Cellular basis for long QT,transmural dispersion of repolarization,and torsade de pointes in the long QT syndrome

Genetic studies have identified four forms of congenital long QT syndrome (LQTS) caused by mutations in ion channel genes located on chromosomes 3 (LQT3), 7 (LQT2), 11 (LQT1), and 21 (LQT5). Preliminary clinical studies have reported different phenotypic electrocardiographic patterns and different sensitivity to pacing or pharmacological therapy for each genotype. A transmural electrocardiogram and transmembrane action potentials from epicardial, M, and endocardial cells were simultaneously recorded from an arterially perfused wedge of canine left ventricle. Isoproterenol (100 nmol/L) in the presence of chromanol 293B (30 μmol/L), an I_Ks blocker (LQT1 model), produced a preferential prolongation of M-cell action potential duration (APD), resulting in an increase in transmural dispersion of repolarization (TDR) and a broad-based T wave, as commonly seen in LQT1 patients. D-Sotalol (100 μmol/L), an I_Kr blocker (LQT2 model), and ATX-II (20 nmol/L), an agent that augments late I_Na (LQT3 model), also produced a preferential prolongation of M-cell APD, an increase in TDR, and low-amplitude T wave with a bifurcated appearance (LQT2), and late-appearing T wave (LQT3), respectively. APD-, QT-, and TDR-rate relations were much steeper in the LQT3 model than in either the LQT1 or LQT2 model, whereas the rate relations in the LQT1 and LQT2 models were both steeper than those under control conditions. Spontaneous and programmed electrical stimulation-induced torsade de pointes (TdP) were observed in all 3 models. Propranolol (1 μmol/L), a beta blocker, completely prevented the effect of isoproterenol to persistently or transiently increase TDR and to induce TdP in the LQT1 and LQT2 models, but facilitated TdP in the LQT3 model. Mexiletine, a class IB Na⁺ channel blocker, dose-dependently (2–20 μmol/L) abbreviated the QT and APD more in the LQT3 model, but decreased TDR and suppressed TdP in the 3 models.     

Interval representative of transmural dispersion of repolarization in children and young adolescents with congenital long QT syndrome.

BACKGROUND: It has been shown experimentally that the interval from the nadir of the initial negative T wave to the end of the T wave is representative of transmural dispersion of repolarization (TDR) when complex T waves are present. In the clinical setting, however, the interval representative of TDR in patients with long QT syndrome (LQTS) is a controversial subject. METHODS AND RESULTS: Five symptomatic patients (3 boys, 2 girls; 3 LQT1, 2 LQT2) were evaluated by a face immersion test before and after treatment to compare the configuration of the T wave. When the notch disappeared after treatment, the single peak of the T wave after treatment coincided with the nadir of the notch before treatment. When the notch remained the same after treatment as before treatment and when the QTc decreased, the corrected interval from the nadir of the notch to the end of the T wave was for the most part shortened. CONCLUSIONS: The present study showed that the interval representative of the TDR in the clinical surface electrocardiogram can be obtained from the nadir of the notch to the end of the T wave in children and adolescents with LQTS, as was shown in the experimental study.     

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.     

Effects of Sodium Channel Block with Mexiletine to Reverse Action Potential Prolongation in In Vitro Models of the Long QT Syndrome

Sodium Channel Block in In Vitro Models of LQTS. 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 I_kr 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 μM) 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 μM) 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 bad 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 μM) totally suppressed early afterdepolarization (EAD) and KAD-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.     

