The M Cell

Spatiotemporal patterns of early afterdepolarizations underlying abnormal T-wave morphologies in a tissue model of the Purkinje-ventricular system

Sudden cardiac death (SCD) is a leading cause of death worldwide, and the majority of SCDs are caused by acute ventricular arrhythmias (VAs). Early afterdepolarizations (EADs) are an important trigger of VA under pathological conditions, e.g., inherited or acquired long QT syndrome (LQTS). However, it remains unclear how EAD events at the cellular level are spatially organized at the tissue level to induce and maintain ventricular arrhythmias and whether the spatial-temporal patterns of EADs at the tissue level are associated with abnormal T-wave morphologies that are often observed in LQTS, such as broad-based, notched or bifid; late appearance; and pointed T-waves. Here, a tissue model of the Purkinje-ventricular system (PVS) was developed to quantitatively investigate the complex spatial-temporal dynamics of EADs during T-wave abnormalities. We found that (1) while major inhibition of ICaL can substantially reduce the excitability of the PVS leading to conduction failures, moderate ICaL inhibition can promote occurrences of AP alternans at short cycle lengths (CLs), and EAD events preferentially occur with a major reduction of IKr (>50%) at long CLs; (2) with a minor reduction of ICaL, spatially synchronized steady-state EAD events with inverted and biphasic T-waves can be "weakened" into beat-to-beat concurrences of spatially synchronized EADs and T-wave alternans, and as pacing CLs increase, beat-to-beat concurrences of localized EADs with late-appearing and pointed T-wave morphologies can be observed; (3) under certain conditions, localized EAD events in the midmyocardium may trigger slow uni-directional electric propagation with inverted (antegrade) or upright (retrograde) broad-based T-waves; (4) spatially discordant EADs were typically characterized by desynchronized spontaneous onsets of EAD events between two groups of PVS tissues with biphasic T-wave morphologies, and they can evolve into spatially discordant oscillating EAD patterns with sustained or self-terminated alternating EAD and electrocardiogram (ECG) patterns. Our results provide new insights into the spatiotemporal aspects of the onset and development of EADs and suggest possible mechanistic links between the complex spatial dynamics of EADs and T-wave morphologies.

The control of cardiac ventricular excitability by autonomic pathways

Central to the genesis of ventricular cardiac arrhythmia are variations in determinants of excitability. These involve individual ionic channels and transporters in cardiac myocytes but also tissue factors such as variable conduction of the excitation wave, fibrosis and source-sink mismatch. It is also known that in certain diseases and particularly the channelopathies critical events occur with specific stressors. For example, in hereditary long QT syndrome due to mutations in KCNQ1 arrhythmic episodes are provoked by exercise and in particular swimming. Thus not only is the static substrate important but also how this is modified by dynamic signalling events associated with common physiological responses. In this review, we examine the regulation of ventricular excitability by signalling pathways from a cellular and tissue perspective in an effort to identify key processes, effectors and potential therapeutic approaches. We specifically focus on the autonomic nervous system and related signalling pathways.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Potential pro-arrhythmic effect of cardiac resynchronization therapy

A decline in mortality due to pump failure has been clearly documented after cardiac resynchronization therapy (CRT), however the impact on sudden cardiac death and the development of malignant ventricular arrhythmias remains questionable. Our study aims to investigate this alleged pro-arrhythmic effect of CRT using surface electrocardiogram (ECG) markers of pro-arrhythmia.

METHODS

Seventy five patients, who received CRT were included in this study. Manual measurement of corrected QT interval (QTc), Tpeak-end (Tp-e) interval, QT dispersion (QTd) and Tpeak-end dispersion during baseline 12 lead surface ECG and after applying atrial-biventricular pacing were done. Arrhythmias post CRT was recorded from ECG, 24 h holter monitoring or pacemaker programmer event recorder.

