Indications for Cardiac Rhythm Management Devices
Ricardo Cardona-Guarache
Gautham Kalahasty
Kenneth A. Ellenbogen
Understanding the indications for device therapy for cardiac arrhythmias should be one of the primary goals of any Cardiology fellow during his or her electrophysiology (EP) rotation. The 2012 Focused Update incorporated into the ACC/AHA/HRS 2008 Guidelines for Device-based Therapy provides an excellent framework on which to base patient management. These guidelines are based on a combination of clinical trial data (with emphasis on randomized controlled trials when available) and expert consensus opinion (when data are limited). A class I indication means that the benefits far outweigh the risks and the procedure should be performed. A class IIa indication means that the benefits outweigh the risks and it is reasonable to perform the procedure. A class IIb indication means that the benefits may outweigh the risks or be equivalent and that the procedure can be considered. A class III indication means that the risks outweigh benefits and that the procedure should not be performed. The use of pacemakers and defibrillators for class I and IIa indications is well supported by clinical data or consensus opinion. Rigid application of the guidelines is not always appropriate or productive, and judicious use of devices for what are considered IIb indications can be reasonable. The goals of this chapter are not to simply restate the guidelines for device implantation, but rather, to provide practical insights for appropriate patient selection and management. Although summarized throughout this chapter, a thorough reading of the ACC/AHA/HRS Guidelines is mandatory for any cardiologist. The currently available devices used for the management arrhythmias include pacemakers, implantable cardioverter-defibrillators (ICDs), subcutaneous ICDs (S-ICDs), wearable cardioverter-defibrillators (WCDs), cardiac resynchronizations pacemakers (CRT-Ps) and defibrillators (CRT-Ds), and implantable loop recorders (ILRs).
PHYSIOLOGIC PACING
A discussion about current indications for implantable devices for cardiac rhythm management is incomplete without addressing the importance of physiologic pacing. The function of a pacemaker is to, as much as possible, approximate normal cardiac
function. Therefore, careful mode selection (AAI, VVI, DDI, DDD, etc.) and proper programming (atrioventricular [AV] delay, hysteresis, mode switch rates, etc.) are needed to optimize the beneficial effects and minimize the potentially detrimental effects of pacing. Although “demand” ventricular pacemakers have been in clinical use since the 1960s and it seems intuitive that dual chamber pacing would be superior to ventricular demand pacing, the body of clinical data needed to support this conclusion took almost 20 years to accumulate. The benefits of dual chamber AV synchronous pacing in patients with sinus node dysfunction (SND) and paroxysmal atrial fibrillation (AF) are now widely accepted. More recently, the potentially adverse effects of ventricular pacing (synchronous or asynchronous) have been recognized and are summarized in Table 10-1. Some of these effects are not unique to ventricular pacing with AV dyssynchrony but may occur with dual chamber pacing and will be discussed below. Even ventricular proarrhythmia (ventricular tachycardia [VT] and ventricular fibrillation [VF]) has been described with both single chamber ventricular and dual chamber pacing. Ventricular remodeling, hemodynamic parameters, quality of life measures, and clinical end points such as incidence of AF, stroke, congestive heart failure (CHF), and mortality have all been investigated. Proper device selection and programming are needed to prevent some of the potentially detrimental effects of pacing. The important clinical trials in pacing and mode selection are summarized in Table 10-2.
function. Therefore, careful mode selection (AAI, VVI, DDI, DDD, etc.) and proper programming (atrioventricular [AV] delay, hysteresis, mode switch rates, etc.) are needed to optimize the beneficial effects and minimize the potentially detrimental effects of pacing. Although “demand” ventricular pacemakers have been in clinical use since the 1960s and it seems intuitive that dual chamber pacing would be superior to ventricular demand pacing, the body of clinical data needed to support this conclusion took almost 20 years to accumulate. The benefits of dual chamber AV synchronous pacing in patients with sinus node dysfunction (SND) and paroxysmal atrial fibrillation (AF) are now widely accepted. More recently, the potentially adverse effects of ventricular pacing (synchronous or asynchronous) have been recognized and are summarized in Table 10-1. Some of these effects are not unique to ventricular pacing with AV dyssynchrony but may occur with dual chamber pacing and will be discussed below. Even ventricular proarrhythmia (ventricular tachycardia [VT] and ventricular fibrillation [VF]) has been described with both single chamber ventricular and dual chamber pacing. Ventricular remodeling, hemodynamic parameters, quality of life measures, and clinical end points such as incidence of AF, stroke, congestive heart failure (CHF), and mortality have all been investigated. Proper device selection and programming are needed to prevent some of the potentially detrimental effects of pacing. The important clinical trials in pacing and mode selection are summarized in Table 10-2.
