Phase-2 Reentry Research Articles

PO-691-05 FIRST EXPERIMENTAL OBSERVATION OF ALTERNANS-INDUCED PHASE-2 REENTRY IN BRUGADA SYNDROME BY OPTICAL MAPPING ON AN EXPLANTED HUMAN HEART, WITH NUMERICAL SIMULATION VALIDATION

: Induction of VF in Brugada syndrome is thought to be produced by a phase-2 reentry resulting from areas with mismatched action potential duration. While this has been shown numerically and in some experiments with MAP recordings, to date there is no full detailed experimental observation of the phase-2 induction.

Why Is Only Type 1 Electrocardiogram Diagnostic of Brugada Syndrome? Mechanistic Insights From Computer Modeling.

Three types of characteristic ST-segment elevation are associated with Brugada syndrome but only type 1 is diagnostic. Why only type 1 ECG is diagnostic remains unanswered. Computer simulations were performed in single cells, 1-dimensional cables, and 2-dimensional tissues to investigate the effects of the peak and late components of the transient outward potassium current (Ito), sodium current, and L-type calcium current (ICa,L) as well as other potassium currents on the genesis of ECG morphologies and phase 2 reentry (P2R). Although a sufficiently large peak Ito was required to result in the type 1 ECG pattern and P2R, increasing the late component of Ito converted type 1 ECG to type 2 ECG and suppressed P2R. Increasing the peak Ito promoted spiral wave breakup, potentiating the transition from tachycardia to fibrillation, but increasing the late Ito prevented spiral wave breakup by flattening the action potential duration restitution and preventing P2R. A sufficiently large ICa,L conductance was needed for P2R to occur, but once above the critical conductance, blocking ICa,L promoted P2R. However, selectively blocking the window and late components of ICa,L suppressed P2R, countering the effect of the late Ito. Blocking either the peak or late components of sodium current promoted P2R, with the late sodium current blockade having the larger effect. As expected, increasing other potassium currents potentiated P2R, with ATP-sensitive potassium current exhibiting a larger effect than rapid and slow component of the delayed rectifier potassium current. The peak Ito promotes type 1 ECG and P2R, whereas the late Ito converts type 1 ECG to type 2 ECG and suppresses P2R. Blocking the peak ICa,L and either the peak or the late sodium current promotes P2R, whereas blocking the window and late ICa,L suppresses P2R. These results provide important insights into the mechanisms of arrhythmogenesis and potential therapeutic targets for treatment of Brugada syndrome. Graphic Abstract: A graphic abstract is available for this article.

Specific decreasing of Na+ channel expression on the lateral membrane of cardiomyocytes causes fatal arrhythmias in Brugada syndrome

Reduced cardiac sodium (Na+) channel current (INa) resulting from the loss-of-function of Na+ channel is a major cause of lethal arrhythmias in Brugada syndrome (BrS). Inspired by previous experimental studies which showed that in heart diseases INa was reduced along with expression changes in Na+ channel within myocytes, we hypothesized that the local decrease in INa caused by the alteration in Na+ channel expression in myocytes leads to the occurrence of phase-2 reentry, the major triggering mechanism of lethal arrhythmias in BrS. We constructed in silico human ventricular myocardial strand and ring models, and examined whether the Na+ channel expression changes in each myocyte cause the phase-2 reentry in BrS. Reducing Na+ channel expression in the lateral membrane of each myocyte caused not only the notch-and-dome but also loss-of-dome type action potentials and slowed conduction, both of which are typically observed in BrS patients. Furthermore, the selective reduction in Na+ channels on the lateral membrane of each myocyte together with spatial tissue heterogeneity of Na+ channel expression caused the phase-2 reentry and phase-2 reentry-mediated reentrant arrhythmias. Our data suggest that the BrS phenotype is strongly influenced by expression abnormalities as well as genetic abnormalities of Na+ channels.

NS5806 Induces Electromechanically Discordant Alternans and Arrhythmogenic Voltage-Calcium Dynamics in the Isolated Intact Rabbit Heart.

