Structural Pathways and Prevention of Heart Failure and Sudden Death

Pathways and Prevention of Heart Failure and Sudden Death. We review the macroscopic and microscopic anatomy of myocardial disease associated with heart failure (HF) and sudden cardiac death (SCD) and focus on the prevention of SCD in light of its structural pathways. Compared to patients without SCD, patients with SCD exhibit 5- to 6-fold increases in the risks of ventricular arrhythmias and SCD. Epidemiologically, left ventricular hypertrophy by ECG or echocardiography acts as a potent dose-dependent SCD predictor. Dyslipidemia, a coronary disease risk factor, independently predicts echocardiographic hypertrophy. In adult SCD autopsy studies, increases in heart weight and severe coronary disease are constant findings. Whereas rates or acute coronary thrombi vary remarkably. The microscopic myocardial anatomy of SCD is incompletely defined but may include prevalent changes or advanced myocardial disease, including cardiomyocyte hypertrophy, cardiomyocyte apoptosis, fibroblast hyperplasia, diffuse and focal matrix protein accumulation, and recruitment of inflammatory cells. Hypertrophied cardiomyocytes express "fetospecific" genetic programs that can account for acquired long QT physiology with risk for polymorphic ventricular arrhythmias. Structural heart disease associated with HF and high SCD risk is causally related to an up-regulation of the adrenergic renin-angiotensin-aldosterone pathway. In outcome trials, suppression of this pathway with combinations of beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin-II receptor blockers, and mineralocorticoid receptor blockers have achieved substantial total mortality and SCD reductions. Contrarily, trials with ion channel-active agents that are not known to reduce structural heart disease have failed to reduce these risks. Device therapy effectively prevents SCD, but whether biventricular pacing-induced remodeling decreases left ventricular mass remains uncertain. (J Cardiovasc Electrophysiol, Vol. 14, pp. 1-12, July 2003)

The aim of this review is first to provide an update on structural and molecular pathways of heart failure (HF) and sudden cardiac death (SCD), which are closely associated complications of advanced myocardial disease. Second, the review addresses SCD prevention in light of the structural pathways of advanced myocardial disease.

Textbooks often define HF as an inability of the heart to maintain adequate tissue perfusion, but they fail to incorporate into their definitions a cardinal characteristic of myocardial disease: loss of vital rhythm control. In three large atrial fibrillation studies totaling 17,437 patients, HF has been assessed to increase the risk of atrial fibrillation 5- to 6-fold.^1-3 SCD has been estimated to account for up to 50% of annual HF deaths.^4-6 Further, in individual studies, the risk of SCD in patients HF has been repeatedly reported to be increased 5- to 6-fold compared with that in HF-free control groups^7-9

The typical cardiologic precursors of SCD were well recognized in the early 1970s by the Framingham investigators. In a 16-year follow-up of 4,120 subjects, more than 50% of all death from coronary disease occurred outside of the hospital, and about 80% of these deaths were sudden. SCD risk correlated positively with hypertension, obesity, and ECG signs of left ventricular hypertrophy (LVH).^1O Subsequently, the Framingham investigators determined that another sign of LVH, QRS prolongation,^11,12 acted as a predictor of SCD.¹³ A recent retrospective study of 669 HF patients confirmed that QRS prolongation and depressed left ventricular ejection fraction (LVEF) were the major independent predictors of sudden-cardiac and all-cause mortality, but the study failed to evaluate LV size.¹⁴ LVH by ECG as a risk factor of SCD has been confirmed in other large studies, notably the Honolulu Heart Program (n = 7,591)¹⁵ and Manitoba studies (n = 3,983).¹⁶ ECG for detecting LVH arguably may lack specificity and sensitivity^11,12; however, the Framingham investigators further characterized SCD/LVH relations using echocardiographic LV mass estimates. In their Off-spring study, echocardiographic LVH in a subgroup of 3,661 subjects was an independent SCD risk factor, with each mass increase of 50 g per meter of body height augmenting adjusted relative risk by 1.45.¹¹ Similarly, in a recent study of 1,033 hypertensive patients, 39 g/m increases in echocardiographic LV mass augmented the risk for major cardiovascular events by 40%.¹⁸ In 480 consecutive patients with hypertrophic cardiomyopathy, the magnitude of echocardiographic hypertrophy over a wide range of maximal LV wall thickness was nearly linearly related to the risk of SCD.¹⁹ In addition, increased echocardiographic LV mass in 126 hypertensive patients has been reported to correlate positively with complex ventricular arhhythmias detected by 24-hour ambulatory ECG monitoring.²⁰ Further, echocardiographic LV mass has been assessed to be more powerful a predictor of survival than LVEF in implantable cardioverter defibrillator (ICD) recipients.²¹ These and other studies indicate that noninvasively detected LV enlargement is a major, if not the major, recognized SCD risk factor of structural heart disease.

In western countries, autopsy studies of adult SCD victims have shown, without exception, increased heart weights compared with variously selected concurrent control groups.^22-29 A second constant feature is severe coronary atherosclerosis.^22-29 The Framingham investigators were among the first to point out that there was a close correlation between heart weight and the severity of coronary artery disease.³⁰ In the recent prospective Uppsala study monitoring 2,322 subjects over 20 years, plasma total cholesterol, low-density lipoprotein (LDL) cholesterol/high-density lipoprotein (HDL)-cholesterol, and triglycerides independently predicted echocardiographic LV mass.³¹ This demonstrates that dyslipidemia independent of hypertension, antihypertensive therapy, and obesity is a risk factor for LVH. Because the severity of dyslipidemia is correlated to that of coronary disease in epidemiologic and angiographic studies, the dyslipidemia/LVH relation demonstrated by the Uppsala survey corroborates the coronary disease/LVH relation of the earlier Framingham studies.^17,30 Similarly, in the Hypertension Genetic Epidemiology Network (HyperGen) study enrolling 342 normotensive and 1,627 hypertensive subjects, hypercholesterolemia contributed to an increased echocardiographic mass, independent of hypertension, obesity, and diabetes.³² In brief, the macroscopic cardiac anatomy of adult SCD in western countries is dominated by the presence of both cardiomegaly and hyperlipidemia/coronary disease. In western populations, coronary disease is often the major single cause of death, and >60% of coronary deaths occur suddenly. In the US population estimated at 270 million, the Centers of Disease Control determined from death certificates that 728,743 coronary heart disease deaths had occurred during 1999 (www.nhlbi.nih.gov).³³ Among these deaths, 607,739 (83%) affected persons older than 65 years and more than 460,000 (>63%) were attributed to SCD.³³