In a secondary analysis of the MOST data, two important findings were reported. Increasing percentage of ventricular pacing was found to be associated with an increasing incidence of AF during both VVIR and DDDR pacing. Also, a greater percentage of ventricular pacing was associated with a greater risk of hospitalization for heart failure. If ventricular pacing occurred more than 40% of the time, there was a twofold increase in the risk of developing CHF. This study suggests that the relative benefits of AV synchronous pacing compared to only ventricular pacing are due to the deleterious effects of right ventricular (RV) pacing rather than the presumed advantages of AV synchronous pacing. The CTOPP and MOST studies had relatively few patients with true atrial based pacing (AAI) without the confounding effect of ventricular pacing. In the MADIT (Multicenter Automatic Defibrillator Implantation Trial) II study, patients who received an ICD had higher survival rates but also demonstrated a trend toward an increased rate of CHF; 73 patients (14.9%) in the conventional-therapy group and 148 in the defibrillator group (19.9%) were hospitalized with heart failure
(p = 0.09). In the DAVID (Dual Chamber and VVI Implantable Defibrillator) Trial, a composite endpoint of time to death and first hospitalization for CHF was compared in ICD patients programmed to receive dual-chamber pacing (DDDR-70) or ventricular backup pacing (VVI-40). At 1 year, 83.9% of the patients in the VVI-40 group were free from the composite endpoint compared to 73.3% of patients in the DDDR-70 group. Hospitalization for CHF occurred in 13.3% of VVI-40 patients compared to 22.6% of DDD-70 patients trending in favor of the VVI-40 group. The VVI-40 group received very little ventricular pacing compared to the DDD-70 group. Although the DAVID study looked only at an ICD population, it had a major impact on the programming of dual chamber pacemakers. By highlighting the deleterious effects of RV pacing, it underscored the importance of mode selection in patients with SND and paroxysmal AF. When a physician programs a pacemaker or an ICD, they should try to minimize RV pacing. All major companies have pulse generators that have incorporated programming parameters/algorithms which allow for maximization of the AV delay to promote intrinsic ventricular conduction. Algorithms also exist that allow for dual chamber pacemakers to automatically switch from single chamber atrial based pacing (AAI/R) to dual chamber AV sequential pacing (DDD/R). When this algorithm is selected, the device operates in AAI/R mode until AV block occurs and then switches to DDD/R mode (Fig. 10-1). A prospective randomized trial (SAVE PACe) of an algorithm that minimizes ventricular pacing (Sweeney et al., 2007) showed a 40% relative risk reduction in the development of persistent AF compared to conventional dual chamber pacing for patients with SND and normal left ventricular (LV) function when this algorithm was utilized. No difference was seen in mortality between the two groups of paced patients.
(p = 0.09). In the DAVID (Dual Chamber and VVI Implantable Defibrillator) Trial, a composite endpoint of time to death and first hospitalization for CHF was compared in ICD patients programmed to receive dual-chamber pacing (DDDR-70) or ventricular backup pacing (VVI-40). At 1 year, 83.9% of the patients in the VVI-40 group were free from the composite endpoint compared to 73.3% of patients in the DDDR-70 group. Hospitalization for CHF occurred in 13.3% of VVI-40 patients compared to 22.6% of DDD-70 patients trending in favor of the VVI-40 group. The VVI-40 group received very little ventricular pacing compared to the DDD-70 group. Although the DAVID study looked only at an ICD population, it had a major impact on the programming of dual chamber pacemakers. By highlighting the deleterious effects of RV pacing, it underscored the importance of mode selection in patients with SND and paroxysmal AF. When a physician programs a pacemaker or an ICD, they should try to minimize RV pacing. All major companies have pulse generators that have incorporated programming parameters/algorithms which allow for maximization of the AV delay to promote intrinsic ventricular conduction. Algorithms also exist that allow for dual chamber pacemakers to automatically switch from single chamber atrial based pacing (AAI/R) to dual chamber AV sequential pacing (DDD/R). When this algorithm is selected, the device operates in AAI/R mode until AV block occurs and then switches to DDD/R mode (Fig. 10-1). A prospective randomized trial (SAVE PACe) of an algorithm that minimizes ventricular pacing (Sweeney et al., 2007) showed a 40% relative risk reduction in the development of persistent AF compared to conventional dual chamber pacing for patients with SND and normal left ventricular (LV) function when this algorithm was utilized. No difference was seen in mortality between the two groups of paced patients.
