Published: August 2010
Broadly defined, cardiac arrhythmias are any abnormality or perturbation in the normal activation sequence of the myocardium. The sinus node, displaying properties of automaticity, spontaneously depolarizes, sending a depolarization wave over the atrium, depolarizing the atrioventricular (AV) node, propagating over the His-Purkinje system, and depolarizing the ventricle in systematic fashion. There are hundreds of different types of cardiac arrhythmias. The normal rhythm of the heart, so-called normal sinus rhythm, can be disturbed through failure of automaticity, such as sick sinus syndrome, or through overactivity, such as inappropriate sinus tachycardia. Ectopic foci prematurely exciting the myocardium on a single or continuous basis results in premature atrial contractions (PACs) and premature ventricular contractions (PVCs). Sustained tachyarrhythmias in the atria, such as atrial fibrillation, paroxysmal atrial tachycardia (PAT), and supraventricular tachycardia (SVT), originate because of micro- or macro re-entry. In general, the seriousness of cardiac arrhythmias depends on the presence or absence of structural heart disease.
The most common example of a relatively benign arrhythmia is atrial fibrillation (see the chapter “Atrial Fibrillation)”. Similarly common are PACs and PVCs, which, although a nuisance, generally are benign in the absence of structural heart disease. In contrast, the presence of nonsustained ventricular tachycardia (VT) or syncope in patients with coronary artery disease (CAD) or severe left ventricular (LV) dysfunction may be a harbinger of subsequent sudden cardiac death and must not be ignored.
Cardiac arrhythmias are common. Symptoms such as dizziness, palpitations, and syncope are frequent complaints encountered by family physicians, internists, and cardiologists. In contrast to these ubiquitous complaints, which are generally benign, sudden cardiac death remains an important public health concern. Statistics from the Centers for Disease Control and Prevention (CDC) have estimated sudden cardiac death rates at more than 600, 000 per year (Fig. 1).1 Up to 50% of patients have sudden death as the first manifestation of cardiac disease. Efforts at decreasing this alarming number have obviously focused on primary prevention, such as reducing cardiac risk factors, but have also led to the proliferation of automatic external defibrillators (AEDs). These devices have been shown to reduce mortality when used quickly in the first few minutes after an arrest.
Regardless of the specific arrhythmia, the pathogenesis of the arrhythmias falls into one of three basic mechanisms: enhanced or suppressed automaticity, triggered activity, or re-entry. Automaticity is a natural property of all myocytes. Ischemia, scarring, electrolyte disturbances, medications, advancing age, and other factors may suppress or enhance automaticity in various areas. Suppression of automaticity of the sinoatrial (SA) node can result in sinus node dysfunction and in sick sinus syndrome (SSS), which is still the most common indication for permanent pacemaker implantation (Fig. 2). In contrast to suppressed automaticity, enhanced automaticity can result in multiple arrhythmias, both atrial and ventricular. Triggered activity occurs when early afterdepolarizations and delayed afterdepolarizations initiate spontaneous multiple depolarizations, precipitating ventricular arrhythmias. Examples include torsades de pointes (Fig. 3) and ventricular arrhythmias caused by digitalis toxicity. Probably the most common mechanism of arrhythmogenesis results from re-entry. Requisites for re-entry include bidirectional conduction and unidirectional block. Micro level re-entry occurs with VT from conduction around the scar of myocardial infarction (MI), and macro level re-entry occurs via conduction through (Wolff-Parkinson-White [WPW] syndrome) concealed accessory pathways.
The signs and symptoms of cardiac arrhythmias can range from none at all to loss of consciousness or sudden cardiac death. In general, more-severe symptoms are more likely to occur in the presence of structural heart disease. For example, sustained monomorphic VT, particularly in a normal heart, may be hemodynamically tolerated without syncope. In contrast, even nonsustained VT may be poorly tolerated and cause marked symptoms in patients with severe LV dysfunction. Complaints such as lightheadedness, dizziness, fluttering, pounding, quivering, shortness of breath, dizziness, chest discomfort, and forceful or painful extra beats are commonly reported with various arrhythmias. Often, patients notice arrhythmias only after checking their peripheral pulses.