Background: NS5806 activates the transient outward potassium current Ito, and has been claimed to reproduce Brugada Syndrome (BrS) in ventricular wedge preparations. Ito modulates excitation-contraction coupling, which is critical in alternans dynamics. We explored NS5806-arrhythmogenic effects in the intact whole heart and its impact on alternans.Methods: Langendorff-perfused rabbit hearts (n = 20) underwent optical AP and Ca mapping during pacing at decremental cycle lengths (CL). Spontaneous arrhythmias and pacing-induced alternans was characterized at baseline (BL), after perfusing with NS5806, before and after adding verapamil (VP), and SEA0400 (SEA, n = 5 each), to modulate Ca-current and Na-Ca exchange, the main AP-Ca coupling mechanisms.Results: NS5806 induced BrS-like ECG features in 6 out of 20 hearts. NS5806 prolonged steady-state (3 Hz) action potential duration (APD) by 16.8%, Ca decay constant by 34%, and decreased conduction velocity (CV) by 52.6%. After NS5806 infusion, spontaneous ventricular ectopy (VE) and AP/Ca alternans occurred. Pacing-induced alternans during NS5806 infusion occurred at longer CL and were AP/Ca discordant from its onset. Spatially discordant alternans after NS5806 infusion had non-propagation-driven nodal line distribution. No spontaneous phase-2 reentry occurred. Under NS5806 + VP, alternans became AP/Ca concordant and only induced in two out of five; NS5806 + SEA did not affect alternans but suppressed spontaneous ectopy.Conclusions: NS5806 disrupts AP-Ca coupling and leads to Ca-driven, AP/Ca-discordant alternans and VE. Despite BrS-like ECG features, no spontaneous sustained arrhythmias or phase-2 reentry occurred. NS5806 does not fully reproduce BrS in the intact rabbit heart.

Reflected Conduction Attributed to Sodium Channel Distribution Within Cardiomyocytes: A Possible Mechanism of Ventricular Fibrillation Induction in Brugada Syndrome

Brugada syndrome is characterized by the ECGs with both a right bundle-branch block pattern and ST-segment elevation in the right precordial leads (V1-V3) and by the higher incidence of sudden cardiac death due to ventricular tachycardia/fibrillation. It is commonly believed that the lethal arrhythmias are elicited by the closely coupled premature ventricular contractions via the reflected conduction (i.e., phase-2 reentry) mechanism. Although loss of function mutation in cardiac sodium (Na) channels have been linked to Brugada syndrome, the relation between the loss of function in Na channels and the arrhythmogenic mechanism remains unclear. We have recently reported that the action potential propagation was greatly affected by the subcellular distribution of Na channels in a physiologically-accurate myofiber model where myocytes were electrically coupled with both gap junctions and intercellular cleft conductor (electric field mechanism). Here, we extended the simulation to the initiation of reflected conduction in Brugada syndrome. We conducted computer simulations of action potential propagation in the myofiber model, and investigated the effects of subcellular distributions of Na channels on the development of reflected conduction. We found that the myofiber model with specific subcellular Na channel distribution (all Na channels were distributed only in the intercalated disks and there were no Na channels along the lateral side of each myocyte) resulted in early repolarization followed by reflected conduction. Subcellular Na channel distribution might be responsible for fibrillation induction in Brugada syndrome.

Transient Outward Potassium Current and its Arrhythmogenic Dynamics in Cardiac Myocytes

The transient outward K+ current (Ito) is a main contributor to repolarization during the cardiac action potential in cardiac myocytes, which may play important roles in arrhythmogenesis in cardiac diseases including ischemia, Brugada syndrome, and heart failure. Previous experimental and computer modeling studies have shown that Ito can cause complex action potential dynamics (alternans and chaos), which can potentiate phase-2 reentry. However, the underlying mechanisms are not well understood. In this study, we carried out computer simulations and developed simplified mathematical models (iterated maps) to reveal the mechanisms of Ito-induced complex action potential dynamics. We showed that: 1) Ito induces short-term cardiac memory via its interactions with a slowly activating K+ current (IKs) and the L-type Ca2+ current (ICa,L), which promotes alternans during the physiological heart rate. This can be a candidate mechanism for T-wave alternans widely seen in patients with Brugada syndrome; 2) Ito gives rise to two different types of non-monotonic restitution curves. In the presence of rapidly recovering Ito, as the diastolic interval increases, the action potential duration is short at short diastolic intervals (DIs), increases sharply at a certain range of DIs, and saturates at large DIs. In the presence of slowly recovering Ito, the action potential duration is normal at short DIs, decreases sharply at a certain range of DIs, and becomes short at large DIs. These restitution curves along with short-term memory induced by Ito can give rise to complex action potential dynamics and chaos. In conclusion, using both computer simulation and simplified mathematical models, we can pinpoint the underlying mechanisms for the complex arrhythmogenic action potential dynamics induced by Ito.