TABLE 10-1 Potential Adverse Effects of Ventricular Pacing (RV) in SND | ||||||||
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TABLE 10-2 Important Clinical Trial in Pacing and Mode Selection | |||||||||||||||||||||||||||||||||||||||||||||
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From the above discussion, it is clear that the definition of physiologic pacing has evolved. A single chamber ventricular based pacemaker should be avoided in patients with paroxysmal AF and SND. It is no longer enough to maintain AV synchrony with a dual chamber atrial based pacemaker. When possible, intrinsic AV conduction should be promoted to minimize the deleterious effects of RV pacing. Therefore, mode selection is important (AAI ↔ DDD, DDI, or DDD with long AV delays). Unresolved questions include the maximum hemodynamically acceptable AV delay and the optimal site for RV pacing.
Common indications for pacing are summarized in Table 10-3. Most of these are class I indications. Class II and class III indications are discussed in the text.
PACING FOR SINUS NODE DYSFUNCTION
SND is the most common indication for pacemaker implantation in the US. SND is also referred to as sick sinus syndrome (SSS) or tachycardia-bradycardia syndrome. SND is characterized by a constellation of symptoms and arrhythmias. The more common symptoms include syncope, presyncope, fatigue, decreased exercise tolerance, and palpitations. Syncope is the most important presenting symptom of SND in elderly patients. Episodes are often unheralded. In elderly patients who present with repeated falls, syncope should be considered a potential precipitating factor. Typical arrhythmias include sinus bradycardia, sinus pauses (usually at AF termination), paroxysmal or permanent AF, atrial flutter, or atrial tachycardias.
Sinus bradycardia can be physiologic, due to high vagal tone or pathologic, due to SND. Even resting heart rates of 40 or 50 bpm while awake or as low as 30 bpm while asleep may be normal for some patients, especially those with excellent cardiovascular fitness. The incidental finding of sinus bradycardia alone is not an indication
for pacemaker implantation. However, if a patient gives a history of recurrent unexplained syncope or easy fatigability and is found to have resting extreme bradycardia, permanent pacemaker implantation should be considered.
for pacemaker implantation. However, if a patient gives a history of recurrent unexplained syncope or easy fatigability and is found to have resting extreme bradycardia, permanent pacemaker implantation should be considered.
 Figure 10-1 Managed ventricular pacing (MVPTM, Medtronic Inc., Minneapolis, MN) is an atrial-based pacing mode that significantly reduces unnecessary RV pacing by primarily operating in an AAI(R) pacing mode while providing the safety of a dual chamber backup mode if necessary. As shown in the figure, the algorithm allows for a single blocked beat before a backup ventricular paced beat is delivered. Mode switch will occur only if two out of four blocked beats occur. Two sequential blocked beats cannot occur due to backup ventricular pacing. |
TABLE 10-3 Indications for Pacing | |||||||||||||||||||||||
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Symptomatic sinus bradycardia may be iatrogenic due to medical therapy such as beta-blockers or calcium channel blockers. Pacemaker implantation is indicated in this situation if no suitable alternative to these medications is advisable. For example, a patient with chronic stable angina or prior myocardial infarction should, in most cases, receive beta-blockers. A pacemaker may be needed to support the use of beta-blockers. High-grade AV block develops with an annual incidence of up to 1.8% per year in patients with a normal QRS duration and with a higher incidence in patients with preexisting bundle branch block. Therefore, in the US, dual chamber pacemakers are almost invariably implanted for SND.
