7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Implantable cardioverter-defibrillators: Overview of indications, components, and functions : Jonathan Piccini, MD, MHS, FACC, FAHA, FHRS, Cara Pellegrini, MD : N A Mark Estes, III, 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: Mar 20, 2023. INTRODUCTION Ventricular fibrillation (VF) is a common cause of sudden cardiac death (SCD) and is sometimes preceded by monomorphic or polymorphic ventricular tachycardia (VT). All sustained ventricular arrhythmias have the potential to be lethal arrhythmias. Although cardiopulmonary resuscitation, including chest compressions and assisted ventilation, can provide transient circulatory support for the patient with cardiac arrest, the only effective approach for terminating VF is electrical defibrillation. Implantable cardioverter-defibrillator (ICD) implantation is generally considered the first-line treatment option for the secondary prevention of SCD and for primary prevention in certain populations at high risk of SCD due to VT/VF. This topic will review the general indications for ICD implantation as well as the components and functionalities of the ICD. The clinical trials documenting the efficacy of an ICD in different clinical settings (including both secondary and primary prevention), complications of ICD placement, optimal ICD programming, and follow-up care of patients with ICDs are discussed separately. (See "Implantable cardioverter-defibrillators: Optimal programming".) (See "Secondary prevention of sudden cardiac death in heart failure and cardiomyopathy".) (See "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF".) (See "Cardiac implantable electronic devices: Long-term complications".) https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 1/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate (See "Cardiac implantable electronic devices: Patient follow-up".) (See "Cardiac implantable electronic devices: Periprocedural complications".) Alternatives and adjunctive therapies to ICD implantation include antiarrhythmic drugs; ablative surgery; catheter ablation; and in advanced cases stellate ganglion resection, noninvasive cardiac radiation, and cardiac transplantation; and are discussed separately. (See "Sustained monomorphic ventricular tachycardia in patients with structural heart disease: Treatment and prognosis", section on 'Radiofrequency catheter ablation'.) (See "Secondary prevention of sudden cardiac death in heart failure and cardiomyopathy".) (See "Pharmacologic therapy in survivors of sudden cardiac arrest".) INDICATIONS The main indications for use of an ICD are as follows [1,2]: Secondary prevention of sudden cardiac death (SCD) in patients with prior sustained ventricular tachycardia (VT), ventricular fibrillation (VF), or resuscitated SCD thought to be due to VT/VF. Primary prevention of SCD in patients at increased risk of life-threatening VT/VF. Secondary prevention Implantation of an ICD is recommended for the secondary prevention of SCD due to life-threatening VT/VF in the following settings [2]: Patients with a prior episode of resuscitated VT/VF or sustained hemodynamically unstable VT in whom a completely reversible cause cannot be identified. This includes patients with a variety of underlying heart diseases and those with idiopathic VT/VF and congenital long QT syndrome, but not patients who have VT/VF limited to the first 48 hours after an acute myocardial infarction (MI). (See "Secondary prevention of sudden cardiac death in heart failure and cardiomyopathy", section on 'Secondary prevention of SCD'.) (See "Ventricular arrhythmias during acute myocardial infarction: Incidence, mechanisms, and clinical features".) (See "Sustained monomorphic ventricular tachycardia in patients with structural heart disease: Treatment and prognosis", section on 'Treatment of associated conditions'.) Patients with episodes of spontaneous sustained VT in the presence of heart disease (valvular, ischemic, hypertrophic, dilated, or infiltrative cardiomyopathies) and other https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 2/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate settings (eg, channelopathies). (See "Sustained monomorphic ventricular tachycardia in patients with structural heart disease: Treatment and prognosis", section on 'ICD therapy' and "Catecholaminergic polymorphic ventricular tachycardia".) Patients with unexplained syncope and high suspicion of VT/VF as the etiology. (See "Syncope in adults: Management and prognosis", section on 'Documented, suspected, or induced ventricular tachycardia'.) A key issue is the prevention of total mortality (not arrhythmic or sudden death). Simply correcting VT/VF may not improve overall mortality. Therefore, patient selection for ICD implantation should take into account both the known risk of SCD due to VT/VF for a specific condition and the risk of total mortality from underlying medical conditions as well. Primary prevention Implantation of an ICD is recommended for the primary prevention of SCD due to life-threatening VT/VF in patients who have received optimal guideline-directed medical therapy. While guideline-directed medical therapy used to be relatively simple and included beta-blocker therapy