Daniel J. Cantillon and Oussama Wazni
DEFINITION OF SUDDEN CARDIAC DEATH
Sudden cardiac death (SCD) is defined by 2006 American College of Cardiology/American Heart Association/Heart Rhythm Society (ACC/AHA/HRS) guidelines as the abrupt cessation of cardiac activity so that the victim becomes unresponsive, without normal breathing or circulation, which progresses to death in the absence of any corrective measures. The definition excludes noncardiac conditions such as pulmonary embolus, intracranial hemorrhage, or airway obstruction. However, it does not exclude nonarrhythmic deaths, as the terminal rhythm is often unknown.
EPIDEMIOLOGY OF SUDDEN CARDIAC DEATH
The incidence of SCD is estimated to be 300,000 to 400,000 per year in the United States. The mortality rate is high, with only 2% to 15% of patients reaching the hospital alive. Fifty percent of these hospitalized patients die before discharge. There is a high recurrence rate of 35% to 50%. More than 75% to 85% of SCDs are associated with ventricular arrhythmias. The most common arrhythmias are ventricular tachycardia (VT) (62%), torsades de pointes (TdP) (13%), primary ventricular fibrillation (VF) (8%), followed by bradycardia (7%). SCD is the first presentation of cardiac disease in 25% of patients. The incidence increases with age at an absolute incidence of 0.1% to 0.2% per year. Men are more commonly affected (3:1). SCD in patients < 35 years of age is most commonly associated with hypertrophic cardiomyopathy (HCM). In patients >35 years old, SCD is most commonly associated with coronary artery disease. Determinants of survival are rapid external defibrillation and bystander cardiopulmonary resuscitation. Hospital-based cooling protocols are being increasingly adopted to improve the neurologic prognosis for sudden death survivors.
RISK FACTORS AND PATHOPHYSIOLOGY
Risk factors include the following:
Prior cardiac arrest: high recurrence rate, up to 35% to 50% at 2 years
Syncope in the presence of coexisting cardiac diseases
Reduced left ventricular (LV) function and congestive heart failure (CHF)
Ventricular premature contractions and nonsustained ventricular tachycardia (NSVT) post-myocardial infarction
Myocardial ischemia and/or documented scar
Conduction system disease
The pathophysiology is determined by trigger factors and an underlying substrate conducive to arrhythmia. The mechanism of SCD can be related to any of the following pathophysiologic mechanisms:
Anatomical reentry around scarred myocardium
Functional reentry using a diseased His–Purkinje system, or areas of nonhomogeneous (anisotropic) conduction
Ischemia, electrolyte imbalance, ion channel abnormalities, surges in neurosympathetic tone, antiarrhythmic drugs
Rapid and irregular ventricular activation (i.e., atrial fibrillation [AF] with rapid ventricular response in Wolff–Parkinson–White [WPW]).
Bradycardic SCD is overall uncommon; however, it remains important in specialized situations like cardiac transplant recipients.
SUDDEN CARDIAC DEATH AND CORONARY ARTERY DISEASE
Coronary artery disease is present in 80% of those with SCD. Approximately 75% have a history of prior myocardial infarction (MI). Sudden death can be the first clinical manifestation in up to 25% of patients with coronary artery disease. Approximately 65% have three-vessel obstructive coronary disease. Risk factors include depressed LV systolic function and frequent ventricular premature depolarizations (VPDs). However, the Cardiac Arrhythmia Suppression Trial (CAST) study demonstrated that suppression of VPDs with class IC medications resulted in higher mortality. Data from large clinical trials have identified groups of patients with coronary artery disease, depressed LV systolic function, and nonsustained ventricular arrhythmias at increased risk for SCD, as discussed later in this chapter.