Bradycardia may also be an unavoidable consequence of the medications used to prevent a rapid ventricular response associated with AF. Beta-blockers, calcium channel blockers, and digoxin are used to control rapid ventricular rates during AF, but the use of these can result in intermittent symptomatic bradycardia or long pauses that can lead to syncope or presyncope. Of note, pauses of up to 3 to 4 s during sleep are not unusual and are not solely an indication for pacing. Rather, bradycardia may be a reflection of the relatively high vagal tone present during sleep. Adjustment of
medication dose or the use of beta-blockers with intrinsic sympathomimetic activity can sometimes mitigate the bradycardia or pauses but can also result in suboptimal rate control when a patient is active and awake. A pacemaker is indicated to facilitate the use of medications that are considered essential and for which there are no other suitable alternatives. Patients with paroxysmal AF and intermittent symptomatic bradycardia or long offset pauses at the terminations of AF should receive a dual chamber pacemaker. Patients with chronic AF and intermittent symptomatic pauses should receive a single chamber ventricular pacemaker. In addition to pharmacologic therapy to control the tachycardia, AV junction ablation may be needed for adequate rate control.
medication dose or the use of beta-blockers with intrinsic sympathomimetic activity can sometimes mitigate the bradycardia or pauses but can also result in suboptimal rate control when a patient is active and awake. A pacemaker is indicated to facilitate the use of medications that are considered essential and for which there are no other suitable alternatives. Patients with paroxysmal AF and intermittent symptomatic bradycardia or long offset pauses at the terminations of AF should receive a dual chamber pacemaker. Patients with chronic AF and intermittent symptomatic pauses should receive a single chamber ventricular pacemaker. In addition to pharmacologic therapy to control the tachycardia, AV junction ablation may be needed for adequate rate control.
The guidelines for pacing emphasize the importance of symptoms attributable to bradycardia. Asymptomatic resting bradycardia, even with heart rates less than 40 bpm while awake, is not an indication for permanent pacing. In the absence of reversible causes, symptomatic bradycardia, regardless of the mechanism, is generally a universally accepted indication for pacing. The clinical challenge in some cases is to establish the correlation between symptoms and bradycardia. This may require the use of monitors such as Holter monitors, event monitors, or ILRs. A formal exercise treadmill test, although not required, may be needed to diagnose chronotropic incompetence. An EP study to evaluate sinus node function is typically of limited benefit. However, in a patient with syncope of unknown origin, it is reasonable to implant a pacemaker if prolonged sinus node recovery times are provoked during an EP study and no other abnormalities are discovered that could be responsible for syncope. Occasionally, an upright tilt table test can reveal SND in the form of carotid sinus hypersensitivity in patients presenting with unexplained syncope. Despite these diagnostic studies, a correlation between symptoms and rhythm cannot be established. Therefore, it is reasonable to implant a pacemaker for patients with heart rates less than 40 bpm who have symptoms consistent with bradycardia (class IIa) such as syncope, even if a direct causal relationship cannot be proven clinically.
Paroxysmal AF is the most common tachyarrhythmia seen in many patients with SND. In appropriate patients with AF, pacemaker implantation and AV junction ablation provide clinical benefit. It should be considered in any patient with suboptimal rate control and in any patient who is at risk of developing or has developed a tachycardia mediated cardiomyopathy. Although this procedure is most often done in patients with chronic AF, it is also appropriate for some patients with paroxysmal AF. The benefits of pacing in patients with a cardiac resynchronization therapy (CRT) device may be maximized in those patients with AF who have undergone AV junction ablation. In patients with chronic AF who are receiving a CRT device, AV junction ablation can be recommended if adequate rate control to allow LV pacing cannot be achieved by medical therapy. This issue is unresolved in patients with paroxysmal AF who receive a CRT device. Pacing algorithms that attempt to promote biventricular pacing even during AF are not effective, and pacing algorithms that attempt to prevent AF have limited value. In patients without a bradycardia indication, permanent pacemaker implantation is not currently recommended solely for the purpose of implementation of AF prevention algorithms. Multisite and novel site (Bachmann’s bundle) pacing strategies do not have broad clinical applications at this time. An exception is the use of short-term multisite pacing in the immediate postoperative period following cardiac surgery.