长QT综合征复极化离散与早期后除极致心律失常机制的跨壁光学标测研究

目的　尖端扭转性室性心动过速 (室速 )是长QT综合征 (LQTS)的致命性心律失常 ,相关机制的细胞学水平研究还不十分清楚。本文报道对复极化离散和触发机制在尖端扭转性室速发生中的作用的研究结果。方法　动脉灌注的犬左心室楔形心肌块 ,经电压敏感性荧光染色后 ,行跨壁包括百余位点跨膜动作电位的同步光学标测研究。d 索他洛尔、海银花 (ATX Ⅱ)分别用来模拟LQT2和LQT3。对照组、LQT2组、LQT3组各 6块。结果　正常对照组的平均动作电位时限 (APD)为 (2 91± 2 7)ms,跨壁复极化的离散(DOR)为 (2 4± 6 )ms;LQT2组APD为 (35 6± 2 0 )ms,DOR为 (35± 9)ms,与对照组相比差异有显著性 (P <0 0 5 ) ;LQT3组APD为 (6 0 9± 92 )ms,DOR为 (12 1± 85 )ms,与对照组相比差异有显著性 (P <0 0 5 )。在LQT3组 ,早期后除极现象可见于所有类型的心肌细胞 ,但以心内膜和中层心肌细胞多见。触发活动常常起源于中层心肌细胞部位。结论　在LQT条件下 ,跨壁动作电位不均一地延长是导致复极化离散的重要原因 ,为心律失常的发生提供了基质。早期后除极诱导的触发活动触发了尖端扭转性室速     

Classification and mechanism of Torsade de Pointes initiation in patients with congenital long QT syndrome.

AIMS: To examine the initiating mode of Torsade de Pointes (TdP) in patients with congenital long QT syndrome (LQTS). METHODS AND RESULTS: We evaluated 111 episodes of TdP recorded on the electrocardiograms of 24 patients with congenital LQTS, and clarified the initiating mode, the three consecutive preceding RR intervals defined as C(2), C(1), and C(0), the timing of initiating premature ventricular contraction (PVC) and the cycle length (CL) of TdP. Three different initiating patterns were observed: (1) a "short-long-short" sequence (SLS) pattern (23 patients, 72 TdP, 65%) defined as one or more short-long cardiac cycles followed by an initiating short-coupled PVC (C(1)>C(2) and C(0)), (2) an "increased sinus rate" (ISR) pattern (8 patients, 28 TdP, 25%) defined as a gradual increase in sinus rate with or without T-wave alternans (C(2)>/=C(1)>/=C(0)), and (3) a "changed depolarization" (CD) pattern (5 patients, 11 TdP, 10%) defined as a sudden long-coupled PVC or fusion beat followed by short-coupled PVC. The C(0) was shorter in ISR than SLS and CD (mean C(0): 488 vs. 587 and 603 ms, respectively; P<0.05). Therefore, the initiating PVC appeared near the T-wave peak of the last beat before onset in ISR, while it occurred after the T-wave peak in SLS and CD. The CL of TdP was shorter in ISR than in SLS (256 vs. 295 ms, P<0.05). CONCLUSIONS: Our data show the existence of three predominant initiating modes of TdP in patients with congenital LQTS and suggests a differential mechanism of initiation of TdP for each mode.     

Increasing gap junction coupling reduces transmural dispersion of repolarization and prevents torsade de pointes in rabbit LQT3 model

Introduction: Increased transmural dispersion of repolarization (TDR) contributes importantly to the development of torsades de pointes (TdP) in long QT syndrome (LQTS). Intercellular electrical coupling via gap junctions plays an important role in maintaining TDR in both normal and diseased hearts. This study examined the effects of antiarrhythmic peptide AAP10, a gap junction enhancer, on TDR and induction of TdP in a rabbit LQT3 model.
Methods and Results: An arterially perfused rabbit left ventricular preparation and sea anemone toxin II (ATX-II, 20 nM) were used to establish a LQT3 model. Transmural ECG as well as action potentials from both endocardium and epicardium were simultaneously recorded. Changes in nonphosphorylated connexin43 (Cx43) were measured by immunoblotting. Compared with the control group, the QT interval, TDR, early afterdepolariztion (EAD), R-on-T extrasystole, and TdP increased sharply with augmented nonphosphorylated Cx43 in the LQT3 group (P < 0.001 for both). Interestingly, compared with the LQT3 group, 500 nM AAP10 reduced QT interval, TDR (P < 0.001 for both), and prevented EAD, R-on-T extrasystole, and TdP (P = 0.003, P = 0.001, P = 0.02) with a parallel decrease in nonphosphorylated Cx43 in the presence of ATX-II (P < 0.001).
Conclusion: Gap junction enhancer AAP10 is capable of abbreviating the QT interval, reducing TDR, and suppressing TdP in a rabbit LQT3 model probably via its effect by preventing dephosphorylation of Cx43. These data suggest that increasing intercellular coupling may reduce TDR and, therefore, prevent TdP in LQTS.     