RESULTS

QTc interval showed significant prolongation after CRT (498.9 ± 50.8 vs. 476.2 ± 41.6 msec, P = 0.0001). Comparing patients with major arrhythmogenic events (MAE) and increased frequency of premature ventricular contractions (PVCs) post CRT pacing to those patients without arrhythmias, there was a significant prolongation of the QTc interval (527 ± 63.29 vs. 496.95 ± 45.2 msec, P = 0.043) and Tp-e interval (94.16 ± 9 vs. 87.41 ± 16.37 msec, P = 0.049). While in the arrhythmogenic group, there was an insignificant decrease in QTd and Tpeak-end dispersion.

CONCLUSION

QTc and Tp-e intervals are a potential predictor of occurrence of MAE and PVCs. On the other hand, Tp-e dispersion and QTd did not show a predictive potential for arrhythmia.

Microvolt T-wave alternans during Holter monitoring in children and adolescents

BACKGROUND

Time-domain microvolt T-wave alternans (TWA) has been described as a noninvasive marker of sudden cardiac death in adults. The incidence of TWA in pediatric populations has not been defined well. The aim of the study was to determine peculiarities of TWA in children.

METHODS

We examined 68 healthy patients-newborns (20) and children in age group of 7-17 years (48)-and 85 pediatric patients: ventricular premature beats-65; dilated cardiomyopathy (DCMP)-2; long QT syndrome (LQTS)-10; Brugada syndrome (BrS)-5, catecholaminergic ventricular tachycardia (CVT)-3. All underwent Holter monitoring (HM) with definition of the peak value of TWA by modified moving average method.

RESULTS

In healthy newborns, TWA was 32 +/- 8 (12-55) microV (HR 123-156 bmp). In healthy children (7-17 years) it was 30 +/- 11 (10-l 55) microV, (HR 64-132 bmp) without any differences between boys and girls. In all group of patients, TWA were significantly higher (P < 0.05) than in healthy. Circadian peak of TWA was found (90%) in a second part of day and at sleep (8%). Among them 60% (LQTS, BrS, and DCPM) had TWA > 55 microV.

CONCLUSION

Time-domain TWA during HM in children was independent of age, gender, and heart rate. In 94% healthy children, values of TWA do not exceed 55 microV but 20-50% children with cardiac pathology had TWA more than 55 microV. Night circadian type of TWA in diseases with risk of life-threatening arrhythmias associated with TWA was more than 55 microV.

Electrocardiographic crotchets or common errors made in the interpretation of the electrocardiogram

Irritating errors are called crotchets. This paper discusses the following electrocardiographic crotchets: memorizing patterns rather than using basic principles, failure to use the electrocardiogram as a diagnostic tool, failure to correlate all available data, failure to appreciate the limitation of the computer interpretation, failure to appreciate the diagnostic value of P-wave abnormalities, the identification and misuse of abnormal Q waves, the misuse of left or right axis deviation of the QRS complexes, the misuse of the amplitude of the QRS complexes as a sign of left ventricular hypertrophy, identification of left ventricular hypertrophy, failure to identify uncomplicated and complicated bundle-branch block, failure to identify secondary and primary T-wave abnormalities, failure to identify secondary and primary S-T abnormalities, and lack of knowledge of the U waves.

Ionic, molecular, and cellular bases of QT-interval prolongation and torsade de pointes

Torsade de pointes (TdP) is a life-threatening arrhythmia that develops as a consequence of a reduction in the repolarization reserve of cardiac cells leading to amplification of electrical heterogeneities in the ventricular myocardium as well as to the development of early after depolarization-induced triggered activity. Electrical heterogeneities within the ventricles are due to differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization that contributes to the inscription of the electrocardiographic T wave. A number of non-antiarrhythmic drugs and antiarrhythmic agents with class III actions and/or the various mutations and cardiomyopathies associated with the long QT syndrome reduce net repolarizing current and amplify spatial dispersion of repolarization, thus creating the substrate for re-entry. This results in a prolongation of the QT interval, abnormal T waves, and development of TdP. Agents that prolong the QT interval but do not cause an increase in transmural dispersion of repolarization (TDR) do not induce TdP, suggesting that QT prolongation is not the sole or optimal determinant for arrhythmogenesis. This article reviews recent advances in our understanding of these mechanisms, particularly the role of TDR in the genesis of drug-induced TdP, and examines how these may guide us towards development of safer drugs.