PACING FOR ACQUIRED OR CHRONIC ATRIOVENTRICULAR BLOCK
AV block is the other major cause of bradycardia resulting in the need for pacemaker implantation. First degree, second degree (types I and II), and third degree AV blocks
may have indications for permanent pacing. The definitions of each of these conduction abnormalities should be known. Advanced second degree AV block refers to the block of two or more consecutive P waves with some conducted beats, which indicates some preservation of AV conduction. In the setting of AF, a prolonged pause (e.g., >5 s) should be considered to be due to advanced second degree AV block. As in SND, the presence of symptoms attributable to bradycardia establishes a class I indication for pacing regardless of the anatomic level of block. Symptoms of AV block not only include fatigue, syncope, and presyncope but also CHF and exercise intolerance. Unlike in patients with SND, there are some types of heart block for which there are class I and IIa indications for pacing even in the absence of symptoms. Bradycardia-dependent torsade de pointes or VT associated with complete or advanced AV block is a class I indication for pacing even without associated symptoms. Complete or advanced AV block with asystole greater than 3 s or escape rates less than 40 bpm is an indication for pacing in symptom-free, awake patients.
may have indications for permanent pacing. The definitions of each of these conduction abnormalities should be known. Advanced second degree AV block refers to the block of two or more consecutive P waves with some conducted beats, which indicates some preservation of AV conduction. In the setting of AF, a prolonged pause (e.g., >5 s) should be considered to be due to advanced second degree AV block. As in SND, the presence of symptoms attributable to bradycardia establishes a class I indication for pacing regardless of the anatomic level of block. Symptoms of AV block not only include fatigue, syncope, and presyncope but also CHF and exercise intolerance. Unlike in patients with SND, there are some types of heart block for which there are class I and IIa indications for pacing even in the absence of symptoms. Bradycardia-dependent torsade de pointes or VT associated with complete or advanced AV block is a class I indication for pacing even without associated symptoms. Complete or advanced AV block with asystole greater than 3 s or escape rates less than 40 bpm is an indication for pacing in symptom-free, awake patients.
First degree AV block is rarely an indication for permanent pacing. However, occasionally patients may experience symptoms due to an extremely long PR interval (>300 ms) even without progression to second or third degree AV block. A very long PR interval can lead to symptoms of “pacemaker syndrome” such as palpitations, chest pain, malaise and fatigue. A dual chamber pacemaker programmed with shorter AV delays may relieve these symptoms, although there is no data to suggest a mortality benefit.
Type I second degree AV block is usually due to block at the AV node (especially when the PR interval is longer than 240 ms) and does not usually progress to complete heart block. PM implantation is indicated if type I AV block is associated with symptoms. Type II AV block due to intra-hisian or infra-hisian block is associated with unpredictable progression to more advanced AV block or third degree AV block. Type II second degree AV block with a wide QRS typically indicates diffuse conduction system disease and constitutes an indication for pacing even in the absence of symptoms. Type II second degree AV block with a narrow QRS complex may not be due to intra-hisian or infra-hisian block. If asymptomatic, these patients may need a diagnostic EP study to determine the site of block.
Another type of AV block requiring permanent pacing occurs after cardiac surgery. Aortic valve, mitral valve, and tricuspid valve surgery are all associated with the need for postoperative permanent pacing. Complete heart block or advanced AV block at any level can be seen. The reported incidence varies from 1% to 5%. Predictors of need for permanent pacing following cardiac surgery include multiple valve surgery, re-operation, prolonged pump time, preexisting bundle branch block, and pre-operative rhythm other than sinus. It is appropriate to wait 3 to 5 postoperative days before implantation of a pacemaker. Despite this conservative observation period, later recovery of AV conduction has been observed, especially in patients with stable junctional rhythms at the time of implantation.
AV block or long sinus pauses during sleep in a patient with sleep disordered breathing is not an indication for PM implantation. These bradyarrhythmias should resolve after treatment of the sleep apnea. AV block during exercise is due to infra-nodal block and if it is not due to ischemia, a PM is indicated. Infiltrative diseases such as sarcoidosis or amyloidosis are progressive and even transient asymptomatic AV block should alert the clinician to the need for PM implantation. Most cardiologists will implant a dual chamber defibrillator in such patients.
Chronic bifascicular block (i.e., RBBB with left anterior fascicular block) reflects significant conduction system disease below the AV node. Unexplained syncope or symptoms suggestive of bradycardia justify pacemaker implantation. Incidental findings during EP study such as infra-hisian AV block during decremental pacing or
HV interval greater than 100 ms are class IIa indications, irrespective of symptoms. An EP study is reasonable in patients with chronic bifascicular block and second degree type I AV block to further characterize the site of AV conduction delay. The incidental finding of asymptomatic “trifascicular block” (RBBB with left anterior fascicular block and prolonged PR interval) is not an indication for pacing.