Basis for the Induction of Tissue-Level Phase-2 Reentry as a Repolarization Disorder in the Brugada Syndrome.

Aims. Human action potentials in the Brugada syndrome have been characterized by delayed or even complete loss of dome formation, especially in the right ventricular epicardial layers. Such a repolarization pattern is believed to trigger phase-2 reentry (P2R); however, little is known about the conditions necessary for its initiation. This study aims to determine the specific mechanisms that facilitate P2R induction in Brugada-affected cardiac tissue in humans. Methods. Ionic models for Brugada syndrome in human epicardial cells were developed and used to study the induction of P2R in cables, sheets, and a three-dimensional model of the right ventricular free wall. Results. In one-dimensional cables, P2R can be induced by adjoining lost-dome and delayed-dome regions, as mediated by tissue excitability and transmembrane voltage profiles, and reduced coupling facilitates its induction. In two and three dimensions, sustained reentry can arise when three regions (delayed-dome, lost-dome, and normal epicardium) are present. Conclusions. Not only does P2R induction by Brugada syndrome require regions of action potential with delayed-dome and lost-dome, but in order to generate a sustained reentry from a triggered waveback multiple factors are necessary, including heterogeneity in action potential distribution, tissue coupling, direction of stimulation, the shape of the late plateau, the duration of lost-dome action potentials, and recovery of tissue excitability, which is predominantly modulated by tissue coupling.

Dependence of phase-2 reentry and repolarization dispersion on epicardial and transmural ionic heterogeneity: a simulation study

Phase-2 reentry (P2R) is a local arrhythmogenic phenomenon where electrotonic current propagates from a spike-and-dome action potential region to re-excite a loss-of-dome action potential region. While ionic heterogeneity has been shown to underlie P2R within the epicardium and has been hypothesized to occur transmurally, we are unaware of any study that has investigated the effects of combining these heterogeneities as they occur in the heart. Thus, we tested the hypothesis that P2R can result by either epicardial or transmural heterogeneity and that the realistic combination of the two would increase the likelihood of P2R. We used computational ionic models of cardiac myocyte dynamics to investigate initiation and development of P2R in simulated tissues with different ionic heterogeneities. In one-dimensional transmural cable simulations, P2R occurred when the conductance of the transient outward current in the epicardial region was near the range for which epicardial action potentials switched intermittently between spike-and-dome and loss-of-dome morphologies. Phase-2 reentry was more likely in two-dimensional tissue simulations by both epicardial and transmural heterogeneity and could expand beyond its local initiation site to create a macroscopic reentry. The characteristics and stability of action potential morphology in the epicardium are important determinants of the occurrence of both transmural and epicardial P2R and its associated arrhythmogenesis.

Brugada Syndrome and Pregnancy: Delving Into the Role of Sex Hormones in Ion Channelopathies