Although severe coronary disease has been a dominant feature of the gross anatomy of adult SCD, reported rates of acute coronary thrombi have been variable, ranging between 4% and 100%.^22,25 As originally emphasized by Spain and Bradess,²⁴ this variability may reflect disparate survival durations after onset of new symptoms. Studies using SCD definitions with long survival durations (6-24 hours) are likely to be contaminated by cases of fatal myocardial infarction.^22,24,25 In recent studies using durations ≤ 1 hour^33,34 and detecting thrombi by cross-sectioning major coronary arteries every 0.5 cm,^22,23 acute thrombi were detected in only ≤ 14% of the victims.^22,23 The authors of these studies concluded that acute coronary occlusion was not an important mechanism of SCD, confirming earlier studies using SCD definitions with brief antemortem symptom durations.^24,26 We previously reported that ICD recipients very rarely (<3%) develop acute coronary syndromes suggestive of atherothromboses in the wake of successful shocks for electrogram-verified rapid highly lethal fast ventricular tachycardia (VT; > 240/min) or ventricular fibrillation (VF).³⁵

Recent SCD autopsy series have often focused on coronary plaque rupture and acute myocardial lesions (infarction), but they provide little quantitative information on the overall microanatomy of the myocardium. We are unable to identify SCD autopsy series defining quantitative structural and immunologic characteristics of cardiomyocytes, such as hypertrophy, DNA ploidy, binucleation index, apoptosis, organelle structure, and cell cycle markers. Information on other cardiac cells, including fibroblasts, myofibroblasts, Purkinje cells, and inflammatory cells, also is limited. One may assume that myocardial cytologic changes in victims of SCD resemble those generally encountered in patients with cardiomegaly and advanced coronary disease, particularly in those with a history of aborted sudden arrhythmic death. Unfortunately, the microanatomy of advanced myocardial disease in the presence of coronary atherosclerosis is itself not as extensive as one might expect. Here we discuss recent pertinent studies retrieved by MEDLINE searches.

Cardiomyocyte Hypertrophy

Beltrami et al.³⁶ performed a quantitative evaluation of the microanatomy of 10 hearts from transplant patients with end-stage "ischemic cardiomyopathy." Measurements for the LV revealed elevated heart weights (+85%), aggregate myocyte mass (+47%), and myocyte volume per nucleus (+103%) compared with 10 hearts from patients assessed not to have died from cardiovascular disease. LV myocytes exhibited hypertrophy characterized by increases in cell length and diameter of 51% and 16%, respectively. Concomitantly, the myocyte number was estimated to have declined by 28% and connective tissue volume to have increased by 28%. These findings support the hypothesis of a net cardiomyocyte cell loss in the presence of hypertrophy and coronary disease. Contrarily, Adler et al.^37-40 repeatedly reported that ischemic heart disease promotes cardiomyocyte regeneration reflected in elevated cardiomyocyte counts (see later).

Hypertrophy in ischemic heart disease often is interpreted as an adaptive response, with hypertrophy of surviving cells "compensating" for cell death from ischemic injury. However, in one of the few quantitative electron microscopic studies of ischemic human myocardium, hypertrophied cells exhibited signs of injury, including mitochondrial changes, myofibrillar lysis, Z-band abnormalities, dilated sarcoplasmic reticulum, glycogen and lipid depositions, and various nuclear deformities.⁴¹ To our knowledge, the functional significance of such changes is uncertain. However, in sheep with experimental myocardial infarcts, hypertrophied cells outside of the infarct zone have revealed similar structural changes. When isolated for functional characterization, these cells exhibited abnormal contractions and relaxations, raising the question of whether cells undergoing hypertrophy in the wake of an ischemic insult represent an adaptive or maladaptive response.⁴² In mice with genetically engineered inhibition of hypertrophy in response to aortic banding, there was little deterioration in LV function compared with control mice undergoing unattenuated hypertrophy.^43,44 The authors of both studies suggested that hypertrophy was neither a requirement nor a supportive mechanism to maintain LV systolic function.^43,44 However, the authors failed to determine whether blunted hypertrophy was associated with decreased myocardial fibrosis and decreased risk of ventricular arrhythmia and SCD. In cats with temporary aortic banding, electro-physiologic characteristics (including inducibility of VT/VF) deteriorated during the progression of hypertrophy but improved during its regression after relief of aortic banding.⁴⁵ Current experimental evidence thus suggests that activation of fetospecific gene programs during precipitous hypertrophy after acute coronary occlusion or aortic banding may produce dysfunctional cell phenotypes with unfavorable mechanical and electrophysiologic properties.

The presence of cardiomyocyte hypertrophy in the hearts of SCD victims has important electrophysiologic implications. Ventricular hypertrophy and HF in various species (human, dog, goat, cat, rat, mouse) is associated with the reinduction of "fetospecific" genetic programs, including suppression of various K currents (I_to, I_ks, I_kr, I_K1, I_kur) and K-channel proteins accounting for these currents (for review see reference 46-48). Suppression of repolarizing K currents in LV hypertrophy and failure provides an explanation for QT prolongation of HP, including long QT physiology with threat of fatal polymorphic ventricular tachyarrhythmias. In addition to prolonging single-cell action potential duration, suppression of repolarizing K currents that affect different cardiac regions differently can predispose to QT dispersion, conduction block, and reentry, thereby accounting for major mechanisms of arrhythmogenesis. The genetic background of patients sustaining HP-induced hypertrophy may render them vulnerable to slowed repolarization mediated by K channel down-regulation, QT-prolonging drugs, or diuretic-induced hypokalemia.⁴⁹ Sodium channel (SCN5A) mutations affecting the early action potential but exerting little proarrhythmia by themselves may have repercussions on the subsequent voltage-sensitive regulation of rectifying K currents undergoing hypertrophy-related remodeling. An important new finding is that such latently proarrhythmic variant genes of sodium channel α-subunits may be much more frequent than anticipated (in one example, > 13% of African-Americans⁴⁹).