NONISCHEMIC CARDIOMYOPATHY
Patients with impaired LV systolic function in the absence of coronary artery disease are at increased risk for ventricular tachyarrhythmias and SCD, particularly those with symptomatic heart failure. In the heart failure population, total mortality is approximately 25% at 2.5 years, with SCD accounting for 25% to 50% of these cases. Mortality due to SCD is much higher in New York Heart Association (NYHA) classes II and III than in class IV patients, who have excess mortality due to pump failure.
Dilated Cardiomyopathy
Fifty percent of deaths in this patient subgroup are arrhythmic. Left ventricular ejection fraction (LVEF) is predictive of sudden death, due to either circulatory failure or fatal arrhythmia. Ventricular ectopy is very common and does not appear to be as predictive of SCD as it is in patients with coronary artery disease. Up to 80% of patients may have NSVT on Holter monitoring. Inducibility for ventricular tachyarrhythmias at the time of electrophysiologic (EP) study is also less predictive when compared to patients with coronary disease and has almost no role in risk stratification. While some modalities such as abnormal microvolt T-wave alternans and heart rate variability have been associated with increased risk in this population, only LVEF ≤35% and the presence of symptomatic heart failure are recommended by practice guidelines for the purposes of risk stratification, particularly regarding selecting candidates for implantable cardioverter-defibrillator (ICD) implantation.
Hypertrophic Cardiomyopathy
HCM is an autosomal dominant disease with incomplete penetrance associated with ever-increasingly discovered genetic mutations. The overall incidence of SCD is 2% to 4% in adults and up to 6% in children. It is the most common cause of SCD in young athletes. Maron et al. have identified factors associated with increased risk such as the presence of NSVT, hypotension with exercise, unexplained syncope, septal thickness >3 cm, and a family history of sudden death in a first-degree relative younger than 50 years old. Among patients with HCM and primary prevention ICDs, appropriate therapy was uncommon among patients with none of these risk factors and occurred in 14% of patients with one risk factor, 11% in patients with two risk factors, and 17% in patients with three or more risk factors. In addition, cardiac magnetic resonance imaging (MRI) has been increasingly utilized for risk stratification in HCM patients as the presence of basal septal scar is associated with VT as well as histopathologic changes. The role of genetic testing for risk stratification has not been established despite the identification of certain high-risk mutations, such as the LAMP2 gene.
Arrhythmogenic Right Ventricular Cardiomyopathy
Progressive fibrofatty right ventricular tissue replacement is the pathologic hallmark of this disease. There is a strongly familial pattern; its prevalence may be up to 20% worldwide for SCD in young patients (U.S. 3%). MRI is the most useful imaging modality to make the diagnosis. Characteristic epsilon waves may be present on the electrocardiogram (ECG), as shown in Figure 27.1. The frequency and the severity of ventricular arrhythmias in this disease are progressive, thus making these patients candidates for ICD implantation.
FIGURE 27.1 ECG reading in ARVD.
INHERITTED AND ACQUIRED CHANNELOPATHIES
Long-QT Syndrome
Long-QT syndrome consists of the inherited abnormalities that prolong cardiac repolarization as measured by the corrected QT (QTc) interval on surface ECG, which confer an increased risk of SCD by polymorphic ventricular tachycardia (PMVT) or TdP. There are numerous identified mutations involving mostly sodium and potassium ion channels (Table 27.1). Abnormal QTc cutoff values are commonly selected as >440 milliseconds in men and >460 milliseconds in women, although actual cardiac events occur on a skewed curve and are highest among patients with QTc > 500 milliseconds. The most common symptoms associated with long QT syndrome are palpitations and syncope, although associations also exist with seizure disorders. Characteristic clinical triggers for arrhythmia events have been described by genotype including exercise or swimming (LQT1), auditory stimuli, or during the postpartum period (LQT2) and during sleep (LQT3). Features associated with higher risk for SCD include the Jervel and Lange-Nielsen syndrome (congenital deafness), syncope or ventricular arrhythmias while on beta-blocker therapy, QTc > 500 milliseconds with an LQT1 or LQT2 genotype, female gender, and family history of SCD. Standard treatment includes beta-blocker therapy, but this remains somewhat controversial in the case of LQT3, where the clinical response rate is lowest. According to 2008 ACC/AHA/HRS device therapy guidelines, patients with syncope or ventricular arrhythmias while on beta-blocker therapy can be considered for primary prevention ICD implantation. The 5-year sudden-death risk in patients on beta-blocker therapy (Long QT Registry) is <1% in asymptomatic patients, 3% in the syncope group, and 13% in the SCD group. Mexiletine, a sodium channel blocker, may be helpful in reducing the burden of ventricular arrhythmias among patients with LQT3, due to the attributable gain-of-function mutation in SCN5A resulting in voltage-gated sodium channels remaining open longer than normal.
