Ian N. Sabir
Gareth D. K. Matthews
Christopher L.-H. Huang- 1The Rayne Institute, St. Thomas' Hospital, London, UK
- 2Physiological Laboratory, University of Cambridge, Cambridge, UK
- 3Department of Biochemistry, University of Cambridge, Cambridge, UK
Sudden cardiac death refers to unexpected death attributable to a cardiac cause occurring within 1 h of the onset of symptoms (NICE, 2006). It often results from cardiac arrhythmias and is a major worldwide cause of morbidity and mortality. Arrhythmias account for 180,000 to 250,000 deaths per year in the United States and ~70,000 deaths per year in the United Kingdom (NICE, 2006; Chugh et al., 2008). These commonly present as ventricular fibrillation often preceded by ventricular tachycardia (Turakhia and Tseng, 2007). Cardiac arrhythmias most frequently result from underlying ischaemic heart disease (Behr et al., 2003). However, ~4% may arise from ion channel abnormalities (Tung et al., 1994; Martin et al., 2012) with their own implications for management (Martin et al., 2011). In all events, cardiac arrhythmias follow disruption of the normal cell excitation and recovery sequence propagating through successive cardiac regions. A sequence of reviews and original articles in Frontiers in Cardiac Electrophysiology together survey genetic, biophysical, physiological and modeling studies bearing upon mechanisms of and possible translational implications for ventricular arrhythmia and sudden cardiac death, referring to other arrhythmic situations, particularly atrial fibrillation, where these throw light upon fundamental mechanisms.
Kapur and Macrae (2013) review developmental events regulating appearance of molecules and structures underlying normal automaticity and conduction responsible for atrial rhythm, providing a necessary background for normal ventricular activation. Jagu et al. (2013) then outline and illustrate outcomes of genetic studies of biochemical processes underlying relationships between genetic background and protein expression whose alterations lead to arrhythmic tendency. Ion channel expression depends upon a sequence of processes beginning with DNA transcription into mRNA and its regulation by promoter sites. Persistence of the resulting mRNA then depends upon its stability and the presence or absence of mRNA splicing. Translation from mRNA into protein is potentially regulated by microRNAs and alternative translation phenomena. Finally, expression of synthesized protein depends upon its assembly, post-translational modification and trafficking to its normal membrane site.
Nielsen et al. (2013) further explore uncertainties in relationships between genetic change and their functional consequences specifically for Brugada Syndrome (BrS). This condition is associated with hundreds of variants in 17 genes, most commonly with SCN5A mutations implicating cardiac voltage-gated sodium channels. However, ~70% of BrS cases cannot currently be explained genetically. Clarification of these relationships would enhance genetic risk stratification taking advantage of multi-gene Next Generation Sequencing. However, Gütter et al. (2013) throw biophysical light on uncertainties in the relationship between genetic and functional properties: voltage-clamp investigations revealed that only one out of three mutant channels associated with clinical long QT syndrome type 3 (LQT3) and three out of six mutant channels associated with BrS showed functional abnormalities in a series of N-terminal, human hNav1.5, mutations.
Nevertheless Nielsen et al. (2013) associate more severe BrS disease phenotypes with large as opposed to small reductions in INa, whose most obvious biophysical effect is to reduce action potential conduction velocity, producing potentially pro-arrhythmic re-entrant substrate. King et al. (2013) review factors affecting cardiac conduction velocity. The underlying local circuit current flows between myocytes depend upon not only on fast Na+ current, but also axial resistance and cellular excitability. These could alter with impaired Na+ channel and gap junction function, and altered tissue geometry following fibrotic change accompanying pathophysiological processes. Such substrate may accompany arrhythmic situations particularly with the triggering events typically associated with disrupted Ca2+ homeostasis exemplified by the altered sarcoplasmic reticular Ca2+ release through RyR2-Ca2+ release channels in catecholaminergic polymorphic ventricular tachycardia. Zhang et al. (2013) summarize recent studies associating RyR2 abnormalities with atrial in addition to ventricular arrhythmias. They further point out that homozygotic RyR2-P2328S hearts show reduced conduction velocities potentially generating arrhythmic substrate, in addition to delayed afterdepolarization and ectopic action potential firing.