胺碘酮对犬左心室M细胞动作电位和心室跨壁复极离散度的影响

目的通过研究胺碘酮对犬左心室M细胞动作电位和心室跨壁复极离散度（TDR）的影响,探讨胺碘酮抗心律失常机制。方法应用标准玻璃微电极技术,在不同频率刺激下,记录应用胺碘酮前后犬左室游离壁内、外膜及中层心肌细胞跨膜动作电位（AP）各种参数的变化。结果:①胺碘酮延长内、外膜层心肌细胞的动作电位时程（APD）,当基础周长（BCL）为1 000 m s时,使用胺碘酮后心内膜层肌细胞的APD90由（254±38）m s延长为（274±37）m s（P<0.01）,外膜层肌细胞的APD90由（205±37）m s延长为（229±16）m s（P<0.01）;但其明显缩短中层肌细胞的APD,其APD90由（331±50）m s缩短为（290±62）m s（P<0.01）。②胺碘酮能显著缩短TDR,当BCL为1 000 m s时,用药后TDR由（126±20）m s减小为（61±11）m s（P<0.01）。结论胺碘酮对左室三层肌细胞电生理具有不同影响,延长内外膜心肌细胞的APD,而缩短中层肌细胞的APD。使左室TDR缩短。     

不同起搏部位对正常犬和长QT间期犬模型在体跨心室壁复极离散的影响

目的以正常犬和长QT间期(LQT)犬模型的在体3层心肌单向动作电位时限(MAPD)和跨室壁复极离散(TDR)、体表心电图上QT间期和Tp-Te间期等为指标,研究不同部位起搏,尤其是左心室外膜参与起搏后心肌复极特性的变化.通过这种模拟临床上心室再同步治疗的方法,旨在观察左心室起搏、双心室起搏是否会增加恶性室性心律失常发生的危险性.方法 8只健康成年犬经导管射频消融希氏束制备三度房室阻滞模型.开胸手术后分别在左心室外膜、右心室内膜和双心室起搏时同步记录体表心电图和心内膜下、中层、心外膜下3层心肌的单相动作电位(MAP),测定QT间期、Tp-Te间期和3层心肌的MAPD、TDR.然后以氯化铯(CsCl)制备LQT犬模型并重复上述实验.结果在正常犬,左心室外膜与双心室起搏后,心内膜下、中层、心外膜下3层心肌的MAPD均有延长,并有TDR的增大(左心室外膜起搏47.16 ms、双心室起搏37.54 ms、右心室内膜起搏26.75 ms,P＜0.001),体表心电图Tp-Te间期的变化与之平行.在LQT犬,在CsCl使3层心肌MAPD延长的基础上,左心室外膜参与起搏进一步明显增大TDR,而CsCl增大TDR的作用被掩盖.结论左心室心外膜与双心室起搏后使心内膜下、中层、心外膜下3层心肌的MAPD均延长并使TDR增大,可能成为导致恶性室性心律失常的基础.在临床上推广应用心室再同步技术治疗充血性心力衰竭患者时,这一问题必须引起高度重视.     

The diagnostic role of T wave morphology biomarkers in congenital and acquired long QT syndrome: A systematic review

Introduction

QTc prolongation is key in diagnosing long QT syndrome (LQTS), however 25%–50% with congenital LQTS (cLQTS) demonstrate a normal resting QTc. T wave morphology (TWM) can distinguish cLQTS subtypes but its role in acquired LQTS (aLQTS) is unclear.

Methods

Electronic databases were searched using the terms “LQTS,” “long QT syndrome,” “QTc prolongation,” “prolonged QT,” and “T wave,” “T wave morphology,” “T wave pattern,” “T wave biomarkers.” Whole text articles assessing TWM, independent of QTc, were included.