TABLE
27.1 Familial Long-QT Syndromes
A prolonged QT interval can also be acquired and secondary to other causes:
Electrolyte derangements
Acute hypokalemia
Chronic hypocalcemia
Chronic hypokalemia
Chronic hypomagnesemia
Medical conditions
Bradyarrhythmias (complete heart block, sick sinus syndrome, bradycardia)
Cardiac (myocarditis, tumors)
Endocrine: hyperparathyroidism, hypothyroidism, pheochromocytoma
Neurologic (cerebrovascular accident, encephalitis, head trauma, subarachnoid hemorrhage)
Nutritional (alcoholism, anorexia nervosa, liquid-protein diet, starvation)
Drugs
Antiarrhythmics: class IA (disopyramide, procainamide), class III (sotalol, dofetilide)
Tricyclic antidepressants (amitriptyline, desipramine)
Antifungals (itraconazole, ketoconazole)
Antihistamines (astemizole, terfenadine)
Antimicrobials (Bactrim, E-mycin, pentamidine)
Neuroleptics (phenothiazines, thioridazine)
Organophosphate insecticides
Promotility agents (cisapride)
Oral hypoglycemics (Glibenclamide)
Brugada Syndrome
The hallmark of this condition is ST-segment elevation in the right precordial leads (Fig. 27.2) attributable to sodium channel defects, including the SCN5A mutation. However, specific genetic mutations are identified in less than half of tested patients. Brugada syndrome is most common in Southeast Asia, affecting mostly men (4:1). The usual mode of inheritance is autosomal dominant. The ST segment may normalize and may be unmasked by drugs (procainamide, flecainide, ajmaline), which may uncover the characteristic ST elevation. Patients with Brugada syndrome are at increased risk of SCD, particularly those with prior torsades/PMVT and unexplained syncope. The role of EP testing in risk stratification remains controversial, although there are some data to support its use in identifying patients more likely to benefit from an ICD. Quinidine, due to blockade of Ito, may have a role in decreasing the burden of ventricular arrhythmias.
FIGURE 27.2 ECG reading in Brugada syndrome.
Short-QT Syndrome
The short-QT syndrome is characterized by QTc intervals <300 milliseconds and associated with SCD by VT and VF. Gain-of-function mutations in the gene for outward potassium currents have been shown to be responsible for this congenital syndrome. HERG (or KCNH2) and KCQN1 gene mutations have been identified in some families. Management consists of implantation of an ICD and possibly quinidine, which can prolong the QT interval and prevent VT.
Idiopathic Ventricular Fibrillation
Primary (idiopathic) VF, by definition, occurs in the absence of the recognized channelopathies and structural heart disease. For decades, this condition has remained poorly understood until work by Haïssaguerre et al. described a high prevalence of an early repolarization pattern detected on the surface ECG among patients with idiopathic VF when compared to a control group (31% vs. 5%). Early repolarization was defined as elevation of the QRS-ST junction (the J point) by >0.1 mV above the baseline in at least two leads, or notching of the terminal QRS in the inferior limb leads, lateral limb leads, or lateral precordial leads. Subsequent experimental data have linked defective modulation of cardiac repolarization to increased risk for ventricular arrhythmias in such patients. These data challenge the long-held notion that early repolarization, commonly present among African American males and athletes, is always a benign finding.