Cellular basis for the repolarization waves of the ECG

One hundred years after Willem Einthoven first recorded the electrocardiogram (ECG), physicians and scientists are still debating the cellular basis for the various waves of the ECG. In this review, our focus is on the cellular basis for the J, T, and U waves of the ECG. The J wave and T wave are thought to arise as a consequence of voltage gradients that develop as a result of the electrical heterogeneities that exist within the ventricular myocardium. The presence of a prominent action potential notch in epicardium but not endocardium gives rise to a voltage gradient during ventricular activation that inscribes the J wave. Transmural and apico-basal voltage gradients developing as a result of difference in the time course of repolarization of the epicardial, M, and endocardial cell action potentials, and the more positive plateau potential of the M cell contribute to inscription of the T wave. Amplification of these heterogeneities results in abnormalities of the J wave and T wave, leading to the development of the Brugada, long QT, and short QT syndromes. The basis for the U wave has long been a matter of debate. One theory attributes the U wave to mechanoelectrical feedback. A second theory ascribes it to voltage gradients within ventricular myocardium and a third to voltage gradients between the ventricular myocardium and the His-Purkinje system. Although direct evidence in support of any of these three hypotheses is lacking, recent studies involving the short QT syndrome have generated renewed interest in the mechanoelectrical hypothesis.

[Osborn J wave. A new "channel pathology"? A case report]

We report, at the time of a hypothermia major, the observation of an anomaly of the repolarisation on the electrocardiogram of surface, called "J wave", and described in an exhaustive way by Osborn, which attached its name there. It corresponds to the picking of the terminal section of the QRS, with heightening in dome, the J point is then elevated compared to the base line. It can be also seen among patients normothermic in physiological or pathological circumstances. Its physiopathology from now on is understood better, the J wave is the result of the difference of potential action between the epicarde and endocarde during phases 1 and 2 of the ventricular repolarisation. This gradient is related to the Ito current, also accused in the "channel pathologies", of which Brugada syndrome.

Relation between left ventricular geometry and transmural dispersion of repolarization

Studies have shown an association between left ventricular (LV) geometry and complex ventricular ectopic activity. Increased transmural dispersion of repolarization (TDR), which correlates to the interval from the peak to the end of the T wave (Tpe) on the surface electrocardiogram, is linked to ventricular tachyarrhythmias. The relation between LV geometry and TDR is unknown. The mean Tpe interval, measured from leads V(1) to V(3) of the surface electrocardiogram, was assessed in 300 patients (50% men) who had normal LV systolic function and QRS duration and were categorized into 3 equal groups, which were matched by age and gender, according to echocardiographically determined LV geometry (normal structure, concentric remodeling, and LV hypertrophy). The Tpe interval was corrected for the QT interval using Tpe/QTc and was compared among the 3 groups. Compared with those who had normal LV structure, the Tpe interval was significantly prolonged in those who had LV hypertrophy and significantly shortened in those who had concentric remodeling (p = <0.0001 for the 2 comparisons). Correcting for the QT interval using Tpe/QTc yielded similar results. Thus, TDR was increased in patients who had LV hypertrophy but decreased in concentric remodeling compared with those who had normal cardiac structure. Although LV hypertrophy represents a maladaptive geometric process that results in an unfavorable electrical substrate, concentric remodeling may represent a structural adaptation that has a more favorable electrical milieu.