and use of an angiotensin receptor blocker or ACE inhibitor, guideline-directed medical therapy now includes several other medications. (See "Primary pharmacologic therapy for heart failure with reduced ejection fraction".) Patients on guideline-directed medical therapy who have high risk of SCD include the following groups of patients [2]: Patients with a prior MI (at least 40 days ago) and left ventricular ejection fraction (LVEF) 30 percent. (See "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF".) Patients with a cardiomyopathy, New York Heart Association (NYHA) functional class II to III table 1), and LVEF 35 percent. Patients with a nonischemic cardiomyopathy generally ( require optimal medical therapy for three months with documentation of persistent LVEF 35 percent at that time. However, the DANISH trial calls into question the role of prophylactic ICDs in some patients with nonischemic cardiomyopathy. It is recommended that patients be evaluated at least three months after revascularization (coronary artery bypass graft surgery [CABG] or stent placement). (See "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF", section on 'Use of an ICD'.) Some patients with heart failure who are candidates for an ICD also have intraventricular conduction delay ( 120 milliseconds) and are candidates for cardiac resynchronization therapy (CRT) with a biventricular pacemaker. Such patients could be treated with a device https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 3/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate with combined ICD and biventricular pacing functions (cardiac resynchronization therapy- defibrillator [CRT-D]). (See 'Cardiac resynchronization therapy' below and "Cardiac resynchronization therapy in heart failure: Indications and choice of system".) Patients with a prior MI, nonsustained VT, and LVEF 40 percent who have VF or sustained VT-induced during electrophysiology study. (See "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF".) Select patients with certain underlying disorders who are deemed to be at high risk for life- threatening VT/VF. This includes: Patients with congenital long QT syndrome who have recurrent symptoms and/or torsades de pointes despite therapy with beta blockers or other high-risk patients. (See "Congenital long QT syndrome: Treatment", section on 'Implantable cardioverter- defibrillator'.) High-risk patients with hypertrophic cardiomyopathy, arrhythmogenic right ventricular (RV) cardiomyopathy, or cardiac sarcoidosis. (See "Hypertrophic cardiomyopathy: Risk stratification for sudden cardiac death", section on 'Risk stratification' and "Management and prognosis of cardiac sarcoidosis", section on 'Implantable cardioverter-defibrillator'.) High-risk patients with Brugada syndrome, catecholaminergic polymorphic VT, and other channelopathies. (See "Catecholaminergic polymorphic ventricular tachycardia", section on 'Implantable cardioverter-defibrillators' and "Brugada syndrome or pattern: Management and approach to screening of relatives".) ICD not recommended ICD therapy is NOT recommended in the following settings [2]: Patients with ventricular tachyarrhythmias due to a completely reversible disorder in the absence of structural heart disease (eg, electrolyte imbalance, drugs, or trauma). Patients who do not have a reasonable expectation of survival with an acceptable functional status for at least one year, even if they otherwise meet ICD implantation criteria. Patients with incessant VT or VF in whom other therapies (eg, catheter ablation) should be considered first. (See "Electrical storm and incessant ventricular tachycardia", section on 'Catheter ablation'.) https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 4/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate Patients with severe psychiatric illnesses that may be aggravated by device implantation. In clinical practice, this situation is very rarely encountered and may apply more to primary prevention than secondary prevention settings. Patients with NYHA Class IV heart failure that is refractory to optimal medical treatment who are not candidates for cardiac transplantation or CRT. (See "Heart transplantation in adults: Indications and contraindications" and "Cardiac resynchronization therapy in heart failure: Indications and choice of system".) Patients with syncope without inducible ventricular tachyarrhythmias and without structural heart disease. Patients with VF or VT amenable to surgical or catheter ablation in whom the risk of sudden cardiac death is normalized after successful ablation (eg, pre-excited atrial fibrillation and subsequent ventricular arrhythmias associated with the Wolff-Parkinson- White syndrome, RV or LV outflow tract VT, idiopathic VT, or fascicular VT in the absence of structural heart disease). (See "Treatment of arrhythmias associated with the Wolff- Parkinson-White syndrome", section on 'Catheter ablation' and "Ventricular tachycardia in the absence of apparent structural heart disease".) ICD implantation should be delayed in patients with active infections or other acute medical issues. If necessary, the patient can be bridged with a wearable cardioverter- defibrillator (WCD) until ICD implantation can be carried out. (See "Wearable cardioverter- defibrillator".) ELEMENTS OF THE ICD The ICD system is comprised of the following elements [3]: Pacing/sensing electrodes Defibrillation electrodes Pulse generator ( picture 1) In contemporary transvenous ICDs, both the pace/sense electrodes and the defibrillation electrodes are located on a single ventricular lead. Decades ago, ICD systems were implanted epicardially, requiring major surgery to affix separate shocking and sensing electrodes to the epicardial surface of the heart. Generators were typically placed in the upper abdomen. The need for epicardial electrodes and abdominal pulse generators has become vanishingly rare. https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 5/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate There is also a subcutaneous ICD (S-ICD) which requires no transvenous leads. (See "Subcutaneous implantable cardioverter defibrillators".) Electrodes Pacing and sensing functions require a pair of electrodes. Contemporary pacemakers and defibrillators usually use leads with two electrodes on the ventricular lead: the distal electrode at the tip of the lead and a second electrode in the shape of a ring, several millimeters back from the tip (ie, true bipolar leads). These bipolar leads provide accurate sensing, with high amplitude, narrow electrograms. Some ICD leads utilize integrated bipolar sensing in which the bipole consists of a single tip electrode and the distal shocking coil electrode. In addition to improved sensing capabilities, bipolar leads reduce the risk of extraneous interference, which could lead to inappropriate device function (eg, inappropriate shocks delivered due to sensing of muscular activity). The defibrillation function of the electrodes requires a relatively large surface area and positioning of the lead to maximize the density of current flow through the ventricular myocardium. Contemporary ICD systems use a "coil" of wire that extends along the ventricular lead as the primary defibrillation electrode. Thus, a single transvenous lead can accomplish all pacing, sensing, and defibrillation functions. In the distant past (and in some unique cases, in persons without vascular access options), epicardial patches were used for defibrillation, but placement required a thoracotomy. Additional defibrillation electrodes improve defibrillation efficacy and reduce the defibrillation threshold. Most contemporary ICD systems have two or three defibrillation electrodes. Along with the distal coil in the right ventricle (RV) on the transvenous lead, some ICD leads have a second defibrillation coil proximal to the RV coil. In addition, with "active can" technology, the metal housing of the ICD serves as one of the shocking electrodes. This configuration requires that the pulse generator be implanted in the pectoral region ( figure 1). The active can and transvenous lead systems can be combined to achieve adequate defibrillation thresholds (minimum energy required for successful defibrillation, which should generally be 10 joules less than the maximum output of the device). (See 'Defibrillation threshold testing' below.) There are three types of pacing offered by current transvenous systems. Single-chamber systems have only an RV lead. Dual-chamber systems have right atrial (RA) and RV leads. Cardiac resynchronization therapy (CRT, also called biventricular) systems have RA, RV, and left ventricular (LV) leads, or in some patients with permanent atrial fibrillation, RV and LV leads. Pulse generator The pulse generator ( picture 1) contains the sensing circuitry as well as the high voltage capacitors and battery. While the initial pulse generators were located in the abdomen, the development of small pulse generators (eg, thickness 15 mm) has permitted https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 6/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate placement in the infraclavicular region of the anterior chest wall in nearly all patients [4]. The majority are placed in a prepectoral (ie, subcutaneous) position, but in some cases, a subpectoral position is advantageous. For most patients with the pulse generator in this location, the impulses generated are transmitted to the myocardium via transvenous leads. Epicardial systems are still available and may be necessary as a result of anatomical limitations to placing a transvenous lead(s). Additionally, a subcutaneous ICD (S-ICD) system is now available in which the pulse generator is placed overlying the left lower lateral ribs. (See 'Choosing the optimal pulse generator location' below.) Battery life in ICD pulse generators has improved over time. For example, devices implanted after 2002 have significantly longer battery lives (5.6 versus 4.9 years) [5]. Single-chamber ICDs implanted since 2002 had the longest battery life (mean 6.7 years). Contemporary ICD devices generally have an expected longevity greater than eight years and CRT- D devices greater than six years, although some ICDs have estimated battery longevity >10 years [6,7]. IMPLANTATION Prior to implanting an ICD, the provider must determine the optimal position for placement of the leads and the pulse generator. Most current ICD systems utilize one or two transvenous leads placed via the