OTHER CARDIAC CONDITIONS ASSOCIATED WITH SUDDEN DEATH
Wolff–Parkinson–White Syndrome
WPW syndrome is caused by accessory atrioventricular connections. There is a 0.1% incidence of SCD per year, with risk related to the conduction properties of the bypass tract. SCD is related to AF conducting rapidly antegrade over the bypass tract into the ventricles, which can then degenerate into VF. The risk is elevated when the shortest R-R interval in AF is <250 milliseconds (240 beats/min), which indicates a pathway capable of rapid antegrade conduction. WPW is now curable with catheter ablation in >95% cases. Asymptomatic patients with WPW in low-risk occupations or with loss of pre-excitation during exercise testing do not require ablation.
Valvular Heart Disease
Any primary valvular pathology associated with depressed LV systolic function confers increased risk for ventricular arrhythmias. Specific valvulopathies such as aortic stenosis, when severe or critical, confer increased risk even when LV systolic function is preserved. Mitral valve prolapse, in rare cases, has been associated with ventricular arrhythmias and SCD also in the setting of preserved LV systolic function. However, literature in this field is limited to case reports and small series data. It remains unclear to what extent valvular correction modifies this risk, and the 2006 ACC/AHA/HRS sudden death guidelines defer treatment recommendations according to the established criteria for mitral valve correction.
Other SCD causes without primary arrhythmia etiology:
Acute aortic dissection, particularly with retrograde extension causing hemopericardium
Mechanical complications following MI such as rupture, tamponade
Congenital heart disease, including coronary anomalies (between aorta and pulmonary artery)
Cyanotic heart disease, right-to-left intracardiac shunts
Commotio cordis (VF associated with chest trauma)
Acute myocarditis
Infiltrative cardiomyopathies such as cardiac amyloid or sarcoid
Chagas disease: multifocal myocarditis, CHF
Muscular dystrophies: myocardial scarring, conduction system disease
EVALUATION AND MANAGEMENT
Sudden Cardiac Death Survivors
A complete history and physical examination focusing on risk factors, medications, illicit substances, and family history should be obtained. Laboratory evaluation should identify any related electrolyte abnormalities, particularly among patients with renal dysfunction. A complete cardiac evaluation includes a 12-lead ECG, ambulatory Holter or inpatient telemetry monitoring, a surface echocardiogram, an ischemia workup (stress testing or coronary angiography), and possibly MRI in selected scenarios such as to evaluate the possibility of arrhythmogenic right ventricular dysplasia (ARVD) or infiltrative cardiomyopathies. Other imaging modalities, such as cardiac positron emission tomography (PET) scans, are selectively utilized to identify proinflammatory conditions such as cardiac sarcoid. In general, ACC/AHA/HRS practice guidelines recommend identification and treatment of reversible causes among sudden death survivors. Among SCD survivors, diagnostic EP studies have limited prognostic value and are not routinely performed except in selected cases such as investigating the etiology of an unknown widecomplex tachycardia. In the majority of SCD survivors, an ICD is indicated in the absence of a transient arrhythmia due to identifiable, reversible causes (i.e., VF within 24 to 48 hours of acute ST-segment elevation MI). In the AVID trial, SCD survivors who received an ICD demonstrated an improved 3-year survival rate of 75.4% when compared to 64.1% with antiarrhythmic drug therapy.
VENTRICULAR TACHYCARDIA
Coronary Artery Disease
Sustained monomorphic VT is most commonly related to scar created by prior MI that can be initiated by spontaneous ventricular ectopy. An acute ischemic event, in contrast, is more commonly associated with PMVT or VF such as in acute ST-segment elevation MI. This distinction becomes blurred when transient ischemia causes an increase in spontaneous ventricular ectopy capable of initiating monomorphic VT in a patient with underlying scar, or PMVT/VF when critically timed VPDs occur during cardiac repolarization.