Certain descriptions of symptoms can raise the index of suspicion and provide clues about the type of arrhythmia. The presence of sustained regular palpitations or heart racing in young patients without any evidence of structural heart disease suggests the presence of a SVT caused by AV nodal re-entry or SVT caused by an accessory pathway. Such tachycardias are often accompanied by chest discomfort, diaphoresis, neck fullness, or a vasovagal type of response with syncope, diaphoresis, or nausea. It has been shown that the hemodynamic consequences of SVT as well as VT can also have an autonomic basis, recruiting vasodepressor reflexes similar to those observed in neurocardiogenic syncope. Isolated or occasional premature beats suggest PACs or PVCs and are benign in the absence of structural heart disease.
Syncope in the setting of noxious stimuli such as pain, prolonged standing, or venipuncture, particularly when preceded by vagal-type symptoms (e.g., diaphoresis, nausea, vomiting) suggests neurocardiogenic (vasovagal) syncope. Occasionally, patients report abrupt syncope without prodromal symptoms, suggesting the possibility of the malignant variety of neurocardiogenic syncope. Malignant neurocardiogenic syncope denotes syncope in the absence of a precipitating stimulus, with a short or absent prodrome, often resulting in injuries, and is associated with marked cardioinhibitory and bradycardic responses spontaneously or provoked by head-up tilt-table testing.2 Sustained or paroxysmal sinus tachycardia, frequently associated with chronic fatigue syndrome and fibromyalgia, suggest the possibility of postural orthostatic tachycardia syndrome (POTS). This syndrome, which may be a form of autonomic dysfunction, currently is unexplained. It is characterized by a markedly exaggerated increased chronotropic response to head-up tilt-table testing and stress testing. POTS often has associated systemic signs, such as muscle aches (fibromyalgia), cognitive dysfunction, and weight loss. Inappropriate sinus tachycardia (IST) syndrome is similar in presentation, but it probably represents a separate disorder with another cause—possibly atrial tachycardias in the sinus node area or dysregulation of sinus node automaticity.
Because a number of tests are available for the diagnosis of cardiac arrhythmias, it is important to proceed with a stepwise approach. The goal is to obtain a correlation between symptoms and the underlying arrhythmia and initiation of appropriate therapy. Additional testing is usually advocated to identify patients with arrhythmias caused by ischemia or who are at risk for sudden cardiac death.
This section assumes a basic knowledge of cardiac arrhythmias and will not focus on specific aspects of arrhythmia identification and diagnosis, except to present the various treatment options available for the many commonly encountered arrhythmias. Excellent texts are available that provide core curriculum material for the identification of cardiac arrhythmias, rate determination, interval measurement, and identification of normal and abnormal P, QRS, and T wave morphologies.
The initial assessment of structural heart disease begins with the history and physical examination. Careful attention to CAD or MIs, risk factors for CAD, and family history of sudden cardiac death are extremely important. Careful scrutiny of the electrocardiogram (ECG) is imperative to look for conduction system delays, QRS widening, previous MI, or PVCs. Cardiac auscultation may detect an irregular rhythm or premature beats. Stress testing, usually with imaging (e.g., stress echocardiography or stress thallium and echocardiography) can demonstrate the presence of CAD, LV dysfunction, or valvular heart disease.
Frequently, patients present with a wide complex tachycardia, possibly VT versus SVT with aberrancy. Various algorithms have been described to facilitate the differentiation of wide complex tachycardias. Brugada and colleagues have synthesized the various schemes into one convenient and simple protocol (Fig. 4). The general rule, however, is that sustained or nonsustained wide complex tachycardia in patients with known CAD or previous MI is VT until proven otherwise.3 Obviously, the initial approach to sustained wide complex tachycardia is to carry out cardioversion if the patient is hemodynamically unstable. In stable patients, assume VT and treat empirically with intravenous medications (e.g., amiodarone, procainamide, lidocaine). If SVT with aberrancy is strongly suspected, diagnostic maneuvers, such as administering adenosine, may be cautiously used.
Ambulatory Holter monitoring has been available for several decades and has proved invaluable in identifying underlying rhythm abnormalities.4 Generally, 24- to 48-hour baseline Holter monitoring is useful in quantitating and qualifying arrhythmias in patients with frequent symptoms (Fig. 5).