Since its initial description in 1992 as a clinical syndrome involving a specific electrocardiogram (ECG) pattern and an increased susceptibility for ventricular arrhythmias and sudden cardiac death (SCD), the Brugada syndrome (BrS) has prompted much research activity aimed at defining the clinical, genetic, and molecular aspects of the disease. Major advances in genetics have allowed the identification of mutations linked to the syndrome in more than 10 different genes thus far. Recognized mutations disturb the expression and/or function of different cardiac ion channels that participate in the cardiac action potential, so the syndrome is now included among the so-called channelopathies. The common functional consequence of these mutations, confirmed by experimental models, is thought to be a heterogeneous imbalance between positive outward and inward currents during early phases of repolarization, which can explain both the ST-segment elevation in the ECG and the susceptibility to ventricular arrhythmias through a phase-2 reentry mechanism. However, important questions concerning the mechanisms and/or clinical manifestations of the BrS remain unanswered. For example, only around 20% to 30% of patients have a positive genetic test, indicating that the disease-causing mutation or mechanism remains unknown for most of them. Also intriguing is the broad phenotypic expression of the syndrome even for a given mutation, which may range from the absence of symptoms or ECG manifestation to the occurrence of SCD at an early age. This latter observation supports the concept that particularities in intrinsic susceptibility and/or the effect of certain modulators (genetic, molecular, or external) might play a role in the final phenotype of patients with BrS. In line with this last concept, the difference in clinical expression between men and women with BrS is a wellestablished observation for which there is no full mechanistic understanding yet. All the main series of BrS concur in reporting a higher symptom and event rate among men than among women with the syndrome. The ECG expression is also more pronounced

Effects of Subcellular Sodium Channel Distribution on the Initiation of Phase-2 Reentry: A Possible Mechanism of Fibrillation Induction in Brugada Syndrome

Cardiac sodium (Na+) channels play critical roles in initiating and propagating action potentials in myocardium. We have recently reported that the action potential propagation was greatly affected by the subcellular distribution of Na+ channels in a physiologically-relevant myofiber model, where myocytes were electrically coupled with both gap-junctions and intercellular cleft conductor. Here, we extended the simulation to the initiation of phase-2 reentry in Brugada syndrome. We conducted computer simulations of action potential propagation in the myofiber model, and investigated the effect of spatial and subcellular distributions of Na+ channels on the phase-2 reentry. In the myofiber model with local decrease in Na+ current density, phase-2 reentry was not reproduced. Surprisingly, in the same myofiber model but with a certain type of subcellular Na+ channel remodeling (all Na+ channels were distributed only in the intercalated disks and there were no Na+ channels along the lateral side of each myocyte), the local decrease in Na+ current density resulted in the marked abbreviation of the action potential duration followed by phase-2 reentry. Changes in subcellular Na+ channel distribution in addition to the spatially-heterogeneous Na+ channel density might be required for fibrillation induction in Brugada syndrome.

Phase-2 reentry in cardiac tissue: Role of the slow calcium pulse

Phase-2 re-entry is thought to underlie many causes of idiopathic ventricular arrhythmias as, for instance, those occurring in Brugada syndrome. In this paper, we study under which circumstances a region of depolarized tissue can re-excite adjacent regions that exhibit shorter action potential duration (APD), eventually inducing reentry. For this purpose, we use a simplified ionic model that reproduces well the ventricular action potential. With the help of this model, we analyze the conditions that lead to very short action potentials (APs), as well as possible mechanisms for re-excitation in a cable. We then study the induction of re-entrant waves (spiral waves) in simulations of AP propagation in the heart ventricles. We show that re-excitation takes place via a slow pulse produced by calcium current that propagates into the region of short APs until it encounters excitable tissue. We calculate analytically the speed of the slow pulse, and also give an estimate of the minimal tissue size necessary for allowing reexcitation to take place.

Reexcitation mechanisms in epicardial tissue: Role of Ito density heterogeneities and INa inactivation kinetics

Dispersion of action potential repolarization is known to be an important arrhythmogenic factor in cardiopathies such as Brugada syndrome. In this work, we analyze the effect of a variation in sodium current ( I Na) inactivation and a heterogeneous rise of transient outward current ( I to) in the probability of reentry in epicardial tissue. We use the Luo–Rudy model of epicardial ventricular action potential to study wave propagation in a one-dimensional fiber. Spatial dispersion in repolarization is introduced by splitting the fiber into zones with different strength of I to. We then analyze the pro-arrhythmic effect of a variation in the relaxation time and steady-state of the sodium channel fast inactivating gate h. We quantify the probability of reentry measuring the percentage of reexcitations that occurs in 200 beats. We find that, for high stimulation rates, this percentage is negligible, but increases notably for pacing periods above 700 ms. Surprisingly, with decreasing I Na inactivation time, the percentage of reexcitations does not grow monotonically, but presents vulnerable windows, separated by values of the I Na inactivation speed-up where reexcitation does not occur. By increasing the strength of L-type calcium current I CaL above a certain threshold, reexcitation disappears. Finally, we show the formation of reentry in stimulated two-dimensional epicardial tissue with modified I Na kinetics and I to heterogeneity. Thus, we confirm that while I to dispersion is necessary for phase-2 reentry, altered sodium inactivation kinetics influences the probability of reexcitation in a highly nonlinear fashion.