axillary, subclavian, or cephalic vein, with attachment to a pulse generator in the subcutaneous tissue in the infraclavicular anterior chest wall. In more recent years, there has been a trend toward single coils rather than dual-coil defibrillation leads. Dual-coil leads were favored earlier in the era of transvenous ICD systems. However, a proximal coil is rarely needed for defibrillation and single-coil leads pose less risk in the future if lead extraction is necessary. An additional defibrillation lead can be placed in the azygos vein, coronary sinus, or subcutaneous tissue if necessary to improve defibrillation. Choosing the optimal pulse generator location Modern devices are small enough to be implanted in the pectoral region of the anterior chest wall; the devices are implanted either subcutaneously or submuscularly, similar to a pacemaker implantation. Although implantation on the left side is preferred, a right-sided implant can be performed [8,9]. The left pectoral position is usually chosen for three reasons: The defibrillation energy requirement is usually lower on the left because of the location of the heart in the left chest Ipsilateral arm movement restrictions shortly after implant are less impactful for the nondominant hand (which for most people is the left) https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 7/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate There is a small risk of arm swelling due to venous occlusion; this is less impactful on the nondominant hand (which for most people is the left hand). Additionally, a subcutaneous ICD (S-ICD) system is available that allows for defibrillation (though no backup pacing aside from immediately post-shock or antitachycardia pacing) without the insertion of a transvenous lead. The pulse generator for the S-ICD system ( picture 2) is implanted in a subcutaneous pocket in the left lateral, mid-axillary thoracic position ( picture 3 and picture 4). (See 'Subcutaneous ICD' below and "Subcutaneous implantable cardioverter defibrillators".) Choosing the optimal lead placement In most de novo ICD implantations, the lead with the pace/sense electrodes is placed transvenously, with the distal electrode positioned on the right ventricular (RV) apical endocardium. Defibrillation energy requirement is generally optimized (ie, lowest) with an RV apical lead position. RV septal lead placement is also an option. In rare cases, usually due to limitations of the venous anatomy and/or a high risk of bacteremia and endovascular infection, the pace/sense electrodes are placed on the epicardium during surgery ( image 1). The electrodes should record a ventricular electrogram of at least 5 mV. These signals should be sufficiently large such that detection of lower amplitude ventricular tachycardia (VT) and ventricular fibrillation (VF) is straight-forward. Dual-chamber ICDs have an additional lead with another pair of pace/sense electrodes in the right atrium for atrial sensing and pacing [10]. Not all patients require an atrial lead. Whether use of an atrial lead reduces the risk of inappropriate shocks for supraventricular rhythms is controversial and debated. (See "Modes of cardiac pacing: Nomenclature and selection", section on 'Pacing modes'.) An S-ICD has been developed with no leads placed in the heart. The subcutaneous lead, which toward its terminal end contains an 8 cm shocking coil electrode, is tunneled from a small midline incision and the pulse generator in the left mid axillary line on the lateral chest wall to a position along the left parasternal margin ( image 2). The S-ICD can sense VT/VF and deliver therapeutic shocks but cannot deliver antitachycardia pacing or pacing for bradycardias. (See 'Subcutaneous ICD' below.) Defibrillation threshold testing Defibrillation threshold testing (DFT) has historically been performed at the time of device implantation, although the necessity for this evaluation with modern devices and randomized trials to date has failed to identify clear benefit [11-17]. Among patients who have had DFT testing, only a small fraction need DFT testing. The following 2015 consensus statement on optimal ICD programming and testing counsels that the following groups may be considered for DFT testing [11]: https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 8/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate DFT testing is reasonable to consider in patients undergoing an initial right pectoral ICD implantation or an ICD pulse generator replacement. The rationale for testing right-sided implants is that defibrillation may be more difficult with a right pectoral pulse generator, given the fact that the heart lies in the left chest. For generator changes, there may be concerns about the integrity of the chronic leads. DFT testing should not be performed in patients with a documented nonchronic cardiac thrombus, atrial fibrillation/flutter without adequate anticoagulation, severe aortic stenosis, unstable angina, recent stroke or transient ischemic attack, or hemodynamic instability. Additionally, many centers avoid DFT testing in patients with very low left ventricular ejection fractions (<15 percent) or severe pulmonary hypertension. Furthermore, the 2015 statement counsels that: DFT