Computational studies permit biophysical information to be compiled into theoretical reconstructions of in vivo physiological changes resulting from a primary loss or gain of function. Thus, carbon monoxide (CO) is produced by a number of different mammalian tissues and exerts significant cardiovascular effects. Computational studies (Trenor et al., 2013) relate known changes in CO-induced alterations in ventricular slowly-inactivating ranolazine-sensitive late Na+, and Ca2+, channel, activity, to potentially pro-arrhythmic after-depolarization-like rhythm disturbances illustrating important elements of their underlying causes. Computational studies also illuminate in vivo physiological outcomes even where primary experimental systems are not available. Adeniran et al. (2013) apply these to the relatively rare, but well-defined clinical short QT syndrome, associated with accelerated ventricular repolarization, arrhythmias and sudden cardiac death. Few experimental SQT1 and SQT3 models, involving altered K+ or Ca2+ channel function are currently available. Nevertheless, application of the ten Tusscher and Panfilov (TP) human ventricular single cell model allowed in silico exploration of the consequences of this condition for Ca2+ release and mechanical output. Combined with cable theory, this further yielded whole organ functional reconstructions. This provided testable predictions implicating stretch-activated current in contractile function. Finally, an original contribution by Finlay et al. (2013) then successfully apply a modeling approach based on one-dimensional cable theory to describe human restitution dynamics incorporating both conduction velocity restitution and action potential restitution, for the first time in man.
Finally, discussions of possible clinical translational developments begin with problems and methodologies associated with diagnosis of sudden cardiac death risk. Vyas and Lambiase (2013) evaluate currently available screening strategies for sudden cardiac death risk in sudden arrhythmic death syndrome families, looking forward to roles for molecular autopsy and genetic testing, and potential future applications of stem cell-based diagnostic strategies. The first of two preliminary clinical studies report findings relating to sphingolipid levels as novel markers for early detection of human myocardial ischaemic injury predisposing to malignant ventricular arrhythmias (Egom et al., 2013). A report in chronic kidney disease patients, in which both early repolarization and sudden cardiac death are common, nevertheless did not associate early repolarization with increased 1-year mortality or entry onto dialysis programs (Hajhosseiny et al., 2013).
This collection of articles thus overviews current knowledge bearing on experimental studies of sudden cardiac death and its associated arrhythmias, and possible translational insights concerning clinical prevention and management.
References
Adeniran, I., Hancox, J. C., and Zhang, H. (2013). In silico investigation of the short QT syndrome, using human ventricle models incorporating electromechanical coupling. Front. Physiol. 4:166. doi: 10.3389/fphys.2013.00166
Behr, E., Wood, D. A., Wright, M., Syrris, P., Sheppard, M. N., Casey, A., et al. (2003). Sudden Arrhythmic Death Syndrome Steering Group. Cardiological assessment of first-degree relatives in sudden arrhythmic death syndrome. Lancet 362, 1457–1459.