For secondary prevention, ICD therapy is recommended for hemodynamically intolerant sustained ventricular tachyarrhythmias >30 seconds in duration or requiring abortive therapy (i.e., shocks). This includes patients with acute MI with events occurring beyond 48 hours and not related to immediately reversible causes (i.e., overinjection of contrast dye during angiography of the right coronary artery). For primary prevention, ICD therapy is recommended in patients beyond 40 days post-MI with LVEF ≤ 35% on optimal medical therapy and with life expectancy >1 year. Key trials in formulating these primary prevention indications include the MADIT-2 trial (ICD benefit for patients post-MI with LVEF < 30% and NYHA class I symptoms) and SCD-HeFT (ICD benefit for LVEF ≤35% and NYHA class II symptoms). In addition, patients may be considered for a primary prevention ICD with prior MI, LVEF <40%, NSVT detected by ambulatory Holter or telemetry and inducible VT with programmed stimulation at the time of EP study, largely on the basis of the MADIT-1 trial (LVEF ≤ 35% with NSVT) and MUSST registry (LVEF ≤ 40% with NSVT).
Current ICD therapy can terminate up to 80% of all spontaneous VT with antitachycardia pacing (ATP). However, up to one-third of patients will still require antiarrhythmic medications to suppress VT and to minimize shocks and ATP. Catheter ablation of reentrant circuits is indicated for patients with VT that is refractory to medications and requiring multiple ICD shocks.
Dilated Cardiomyopathy
More than a quarter of patients with dilated cardiomyopathy (DCM) have NSVT on Holter monitoring during a 24-hour period. ICD implantation is recommended for secondary prevention in patients with DCM and prior sustained VT/VF, and also as primary prevention for patients with LVEF ≤ 35% with NYHA class II symptoms based on the SCD-HeFT trial. The 2008 device therapy guidelines also allow a primary prevention ICD to be offered to patients with DCM, LVEF ≥ 35%, and NYHA class I symptoms, although this is based on weaker evidence. In addition to scar-related VT, patients with DCM are particularly susceptible to bundle-branch reentry VT. This is a VT most commonly occurring with left bundle branch bundle (LBBB) morphology. Electrophysiologic testing reveals abnormal conduction in the His–Purkinje system as measured by a prolonged HV interval in sinus rhythm. Most frequently, the right bundle is used as the antegrade limb and the left bundle as the retrograde limb of the tachycardia, and the right bundle is typically targeted for catheter ablation.
Ventricular Tachycardia and the Structurally Normal Heart
Outflow-Tract Ventricular Tachycardia
VT occurring in patients without structural heart disease most commonly originates from discrete foci in the right and the left ventricular outflow tracts (LVOTs). The right ventricular outflow tract (RVOT) is more common than the LVOT by a ratio of 9:1. However, data by Iwai et al. suggest these tachycardias share identical electrophysiologic mechanisms and clinical behavior due to embryologic origin in the maturation of the outflow tract. Outflow-tract tachycardias can occur as sustained monomorphic VT, frequent salvos of NSVT, or frequent symptomatic VPDs. Reported mechanisms include triggered activity due to delayed-after-depolarization (during phase 4). This mechanism is unique when compared with VT caused by reentry or enhanced automaticity. Unlike catecholaminergic polymorphic ventricular tachycardia (CPMVT), outflow-tract VTs are always monomorphic and most commonly elicited by either the warm-up or the cooldown phases of exercise that can be mimicked in the EP lab during the “wash-out” phase of an isoproterenol infusion.