testing can performed in patients receiving an S-ICD. (See "Subcutaneous implantable cardioverter defibrillators".) DFT testing can be omitted in patients undergoing a left pectoral transvenous ICD implantation with a RV apical lead that is functioning appropriately. Some electrophysiologists feel that universally omitting DFT testing might compromise the safety within certain subsets of patients, especially those patients with high DFTs who would benefit from a higher energy device and/or additional leads. A distinction should be made, however, between DFT testing at initial implantation and at the time of generator replacement. DFT testing at the time of generator replacement is useful in subsets of patients with leads that have a hazard alert or in patients at higher risk of DFT changes (eg, obese patients, patients with heart failure symptoms, patients on amiodarone, etc) [18]. Early ICD systems frequently required lead system adjustment at the time of implantation in order to achieve an adequate safety margin (arbitrarily set at 10 joules or greater). As technology improved, thresholds were substantially reduced [19]. As a result, it is very unusual for defibrillator systems to require modification at the time of implantation ( figure 1). Data regarding DFT testing on the more modern single-coil systems are limited since the available cohort and registry data predate the development of single-coil systems. One paired randomized study of 216 patients with a mix of ICD indications and ICD manufacturers found no difference in first shock efficacy, which was >90 percent for either system [20]. On average, omitting the proximal coil in a single-coil system likely increases the DFT a few joules, which usually does not impact the safety margin but could be significant in some patients [21]. https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 9/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate Several studies have illustrated the impact of DFT testing at the time of ICD implantation with the current generation of devices, generally showing no significant difference in outcomes [14- 17,22,23]. In the Shockless Implant Evaluation (SIMPLE) trial, a single-blind, multicenter, noninferiority study of 2500 patients receiving an initial ICD in the left pectoral region for standard primary or secondary prevention indications, patients were randomized 1:1 to either have or not have DFT testing at the time of ICD implantation and were followed for an average of 3.1 years [22]. For the composite primary outcome of arrhythmic death or failed appropriate shock, no DFT testing was identified as noninferior to DFT testing (with a trend toward superiority), as patients in the no DFT testing group had a lower incidence of the primary outcome (7 percent per year versus 8 percent per year in the DFT testing group), with no significant differences in the secondary safety outcomes noted between the two groups. Similarly, in the NORDIC ICD trial, which also randomized patients receiving a first ICD to have or not have DFT testing at the time of ICD implantation, no DFT testing was identified as noninferior to DFT testing (also with a trend toward superiority) and was also associated with a trend toward fewer procedure-related adverse events [23]. In a systematic review and meta- analysis of 13 studies involving 9740 patients undergoing initial ICD implantation, there was no significant difference in mortality or adverse outcomes between patients with and without DFT testing [24]. In the absence of randomized data or society guidelines, many electrophysiologists perform DFT for most S-ICD implantations; it is also strongly encouraged for S-ICD generator replacement procedures. However, observational data have not shown that DFT at the time of initial S-ICD implantation is associated with a lower rate of ineffective shocks or cardiovascular mortality [25,26]. Among 566 propensity-matched patients with S-ICDs implanted across 17 European centers, there was no significant difference in the composite of ineffective shocks and cardiovascular mortality in those who underwent or did not undergo defibrillation testing [25]. Similarly, a multi-center Italian study of 650 propensity-matched patients found no significant difference in the composite of all-cause death and ineffective S-ICD therapy, as well as a secondary composite endpoint of all-cause death, ineffective shock, inappropriate shock, and complication [26]. Periprocedural monitoring Nearly all patients who undergo ICD implantation will have the device placed using local anesthesia at the site of the pulse generator insertion, with intravenous sedation provided most commonly by nurse anesthetists and/or anesthesiologists. If patients undergo DFT testing following device implantation, a "deeper" level of sedation may be required, but in most cases DFT testing can be performed without requiring general anesthesia. Following ICD implantation, a posteroanterior (PA) and lateral chest radiograph should be obtained to establish the position of the pulse generator and the associated lead(s) https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 10/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate and to exclude any apparent complications, including