Hypertrophy may provide a unifying mechanism for the risk of SCD in a variety of myocardial diseases, including coronary artery disease, hypertensive heart disease, hypertrophic and congestive cardiomyopathy, thyrotoxicosis, and perhaps athlete's heart.⁵⁰ Hypertrophy in coronary disease and HF is one of the structural factors that can account for increased SCD risk without the need to invoke occlusive atherothromboses that have proved inconsistent in SCD autopsy series.^22,25,50

Cardiomyocyte Apoptosis

Apoptosis is mediated by different mechanisms that may or may not involve caspases (proteases initiating and executing cellular disassembly) and mitochondria-derived proapoptotic factors.^51-56 It has been proposed that early and persistent activation of cardiomyocyte apoptosis contributes importantly to the progression of clinical HF. Olivetti et al.⁵⁷ examined 36 explanted hearts from transplant recipients and evaluated apoptosis with multiple assays. Histometric analyses suggested that apoptosis involved 0.23% of the cardiomyocytes, an apoptosis frequency believed capable of accelerating heart failure progression.⁵⁷ In another study of eight patients with arrhythmogenic right ventricular dysplasia, Mallat et al.⁵⁸ detected apoptosis in 6 of the 8 hearts. These and other pioneering clinical pathologic studies⁵⁹ suggest that myocardial disease states conferring high SCD risk are associated with important cell loss from apoptosis.

The Bcl-2 peptide family has been implicated in the regulation of the outer membrane mitochondrial permeability that controls the mitochondrial release of proapoptotic factors such as cytochrome c, Smac/Diablo, and apoptosis-inducing factor (AIF). Bcl-2 peptides possess up to four homology domains (BH1, BH2, BH3, BH4) and can be divided into subclasses according to their domain sets and proapoptotic or anti-apoptotic activities. One subclass of proapoptotic peptides, including BNIP3 and NIX, contains a single BH3 domain (so called "BH3-only peptides"). The genes of BNIP3 and NIX possess in their promoter region a hypoxia response element (HRE) targeted by the newly characterized hypoxia-inducible transcription factors (HIFs).⁶⁰ HIF-1α subunits are stable and confer to transcription during hypoxia exclusively. Regula et al.⁶¹ and Kubasiak et al.⁶² recently demonstrated that cardiomyocyte apoptosis is induced by hypoxia through up-regulation of BNIP3. These new findings provide a potential explanation for the disruption of myocardial syncytial integrity and rhythm control during ischemia and hypoxia.

Activation of caspases influences processes other than apoptotic cell death. Caspase-1 (also known as interleukin [IL]-1β converting enzyme [ICE]) generates active interleukin IL-1β and IL-18, which are inflammatory peptides postulated to play an important role in the progression of HF.⁵⁵ Further, caspase-3 attacks myofibrillar proteins, particularly the ventricular essential myosin light chain (vMLC1), a mechanism that may explain contractile deterioration during apoptotic responses activated by myocardial ischemia and hypoxia.^63,64 Apoptosis-specific pathways may thus accelerate myocardial deterioration, which in turn may promote HF-associated arrhythmogenesis. However, the crucial question persists as to whether myocardial apoptosis in clinical HF reflects primarily adaptive responses facilitating remodeling during organ growth mediated by hypertrophy (cardiomyocytes) and hyperplasia (fibroblasts), or whether it is maladaptive from an imbalance between anti-apoptotic and proapoptotic factors such as excessive hypoxia-induced expression of BNip3. If apoptosis is predominantly adaptive, therapeutic attack of proapoptotic pathways (anti-tumor necrosis factor [TNF] interventions, caspase blockers) might not be salutary or might even be harmful. The negative outcome of anti-TNF trials for HF and the occurrence of seven deaths in patients receiving a humanized, mouse-derived engineered monoclonal antibody to TNF (infliximab, Remicade®) are not completely reassuring in this context.^65,66 In view of these disappointing clinical experiences, the significance of apoptosis in HF-related arrhythmogenesis remains debatable.

Cardiomyocyte Mitosis

For many years, Adler et al.^37-40 defended the thesis that hypertrophied adult human myocardium in the presence of coronary artery disease was undergoing hyperplasia of cardiomyocytes (order of magnitude doubling of cardiomyocyte number). However, they never took advantage of modern genetic and immunohistochemical techniques to prove the occurrence of myocyte mitosis. Contrarily, Beltrami et al.⁶⁷ used an immunostain for Ki67, a peptide used clinically for nearly 20 years as an index of cell proliferation in common cancers.⁶⁸ They assessed cardiomyocyte proliferation in the hearts of patients who died 4 to 12 days after acute myocardial infarction. In combination with two other immunostains-an alpha-sarcomeric actin stain to identify (actin-rich) muscle cells and a tubulin stain to visualize mitotic spindles-the authors concluded that adult human cardiomyocytes underwent karyokinesis and cytokinesis both in the border zone of infarcts and at a distance from them.⁶⁷ The authors theorized that their stains might indicate a significant growth potential of myocardium, accounting for physiologically relevant tissue regeneration as postulated by Adler.⁶⁷ This represents a provocative interpretation of the stains, because the postmitotic fate of the cells remained a black box. It has been generally recognized that adult tissues with limited mitotic potential, such as neurons⁶⁹and lens cells,⁷⁰ may undergo limited (single) mitoses that result in postmitotic (apoptotic) cell death, partly under the influence of the E2F1/E2F2 transcription factors. The kinetic result is then a net cell loss, not "regeneration" as implied by Beltrami et al.⁶⁷ Further, in an organ with polyploidy and multinucleation, such as adult human myocardium, the significance of the accumulation of Ki67 as a cytokinesis (completed mitosis) marker has yet to be precisely defined. As with cells undergoing hypertrophy or apoptosis, cells entering the cell cycle would be an obstacle to differentiated cell-to-cell communication necessary for syncytial electrophysiologic integration.