For patients who have symptoms occurring on a weekly or monthly basis, Holter monitoring may not establish the diagnosis unless the patient fortuitously experiences an event during recording. Event recording monitoring systems, also called loop recorders (e.g., King of Hearts, Instromedix, Rosemont, Ill) can be worn for longer intervals (usually a month) and can document infrequent arrhythmia episodes and provide symptom-to-arrhythmia correlation. These devices are automatically activated or patient-activated and use telephone modem technology to transmit the electrocardiographic rhythm strips. They use continuous loop technology (retrograde memory) so that in the event of a symptom, the patient activates the device by pushing a button and records an electrocardiographic rhythm strip several minutes before the event. When prolonged external ambulatory event monitors fail to document an arrhythmia, an implantable device (Reveal, Medtronic, Minneapolis, Minn) can be used in patients with recurrent enigmatic syncope or arrhythmias, in whom conventional testing has not yielded a diagnosis. This device, with a battery life of 14 to 22 months, is implanted subcutaneously and continuously scans for arrhythmias (Fig. 6). The device automatically records and stores tachycardia or bradycardia events and can be patient-activated. Insurance reimbursement for the Reveal device requires extensive conventional diagnostic testing, including negative event monitors, tilt-table testing, and an electrophysiologic study (EPS). Preliminary reports of implantable event monitor studies have shown a significant reduction in time to diagnosis and decreased overall costs when used in patients with syncope and no structural heart disease.
Although initially touted as an important screening test for patients with syncope or ventricular arrhythmia risk, the signal-averaged ECG (SAECG) now has a limited role.5 The presence of low-amplitude late potentials, indicating a positive signal-averaged ECG, suggests an underlying abnormality in ventricular repolarization seen with a discrete scar and can be associated with ventricular ectopy and spontaneous VT (Fig. 7). However, the SAECG may be abnormal in patients with no evidence of structural heart disease and in patients with conduction disturbances (e.g., right bundle branch block [RBBB]) and therefore, a positive study has an uncertain specificity and sensitivity. In contrast, the SAECG can be helpful in screening patients or family members for arrhythmogenic right ventricular dysplasia (ARVD). Similarly, T wave alternans may have an important role for risk stratification in patients with LV dysfunction and complex ventricular arrhythmias. It has long been recognized that abnormalities in the ST segment and T wave may precede the onset of ventricular arrhythmias. Presumably, changes in autonomic activity, as well as repolarization, may facilitate the provocation of lethal ventricular arrhythmias in susceptible patients. Rosenbaum and colleagues6 have reported that abnormal T wave alternans may be an important marker for assessing patients and determining their risk for sudden cardiac death (SCD). T wave alternans can be measured by stress testing and ambulatory monitors (Fig. 8).
Wireless technologies have now been introduced that are capable of long-term cardiac telemetric monitoring for cardiac arrhythmias, both in the home environment and on an ambulatory basis. External monitoring systems can be worn continuously by the patient and use hard-wired telephone modem connections or wireless cellular network technology. These monitors automatically detect cardiac arrhythmias and transmit the telemetry strip to a central cardiac monitoring station, which alerts the patient, physician, or emergency response systems. These devices are capable of patient activation, but they also have automatic logic algorithms for detecting arrhythmias similar to those incorporated in defibrillators. This wireless technology has become available on implanted devices, such as pacemakers and defibrillators (Biotronik, Lake Oswego, Ore). These devices monitor for arrhythmias and detect pacemaker or defibrillator activity or device malfunction. Ambulatory cardiac monitoring provides an attractive alternative to prolonged hospitalization and may ultimately lower health care costs and reduce mortality.
Electrophysiologic testing has become an important standard for identifying high-risk patients who have nonsustained VT, such as those with previous MI and LV dysfunction (Fig. 9).5, 7 Inducible, sustained, monomorphic VT predicts substantial risk for subsequent, spontaneous, clinically sustained VT and ventricular fibrillation (VF). Electrophysiologic testing is the gold standard for evaluating patients with recurrent syncope and can help identify underlying His-Purkinje disease, inducible VT, SVT, and sinus node dysfunction (Box 1).8
|Box 1: Indications for Electrophysiologic Testing for Syncope
From Zipes DP, DiMarco JP, Jackman WM, et al: Guidelines for clinical intracardiac electrophysiological and catheter ablation procedures. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Clinical Intracardiac Electrophysiologic and Catheter Ablation Procedures), developed in collaboration with the North American Society of Pacing and Electrophysiology. J Am Coll Cardiol 1995;26:555-573.
