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. 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Topic 975 Version 30.0 https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 19/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate GRAPHICS Defibrillation waveforms in implantable cardioverter- defibrillators Figure A shows the monophasic, exponentially decaying pulse was the waveform used in the first generation of ICDs. Figure B shows the biphasic waveform, which is generated with a single capacitor by switching the output polarity during discharge. Each division is 2 milliseconds. ICD: implantable cardioverter-defibrillator. Graphic 51842 Version 3.0 https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 20/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Mechanisms of reentry in cardiac arrhythmias Schematic representation of possible reentrant circuits. The thick black arrow represents the circulating impulse; thin black lines represent advancing wavefronts in completely refractory tissue; speckled areas are partially refractory tissue; white areas are fully excitable tissue. A is the original model of circus movement around a fixed obstacle. There is a fully excitable gap, and the length and location of the circuit are fixed. B represents circus movement around 2 fixed anatomic obstacles. A fully excitable gap is present. C represents rapidly conducting bundles forming closed loops that serve as preferential circuits through which the impulse may travel. D is the leading circle type of reentry which does not require an anatomic obstacle. Instead, the impulse propagates around a functionally refractory core and among neighboring fibers that have different electrophysiologic properties. Since the refractoriness of the core is variable, the circuit size changes but will be the smallest possible circuit that can continue to propagate an impulse. Functional circuits tend to be small, rapid, and unstable. E represents reentry around a fixed anatomic obstacle, but a fully excitable gap is absent. F demonstrates an area of slowed conduction (hatched lines) between anatomic boundaries, while in G all areas of slowed conduction neighbor an anatomic obstacle. H represents anisotropic reentry. There are differences in the conduction of a single impulse in various fibers as a result of differences in their orientation. https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 21/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Courtesy of Philip J Podrid, MD, FACC. Graphic 52134 Version 5.0 https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 22/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Sites of reentry in supraventricular tachyarrhythmias Reentry may occur around a fixed anatomic obstacle or may be functional, developing in the absence of an anatomic obstacle and resulting from the intrinsic heterogeneity of electrophysiologic properties of the myocardial tissue. Reentrant circuits leading to a supraventricular tachyarrhythmia may develop in various parts of the heart: within and around the sinoatrial node (sinus node reentry); within the atrial myocardium (atrial tachycardia, atrial flutter, or atrial fibrillation); within the atrioventricular (AV) node due to the presence of a slow and fast pathway (atrioventricular nodal reentrant tachycardia); or involving the AV node and an accessory pathway (AP) (atrioventricular reentrant tachycardia). LAF: left anterior fascicle; LPF: left posterior fascicle. Graphic 82249 Version 4.0 https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 23/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Options for hands-free pacemaker/defibrillator pad positioning Positioning options for hands-free pacemaker/defibrillator pads showing anterior/lateral positioning (left) and anterior/posterior positioning (right). Graphic 103268 Version 2.0 https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 24/25 7/6/23, 2:49 PM Basic principles and technique of external electrical cardioversion and defibrillation - UpToDate Contributor Disclosures Bradley P Knight, MD, FACC Grant/Research/Clinical Trial Support: Abbott [Electrophysiology]; Atricure [Electrophysiology]; Biosense Webster [Electrophysiology]; Biotronik [Electrophysiology]; Boston Scientific [Electrophysiology]; BSCI [Electrophysiology]; MDT [Electrophysiology]; Philips [Electrophysiology]. Consultant/Advisory Boards: Abbott [Electrophysiology]; Atricure [Electrophysiology]; Biosense Webster [Electrophysiology]; Biotronik [Electrophysiology]; Boston Scientific [Electrophysiology]; BSCI [Electrophysiology]; CVRx [Heart failure]; MDT [Electrophysiology]; Philips [Electrophysiology]; Sanofi [Arrhythmias]. Speaker's Bureau: Abbott [Electrophysiology]; Biosense Webster [Electrophysiology]; Biotronik [Electrophysiology]; Boston Scientific [Transeptal catheterization]; BSCI [Electrophysiology]; MDT [Electrophysiology]. All of the relevant financial relationships listed have been mitigated. Richard L Page, MD No relevant financial relationship(s) with ineligible companies to disclose. Nisha Parikh, MD, MPH No relevant financial relationship(s) with ineligible companies to disclose. Contributor disclosures are reviewed for conflicts of interest by the editorial group. When found, these are addressed by vetting through a multi-level review process, and through requirements for references to be provided to support the content. Appropriately referenced content is required of all authors and must conform to UpToDate standards of evidence. Conflict of interest policy https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation/print 25/25