Phase - 2 Reentry in Man

Ventricular extrasystoles are characterized by a fixed coupling interval to the last QRST-complex preceding it. We hypothesized that this QRST-complex differed from QRST-complexes of other sinus beats not followed by ventricular extrasystoles. Further, we investigated whether phase-2 reentry, demonstrated recently in animal experiments to initiate ventricular extrasystoles, ventricular tachycardia and ventricular fibrillation, also could play a role in humans. We examined 18 patients with ventricular extrasystoles and/or ventricular tachycardia by signal averaging of the ECG (group A) or by single beat analysis of intracardiac electrograms (group B). Group A consisted of 6 patients without structural heart disease and one patient with the Brugada syndrome. Six of the 7 patients had right ventricular outflow tract ventricular extrasystoles. In 6 of the 7 patients we demonstrated significant ST-elevation and/or T-wave changes in the sinus beat preceding ventricular extrasystoles compared to the second last sinus beat in one or more of the three orthogonal leads X, Y and Z. Group B consisted of 11 patients undergoing radiofrequency ablation. Eight of the 11 patients had right ventricular outflow tract extrasystoles. In 9 of the 11 patients, single beat analysis of unipolar and bipolar electrograms recorded close to successful ablation sites demonstrated similar changes i.e. ST-elevation (median peak voltage gradient 150 microvolt (range 0-1700) and T-wave changes in the sinus beat prior to ventricular ectopy. In addition J-point elevation was demonstrated in several cases. In total, significant changes were demonstrated in 15 of the 18 patients studied (83%). The J-point elevation, the ST-elevation and the T-wave changes documented in the last sinus beat prior to ventricular extrasystoles are in agreement with phase-2 reentry suggesting that this may be the responsible mechanism for ventricular extrasystoles and/or ventricular tachycardia/fibrillation The phenomenon has hitherto been demonstrated in animal experiments only.

Link Between SCN5A Mutation and the Brugada Syndrome ECG Phenotype Simulation Study

The specific changes in the gating kinetics of the sodium current (I(Na)) responsible for its phenotype have remained to be elucidated. In the present study the effect of changes in the gating kinetics of I(Na) on early repolarization (ER) and initiation of phase 2 reentry (P2R) were evaluated in a theoretical epicardial ventricular fiber model. Miyoshi-I(CaL) was incorporated into the modified Luo-Rudy dynamic (LRd) model. Dispersion at Ito-density was set within a theoretical fiber composed of serially arranged epicardial cells with gap junctions. The following changes in I(Na) kinetics were made: (1) a-10 mV shift in steady-state inactivation, (2) a+10 mV shift in steady-state activation curve, (3) a small inactivation time constant (DEC); P2R and ER were observed. A conduction disturbance within the fiber was simulated and only when the I(Na)-density was decreased did DEC, especially, show a marked increase in the likelihood of causing ER and P2R. Conduction disturbance significantly increased the likelihood causing ER or P2R. In this one-dimension model with Ito-density dispersion, DEC-I(Na) precipitates I(Na)-blocker inducible ER. This suggests that the characteristic ST-segment elevation in the Brugada syndrome with SCN5A mutation can be interpreted in part by DEC-I(Na). Concomitant conduction disturbance may be required to cause P2R at physiological Ito density.