The classic ECG pattern for RVOT VT is a LBBB with precordial R-wave transition in V2–V3, and an inferior limb lead axis, with tall R waves in II, III, and aVF. LVOT VT can occur either with a RBBB morphology, inferior axis, or a LBBB morphology, inferior axis with earlier precordial R-wave transition by V2 (Fig. 27.3). In the EP lab, outflow-tract tachycardia may be induced with programmed stimulation and has a characteristic pharmacologic response of adenosine sensitivity in most cases. Mapping and ablation of the site of earliest origin is highly successful. Left-sided foci are also commonly ablated from left and right coronary cusps above the aortic valve. Outflow-tract VTs are not associated with SCD, and catheter ablation is curative. Therefore, ICD therapy is not indicated (class III recommendation by 2008 guidelines).
FIGURE 27.3 ECG reading in RVOT VT.
Idiopathic Left Ventricular Tachycardia
This is a paroxysmal VT that occurs predominantly in men, most commonly involving the LV anterior and posterior fascicles. It is characterized by the following triad: (a) inducibility by atrial pacing or premature complexes, (b) RBBB morphology most commonly with left anterior hemiblock pattern, and (c) absence of structural heart disease. This type of VT is highly sensitive to calcium channel blockers like verapamil. Ventricular activation at the earliest site is usually preceded by highfrequency potentials termed Purkinje potentials. Ablation at these sites is highly successful in terminating this arrhythmia. In the absence of concomitant structural heart disease, true fascicular VT is not associated with SCD and thus not recommended for ICD implantation according to the 2008 guidelines.
ACKNOWLEDGMENTS
The authors wish to acknowledge the contributions of J. David Burkhart, MD to an earlier version of this chapter.
SUGGESTED READINGS
Epstein AE, DiMarco JP, Ellenbogen KA, et al. ACC/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities. J Am Coll Cardiol. 2008;51:E1–E62.
Faramarz, Samie H, Jalife J. Mechanisms underlying ventricular tachycardia and its transition to ventricular fibrillation in the structurally normal heart. Cardiovasc Res. 2001;50:242–250.
Haïssaguerre M, Derval N, Sacher F, et al. Sudden cardiac death associated with early repolarization. N Eng J Med. 2008; 358(19):2016–2023.
Iwai S, Cantillon DJ, Kim RJ, et al. Right and left ventricular outflow tract tachycardias: evidence for a common electrophysiologic mechanism. J Cardiovasc Electrophysiol. 2006;17(10):1052–1058.
Maron BJ, Spirito P, Shen W, et al. Implantable cardioverter- defibrillators and sudden death in hypertrophic cardiomyopathy. JAMA. 2007;298(4):405–412.
Nogami A. Idiopathic left ventricular tachycardia: assessment and treatment. Cardiac Electrophysiol Rev. 2002;6:448–457.
Wever EFD, Robles de Medina EO. Sudden death in patients without structural heart disease. J Am Coll Cardiol. 2004;43(7):1137–44.
Zipes DP, Camm AJ, Borggrefe M, et al. ACC/AHA/ESC 2006 Guidelines for Management of Patients with Ventricular Arrhythmias and Prevention of Sudden Cardiac Death. J Am Coll Cardiol. 2006;48:e247–e346.
QUESTIONS AND ANSWERS
Questions
1. Clinical features associated with increased risk for sudden cardiac death (SCD) among patients with hypertrophic cardiomyopathy (HCM) include all of the following except:
a. Septal thickness >3 cm
b. Syncope or hypotension associated with exercise
c. Dynamic left ventricular outflow-tract (LVOT) gradient >100 mm Hg by Doppler echocardiography
d. Nonsustained ventricular tachycardia (NSVT)
2. A primary prevention implantable cardioverter- defibrillator (ICD) is most strongly indicated in which of following patients?
