br Mechanisms of arrhythmias Several schemes have been used

Mechanisms of arrhythmias Several schemes have been used to classify the mechanisms of cardiac arrhythmias. Traditionally, these have been divided into nonreentrant and reentrant activity [11]. An alternative scheme divided them into those occurring at the cellular and tissue levels [12]. A dynamics-based classification, focusing on the trigger-tissue substrate interactions, divided arrhythmogenic mechanisms into unstable calcium cycling, reduced repolarization reserve, and excess repolarization reserve [13].
Focal activity Focal activity can arise from enhanced automaticity or triggered activity (Fig. 1).
Reentry Re-entry occurs when an CM-272 fails to extinguish itself and reactivates a region that has recovered from refractoriness [37]. It can be divided into two types: (i) reentry that occurs in the presence of an obstacle, around which an action potential can travel (circus-type); and (ii) reentry that occurs without an obstacle (reflection or phase 2).
Trigger versus substrate? Traditional views have considered trigger and substrate as independent entities [68]. Examples of triggers are EADs and DADs, which can induce ectopic beats. Substrate refers to electrophysiological abnormalities predisposing to reentry, including increased dispersion of conduction or repolarization, and abnormal restitution [69]. However, evidence suggests that the trigger itself can produce the necessary substrate for reentry, e.g., DADs inducing unidirectional block [70]. Conversely, reentry may be the triggering event for an arrhythmia. Thus, phase 2 reentry can produce closely coupled ectopic beats capable of initiating circus-type reentry in Brugada syndrome [64].
Future directions Novel antiarrhythmic agents that have been developed are mainly for the management of atrial fibrillation, targeting atrial-specific ion channels [71]. An example is vernakalant, a multichannel blocker of the ultra-rapid (IKur) and acetylcholine-activated (IK, ACh) potassium currents. This has minimal effects on the ventricles [72], and is thus less likely to induce malignant ventricular arrhythmias. However, there has been no recently licensed drug for the treatment of ventricular arrhythmias, although ranolazine, a late INa inhibitor licensed for angina, has demonstrated efficacy in experimental studies [73]. There is a need to develop new drugs for several reasons. First, implantable cardioverter-defibrillators (ICDs) are not able to prevent the occurrence of ventricular arrhythmias, but can only terminate them, and are not without significant morbidity. Yet, they are the only form of treatment shown to increase life expectancy. Secondly, current pharmacological agents possess significant cardiac and extracardiac side effects, the most concerning of which are their proarrhythmic properties [74]. A potential solution is the rational design of agents that target the abnormal proteins concerned. Thus, EADs observed in long QT syndrome type 3 could be prevented by drugs inhibiting the late INa[75]. DADs underlying CPVT may be abolished by blocking the ryanodine receptors, which mediate calcium release from the sarcoplasmic reticulum [76], or the sodium–calcium exchanger [77]. Cardiac-specific KATP channels, which are activated by ATP depletion during ischemia, are currently under investigation [78]. Furthermore, understanding the physiology of arrhythmogenesis, for example, the substrates that sustain reentry, can guide drug development. Thus, heterogeneities in action potential conduction or repolarization, observed in a number of ion channelopathies, may be reduced by gap junction openers [79].
Conflict of interest Disclosure of potential conflicts of interest: GT received funding from Xention Discovery for his research. This did not influence the content of this review article in any manner.
Acknowledgments
Introduction Rapid ventricular contractions caused by various tachycardias, such as atrial fibrillation (AF), atrial flutter (AFL), atrial tachycardia (AT), ventricular tachycardia, or even frequent ventricular ectopic beats, can induce left ventricular (LV) systolic dysfunction in clinical settings [1,2]. Tachypacing is one of the most useful methods to induce LV dysfunction in an animal model of heart failure (HF) [3] as a consequence of the extracellular (Ca2+ overload) and cellular modification of various ion channels and transporters [4,5], and beta-adrenergic receptors [3,6,7]. Beta-blockers have both negative chronotropic action and a pleiotropic effect on cardioprotection, leading to the recovery of impaired LV function in various structural and rhythm disorders, including idiopathic cardiomyopathy [8] and ventricular tachycardia [9]. The antiarrhythmic effects of β-blockers are well-known, although their negative inotropic and even proarrhythmic action in some cases with advanced HF [10] may be non-negligible.