Cellular and ionic basis for the sex-related difference in the manifestation of the Brugada syndrome and progressive conduction disease phenotypes

The Brugada syndrome (BS) has been linked to mutations in SCN5A. Despite equal hereditary transmission of the mutation between the sexes, the syndrome is 8 to 10 times more likely to occur in males. As recently reported, SCN5A mutations such as G1406R lead to development of BS phenotype principally in males and conduction disease phenotype in females. We hypothesized that these differences may be related to a larger transient outward current (Ito)-mediated right ventricular (RV) epicardial (Epi) action potential (AP) notch in males versus females, resulting in a higher incidence of all-or-none repolarization at the end of phase 1 and phase 2 re-entry (P2R) when challenged with sodium and calcium channel block. Using canine RV wedge preparations, we developed an experimental model of the BS using terfenadine to depress the AP dome in RV Epi. Results: RV Epi AP phase 1 amplitude corrected to phase 2 amplitude was 12% smaller in males (n = 18) compared to females (n = 8, P< .05) at a cycle length of 2,000 ms. When exposed to 5 μM terfenadine for up to 2 hours, 6 of 7 male but only 2 of 7 female preparations exhibited spontaneous P2R, generating a closely coupled extrasystole. Two of 6 male and 1 of 2 female preparations displaying P2R developed polymorphic VT/VF. Female and male preparations that failed to develop P2R displayed progressive conduction impairment with continued exposure to terfenadine and developed polymorphic and monomorphic VT/VF when paced at rapid rates. Male preparations pretreated with 4-aminopyridine to inhibit Ito displayed progressive conduction impairment but not Brugada syndrome. Conclusion: Our data suggest that the presence of a more prominent I to-mediated notch in the Epi of males predisposes males to the development of the Brugada phenotype and that a smaller Epi notch in females relegates them to development of progressive conduction problems under conditions in which inward currents are compromised.

A mathematical model of phase 2 reentry: role of L-type Ca current.

Phase 2 reentry (P2R) is known to be one of the mechanisms of malignant ventricular arrhythmias, especially those associated with Brugada syndrome. However, little is known about the underlying mechanism for P2R. Our aim in this study was to simulate P2R in a mathematical model to enable us to understand its mechanism and identify a potential therapeutic target. A mathematical model of the L-type Ca current was composed according to whole cell current data from guinea pig ventricular myocytes recorded at 37 degrees C. Our mathematical model was incorporated into the modified Luo-Rudy phase 2 model. We set a dispersion in transient outward current (I(to)) density within the theoretical fiber, composed of 80 serially arranged epicardial cells with gap junctions and then observed the P2R. The dispersion in I(to) density within an only 0.8-cm epicardial theoretical fiber generated P2R with our Ca channel but not with the original model. When the P2R developed in the theoretical fiber, the calculated extracellular field potential showed coved-type ST segment elevation. We succeeded in generating P2R in our model for the first time. The local epicardial P2R may contribute the genesis of coved-type ST segment elevation in the Brugada syndrome.

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Dispersion of action potential repolarization is known to be an important arrhythmogenic factor in cardiopathies such as Brugada syndrome. In this work, we analyze the effect of a variation in sodium current (INa) inactivation and a heterogeneous rise of transient outward current (Ito) in the probability of reentry in epicardial tissue. We use the Luo–Rudy model of epicardial ventricular action potential to study wave propagation in a one-dimensional fiber. Spatial dispersion in repolarization is introduced by splitting the fiber into zones with different strength of Ito. We then analyze the pro-arrhythmic effect of a variation in the relaxation time and steady-state of the sodium channel fast inactivating gate h. We quantify the probability of reentry measuring the percentage of reexcitations that occurs in 200 beats. We find that, for high stimulation rates, this percentage is negligible, but increases notably for pacing periods above 700 ms. Surprisingly, with decreasing INa inactivation time, the percentage of reexcitations does not grow monotonically, but presents vulnerable windows, separated by values of the INa inactivation speed-up where reexcitation does not occur. By increasing the strength of L-type calcium current ICaL above a certain threshold, reexcitation disappears. Finally, we show the formation of reentry in stimulated two-dimensional epicardial tissue with modified INa kinetics and Ito heterogeneity. Thus, we confirm that while Ito dispersion is necessary for phase-2 reentry, altered sodium inactivation kinetics influences the probability of reexcitation in a highly nonlinear fashion.