Results

Seventeen studies met criteria. TWM measurements included T-wave amplitude, duration, magnitude, Tpeak-Tend, QTpeak, left and right slope, center of gravity (COG), sigmoidal and polynomial classifiers, repolarizing integral, morphology combination score (MCS) and principal component analysis (PCA); and vectorcardiographic biomarkers. cLQTS were distinguished from controls by sigmoidal and polynomial classifiers, MCS, QTpeak, Tpeak-Tend, left slope; and COG x axis. MCS detected aLQTS more significantly than QTc. Flatness, asymmetry and notching, J-Tpeak; and Tpeak-Tend correlated with QTc in aLQTS. Multichannel block in aLQTS was identified by early repolarization (ERD_30%) and late repolarization (LRD_30%), with ERD reflecting hERG-specific blockade. Cardiac events were predicted in cLQTS by T wave flatness, notching, and inversion in leads II and V₅, left slope in lead V₆; and COG last 25% in lead I. T wave right slope in lead I and T-roundness achieved this in aLQTS.

Conclusion

Numerous TWM biomarkers which supplement QTc assessment were identified. Their diagnostic capabilities include differentiation of genotypes, identification of concealed LQTS, differentiating aLQTS from cLQTS; and determining multichannel versus hERG channel blockade.     

Sympathetic stimulation produces a greater increase in both transmural and spatial dispersion of repolarization in LQT1 than LQT2 forms of congenital long QT syndrome.

OBJECTIVES: The study compared the influence of sympathetic stimulation on transmural and spatial dispersion of repolarization between LQT1 and LQT2 forms of congenital long QT sYndrome (LQTS). BACKGROUND: Cardiac events are more associated with sympathetic stimulation in LQT1 than in LQT2 or LQT3 syndrome. Experimental studies have suggested that the interval between Tpeak and Tend (Tp-e) in the electrocardiogram (ECG) reflects transmural dispersion of repolarization across the ventricular wall. METHODS: We recorded 87-lead body-surface ECGs before and after epinephrine infusion (0.1 microg/kg/min) in 13 LQT1, 6 LQT2, and 7 control patients. The Q-Tend (QT-e), Q-Tpeak (QT-p), and Tp-e were measured automatically from 87-lead ECGs, corrected by Bazett's method (QTc-e, QTc-p, Tcp-e), and averaged among all 87-leads and among 24-leads, which reflect the potential from the left ventricular free wall. As an index of spatial dispersion of repolarization, the dispersion of QTc-e (QTc-eD) and QTc-p (QTc-pD) were obtained among 87-leads and among 24-leads, and were defined as the interval between the maximum and the minimum of the QTc-e and the QTc-p, respectively. RESULTS: Epinephrine significantly increased the mean QTc-e but not the mean QTc-p, resulting in a significant increase in the mean Tcp-e in both LQT1 and LQT2, but not in control patients. The epinephrine-induced increases in the mean QTc-e and Tcp-e were larger in LQT1 than in LQT2, and were more pronounced when the averaged data were obtained from 24-leads than from 87-leads. Epinephrine increased the maximum QTc-e but not the minimum QTc-e, producing a significant increase in the QTc-eD in both LQT1 and LQT2 patients, but not in control patients. The increase in the QTc-eD was larger in LQT1 than in LQT2 patients. CONCLUSIONS: Our data suggest that sympathetic stimulation produces a greater increase in both transmural and spatial dispersion of repolarization in LQT1 than in LQT2 syndrome, and this may explain why LQT1 patients are more sensitive to sympathetic stimulation.     

