed in detail separately. (See "Cardioversion for specific arrhythmias", section on 'Energy selection'.) SOCIETY GUIDELINE LINKS Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Basic and advanced cardiac life support in adults" and "Society guideline links: Cardiac arrest in adults".) INFORMATION FOR PATIENTS UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." th th The Basics patient education pieces are written in plain language, at the 5 to 6 grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more th th sophisticated, and more detailed. These articles are written at the 10 to 12 grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon. Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.) Beyond the Basics topics (see "Patient education: Cardioversion (Beyond the Basics)") https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 12/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate SUMMARY Electrical cardioversion and defibrillation have become routine procedures in the management of patients with cardiac arrhythmias. Cardioversion is the delivery of energy that is synchronized to the QRS complex, while defibrillation is the non-synchronized delivery of a shock during the cardiac cycle. (See 'Introduction' above.) Cardioversion terminates arrhythmias by delivering a synchronized shock that depolarizes the tissue involved in a reentrant circuit ( figure 2 and figure 3). By depolarizing all excitable tissue of the circuit and making the tissue refractory, the circuit is no longer able to propagate or sustain reentry. (See 'Electrophysiology of defibrillation and cardioversion' above.) Despite its widespread clinical use, controversy persists concerning the electrophysiologic mechanisms by which electrical cardioversion or defibrillation terminates atrial or ventricular fibrillation, arrhythmias which involve multiple microreentrant circuits. Most investigators agree that defibrillation occurs when a certain amount of current density reaches the myocardium. However, it is unclear what amount of current density is needed and what energy setting is necessary to achieve a homogeneous current density. (See 'Electrophysiology of defibrillation and cardioversion' above.) A variety of device-related factors will influence the chances of successful cardioversion and/or defibrillation, including electrode size and position, hand-held versus self-adhesive patch electrodes, and monophasic versus biphasic waveforms. (See 'Device-related variables' above.) A variety of patient-related factors also influence the chances of successful cardioversion and/or defibrillation, including transthoracic impedance, type of arrhythmia, use of antiarrhythmic drugs, and the duration of arrhythmia. (See 'Patient-related variables' above.) While the anteroposterior electrode position is suggested for most patients, for persons with an underlying cardiac implantable electronic device (CIED) such as a permanent pacemaker or an implantable cardioverter-defibrillator, we specifically recommend placing the external electrode pads in the anteroposterior position and avoiding any contact with the skin overlying the CIED. This placement should maximize the shock efficacy and minimize the likelihood of damage to the CIED. (See 'Electrode position' above and 'Patients with an underlying cardiac implantable electronic device' above.) https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 13/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Use of UpToDate is subject to the Terms of Use. REFERENCES 1. ZOLL PM, LINENTHAL AJ, GIBSON W, et al. Termination of ventricular fibrillation in man by externally applied electric countershock. N Engl J Med 1956; 254:727. 2. LOWN B, AMARASINGHAM R, NEUMAN J. New method for terminating cardiac arrhythmias. Use of synchronized capacitor discharge. JAMA 1962; 182:548. 3. ALEXANDER S, KLEIGER R, LOWN B. Use of external electric countershock in the treatment of ventricular tachycardia. JAMA 1961; 177:916. 4. Zoll PM, Linentha AJ. Termination of refractory tachycardia by external countershock. Circulation 1962; 25:596. 5. PAUL MH, MILLER RA. External electrical termination of supraventricular arrhythmias in congenital heart disease. Circulation 1962; 25:604. 6. Kerber RE, Martins JB, Kienzle MG, et al. Energy, current, and success in defibrillation and cardioversion: clinical studies using an automated impedance-based method of energy adjustment. Circulation 1988; 77:1038. 7. Lerman BB, Halperin HR, Tsitlik JE, et al. Relationship between canine transthoracic impedance and defibrillation threshold. Evidence for current-based defibrillation. J Clin Invest 1987; 80:797. 8. Kerber RE, Jensen SR, Gascho JA, et al. Determinants of defibrillation: prospective analysis of 183 patients. Am J Cardiol 1983; 52:739. 9. Lerman BB, DiMarco JP, Haines DE. Current-based versus energy-based ventricular defibrillation: a prospective study. J Am Coll Cardiol 1988; 12:1259. 10. Plonsey R, Barr RC. Effect of microscopic and macroscopic discontinuities on the response of cardiac tissue to defibrillating (stimulating) currents. Med Biol Eng Comput 1986; 24:130. 11. Mower MM, Mirowski M, Spear JF, Moore EN. Patterns of ventricular activity during catheter defibrillation. Circulation 1974; 49:858. 12. Witkowski FX, Penkoske PA, Plonsey R. Mechanism of cardiac defibrillation in open-chest dogs with unipolar DC-coupled simultaneous activation and shock potential recordings. Circulation 1990; 82:244.