Implantation of a permanent pacemaker requires specific levels of evidence and indications based on American College of Cardiology–American Heart Association (ACC/AHA guidelines.9 Class I and Class II indications are appropriate for the implantation of a permanent pacemaker (PPM). Correlation of symptoms with underlying bradyarrhythmias or heart block is required. Rarely, intuitive or empirical pacemaker implantation is performed. The implantable cardioverter-defibrillator (ICD) is indicated for sustained VT or VF, survivors of sudden cardiac death (AVID trial [Antiarrhythmics Versus Implantable Defibrillators], secondary prevention), 10 or inducible, sustained, monomorphic VT (MADIT I [Multicenter Automatic Defibrillator Implantation Trial], primary prevention).11 Based on the results of the MADIT II study, ICDs will routinely be implanted in patients with LV dysfunction, ejection fraction (EF) of less than 35%, and a previous MI.12 Emerging indications for implantation of ICDs include patients with syncope who have dilated cardiomyopathy and patients who have hypertrophic obstructive cardiomyopathy (HOCM) and are believed be at high risk for sudden cardiac death (nonsustained VTs, syncope, and family members who have experienced sudden cardiac death).
Since their introduction 40 years ago, pacemakers have advanced in sophistication, reliability, and longevity. Current pacemakers are expected to last at least 10 years and leads much longer. Although lead technology is continuously undergoing improvement, leads still fail because of material breakdown, fatigue, and manufacturing defects and may require removal and replacement (Fig. 10). Leads and devices may also need to be removed secondary to infection. Chronic leads are often heavily fibrosed by endovascular tissue. Lead extraction requires sophisticated equipment, such as lasers, and experienced operators for safe removal. ICD battery life is currently 5 to 7 years and continues to improve. Follow-ups of PPMs and ICDs are usually every 6 to 12 months, with comprehensive testing of pacing and sensing thresholds. Pacemakers can be dual chamber and have rate-response capability. Rate responsiveness simulates the chronotropic response of the sinus node and uses minute ventilation or, more commonly, motion to estimate the needed heart rate. Pacemakers and ICDs have extensive telemetric capacity, allowing retrieval of event, trend, battery, and lead data. PPMs and ICDs can also transmit limited data on the telephone. An ICD can obviously terminate VT or VF with a shock (Fig. 11), but it can also terminate sustained VT with antitachycardic pacing (ATP; Fig. 12).
Defibrillators can be single chamber or dual chamber and can have rate responsiveness as well (Fig. 13). The results of the Dual Chamber and VVI Implantable Defibrillator (DAVID) study have demonstrated that dual-chamber pacing ICDs in patients with decreased LV function lead to an increased incidence of congestive heart failure (CHF) and increased mortality.13 The presumed mechanism is by creating a functional left bundle branch block (LBBB), which can lead to cardiac desynchronization and heart failure. In contrast, restoring cardiac resynchronization with biventricular pacing can improve congestive heart failure symptoms and has led to an explosion of referrals to implant patients with Classes II to IV CHF and severe LV dysfunction.14 For the primary prevention of sudden death in patients with severe LV dysfunction, normal intact sinus node, AV node, and conduction, and no perceived or anticipated indication for pacing, the preferred ICD device is a single-chamber device (ventricular paced and inhibited [VVI] pacemaker). Dual-chamber ICDs should be reserved for patients with an abnormal SA or AV node or conduction system or those with frequent supraventricular arrhythmias, such as atrial fibrillation, to avoid spurious shocks secondary to rapid ventricular responses. When dual-chamber pacing is required, and the LV is severely impaired, consideration should be given to a biventricular ICD.
