**2. Sudden cardiac death and various forms of cardiomyopathy**

Most cardiomyopathies with primary myocardial pathology predispose to sudden cardiac death. These include dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), left ventricular noncompaction and arrhythmogenic right ventricular cardiomyopathy (ARVC). Apart from the primary pathologies involving the myocardium, various other conditions can affect the myocardium secondarily due to myocardial stress, ischemia and

© 2013 Kumar and Mounsey; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

body as a compensatory hemodynamic mechanism. The efficacy of neuro-humoral blockers like beta-blocker and renin-angiotensin-aldosterone axis blockers in effectively reducing the risk of sudden cardiac death in cardiomyopathy, and relation of the risk of sudden cardiac death to degree of hemodynamic jeopardy with cardiomyopathy suggest the latter mechanism. In the following sections we will discuss the current knowledge of mechanisms of sudden

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Ischemic cardiomyopathy is by far the most common cardiomyopathy leading to SCD Commonest cause of SCD in these patients is ventricular tachyarrhythmia. Beyond the early post-MI period, when recurrent MI and associated complications (mechanical and arrhythmic) are more likely, almost three-fourth of patient deaths among those with prior MI (more than three months old) and LV dysfunction are sudden and presumably arrhythmic, most likely due to ventricular arrhythmias. [2] Susceptibility to ventricular arrhythmia in these patients has multiple mechanisms. Scar resulting from myocardial infarction provides substrate for reentrant ventricular tachycardia. Re-entry circuits involve areas of residual viable relatively slowly conducting myocardial tissue inside the scars. These tracks of slowly conducting myocardial tissue inside a scar, called isthmus, connecting two healthy or relatively healthy areas form a full circuit for re-entrant arrhythmia. Patients with larger myocardial scar are more likely to have reentrant circuit. [3] Moreover, larger scars also lead to more ventricular remodeling and LV dysfunction, leading to activation of compensatory neuro-humoral factors in the setting of left ventricular dysfunction and heart failure. These factors lead to changes in repolarization and conduction properties of myocardial cells and abnormalities in intracellular calcium homeostasis which are potentially arrhythmogenic by promoting triggered activity and facilitating reentry. [4] Moreover, patients with ischemic cardiomyopathy have areas of ischemic myocardium which predispose to the arrhythmia by changes in the myocyte automaticity, excitability and refractoriness leading to dispersion of repolarization. The border zones of the myocardial scars are important substrates for arrhythmia as they are composed of fibrotic tissue as well as viable myocardium which are often ischemic. Heterogeneity of infarct tissue as assessed by magnetic resonance imaging has been shown to predispose to arrhythmia. Additionally, myocardial infarction leads to disturbances in the autonomic innervation of the myocardium in the area surrounding the post-infarct scar which makes the surrounding myocardium more susceptible to arrhythmia due to prolongation of refractory periods in the denervated myocardium. [5] Apart from these, a patient with ischemic heart

death in various forms of cardiomyopathies.

disease is predisposed to SCD due to acute coronary syndrome.

Studies of HCM patients with ICDs have suggested that ventricular arrhythmias are the major causes of SCD in this group of patients, [6], [7] although availability of back up pacing for bradyarrhythmia precludes the ability of an ICD study to exclude the possibility of a bradyar‐ rhythmic etiology. [6] Bradyarrhythmias are, however, reported rarely in HCM so this seems an unlikely possibility. Multiple pathologic, molecular and physiologic mechanisms could

**3.2. Hypertrophic cardiomyopathy**

**3.1. Ischemic cardiomyopathy**

**Figure 1.** Absolute numbers of events and event rates of SCD in the general population and in specific subpopulations over 1 y. General population refers to unselected population age greater than or equal to 35 y, and high-risk sub‐ groups to those with multiple risk factors for a first coronary event. Clinical trials that include specific subpopulations of patients are shown in the right side of the figure. AVID \_ Antiarrhythmics Versus Implantable Defibrillators; CASH \_ Cardiac Arrest Study Hamburg; CIDS \_ Canadian Implantable Defibrillator Study; EF \_ ejection fraction; HF \_ heart fail‐ ure; MADIT \_ Multicenter Automatic Defibrillator Implantation Trial; MI \_ myocardial infarction; MUSTT \_ Multicenter UnSustained Tachycardia Trial; SCD-HeFT \_ Sudden Cardiac Death in Heart Failure Trial. (Adapted with permission from reference 165)

infiltration. These conditions though not strictly classifiable as cardiomyopathies, but are important and common causes of SCD in the setting of myocardial dysfunction. They include ischemic heart disease, hypertension, valvular heart disease, and myocardial involvement with conditions like sarcoidosis, amyloidosis.