QT interval in patients with unstable angina and non-Q wave myocardial infarction

BACKGROUND

Non-Q wave myocardial infarction (NQMI) and unstable angina (UAP) have similar clinical presentations and similar ST-T changes on the electrocardiogram. The purpose of this study was to assess whether changes in QT interval might help differentiating between these entities.

METHODS

The QT intervals of 52 patients hospitalized with NQMI were compared to those of 52 patients hospitalized for UAP. All patients had repeated ECG for at least 4 days.

RESULTS

Maximal QTc in patients with NQMI was significantly longer than in patients with UAP (475 vs 439 ms, P < 0.0001). QTc on the admission ECG was 450 ms in patients with NQMI compared to 417 ms in UAP (P < 0.005). QTc > 460 ms was present in 48% patients with NQMI and in 19% of UAP patients. Maximal QT prolongation was observed within 36 hours of admission with return to normal within 96 hours. QT dispersion was within normal range, being longer in patients with NQMI than patients with UAP (55 vs 43 ms, P < 0.003). QT prolongation was not associated with increased frequency of arrhythmia. The cause of QT prolongation in NQMI may be related to the damage of subendocardial layer exposing the M cells layer which markedly prolong action potential duration.

CONCLUSION

Transient QT prolongation is observed in about half of patients with NQMI. These ECG changes may help differentiating between patients with NQMI and UAP already on admission.

Novel arrhythmogenic mechanism revealed by a long-QT syndrome mutation in the cardiac Na(+) channel

Variant 3 of the congenital long-QT syndrome (LQTS-3) is caused by mutations in the gene encoding the alpha subunit of the cardiac Na(+) channel. In the present study, we report a novel LQTS-3 mutation, E1295K (EK), and describe its functional consequences when expressed in HEK293 cells. The clinical phenotype of the proband indicated QT interval prolongation in the absence of T-wave morphological abnormalities and a steep QT/R-R relationship, consistent with an LQTS-3 lesion. However, biophysical analysis of mutant channels indicates that the EK mutation changes channel activity in a manner that is distinct from previously investigated LQTS-3 mutations. The EK mutation causes significant positive shifts in the half-maximal voltage (V(1/2)) of steady-state inactivation and activation (+5.2 and +3.4 mV, respectively). These gating changes shift the window of voltages over which Na(+) channels do not completely inactivate without altering the magnitude of these currents. The change in voltage dependence of window currents suggests that this alteration in the voltage dependence of Na(+) channel gating may cause marked changes in action potential duration because of the unique voltage-dependent rectifying properties of cardiac K(+) channels that underlie the plateau and terminal repolarization phases of the action potential. Na(+) channel window current is likely to have a greater effect on net membrane current at more positive potentials (EK channels) where total K(+) channel conductance is low than at more negative potentials (wild-type channels), where total K(+) channel conductance is high. These findings suggest a fundamentally distinct mechanism of arrhythmogenesis for congenital LQTS-3.

Transmural heterogeneity of ventricular repolarization under baseline and long QT conditions in the canine heart in vivo: torsades de pointes develops with halothane but not pentobarbital anesthesia

INTRODUCTION

In vitro studies have provided evidence for the existence of M cells. The present study examines the contribution of the M cell to transmural dispersion of repolarization (TDR) and to the development of torsades de pointes (TdP) in the canine heart in vivo in animals anesthetized with either pentobarbital or halothane.

METHODS AND RESULTS

Monophasic action potentials (MAPs) were recorded from 4 to 7 transmural sites, before and after d-sotalol. Cells displaying the longest MAP duration (MAPD) generally were localized to the deep subendocardium to mid-myocardium (M region) in the anterior wall of the left ventricle. d-Sotalol preferentially prolonged the MAPD of the M region, increasing TDR significantly more (P < 0.05) in animals anesthetized with halothane (31+/-5 to 88+/-17 msec) than in those receiving pentobarbital (24+/-9 to 53+/-7 msec; basic cycle length 1,500 msec). In halothane-anesthetized dogs, a remarkable transient increase in M cell MAPD followed interpolation of one or more extrasystole(s), leading to a transient increase in TDR and TdP. TdP was never observed with pentobarbital anesthesia.