Fibroblast Activation and Matrix Protein Accumulation

In transgenic mice, a great variety of sarcomeric and cytoskeletal protein mutations determining cardiac hypertrophy are complicated by myocardial fibrosis.^71-73 Sudden death in transgenic mice with HF appears to have occurred in phenotypes exhibiting myocardial fibrosis, irrespective of whether the cardiac changes were interpreted as hypertrophic or dilated cardiomyopathy.^71-73 Fibrosis is so common that one starts to wonder whether appreciable cardiac hypertrophy can occur in the absence of fibrosis as reflected by increases in myocardial collagen (hydroxyproline) concentration. Fibrosis appears to be one of the features that distinguishes "physiologic" (adaptive) from pathologic and proarrhythmic ("maladaptive") hypertrophy.⁷² Therefore, understanding the activation of connective tissue accumulation during hypertrophy is of pivotal importance.^71-73

In rats and mice performing treadmill exercise, hypertrophy is unassociated with up-regulated expressions of marker genes of pathologic hypertrophy, including collagens I and III, smooth muscle actins, beta myosin heavy chain, and natriuretic factors (see bibliographic citations in references 74-76). Further, apoptosis of cardiomyocytes is not demonstrable.⁷⁴ Myocardial hypertrophy achieved by exercising caged rodents is, however, often modest. Contrarily, athletes engaged in endurance sports may exhibit rather striking increases in LV mass (>60%). For instance, professional cyclists have been repeatedly estimated to have LV masses >400 g.⁷⁷ Unfortunately, little is known about the longevity, risk of SCD, and myocardial connective tissue expression in such elite endurance athletes. Besides exercise-induced hypertrophy, another syndrome documenting that linkage of hypertrophy and myocardial fibrosis is not obligatory is hyperthyroidism. Hypertrophy without fibrosis or decreased steady-state hydroxyproline concentration in hyperthyroid states has been well documented.^78-80 The literature on exercise- and thyroxin-induced hypertrophy strongly suggests that cardiac hypertrophy without fibrosis is physiologically achievable, irrespective of postulated paracrine interactions between cardiomyocytes and fibroblasts that would obligate the concomitance of hypertrophy and fibrosis (partly reviewed in references 81 and 82).

Major candidate mechanisms that may account for cardiac fibroblast activation in coronary artery disease and HF are mechanical stimuli (fibroblasts as mechanoreceptors, reviewed in references 81 and 83) and local hypoxia (fibroblast as O2 sensors, reviewed in reference 84-86). The myocardium undergoing ischemic (hypoxic) systolic bulging creates conditions for early fibroblast activation by sensing both mechanical and hypoxic stimuli. Further, perivenular (end capillary) adventitial fibroblasts are strategically positioned to detect local hypoxic distress, and their subsequent activation might invite perivascular fibrosis. The stimulation of fibrogenic responses by hypoxia in myocardium, kidney, skin, and other organs has been well recognized for many years.⁸⁶ Recently, multiple genes involved in matrix protein deposition have been recognized to be under regulation of the HIFs, a finding explaining the phenomenon of hypoxic fibrosis. Examples of HIF-regulated gene products are prolyl and lysyl hydroxylases (collagen cross-linking) and inhibitors of metalloproteinases (TIMP-l).^85,87

Fibroblasts are pluripotential cells capable of releasing a variety of cytokines (IL-1, IL-6, TNF), chemokines (IL-8, MCP-1), and growth factors (transforming growth factor [TGF]-β1, fibroblast growth factor) in response to mechanical, hypoxic, and paracrine stimuli. These stimuli promote transdifferentiation of fibroblasts into myofibroblasts. Myofibroblasts are characterized by the expression of smooth muscle cytoskeletal markers (α-smooth muscle actin) and are key to the persistent accumulation of matrix proteins. In cell culture, TGF-β1 stimulates fibroblast transdifferentiation to myofibroblast.^81,83,88 In myocardium, for unclear reasons, myofibroblasts, once generated after ischemic insults seem to be persistent for long times, eluding apoptosis as seen in the final phase of normal wound healing.^81,82,88 Release of chemokines by stimulated fibroblasts play an important role in the initial recruitment of mononuclear inflammatory cells, particularly monocyte-derived macro phages under the influence of MCP-1. As part of the cross-talk between fibroblasts and recruited inflammatory cells, macrophage-derived TGF-β1 stimulates the transdifferentiation of fibroblasts into myofibroblasts.^81,83,88 Although specific details of the paracrine interactions among cardiomyocytes, fibroblasts, and recruited inflammatory cells appear somewhat contradictory in different mouse studies.^82,89-91 there is little doubt that the renin-angiotensin system plays a pivotal role in stimulating both cardiomyocyte hypertrophy and fibrogenic myofibroblasts. In support of this conclusion are in vitro experiments indicating that rat cardiac fibroblasts in culture respond to exogenous angiotensin II (AII; 100 nM) or aldosterone (1 nM) with increases in collagen synthesis.⁹² A recent report provides evidence that human ventricular myocardium can express aldosterone synthase and that local profibrotic aldosterone synthesis is up-regulated in heart failure.⁹³ AII also may act by facilitating norepinephrine release from noradrenergic nerve endings, with norepinephrine overflow acting as a potentiator of TGF-β1⁹⁴ and activator of the renin-angiotensin-aldosterone system (RAAS).^95,96 Activation of the adrenergic and RAAS systems in the heart and other organs is interactive at multiple levels.^94-96 For this reason, antagonizing adrenergic and angiotensinergic effects should not be considered independent therapeutic interventions.