a. Male patient with syncope, QTc 440 milliseconds, and LQT1 genotype not previously treated with beta-blockers
b. Young patient with syncope, NSVT, and diagnostic criteria for arrhythmogenic right ventricular cardiomyopathy (ARVC), including cardiac magnetic resonance imaging (MRI)
c. Asymptomatic patient with newly diagnosed nonischemic dilated cardiomyopathy (DCM), left ventricular ejection fraction (LVEF) 35%
d. Young patient without structural heart disease and monomorphic ventricular tachycardia (VT) (left bundle branch bundle [LBBB] morphology, right inferior axis, precordial R-wave transition in V3)
3. A diagnostic EP study is least useful in which of the following clinical scenarios?
a. Risk stratification in a patient with coronary artery disease, LVEF 38% with NSVT
b. Risk stratification in a patient with DCM, LVEF 20%, and heart failure symptoms
c. Risk stratification and evaluation for arrhythmia mechanism in a patient with a Brugada pattern electrocardiogram (ECG) and unexplained syncope
d. Evaluation of arrhythmia mechanism and possible ablative therapy for a patient with symptomatic wide-complex tachycardia of unknown etiology
4. Bundle branch reentry VT is most commonly associated with:
a. Enhanced automaticity in the right bundle
b. Enhanced automaticity in the left bundle
c. Supranormal conduction in the His bundle
d. Abnormally slow conduction in the His-Purkinje system
5. The ECG shown is consistent with:
a. Acute anteroseptal MI
b. Abnormal SCN5A channel
c. Abnormal KCQN1 channel
d. Old anteroseptal MI with an aneurysm
Answers
1. Answer C: Clinical features associated with SCD among patients with HCM include NSVT, hypotension associated with exercise, unexplained syncope, septal thickness >3 cm, and a family history of sudden death in a first-degree relative younger than 50 years old. Dynamic LVOT gradients by Doppler echocardiography, even when elevated, are not a commonly applied as a guidelines-based risk stratification tool for SCD.
2. Answer B: ARVC is a progressive disease involving fibrofatty infiltration, and patients with VT are highly likely to have recurrences despite medical therapy. Among patients with symptomatic long QT syndrome, response to beta-blocker therapy is highest among patients with the LQT1 genotype. The SCD risk is very low among male patients treated with beta-blockers, LQT1 genotype, and a QT interval in this range. Among patients with DCM, the 2008 guidelines do not specify a treatment duration requirement, unlike CMS reimbursement criteria. However, ICD implantation is recommended for patients with LVEF ≤ 35% and symptomatic heart failure (class I recommendation for NYHA functional class II or greater, and class II recommendation for NYHA functional class I). In choice d, all of these features are associated with VT originating from the right ventricular outflow tract (RVOT), which is curable by catheter ablation and not associated with SCD. ICD implantation is a class III recommendation among such patients.
3.Answer B: A diagnostic EP study is least predictive for future ventricular tachyarrhythmia events among patients with nonischemic cardiomyopathy, and would not alter management in the patient described in choice b, who is recommended for a primary prevention ICD. The patient in choice a, however, meets guidelinesbased criteria for further risk stratification using EP study to evaluate candidacy for a primary prevention ICD. EP testing for risk stratification among patients with the Brugada syndrome remains controversial. However, EP testing is not completely unreasonable to evaluate a possible arrhythmia mechanism for any patient with unexplained syncope. It is certainly not the weakest indication among the choices listed. In keeping with this concept, choice d actually highlights the strength of an EP study, which is to define the precise arrhythmia mechanism in a patient with a poorly tolerated, wide complex tachycardia that cannot be definitively diagnosed by other clinical criteria (i.e., distinguishing VT from supraventricular tachycardia (SVT) with aberrant conduction or pre-excited tachycardia). This is particularly true where catheter ablation can be curative.
4. Answer D: Abnormally slow conduction in the His-Purkinje system sets up the conditions of reentry required to sustain this kind of tachycardia.
5. Answer B: The ECG is consistent with abnormal SCN5A channel, which causes the Brugada syndrome.