CONCLUSION

Our results demonstrate that transmural heterogeneity of repolarization is amplified under acquired long QT conditions and that the increase in TDR underlies the development of TdP in halothane- but not pentobarbital-anesthetized dogs. The data support an important contribution of M cells to TDR and to the development of TdP in the canine heart in vivo. Our data also highlight the importance of acceleration-induced prolongation of MAPD (a phenomena observed principally in M cells) in the development of TdP.

The M cell: its contribution to the ECG and to normal and abnormal electrical function of the heart

The discovery and characterization of the M cell, a unique cell type residing in the deep layers of the ventricular myocardium, has opened a new door in our understanding of the electrophysiology and pharmacology of the heart in both health and disease. The hallmark of the M cell is the ability of its action potential to prolong much more than that of other ventricular myocardial cells in response to a slowing of rate and/or in response to agents that act to prolong action potential duration. Our goal in this review is to provide a comprehensive characterization of the M cell, its contribution to transmural heterogeneity, and its role in the normal electrical function of the heart, in the inscription of the ECG (particularly the T wave), and in the development of QT dispersion, T wave alternans, long QT intervals, and cardiac arrhythmias, such as torsades de pointes. Our secondary goal is to address the controversy that has arisen relative to the functional importance of the M cell in the normal heart. The controversy derives largely from the failure of some investigators to demonstrate transmural heterogeneity of repolarization in the dog in vivo under control conditions and after administration of quinidine. The inability to demonstrate transmural heterogeneity under these conditions may be due to the use of bipolar recording techniques that, in our experience, seriously underestimate transmural dispersion of repolarization (TDR). The use of sodium pentobarbital and alpha-chloralose as anesthesia also is problematic, because these agents reduce or eliminate TDR by affecting a variety of ion channel currents. Finally, attempts to amplify transmural dispersion of repolarization with an agent such as quinidine must take into account that relatively high concentrations can result in effects opposite to those desired due to drug inhibition of multiple ion channels. These observations may explain the inability of earlier studies to detect the M cell.

Ion channels and ventricular arrhythmias: cellular and ionic mechanisms underlying the Brugada syndrome

Brugada syndrome is characterized by ST segment elevation in the right precordial leads, V1-V3 (unrelated to ischemia or structural disease), normal QT intervals, apparent right bundle branch block, and sudden cardiac death, particularly in men of Asian origin. An autosomal dominant mode of inheritance with variable expression has been described. The only gene thus far linked to the Brugada syndrome is the cardiac sodium channel gene, SCN5A. The possible cellular and ionic basis for these features of the Brugada syndrome are discussed. Strong sodium channel block, among other modalities, has been shown to be capable of inducing epicardial and transmural dispersion of repolarization, thus providing the substrate for the development of phase 2 and circus movement reentry, which underlies ventricular tachycardia/ventricular fibrillation.

Characteristics and distribution of M cells in arterially perfused canine left ventricular wedge preparations

BACKGROUND

Much of the characterization of the M cell to date has been accomplished using isolated tissues and cells. This study uses an arterially perfused wedge preparation to examine the characteristics and distribution of M cells within the anterior wall of the canine left ventricle under more physiological conditions.

METHODS AND RESULTS

Floating microelectrodes were used to record transmembrane action potentials simultaneously from epicardial, M region, and endocardial or subendocardial Purkinje sites in isolated arterially perfused canine left ventricular wedge preparations. A transmural ECG was recorded concurrently. M cells with the longest action potentials were found in the deep subendocardium in wedge preparations isolated from the anterior wall of the left ventricle. Fairly smooth transitions in action potential duration (APD) were observed except in the region between epicardium and deep subepicardium. Tissue resistivity increased 2.8-fold in this region and much more modestly in the deep subendocardium. Dispersion of APD90 across the left ventricular wall averaged 51+/-19 and 64+/-25 ms at basic cycle lengths of 1000 and 2000 ms, respectively, whereas transmural dispersion of repolarization time was smaller (34+/-18 and 45+/-25 ms), owing to the endocardial to epicardial activation sequence.