Major difficulties in evaluating the interactions of multiple profibrotic factors include the participation of multiple cells (myocytes, fibroblasts, inflammatory cells, neurons [presynaptic regulation], and vascular cells); variable superimpositions of local and systemic effects; generation of AII from locally produced angiotensinogen or angiotensin I (AI) under the influence of multiple cardiac peptidases (angiotensin-converting enzyme (ACE), ACE-II, chymase, cathepsin G, elastase, TPA); and complexity of TGF-β1 signaling exerting both proproliferative and antiproliferative effects.⁹⁷ Additional difficulties arise from the interactions between profibrogenic and antifibrogenic factors. We mentioned thyroid hormone as one factor that reduces steady-state accumulation of myocardial collagen.^78-80 Factors that appear to play a crucial role in antagonizing fibrogenic activity in myocardium undergoing hypertrophy are the natriuretic peptides (atrial natriuretic protein [ANP], brain natriuretic protein [BNP]). These cardiac hormones, ventricular cardiomyocyte-derived BNP in particular, are known for their vasodilatory and natriuretic activities. However, in gene knockout mice, targeted disruptions of the BNP gene (Nppb [-/-] mice)^98-100 or its receptor (guanylyl cyclase A receptor gene, GC-A [-/-] mice)¹⁰¹ result in cardiomyocyte hypertrophy and marked myocardial fibrosis associated with increases in myocardial TGF-β. These changes are out of proportion to modest concomitant hemodynamic changes attributable to BNP deficiency (hypertension).^98-101 Clinically, elevations in the plasma concentration of BNP correlate well with the severity of HF, making BNP a useful prognostic marker.¹⁰² Because of the well-established relation between HF and SCD risk, BNP plasma levels act in multivariate analysis also as an independent predictor of SCD superior to other predictors, including ejection fraction.¹⁰³ Therapeutic trials involving a total of about 2,000 HF patients demonstrated that intravenously administered recombinant BNP (nesiritide) alleviates symptoms and signs of HF, including reduced activations of the sympathetic and renin-angiotensin systems.¹⁰⁴ Transgenic experiments and clinical trials indicate that BNP acts as a cardiomyocyte-derived counterregulatory agent against myocardial hypertrophy and fibrosis.^{98-101,102,104} Therefore, the critical question is whether long-term nesiritide therapy or pharmacologic inhibition of BNP degradation (suppression of neutral endopeptidase with now available oral inhibitors) might help to reverse pathologic fibrotic hypertrophy and thereby reduce the risk of its most serious complication, SCD. We hope that development of nonpeptide BNP mimetic agonists will be successful.

Suppression of the Adrenergic
Renin-Angiotensin-Aldosterone Pathway

The experimental and clinical literature that has accumulated over the past 30 years provides incontrovertible evidence for a close relation between activation of the adrenergic RAAS and the progression of structural and functional myocardial changes of HF.^105-117 Stretching cardiomyocytes in culture triggers protein kinase cascades (protein kinase C, Raf-1 kinase, MAP kinases) important for the activation of hypertrophy programs (partly discussed in reference 107). Thc stretch-elicited increase in protein synthesis is associated with up-regulated expression by the isolated cardiomyocytes of all components of the RAAS. In the presence of an ACE receptor (AT1) blocker, the stretch-elicited protein synthesis is suppressed, which demonstrates the importance of cardiomyocyte-released AII acting on AT1 receptors.¹⁰⁷ Another autocrine mechanism, endothelin-1 release acting through endothelin-A receptors, and Na/H exchange activation contribute to the angiotensinergic protein synthetic effect.¹⁰⁷ The historical experiments of Davis et al. showed that dogs with intrathoracic inferior vena cava constriction develop a syndrome exhibiting most, if not all, of the features of heart failure.¹¹³ Importantly, they showed that the experimental HF syndrome was associated with activation of the RAAS with typical repercussions on myocardial structure. In 1979, Davis et al. demonstrated for the first time that HP in vena cava-constricted dogs could be effectively suppressed by a newly available oral drug, SQ 14225 (now known as captopril).¹¹³ Extensive clinical studies subsequently confirmed the importance of the RAAS in the pathophysiology of HF. Inhibitors of renin activation (beta-blockers^95,96,110), inhibitors of the generation of AII (ACE inhibitors¹¹¹), AII receptor (AT1) blockers (ARBs ¹¹²), and mineralocorticoid receptor blockers (spironolactone, eplerenone^105,106) were all shown to improve significantly the outcome of clinical HF. The role of the RAAS in studies pertaining to myocardial hypertrophy and HF fulfills the three Koch criteria for causative involvement: (1) agent ( = up-regulated beta-agonists: renin-AII-aldosterone pathway) is isolated from the organism (elevated plasma levels); (2) agent produces specific lesions (cardiac hypertrophy and fibrosis); and (3) agent occurs within the lesions (i.e., up-regulated expression within cardiac myocytes and fibroblasts undergoing growth responses). Finally and most important in our context, antagonization of the agent suppresses lesion generation and clinical deterioration. If dysregulation of the adrenergic-AII-aldosterone system causes fibrotic myocardial hypertrophy and if the latter is related to increased SCD risk, one may hypothesize that interventions aimed at antagonizing this system should reduce SCD risk.