pneumothorax and lead dislodgment. Patients should also have a 12-lead electrocardiogram (ECG) recorded during pacing to document the ECG appearance of the QRS complex. The monitoring associated with procedural sedation, as well as additional periprocedural observation and timing of discharge post-procedure, is discussed in detail separately. (See "Procedural sedation in adults: General considerations, preparation, monitoring, and mitigating complications", section on 'Monitoring' and "Cardiac implantable electronic devices: Periprocedural complications", section on 'Periprocedural monitoring'.) Complications There are a variety of potential complications associated with ICDs, both at and around the time of implantation as well as long-term over the life of patients and their device(s). Both the periprocedural and long-term complications associated with ICDs are discussed in detail separately. (See "Cardiac implantable electronic devices: Periprocedural complications" and "Cardiac implantable electronic devices: Long-term complications".) ICD FUNCTIONS ECG monitoring and storage Contemporary ICDs have more extensive storage and monitoring capacities, thereby allowing more expedient patient management, often without requiring a face-to-face visit. Some examples: Recording and display of stored electrograms from tachyarrhythmia events. This can be very helpful for the detection of "silent" or asymptomatic arrhythmias where management of the patient is likely to change (eg, brief episodes of rate-controlled atrial fibrillation). Telemetry capabilities that permit easier analysis when patients receive shocks. Remote monitoring capabilities via telephone or internet that allow clinicians to review ICD parameters and events without requiring the patient to come to the office or hospital. (See "Cardiac implantable electronic devices: Patient follow-up".) Antitachycardia pacing Ventricular tachycardia (VT), particularly reentrant VT associated with scar from a prior myocardial infarction, can sometimes be terminated by pacing the ventricle. When a paced impulse enters the reentrant circuit during a tachycardia, it can depolarize a segment of the circuit, leaving that segment refractory when the reentrant wave returns, thus terminating the tachycardia. Antitachycardia pacing, or overdrive pacing, refers to the delivery of short bursts (eg, eight beats) of rapid ventricular pacing to terminate VT ( waveform 1). Although a variety of https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 11/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate algorithms exist, antitachycardia pacing is usually programmed to be delivered at a rate that is slightly faster (eg, at a cycle length 10 to 12 percent shorter) than the rate of the detected tachycardia. Subcutaneous ICDs (S-ICDs) cannot deliver antitachycardic pacing. (See "Implantable cardioverter-defibrillators: Optimal programming", section on 'Antitachycardia pacing'.) Though employed substantially less frequently, antitachycardia pacing can also terminate some atrial tachyarrhythmias, and these features have been incorporated in some contemporary ICD systems. Cardioversion/defibrillation A shock that is synchronized to be delivered at the peak of the R wave is referred to as cardioversion. Because VT is an organized electrical rhythm, the delivery of an electrical shock during the vulnerable period of repolarization can cause VT to degenerate into ventricular fibrillation (VF). Synchronized cardioversion prevents shock delivery during the vulnerable period. Although ICDs can be programmed to deliver synchronized shocks at a range of energies up to the maximum output of the device (usually 30 to 40 joules), synchronized cardioversion can often terminate VT with relatively low energy (eg, 10 joules or less). (See "Implantable cardioverter-defibrillators: Optimal programming", section on 'Cardioversion'.) An unsynchronized shock (ie, a shock delivered randomly during the cardiac cycle) is referred to as defibrillation. Clinicians can program ICDs to deliver unsynchronized shocks for very rapid ventricular arrhythmias (eg, heart rate greater than 200 beats/min). Because VF is not an organized rhythm, synchronized cardioversion is neither possible nor necessary. Similarly, it can be difficult to synchronize with very rapid VTs, and such rapid rhythms are unlikely to be hemodynamically tolerated. ICDs are typically programmed to deliver synchronized shocks at energies approaching the maximum output of the device (usually 30 to 40 joules) ( waveform 2). (See "Implantable cardioverter-defibrillators: Optimal programming", section on 'Defibrillation'.) Bradycardia pacing All contemporary transvenous ICDs are capable of pacing; however, current S-ICDs can deliver pacing for only 30 seconds post shock delivery and not standard bradycardia pacing. Many patients with an ICD have a conventional indication for cardiac pacing [27]. Separate ICDs and pacemakers can lead to device-to-device interactions, particularly with older models, potentially resulting in inappropriate shocks and underdetection of VT/VF [28-31]. With rare exceptions, patients should have only one transvenous or epicardial device, although the combined use of a leadless pacemaker with an S-ICD is under