CONCLUSIONS

We conclude that the qualitative differences between the 3 ventricular cell types previously described in isolated tissues and cells are maintained in intact canine left ventricular wall preparations in which the myocardial cells are electrically well coupled. As anticipated, differences in APD are quantitatively smaller because of electrotonic interactions among the 3 cell types. Our data indicate that transmural dispersion of repolarization is the result of intrinsic differences in APD of cells spanning the ventricular wall as well as a heterogeneous distribution of tissue resistivity across the wall.

Comparison of the cellular electrophysiological characteristics of canine left ventricular epicardium, M cells, endocardium and Purkinje fibres

Electrophysiological differences among M cells, epicardium, endocardium and Purkinje fibres of the canine ventricle were studied over a wide range of stimulation cycle lengths, and the pharmacological response of these cell types to the sodium channel blocker tetrodotoxin, calcium channel blocker nifedipine and ATP-sensitive potassium channel activator pinacidil was compared. The experiments were carried out by applying standard intracellular microelectrode technique in isolated dog left ventricular preparations. The results confirmed the existence of M cells in the canine ventricle, in addition, the distribution of the rate of rise of the action potential upstroke and action potential amplitude values reflecting probably the inhomogeneity of the fast sodium current in these cells was revealed. It was also demonstrated that M cells differ from Purkinje fibres in some aspects which were not expected from previous investigations: (1) The early portion of the action potential duration restitution curve in M cells is more similar to that of endocardial and epicardial cells than to Purkinje fibres. (2) The plateau phase of the action potentials in Purkinje fibres developed at a more negative potential range than that in the other cell types studied. (3) The pharmacological response to tetrodotoxin and pinacidil in M cells resembles to that in the endocardial and epicardial cells more than in the Purkinje fibres. Our results provide further evidence in support of the existence of M cells but also indicate that there are important electrophysiological as well as pharmacological differences between M cells and Purkinje fibres.

Amiodarone reduces transmural heterogeneity of repolarization in the human heart

OBJECTIVES

The present work was designed to test the effects of amiodarone therapy on action potential characteristics of the three cell types observed in human left ventricular preparations.

BACKGROUND

The electrophysiologic basis for amiodarone's exceptional antiarrhythmic efficacy and low proarrhythmic profile remains unclear.

METHODS

We used standard microelectrode techniques to investigate the effects of chronic amiodarone therapy on transmembrane activity of the three predominant cellular subtypes (epicardial, midmyocardial [M] and endocardial cells) spanning the human left ventricle in hearts explanted from normal, heart failure and amiodarone-treated heart failure patients.

RESULTS

Tissues isolated from the ventricles of heart failure patients receiving chronic amiodarone therapy displayed M cell action potential duration (404+/-12 ms) significantly briefer (p < 0.05) than that recorded in tissues isolated from normal hearts (439+/-22 ms) or from heart failure patients not treated with amiodarone (449+/-18 ms). Endocardial cells from amiodarone-treated heart failure patients displayed longer (p < 0.05) action potential duration (363+/-10 ms) than endocardial cells isolated from normal hearts (330+/-6 ms). As a consequence, the heterogeneity of ventricular repolarization in tissues from patients treated with amiodarone was considerably smaller than in the two other groups, especially at long pacing cycle lengths.

CONCLUSIONS

These findings may explain, at least in part, the reduction of ventricular repolarization dispersion and the lower incidence of torsade de pointes observed with chronic amiodarone therapy as compared with other class III agents.