Table 1 lists successful clinical interventions supporting this hypothesis. Interpreting the table, the following points appear important to us:

1. Beta-blockers have been clearly demonstrated to inhibit the RAAS axis in the clinical setting.^95,96 The crosstalk between the adrenergic and angiotensinergic systems reflects in part direct angiotensin receptor 1 (AT1)-mediated effects on central and peripheral sympathetic neurons and adrenomedullary cells.¹¹⁵
   
   
2. Substantial suppression of the RAAS pathway by beta-blockers on top of ACE inhibitors (ACE-I) in CIBIS-II, MERIT-HF, and COPERNICUS suggests that ACE inhibitors by themselves do not suppress optimally the RAAS for HF treatment (discussed in references 95, 96, and 110). For instance, in the tabulated SOLVD trial,^118-120 addition of beta-blockers on top of enalapril reduced total and sudden mortality rates. Effective blockade of intrinsic cardiac RAAS by currently available ACE inhibitors is not achieved, because cardiac cells are known to generate AII by various peptidases, including ACE II, chymase, cathepsin G, elastase, TPA, and others. ACE-II, which is abundant in heart,⁹¹ is not inhibited by ACE inhibitors despite considerable homology with ACE. These facts raise the question as to whether ATI receptor blockers (ARBs) may exert more potent direct myocardial effects (including antiarrhythmic effects) than ACE inhibitors. In this context, the assertion by the investigators of the OPTIMAAL trial¹²¹ that an ACE inhibitor (captopril) is a first-choice drug post infarction over an arbitrarily dosed short-acting ARB cannot be accepted without reservation. In the OPTIMAAL trial, single-dose losartan was started at a rather low dose (12.5 mg/day), and, more importantly, after slow up-titration of losartan to a low single-dose maintenance dose of 50 mg/day, there was no divergence over 30 months between the Kaplan-Meyer survival curves for the two treatment groups (see Fig. 4 of the trial report)
   
   
3. The remarkable efficacy of spironolactone (aldactone) treatment on top of ACE-I therapy (RALES¹⁰⁵) suggests that ACE-I treatment for HF is insufficient to suppress optimally undesirable mineralocorticoid effects
   
   
4. In view of points 2 and 3, RAAS antagonization using drugs such as beta-blockers, ARBs, ACE-I, and spironolactone in various combinations has theoretical appeal (discussed in reference 115) and needs testing for long-term prevention of SCD.
   
   
5. Substudies of the HOPE and ONTARGET trials support the concept that antagonizing the RAAS with ACE-I may suppress atherosclerosis, a major pathway of proarrhythmic structural heart disease (discussed in reference 111).
   
   
6. Table I suggests that trials not primarily designed as antiarrhythmic trials were more effective in suppressing SCD than any large controlled drug trial designed to test ion channel-active agents with putative chronic antiarrhythmic effects (see later).
   
   

A neglected aspect of the renin-angiotensin system in HF is the brain RAAS-arginine vasopressin pathway. Components of the RAAS, including a novel renin isoform, angiotensinogen, ACE, ACE-II, and AII receptors type 1 and 2, all are expressed in relevant regions of the brain.¹¹⁶ Intracerebroventricular injection of AII has long been known to induce hypertension by promoting the release of arginine vasopressin from posterior piruitary cells.¹¹⁷ Like other agents with vasoconstrictive and mitogenic effects, such as norepinephrine, AII, and endothelin,¹²² vasopressin might be proarrhythmic. Although there are case reports on the arrhythmogenic ("torsadogenic") effects of vasopressin,^123-125 to our knowledge the possible role of hypervasopressinemia of HF in triggering arrhythmias has not been investigated. Conivaptan is a new oral vasopressin antagonist with actions both on V(1a) and V(2) receptors that mediate vasoconstriction and antidiuresis (antiaquaresis), respectively. Recent studies suggest that conivaptan exerts beneficial vasodilating and aquaretic effects in experimental HF, but unfortunately they provide little information on possible antiarrhythmic actions.^126,127

SCD Prevention by Risk Factor Intervention

As mentioned earlier, in western societies heart disease with high risk for SCD depends importantly upon genetically and environmentally linked disease factors, particularly dyslipidemia, hypertension, obesity, and diabetes/hyperinsulinism. These factors typically are associated with the structural cardiac changes discussed earlier, namely cardiomegaly and coronary disease. Therefore, beyond interventions modulating specific steps of the adrenergic-RAAS cascade, prevention of SCD requires a general strategy of cardiovascular risk factor intervention, including prevention/treatment of hyperlipidemia, hypertension, and obesity (the recent increase in juvenile SCD in the United States has been tentatively ascribed to increased childhood obesity and associated type II diabetes³³). Treatment targets include low measurements for body mass index (BMI <25 kg/m²), arterial pressure (simply, as low as tolerable), plasma non-HDL-cholesterol (<130 mg/dL, or <100 mg/dL for LDL-cholesterol), fasting plasma triglycerides (<150 mg/dL), postprandial glycemia (<160 mg/dL), and HbA_1e <7%.

It is widely held that western-style diets are a risk factor for coronary artery disease. An interesting dietary manipulation in the context of SCD prevention is the provision of diets rich in n-3 (or ω-3) polyunsatUrated fatty acids (n-3 PUFAs), including linolenic acid (18:3n-3), eicosapentaenoic acid (EPA, 20:5n-3), docosapentaenoic acid (DPA, 22:5n-3), and docosahexaenoic acid (DHA. 22:6n-3). An important source of long chain n-3 PUFAs is the phytoplankton and marine organisms feeding on it, such as fish. Recommended adult human daily intake of EPA plus DHA is on the order of 1.5 g. corresponding to about 5g of fish oil per day or three fish "servings" per week.^128-132 An action of n-3 PUFAs that explains their potency is their direct or indirect effects on transcription factors such as sterol regulatory element-binding protein (SREB-1) and peroxisome proliferator-activated receptors α (with secondary actions on inflammation control via the key transcription factors NF-κB, STATs, and AP-1).^128,129 Effects include down-regulations of lipogenic (very-low-density lipoprotein [VLDL] synthesis suppression) and proinflammatory genes, such as cytokines (IL-1β, IL-2, IL-6, IL-18, TNF), chemokines (IL-8, MCP-1), and proinflammatory enzymes (COX-2, phospholipases A2,lipooxygenases. iNOS). Contrarily, effects include up-regulations of genes essential for lipid catabolism (oxidation), immune functions (immunoglobulin peptides). and oxidation control (glutathione transferases). Generally, genetic effects are antiadipogenic, antidiabetic (insulin-sensitizing), and anti-inflammatory, all highly relevant to the treatment of atherosclerosis, HF, and related arrhythmia risk. PUFAs also can exert direct effects by acting as substrates. Entry of EPA (n-3) instead of arachidonate (n-6) into the cyclo-oxygenase and lipo-oxygenase pathways promotes variant eicosanoid signaling favoring anti-inflammatory/antithrombotic effects.¹³¹ Structural incorporation ofPUFAs into microdomains ("rafts") of surface membranes may modulate membrane protein functions.¹³⁰