Radiofrequency catheter ablation (RFA) has been a revolutionary advance in the treatment of cardiac arrhythmias.7, 8, 15 For the first time, RFA has provided an opportunity to cure a specific cardiac disease completely. Introduced over 2 decades ago as DC ablation, and then again in the late 1980s with radiofrequency, RFA has proved to be a safe, efficacious, and cost-effective treatment for specific cardiac arrhythmias such as atrioventricular nodal re-entry tachycardia (AVNRT), orthodromic reciprocating tachycardia associated with WPW syndrome and concealed accessory pathways, normal heart VT (particularly right ventricular outflow tract tachycardia or fascicular tachycardia), and atrial flutter.7, 8, 16 In addition, RFA can provide adjuvant therapy for ischemic VT when the patient is experiencing frequent ICD shocks or failing antiarrhythmic therapy. Finally, pulmonary vein isolation as a treatment option for symptomatic, drug-refractory, paroxysmal, or persistent atrial fibrillation is gaining widespread acceptance and is undergoing intense clinical scrutiny.17 However, rate control and chronic anticoagulation are acceptable alternatives for asymptomatic or mildly symptomatic patients with atrial fibrillation according to the results of the Atrial Fibrillation Follow-up Investigation of Rhythm Management (AFFIRM) study.18
AVNRT is the most common of the SVTs (Fig. 14). Onset is usually in the third to fifth decade of life; the patient may present with a sustained, rapid, tachycardia rate of 180 to 240 beats/min. AVNRT originates from a micro re-entry around the fast and slow pathways of the AV node. Typically, AVNRT shows a narrow complex tachycardia without apparent P waves. Vagal maneuvers or adenosine can terminate AVNRT. Radiofrequency ablation has proved extremely effective at curing AVNRT, with success rates higher than 95%. Complication rates are low and, with successful modification of the AV node, specifically ablation of the slow pathway, the need for a permanent pacemaker is rare.
The presence of an accessory pathway (Kent bundle) in various locations around the tricuspid or mitral annulus results in a characteristic delta wave pattern on the ECG (Fig. 15). Macro re-entry tachycardia, called orthodromic reciprocating tachycardia (ORT) or AV reciprocating tachycardia (AVRT), occurs when the AV node is used in an antegrade direction and the accessory pathway is used in a retrograde direction. Typically, AVRT is a narrow complex tachycardia, but it may have small retrograde P waves visible between the QRS and T waves. When the accessory pathway is used in an antegrade direction, antidromic reciprocating tachycardia (ART), a wide complex tachycardia, occurs, which mimics VT. Atrial fibrillation is common with WPW syndrome. It is speculated that constant retrograde re-entry into the atrium during ventricular depolarization is responsible. Because of the potential for rapid conduction over an accessory pathway with atrial fibrillation and WPW, extreme caution must be exercised with AV nodal blocking agents, particularly digoxin, and calcium channel blockers. Although rare, atrial fibrillation with rapid ventricular response over an accessory pathway can initiate ventricular fibrillation, leading to sudden death. Acute treatment of atrial fibrillation and WPW consists of cardioversion and occasionally intravenous procainamide. The most common location for an accessory pathway is in the left ventricular free wall, but it also can be posteroseptal or right sided. Radiofrequency catheter ablation has been successful in ablating and curing WPW. Success rates approaching 97% have been safely achieved in experienced centers. For symptomatic WPW, particularly in young patients, RFA is considered to be the treatment of choice.
Radiofrequency ablation has also been extremely useful in curing typical atrial flutter (Fig. 16), which is identified by an atrial rate of 240 beats/min or higher and characteristic negative sawtooth flutter waves identified on the ECG, typically in inferior leads (II, III, and aVF). Mapping studies have revealed that typical flutter occurs with a counterclockwise rotation of atrial activation descending on the right atrial free wall, traversing the isthmus (zone between the coronary sinus orifice and tricuspid leaflet) and ascending the intra-atrial septum. Disruption of conduction over the isthmus by radiofrequency ablation can successfully eliminate the potential for typical flutter. In up to 25% of cases, patients continue to have atrial tachyarrhythmias, especially atrial fibrillation. Nevertheless, RFA is an acceptable first-line therapy for symptomatic atrial flutter.
The CAST study (Cardiac Arrhythmia Suppression Trial), published in 1989, radically changed the use of antiarrhythmic medications.19 CAST was designed to test the hypothesis that antiarrhythmic medication suppression of PVCs and nonsustained VT would improve mortality in patients following an MI who had decreased LV function. The medications selected—moricizine, flecainide, and encainide—were known to have potent ventricular arrhythmia suppression properties. However, CAST demonstrated an increase in mortality in patients treated with antiarrhythmic medications compared with placebo (Fig. 17). It was suspected that the increased mortality resulted from the proarrhythmic effects of these drugs, especially in the presence of ischemia and LV dysfunction.20 Therefore, type 1C drugs (Table 1) are contraindicated in patients with CAD and ischemia. Because of the CAST findings, there is concern that increased mortality could occur with other antiarrhythmics, especially when administered for relatively benign arrhythmias (e.g., atrial fibrillation, PVCs). Quinidine was subsequently shown to increase mortality when administered to patients with atrial fibrillation.21
|I||Sodium channel blockers|
|IA||Depress phase 0 of action potential; delay conduction, prolong repolarization—phase III or IV (quinidine, procainamide, disopyramide)|
|IB||Little effect on phase 0 of action potential in normal tissues; depress phase 0 in abnormal tissues; shorten repolarization or little effect (lidocaine, tocainide, mexiletine, diphenyl-hydantoin)|
|IC||Depress phase 0 of action potential; markedly slow conduction in normal tissues (flecainide, propafenone, moricizine)|
|II||β-Adrenergic blocking agents (acebutolol, atenolol, bisoprolol, carvedilol, metoprolol, nadolol, pindolol, propranolol)|
|III||Prolong action potential duration by increasing repolarization and refractoriness (amiodarone, sotalol, bretylium, dofetilide, azimilide, ibutilide)|
|IV||Calcium channel blockers (diltiazem, verapamil)|
From Chaudhry G, Muqtada MD, Haffajee CI: Antiarrhythmic agents and proarrhythmia. Crit Care Med 2000;28:N158-N164.