Chugh, S. S., Reinier, K., Teodorescu, C., Evanado, A., Kehr, E., Al Samara, M., et al. (2008). Epidemiology of sudden cardiac death: clinical and research implications. Prog. Cardiovasc. Dis. 51, 213–228. doi: 10.1016/j.pcad.2008.06.003
Egom, E. E., Mamas, M. A., Chacko, S., Stringer, S. E., Charlton-Menys, V., El-Omar, M., et al. (2013). Serum sphingolipids level as a novel potential marker for early detection of human myocardial ischaemic injury. Front. Physiol. 4:130. doi: 10.3389/fphys.2013.00130
Finlay, M. C., Xu, L., Taggart, P., Hanson, B., and Lambiase, P. D. (2013). Bridging the gap between computation and clinical biology: validation of cable theory in humans. Front. Physiol. 4:213. doi: 10.3389/fphys.2013.00213
Gütter, C., Benndorf, K., and Zimmer, T. (2013). Characterization of N-terminally mutated cardiac Na+ channels associated with long QT syndrome 3 and Brugada syndrome. Front. Physiol. 4:153. doi: 10.3389/fphys.2013.00153
Hajhosseiny, R., Rajani, R., Khavandi, K., Sebag, F. A., Mashayekhi, S., Wright, M., et al. (2013). The prevalence of electrocardiographic early repolarization in an adult cohort with chronic kidney disease and its impact upon all-cause mortality and progression to dialysis. Front. Physiol. 4:127. doi: 10.3389/fphys.2013.00127
Jagu, B., Charpentier, F., and Toumaniantz, G. (2013). Identifying potential functional impact of mutations and polymorphisms: linking heart failure, increased risk of arrhythmias and sudden cardiac death. Front. Physiol. 4:254. doi: 10.3389/fphys.2013.00254
Kapur, S., and Macrae, C. A. (2013). The developmental basis of adult arrhythmia: atrial fibrillation as a paradigm. Front. Physiol. 4:221. doi: 10.3389/fphys.2013.00221
King, J. H., Huang, C. L.-H., and Fraser, J. A. (2013). Determinants of myocardial conduction velocity: implications for arrhythmogenesis. Front. Physiol. 4:154. doi: 10.3389/fphys.2013.00154
Martin, C. A., Huang, C. L.-H., and Matthews, G. D. (2011). Recent developments in the management of patients at risk for sudden cardiac death. Postgrad. Med. 123, 84–94. doi: 10.3810/pgm.2011.03.2266
Martin, C. A., Matthews, G. D., and Huang, C. L.-H. (2012). Sudden cardiac death and inherited channelopathy: the basic electrophysiology of the myocyte and myocardium in ion channel disease. Heart 98, 536–543. doi: 10.1136/heartjnl-2011-300953
National Institute for Health and Clinical Excellence (NICE). (2006). Arrhythmia - implantable cardioverter defibrillators (ICDs) (review). London: Technical Appraisals. TA95. Available online at: http://guidance.nice.org.uk/TA95.
Nielsen, M. W., Holst, A. G., Olesen, S. P., and Olesen, M. S. (2013). The genetic component of Brugada syndrome. Front. Physiol. 4:179. doi: 10.3389/fphys.2013.00179
Trenor, B., Cardona, K., Saiz, J., Rajamani, S., Belardinelli, L., and Giles, W. R. (2013). Carbon monoxide effects on human ventricle action potential assessed by mathematical simulations. Front. Physiol. 4:282. doi: 10.3389/fphys.2013.00282
Tung, R. T., Shen, W. K., Hammill, S. C., and Gersh, B. J. (1994). Idiopathic ventricular fibrillation in out-of-hospital cardiac arrest survivors. Pacing Clin. Electrophysiol. 17, 1405–1412. doi: 10.1111/j.1540-8159.1994.tb02460.x
Turakhia, M., and Tseng, Z. H. (2007). Sudden cardiac death: epidemiology, mechanisms, and therapy. Curr. Probl. Cardiol. 32, 501–546. doi: 10.1016/j.cpcardiol.2007.05.002
Vyas, V., and Lambiase, P. D. (2013). The investigation of sudden arrhythmic death syndrome (SADS)-the current approach to family screening and the future role of genomics and stem cell technology. Front. Physiol. 4:199. doi: 10.3389/fphys.2013.00199
Keywords: sudden cardiac death, ventricular arrhythmia, ion channels, action potentials, conduction velocity, re-entrant substrate, Brugada Syndrome, catecholaminergic polymorphic ventricular tachycardia
Citation: Sabir IN, Matthews GDK and Huang CL-H (2013) Sudden arrhythmic death: from basic science to clinical practice. Front. Physiol. 4:339. doi: 10.3389/fphys.2013.00339
Received: 30 October 2013; Accepted: 03 November 2013;
Published online: 25 November 2013.
Edited by:
Ruben Coronel, Academic Medical Center, NetherlandsCopyright © 2013 Sabir, Matthews and Huang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: clh11@cam.ac.uk