Numerous variably well-controlled clinical studies have given increasing credence to the notion that n-3 PUFAs exert antiarrhythmic and SCD preventive effects (reviewed in reference 149). As mentioned earlier, molecular actions of PUFAs, such as antagonization of inflammatory responses, or arguably membrane effects postulated to "stabilize" ion channels might prevent myocardial disease progression and/or exert direct antiarrhythmic effects.¹³² Recently, two large studies have corroborated the concept of PUFA-mediated antiarrhythmia.^133,134

One important risk factor intervention is the control of hyperlipidemia with diet and hypolipidemic agents. Treatment with statins (HMG CoA reductase inhibitors) has been documented to reduce mortality from any cause and cardiovascular disease (and possibly SCD and HF, discussed partly in reference 135). Statins influence signaling through small GTP binding proteins (such as Rho, Ras, and Rac) whose anchoring to membrancs depends upon lipids covalently attached to them (lipidation). These anchoring lipids are partly derived from the presterol cholesterol synthesis pathway (isoprenoids), and their availability influenced by the statin drugs (see reference 136 for bibliographic citations). Suppression of signals mediated by the GTP-binding peptides may help to limit AII-dependent myocardial remodeling and inflammation, thereby limiting hypertrophy as an anatomic risk factor of SCD. In rats subjectcd to left coronary ligation, cerivastatin markedly reduced postinfarction LV hypertrophy.¹³⁶ Some statins may inhibit inflammatory responses by a different mechanism, direct complexing with α1β1 integrin (=integrin LFA-1).

Although hypercholesterolemia appears to be a risk factor for increased LV mass^31,32 and LVH is a clear-cut risk factor for death from coronary artery disease, guidelines for the treatment of coronary heart disease thus far have ignored cardiac structural criteria. However, myocardial mass may represent an "integrative cardiovascular risk factor" reflecting dyslipidemia, hypertension, obesity, and diabetes.

SCD Prevention by Ion Channel-Active Drugs

As discussed earlier, hypertrophy is associated with ion channel remodeling that involves derepression of fetospecific expression patterns. One possibility would be to treat patients at risk for SCD with drugs that would selectively correct abnormal transmembrane currents, but no such "ortho-electric" pharmacotherapy has been tested successfully. Specifically, the utility of currently available K-channel openers for the treatment of acquired cardiac hypertrophy/heart failure-associated long QT syndromes remains uncertain.¹³⁷

In sharp contrast to the trials listed in Table 1, recent large trials of antiarrhythmic agents failed to reduce both total mortality and SCD.^138-141 Worse, in the CAST trial¹³⁸ testing agents with putative selectivity for sodium channels and in the SWORD trial¹³⁹ testing a potassium channel blocker (d-sotalol), total mortality rates were increased, which popularized the concept of "proarrhythmia." The problem with potassium channel blockers is that they may aggravate the prolonged QT physiology of HF and invite life-threatening polymorphic ventricular tachyarrhythmias. In the EMIAT¹⁴⁰ and CAMIAT^I41 trials enrolling coronary disease patients with myocardial infarction, amiodarone failed to reduce total mortality, which is disconcerting considering that arrhythmic death is a dominant cause of mortality from coronary disease (see earlier⁴). In a subsequent meta-analysis¹⁴² that included EMIAT and CAMIAT, 13 disparate partly incompletely controlled amiodarone treatment studies were selected for analysis. Among the 13 studies, 4 showed decreases in total mortality risk of 52%,55%, 59%, and 61%, which is difficult to interpret in wake of the large negative EMIAT¹⁴⁰ and CAMIAT trials.¹⁴¹ In contrast, in 3 other studies, increases in total mortality risk of 19%, 54%, and 94% were reported. The overall meta-analytic mortality reduction of 13% computed by the authors is unimpressive numerically and unconvincing statistically. In the substudy on causes of death in the AVID trial (N = 1,013 patients),¹⁴³ noncardiac mortality in the non-ICD group (85% receiving amiodarone) significantly exceeded that in the ICD group, but this difference was eliminated by censoring pulmonary deaths. Therefore, excess noncardiac mortality in the non-ICD (amiodarone) group may have reflected amiodarone-induced fatal lung toxicity. A conservative use of currently approved antiarrhythmic agents for patients with coronary disease and increased LV mass is to reserve them for the prevention of spurious (atrial fibrillation-related) and appropriate shocks in ICD recipients.¹⁴⁴ The extent to which arrhythmic death, despite appropriately programmed and delivered ICD therapy (a frequent mechanism of death according to the AVID trial¹⁴³), might be related to adjunctive treatment with antiarrhythmic drugs awaits further analysis. Unfortunately, major trials with ion channel-active agentS have provided little or no informiltion on the possible effects of these agents on cardiac structure, including expression of ion channel isoforms and LV mass or fibrosis.