investigation. Generally, however, when a patient with a pacemaker develops an indication for ICD implantation, the pacemaker is removed and replaced with an ICD. https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 12/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate For patients with known atrioventricular (AV) block or sinus node dysfunction, or those who are receiving left ventricular (LV) pacing as part of cardiac resynchronization therapy (CRT), the device will be programmed accordingly. (See "Modes of cardiac pacing: Nomenclature and selection", section on 'Pacing modes'.) For those without pre-existing AV block or sinus node dysfunction and who presumably do not require regular ventricular pacing, the ICD will typically be programmed to minimize the amount of pacing provided (eg, pace only for intrinsic rates less than 40 beats/min). (See "Overview of pacemakers in heart failure" and "Modes of cardiac pacing: Nomenclature and selection", section on 'Modes to minimize ventricular pacing'.) In addition to the usual indications for pacing, the ability to provide pacing also protects against bradyarrhythmias that can follow a tachycardia or shock, and also against ventricular arrhythmias that are bradycardia-dependent [32]. Because of the unique physiology following a ventricular tachyarrhythmia and device shock, ICDs allow for distinct post-shock pacing programming (usually at higher outputs). S-ICDs are able to provide this brief pacing function. Cardiac resynchronization therapy CRT, which utilizes biventricular pacing, is an effective treatment for symptomatic heart failure in some patients with LV dyssynchrony. CRT is currently recommended in patients with advanced heart failure (usually NYHA class III or IV), severe systolic dysfunction (LV ejection fraction 35 percent), and intraventricular conduction delay (QRS >120 milliseconds). The evidence of benefit is greatest in patients with left bundle branch block and a QRS duration >150 milliseconds. Pacing of the LV is most frequently achieved by transvenous insertion of an electrode into a cardiac vein via the coronary sinus. Surgical placement of an epicardial lead is also an option in patients following failed efforts at transvenous lead placement, or in patients undergoing cardiac surgery for another reason. Conduction system pacing is another means of synchronizing ventricular stimulation; its impact, compared with traditional CRT with an coronary sinus lead, is being actively investigated. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system" and "Cardiac resynchronization therapy in heart failure: System implantation and programming" and "Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Rationale for CRT'.) Improvement in heart failure can reduce the frequency of ventricular arrhythmias, raising the possibility that biventricular pacing may have an adjunctive role with an ICD by reducing the need for ICD therapy. Although an initial series of 32 patients found such an effect [33], this benefit was not confirmed in the much larger MIRACLE ICD trial of 369 patients [34]. Although the addition of biventricular pacing to ICD therapy was associated with significant improvements https://www.uptodate.com/contents/implantable-cardioverter-defibrillators-overview-of-indications-components-and-functions/print 13/32 7/6/23, 10:53 AM Implantable cardioverter-defibrillators: Overview of indications, components, and functions - UpToDate in symptoms and quality of life, there was no reduction in the number of appropriate or inappropriate shocks. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system" and "Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Rationale for CRT'.) Perioperative ICD functioning During surgical procedures, the function of ICDs may be affected by electromagnetic interference (EMI), most commonly due to use of an electrosurgery unit (ESU). ICDs with integrated bipolar sensing configuration may be more susceptible to EMI than those with true bipolar sensing. Very rarely, direct damage from cautery to the ICD may alter its ability to deliver pacing or shocks or reset the ICD to an alternate or backup mode. The much more common concern is that the device might misinterpret the cautery as tachyarrhythmia, leading to withholding of bradycardia pacing and perhaps inappropriate ICD shocks. A full discussion regarding the perioperative management of patients with an ICD, including optimal monitoring and cardiac implantable electronic device programming, is presented separately. (See "Perioperative management of patients with a pacemaker or implantable cardioverter-defibrillator".) NONVASCULAR CARDIOVERTER-DEFIBRILLATOR Transvenous ICDs have a number of short- and long-term complications that may potentially be avoided with nonvascular systems. (See "Cardiac implantable electronic devices: Periprocedural complications", section on 'Transvenous lead systems' and "Cardiac implantable electronic devices: Long-term complications".) Wearable cardioverter-defibrillator Some patients who are at risk for sudden cardiac death do not meet established criteria for implantation of an ICD or may require only short-term protection (such as patients awaiting subsequent ICD insertion or cardiac transplantation). In