Cellular basis for QT dispersion

The cellular basis for the dispersion of the QT interval recorded at the body surface is incompletely understood. Contributing to QT dispersion are heterogeneities of repolarization time in the three-dimensional structure of the ventricular myocardium, which are secondary to regional differences in action potential duration (APD) and activation time. While differences in APD occur along the apicobasal and anteroposterior axes in both epicardium and endocardium of many species, transitions are usually gradual. Recent studies have also demonstrated important APD gradients along the transmural axis. Because transmural heterogeneities in repolarization time are more abrupt than those recorded along the surfaces of the heart, they may represent a more onerous substrate for the development of arrhythmias, and their quantitation may provide a valuable tool for evaluation of arrhythmia risk. Our data, derived from the arterially perfused canine left ventricular wedge preparation, suggest that transmural gradients of voltage during repolarization contribute importantly to the inscription of the T wave. The start of the T wave is caused by a more rapid decline of the plateau, or phase 2 of the epicardial action potential, creating a voltage gradient across the wall. The gradient increases as the epicardial action potential continues to repolarize, reaching a maximum with full repolarization of epicardium; this juncture marks the peak of the T wave. The next region to repolarize is endocardium, giving rise to the initial descending limb of the upright T wave. The last region to repolarize is the M region, contributing to the final segment of the T wave. Full repolarization of the M region marks the end of the T wave. The time interval between the peak and the end of the T wave therefore represents the transmural dispersion of repolarization. Conditions known to augment QTc dispersion, including acquired long QT syndrome (class IA or III antiarrhythmics) lead to augmentation of transmural dispersion of repolarization in the wedge, due to a preferential effect of the drugs to prolong the M cell action potential. Antiarrhythmic agents known to diminish QTc dispersion, such as amiodarone, also diminish transmural dispersion of repolarization in the wedge by causing a preferential prolongation of APD in epicardium and endocardium. While exaggerated transmural heterogeneity clearly can provide the substrate for reentry, a precipitating event in the form of a premature beat that penetrates the vulnerable window is usually required to initiate the reentrant arrhythmia. In long QT syndrome, the trigger is thought to be an early afterdepolarization (EAD)-induced triggered beat. The likelihood of developing EADs and triggered activity is increased when repolarizing forces are diminished, making for a slower and more gradual repolarization of phases 2 and 3 of the action potential, which translates into broad, low amplitude and sometimes bifurcated T waves in the electrocardiogram. Our findings suggest that regional differences in the duration of the M cell action potential may be the basis for QT dispersion measured at the body surface under normal and long QT conditions. The data indicate that the interval delimited by the peak and the end of the T wave represents an accurate measure of regional dispersion of repolarization across the ventricular wall and as such may be a valuable index for assessment of arrhythmic risk. The presence of low amplitude, broad and/or bifurcated T waves, particularly under conditions of long QT syndrome, is indicative of diminished repolarizing forces and may represent an independent variable of arrhythmic risk, forecasting the development of EAD-induced triggered beats that can precipitate torsade de pointes. Although the QT interval, QT dispersion, the T wave peak-to-end interval, and the width and amplitude of the T wave often change in parallel, they contain different information and should not be expected to be e

Effects of sodium channel block with mexiletine to reverse action potential prolongation in in vitro models of the long term QT syndrome

INTRODUCTION

Recent clinical studies have reported a greater effectiveness of sodium channel block with mexiletine to abbreviate the QT interval in patients with the chromosome 3 variant (SCN5A, LQT3) of the long QT syndrome (LQTS) than those with the chromosome 7 form of the disease (HERG, LQT2), suggesting the possibility of gene-specific therapy for the two distinct forms of the congenital LQTS. Experimental studies using the arterially perfused left ventricular wedge preparation have confirmed these clinical observations on the QT interval but have gone on to further demonstrate a potent effect of mexiletine to reduce dispersion of repolarization and prevent torsades de pointes (TdP) in both LQT2 and LQT3 models. A differential action of sodium channel block on the three ventricular cell types is thought to mediate these actions of mexiletine. This study provides a test of this hypothesis by examining the effects of mexiletine in isolated canine ventricular epicardial, endocardial, and M region tissues under conditions that mimic the SCN5A and HERG gene defects.