Since the publication of the CAST study, many other reports have confirmed the proarrhythmic effects of antiarrhythmic medication when used capriciously. This has led to specific guidelines for the use of antiarrhythmic medications, especially those that prolong the QT interval and increase proarrhythmia. Usually, types IA and III medications are initiated in the hospital with telemetry monitoring. Type IC agents, however, are relatively safe when used in a normal heart. Similarly, amiodarone, because of its long half-life (43 days to months) and low incidence of proarrhythmia, usually can be initiated at low doses in an outpatient setting in the absence of severe LV dysfunction or bradycardia. Based on the results of the CAST study, the U.S. Food and Drug Administration (FDA) and pharmaceutical industry took unprecedented measures to ensure appropriate prescription practices and credentialing of ordering physicians when the new antiarrhythmic medication, dofetilide (Tikosyn), was released for use in patients with atrial fibrillation.
Occasionally, sustained and nonsustained VT can occur in the absence of structural heart disease, so-called normal heart VT. In general, prognosis is good, with a low risk for sudden cardiac death. Examples include right ventricular outflow tract (RVOT) and left ventricular outflow tract (LVOT) VT, fascicular VT, idiopathic left VT, repetitive monomorphic VT, and sinus of Valsalva VT22 (Fig. 18). Treatment usually is with beta blockers or calcium channel blockers and, if refractory, RFA. Treatment is aimed at symptom suppression.
Arrhythmogenic right ventricular dysplasia (ARVD) is a genetic disease in which right ventricular normal architecture is disrupted by progressive infiltration and transformation into fatty fibrous material. This creates the potential for chaotic depolarization and VT. Symptoms include palpitations, VT, and sudden cardiac death. Diagnosis is suspected by an abnormal ECG showing RBBB, juvenile T wave pattern (inverted precordial T waves), and epsilon waves (prominent deflections in the ST segment, often best seen at higher recording speeds; Fig. 19). The SA ECG is often abnormal, with the presence of low-amplitude late potentials. The ECG reveals RV dysfunction and aneurysms. The diagnosis of ARVD is confirmed with an abnormal computed tomography (CT) or magnetic resonance imaging (MRI) scan showing the typical fatty infiltration (Fig. 20). Symptomatic patients are screened for ventricular arrhythmias with EPS and, if positive, receive an ICD.
Long QT syndrome (LQTS) is a genetically transmitted disorder causing metabolic abnormalities of cardiac myocyte sodium and potassium channel depolarization (channelopathy), causing prolongation of the QT interval. This prolongation increases susceptibility to spontaneous polymorphic VT, torsades de pointes, and VT (Fig. 21). Treatment consists of atrial pacing, beta blockers, specific antiarrhythmics to improve repolarization, and ICDs in high-risk patients.
Brugada syndrome is a relatively rare cause of VT and fibrillation. It is characterized by an abnormal ECG exhibiting a right bundle branch block pattern and ST segment elevation in the precordial leads (Fig. 22). Treatment requires implantation of an ICD in patients with syncope and complex ventricular arrhythmias.
Bundle branch re-entry (BBR) VT, in which re-entry around components of the His-Purkinje system and one of the bundle branches results in a VT that resembles the native QRS. This type of VT is often seen in dilated cardiomyopathy. The major prerequisite is a baseline conduction delay, usually LBBB. Confirmation of this type of VT requires an EPS demonstrating His-Bundle participation in the VT. BBR VT can be eliminated by right bundle branch ablation and usually does not result in complete heart block. Patients still require an ICD because other morphologies of VT are usually present or probable.