HV interval greater than 100 ms are class IIa indications, irrespective of symptoms. An EP study is reasonable in patients with chronic bifascicular block and second degree type I AV block to further characterize the site of AV conduction delay. The incidental finding of asymptomatic “trifascicular block” (RBBB with left anterior fascicular block and prolonged PR interval) is not an indication for pacing.
The need for permanent PM implantation in the recovery phase of an acute myocardial infarction (AMI) is not necessarily related to the presence or absence of symptoms attributable to AV block. Furthermore, the requirement for temporary pacing in AMI does not by itself constitute an indication for permanent pacing. Alternating bundle branch block or second degree AV block associated with bundle branch block, with or without symptoms, is an ominous prognostic sign and permanent pacing is indicated. Consideration should be given to the location of the AMI. Heart block associated with an inferior wall MI is typically transient and does not usually require permanent pacing. Heart block associated with an anterior wall MI suggests extensive damage to the conduction system and permanent pacing is usually indicated. With myocardial infarctions, the ejection fraction needs to be measured and a decision regarding pacemaker versus ICD must be made. The approach to such patients is discussed in recent guidelines (Epstein et al., 2013).
INDICATIONS FOR PACING NOT RELATED TO SND OR AV BLOCK
There are several other conditions for which a permanent PM may be indicated. However, collectively these diagnoses represent a small fraction of the total number of pacemaker implants in the US. These diagnoses include neurocardiogenic syndrome, sleep apnea, carotid sinus hypersensitivity syndrome, neuromuscular disorders, and bradycardia in the transplanted heart.
Neuromuscular disorders such as myotonic dystrophy and Emery-Dreifuss muscular dystrophy are associated with an unpredictable progression to complete heart block. While second and third degree AV block are clear indications for pacing, pacemaker implantation may be considered in patients with first degree AV block, irrespective of the symptoms.
Sleep apnea syndrome is a very common respiratory disturbance occurring during sleep, especially in patients with CHF. Atrial overdrive pacing above the average baseline nocturnal heart rate has not consistently been shown to improve sleep apnea. Overdrive pacing as a treatment modality for sleep apnea is not an accepted indication for permanent pacing. However, both the obstructive and central forms are associated with significant bradyarrhythmias (sinus bradycardia, sinus arrest, and AV block) during sleep. There is a high degree of intra-patient and inter-patient variability in the severity of nocturnal bradyarrhythmia. The main objective in the management of these patients is to treat the sleep apnea. If patients have persistent extreme (i.e., 10 s) asystole during sleep despite treatment with nasal continuous positive airway pressure, many clinicians would recommend permanent pacing.
Initial hemodynamic studies of pacing in patients with hypertrophic obstructive cardiomyopathy suggested pacing may result in a decrease in outflow gradient. Larger randomized clinical trials have failed to show consistent improvement in quality of life or survival. Therefore, pacing to relieve the outflow gradient in medically refractory hypertrophic obstructive cardiomyopathy is a class IIb indication. Permanent pacing is indicated in these patients if it is needed only for standard bradycardia indications. The reader is referred to the guidelines for specific details. Again many of these patients will receive defibrillators instead of pacemakers.
INDICATIONS FOR INTERNAL CARDIOVERTER-DEFIBRILLATOR IMPLANTATION
Sudden cardiac death (SCD) is defined as death attributable to a cardiac cause that occurs soon after the onset of symptoms. This definition is more selective for arrhythmia related death when the time course is restricted to 1 hour from the onset of symptoms. Of course, other processes can lead to sudden death within less than an hour, including pulmonary embolism and AMI. Despite efforts to determine the cause of death, it cannot always be determined with certainty. Therefore, clinical trials have used total mortality as endpoints for evaluating the benefits of ICD therapy.
Indications for ICD implantation are divided into secondary prevention of cardiac arrest (or sustained VT) and primary prevention of SCD. Secondary and primary prevention indications are summarized in Table 10-4. A secondary prevention indication exists when a patient has survived an episode of sudden cardiac arrest or sustained VT. Primary prevention of SCD refers to the use of ICDs in individuals who are at risk for but have not yet had an episode of sustained VT, VF, or resuscitated cardiac arrest (Fig. 10-2).
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