SCD Prevention by Implantable Device Therapy

More promising than current ion channel-targeting drug therapy for the prevention of SCD are the results with implantable devices. Several randomized trials in patients with a history of life-threatening arrhythmias, including the Dutch Study (1995), AVID (1997), CIDS (2000), and CASH (2000), have confirmed that therapy with ICDs is superior to conventional therapy for the prevention of arrhythmic death (reviewed in reference 145). Other trials, such as MADIT I (1996), MUSTT (1999), CABG PATCH (1997), AMIOVIRT (2002), and MADIT II (2002), as well as ongoing trials (SCD-HeFT, DEFINITE, BEST ICD, DINAMIT), were designed to assess the therapeutic efficacy of ICDs in patients at high risk for SCD (HF,low ejection fraction) but without a history of life-threatening ventricular tachyarrhythmias. In the MADIT II trial,¹⁴⁶ coronary artery disease patient, (N = 1,232) with a mean LVEF of 23% were randomly assigned in a 3:2 ratio to ICD therapy (n = 742) or conventional therapy (n = 490). A history of life-threatening VTNF or inducibility of VTNF at electrophysiologic testing was not required. The study was stopped after an average follow-up of 20 months, when mortality rates in the ICD and conventional groups averaged 14.2% and 19.8%, respectively. This amounted to a 31% risk reduction in death from any cause in favor of ICD therapy. ICD therapy making available complex pacemaker therapy has reopened old questions about optimal pacing strategies. In the recent DAVID trial,¹⁴⁷ in patients without indication for antibradyarrhythmia pacing and with a mean LVEF of 27%, the probability of death or hospitalization appeared less likely with backup VVI pacing (lower rate 40/min) compared with DDR dual-chamber pacing (lower rate 70/min).

Current evidence from several studies support the concept that biventricular pacing improves acutely symptoms of HF, effort tolerance, and LV performance in HF patients with QRS durations exceeding 120 msec.^148-155 As mentioned earlier, QRS prolongation of this magnitude, which occurs in about one third of HF patients, is an independent predictor of total and arrhythmic death.¹⁴ In the MADIT II trial, multivariate analysis revealed that QRS prolongation > 120 msec was an independent predictor of death in the non-ICD control arm.³² Importantly, patients with QRS prolongation > 120 msec (n = 364) benefited the most from ICD-based antiarrhythmic therapy, exhibiting over 3 years a 63% mortality reduction compared with non-ICD controls (P = 0.004).¹⁵⁶ Long-term (3-12 months) improvements in systolic LV performance assessed by echocardiographic techniques including "tissue tracking" (tissue Doppler imaging) have been reported recently.^151,152 Important cardiomechanical effects include contraction of the LV free wall as early as that of the septum ("resynchronization"), decreases in both end-diastolic and end-systolic volumes, improved ejection fraction, decreased mitral regurgitation, shortened isovolumic contraction time, and increased diastolic filling time with probable facilitation of diastolic myocardial perfusion. The authors concluded that chronic biventricular pacing achieves "reverse remodeling," but perhaps the most direct demonstration of tissue remodeling, regression of LV mass, unfortunately was not reported.^151,152 Of considerable interest is that echocardiographically monitored gains in LV function during biventricular pacing for 3 months were lost during subsequent omission of pacing for 4 weeks.¹⁵¹ Early reports suggested that periods with biventricular pacing compared with those withholding pacing were associated with decreased rates of appropriate ICD antitachyarrhythmia therapies or VT/VF inducibility.^153-155 However, a follow-up analysis of patients enrolled in the CONTAK CD study¹⁵⁷ failed to confirm earlier findings.¹⁵³ The preliminary results of the COMPANION trial¹⁵⁸ are encouraging. Congestive cardiomyopathy patients (n = 1,600) with LVEF <35% and QRS duration > 120 msec were randomized to three open-label treatment arms: no device therapy, biventricular pacing without ICD backup, and biventricular pacing with ICD backup.¹⁵⁸ Total mortality rates in these arms were 19%, 15%, and 11%, respectively, confirming the superiority of biventricular pacing with ICD backup. We hope that device trials in the future will relate clinical outcome to structural cardiologic parameters, particularly estimate of LV mass by echocardiography or magnetic resonance imaging.

Endpoints for the Efficacy of Therapies for Prevention of SCD

Predictors of arrhythmic death in HF have been a focus of clinical research for many years. Readily determined predictors might be particularly useful for follow-up of patients during therapies aimed at reducing SCD risk or frequent shocks in ICD recipients. Proposed predictors include measures of total body performance (maximum oxygen uptake, various exercise protocols), cardiomechanical performance (ejection fraction), ECG criteria (signal-averaged ECG, QRS duration, QTc duration, QT-interval dispersion, T wave alternans), cardiac electrophysiologic characteristics (electro-physiologic study and VT/VF inducibility), arrhythmic history including history of ICD therapies in ICD recipients, neurophysiologic regulation (heart rate variability, baroreflex sensitivity, Valsalva response), and a multitude of circulating factors, such as inflammatory peptides (C-reactive protein [CRP], IL-18, TNF), cardiac peptides (BNP), endothelins, AII, catecholamines, markers of collagen turnover, and even n-3 fatty acids. In a recent meta-analysis of tests in use for arrhythmia risk stratification, including history of serious arrhythmia, signal-averaged ECG, heart rate variability, LVEF, and electrophysiologic study, tests ranged from 50%-60% for sensitivity, 75%-85% for specificity, 15%-25% for positive predictive value, and 94%-96% for negative predictive value.¹⁵⁹ Recently, three peptides, BNP,^102,103 CRP,^160,161 and IL-18¹⁶² have attracted attention as predictors of cardiovascular death. In a substudy (n = 129 patients) of the Simvastatin 4S trial (N = 4,444 in original 4S), CRP predicted death in patients with "stable ischemic heart disease" which was interpreted as an association between proinflammatory CRP levels and coronary plaque destabilization.¹⁶¹ Similarly, Blankenberg et al.¹⁶² believed that IL-18 as a predictor of cardiovascular mortality reflected an inflammatory destabilization of coronary plaques. Discussing apoptosis, we mentioned that IL-18 and IL-1β, two proinflammatory cytokines, were generated under the action of caspase-1 (also known as IL-1β converting enzyme [ICE]). If these cytokines play a role in destabilizing coronary heart disease or provoking SCD, it would be interesting to limit their syntheses with oral caspase-1 inhibitors such as pralnacasan.¹⁶³ We previously discussed the evidence that proinflammatory agents act as arrhythmogens independent of atherothrombotic events.¹⁶⁴ It will be of interest to develop microarray test kits targeting multiple gene expressions presumed to act as SCD risk markers and to relate multi marker profiles to clinical data including cardiac structure.

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