In the absence of structural heart disease, benign arrhythmias, such as PACs, SVT, PVCs, and atrial fibrillation, have been shown to have an excellent prognosis.10 Radiofrequency ablation for SVT has a higher than 95% success rate, with no long-term adverse side effects. The use of ICDs has improved survival in primary and secondary prevention trials. Secondary prevention trials, such as the AVID study, enrolled patients who had a life-threatening arrhythmia and who were successfully resuscitated.10 The defibrillator proved superior to medical therapy, usually amiodarone, in preventing sudden death. Primary prevention trials have focused on high-risk groups who have not already experienced an untoward ventricular arrhythmia. The MADIT and MUST (Multicenter Unsustained Tachycardia) trials have confirmed superiority of the ICD and survival in patients with inducible, sustained, monomorphic VT during an EPS and a history of CAD and MI.11, 23 The MADIT-II study has suggested that the EPS is superfluous for risk stratification in patients with LV dysfunction (EF <35%) and a history of CAD, regardless of whether ventricular arrhythmias are present or absent.12 Patients who received an empirical ICD had a significant survival benefit.
Following the publication of MADIT-II results, there was considerable debate about its applicability for clinical practice. A major concern was that widespread adoption of MADIT-II ICD criteria could bankrupt an already stressed health care system. Subsequently, many consensus panels convened to determine the appropriateness of the MADIT-II criteria for ICD therapy. Initially, some insurance carriers required that in addition to the MADIT-II criteria (LV dysfunction, previous MI, EF <35%), the patient should also have a widened QRS interval to qualify for a defibrillator. This was based on ancillary studies showing that a wide QRS could be an independent predictor of mortality. This stringent criterion existed for approximately 1 year, until it was abandoned.
Currently, any patient meeting MADIT-II criteria with an EF lower than 35% and LV dysfunction caused by MI is a candidate for a defibrillator (Box 2). In addition, indications for ICD implantation have been expanded to include patients with a dilated cardiomyopathy and an LV EF of 35% or lower, based on the results of the SCD-HeFT (Sudden Cardiac Death in Heart Failure Trial) and DEFINITE (Defibrillators in Non-Ischemic Cardiomyopathy Treatment Evaluation) studies.24, 25 Furthermore, patients with dilated and ischemic cardiomyopathies, with a wide QRS (typically >130 msec) and recurrent heart failure, with functional class III or IV symptoms, are ideal candidates for the implantation of a biventricular ICD system. Ongoing studies, such as the RethinQ (Resynchronization Therapy in Narrow QRS) trial and MADIT-CRT, are looking at the role of biventricular pacing in patients with a narrow QRS and those with functional Class I or II symptoms, respectively. If these two studies yield positive results for survival in these subclassifications, it will certainly add to the unprecedented growth of ICD implantation. Anticipating the tremendous need for ICD services, current development is focused on leadless ICD systems, in which a subcutaneous placement of the ICD device could be performed more simply by electrophysiology specialists, as well as by other physicians.
|Box 2: Indications for Implantable Cardioverter-Defibrillator
|Boutique Indications For ICD|
DEFINITE, Defibrillators in Non-Ischemic Cardiomyopathy Treatment Evaluation; EF, ejection fraction; ICD, implantable cardioverter-defibrillator; MADIT-II, Multicenter Automatic Defibrillator Implantation Trial 2; RethinQ, Resynchronization Therapy in Narrow QRS; SCD-HeFT, Sudden Cardiac Death in Heart Failure Trial; VF, ventricular fibrillation; VT, ventricular tachycardia.
Recently, media attention has been focused on the potential for ICD malfunction or device failure.26 Most manufacturers have recognized potential design flaws, such as premature battery depletion, oversensing or undersensing of ventricular arrhythmias, and crosstalk. In general, since the introduction of the defibrillator nearly 3 decades ago, the devices have been extremely reliable. The failure rates as reported have been extremely low and have not appreciably increased. However, the burgeoning use of ICDs has led to an awareness of manufacturing defects although, as noted, their incidence has remained relatively low. Many potential device recalls can be managed conservatively with expedited and intensified follow-up of battery status and the use of home telephonic monitoring modalities, such as CareLink (for more information, see www.medtronic.com/carelink).