METHODS AND RESULTS

We used standard microelectrode techniques to record transmembrane activity from endocardial, epicardial, mid-myocardial, and transmural strips isolated from the canine left ventricle. d-Sotalol, an IKr blocker, was used to mimic the HERG defect (LQT2), and ATX-II, which increases late Na channel current, was used to mimic the SCN5A defect (LQT3). d-Sotalol (100 microM) preferentially prolonged the action potential of the mid-myocardial M cell (APD90 increased from 340 +/- 65 to 623 +/- 203 msec) as did ATX-II (10 to 20 nM; APD90 increased from 325 +/- 51 to 580 +/- 178 msec; basic cycle length = 2000 msec), thus causing a marked increase in transmural dispersion of repolarization (TDR). Mexiletine (2 to 20 microM) dose-dependently reversed the ATX-II-induced prolongation of APD90 in all three cell types. Mexiletine also reversed the d-sotalol-induced prolongation of the M cell action potential duration (APD), but had little effect on the action potential of epicardium and endocardium. Due to its preferential effect to abbreviate the action potential of M cells, mexiletine reduced the dispersion of repolarization in both models. Low concentrations of mexiletine (5 to 10 microM) totally suppressed early afterdepolarization (EAD) and EAD-induced triggered activity in both models.

CONCLUSIONS

Our results indicate that the actions of mexiletine are both cell and model specific, but that sodium channel block with mexiletine is effective in reducing transmural differences in APD and in abolishing triggered activity induced by d-sotalol and ATX-II. The data suggest that mexiletine's actions to reduce TDR and prevent the induction of spontaneous and programmed stimulation-induced TdP in these models are due to a preferential effect of the drug to abbreviate the APD of the M cell and to suppress the development of EADs. The data provide further support for the hypothesis that block of the late sodium current may be of value in the treatment of LQT2 as well as LQT3 and perhaps other congenital and acquired (drug-induced) forms of LQTS.

Chronic amiodarone reduces transmural dispersion of repolarization in the canine heart

INTRODUCTION

Amiodarone is a potent antiarrhythmic agent used in the management of both atrial and ventricular arrhythmias. In addition to its beta-blocking properties, amiodarone is known to block the sodium, potassium, and calcium channels in the heart. Its complex electropharmacology notwithstanding, the reasons for the high efficacy of the drug remain unclear. Also not well understood is the basis for the low incidence of proarrhythmia seen with amiodarone relative to other agents with Class III actions. The present study was designed to examine the effects of chronic amiodarone in epicardial, endocardial, and M cells of the canine left ventricle.

METHODS AND RESULTS

We used standard microelectrode techniques to record transmembrane activity from endocardial, epicardial, mid-myocardial, and transmural strips isolated from the canine left ventricle. Tissues were obtained from mongrel dogs receiving amiodarone orally (30 to 40 mg/kg per day) for 30 to 45 days or from untreated controls. Chronic amiodarone produced a greater prolongation of action potential duration in epicardium and endocardium, but less of an increase, or even a decrease at slow rates, in the M region, thereby reducing transmural dispersion of repolarization. In addition, chronic amiodarone therapy suppressed the ability of the IKr blocker, d-sotalol, to induce a marked dispersion of repolarization or early afterdepolarization activity.

CONCLUSION

Our data demonstrate for the first time a direct effect of chronic amiodarone treatment to differentially alter the cellular electrophysiology of ventricular myocardium so as to produce an important decrease in transmural dispersion of repolarization, especially under conditions in which dispersion is exaggerated. These results may contribute to our understanding of the effectiveness of amiodarone in the treatment of life-threatening arrhythmias as well as to our understanding of the low incidence of proarrhythmia attending therapy with chronic amiodarone in comparison with other Class III agents.