Rate adaptation of QT intervals during and after exercise in children with congenital long QT syndrome

Objectives To improve the diagnostic criteria of the congenital long QTsyndrome in borderline cases we examined rate adaptation ofventricular repolarization phases during exercise and subsequentrecovery in children with the long QT syndrome and controls. Methods Nineteen children with definite long QT syndrome and 19 healthycontrols underwent exercise testing. QT intervals were measuredto the apex (early QT), to the end (total QT) and from apexto the end of the T wave (late QT) at heart rates from 90 bysteps of 10 to 150 beats.min^–1. Results In 11/19 long QT syndrome patients (61%) and 2/19 controls (12%)the total QT lengthened during the recovery phase compared withexercise (P=0·005) at the lowest comparable heart rate.No difference was found between the groups during exercise.The sensitivity of rate adaptation of repolarization intervalswas analysed by calculating linear regression slopes relatingthe QT intervals to the heart rates. During recovery, slopesrelating the total QT to heart rate were steeper in long QTsyndrome patients than those in controls (–2·50±0·82vs –1·79±0·47, P=0·003). TotalQT/heart rate slopes differed between exercise and recoveryphases in the long QT syndrome group only (–1·77±0·71vs 2·50±0·82, P=0·009). In longQT syndrome patients, the difference in total QT/heart rateslopes was mainly because the late QT/heart rate slopes indicatinginhomogeneity of repolarization were steeper during recovery(–1·27±0·74) than during exercise(–0·46±0·29, P<0·0001). Conclusions After exercise in long QT syndrome children the QT intervallengthens abnormally and inhomogeneity of repolarization increases.Evaluation of the QT interval, and especially its late portionafter exercise, may help in establishing the diagnosis of longQT syndrome.     

Predictive factors of ventricular fibrillation triggered by pause-dependent torsades de pointes associated with acquired long QT interval: role of QT dispersion and left ventricular function

INTRODUCTION: Death due to acquired torsades de pointes usually is caused by ventricular fibrillation (VF), but the contributing factors to VF triggered by pause-dependent torsades de pointes are not understood. METHODS AND RESULTS: We evaluated 91 patients who fulfilled four criteria: (1) pause-dependent torsades de pointes; (2) prolonged QT interval and/or corrected QT (QTc) (>0.44 sec); (3) long-short initiation sequence; and (4) conditions known to induce pause-dependent torsades de pointes. There were 38 patients with a documented VF (group I) and 53 without VF (group II). Absolute and relative dispersions of QT and QTc were calculated based on the 12-lead standard ECG. Group I differed from group II with regard to myocardial infarction history (32% vs 13%; P = 0.035), left ventricular ejection fraction (44% +/- 14% vs 65% +/- 9%; P < 0.0001), presence of structural heart disease (100% vs 20.8%; P < 0.0001), QT mean (591 +/- 73 msec vs 514 +/- 78 msec; P < 0.0001), QTc mean (563 +/- 76 msec vs 508 +/- 90 msec; P = 0.002), absolute QT dispersion (166 +/- 56 msec vs 84 +/- 49 msec; P < 0.0001), relative QT dispersion (9.9% +/- 3.5% vs 6.3% +/- 3.2%; P < 0.0001), absolute QTc dispersion (158 +/- 57 msec vs 81 +/- 44 msec; P < 0.0001), and relative QTc dispersion (9.9% +/- 3.6% vs 6.2% +/- 3%; P < 0.0001). Multiple regression analysis showed that ejection fraction (P = 0.0001), presence of structural heart disease (P < 0.0001), and relative QTc dispersion (P = 0.038) were the only independent predictors of VF. CONCLUSION: Left ventricular function, presence of structural heart disease, and QTc relative dispersion should be evaluated carefully in patients with conditions susceptible to inducing torsades de pointes.     

先天性长QT综合征HERG基因真核表达质粒的构建及体外表达

目的构建携带HERG目的基因的真核表达重组质粒pcDNA3-HERG,并观察HERG基因在心肌细胞中的表达情况。方法应用基因重组技术,将原核克隆载体pGEM-HERG中HERG基因亚克隆到真核表达载体pcDNA3,以酶切和聚合酶链反应(PCR)方法鉴定重组质粒pcDNA3-HERG的正确性;以Lipofectamine为介导将重组质粒与荧光真核表达载体pRK5-GFP共转染培养的乳兔心肌细胞,转染48h后荧光显微镜下观察计算转染效率并采用Western Blot和全细胞膜片钳技术检测HERG基因的表达情况。结果酶切和PCR结果均证实pcDNA3-HERG的正确性,以脂质体为介导,其转染心肌细胞的效率为(15.2±2.6)%;Western Blot和膜片钳技术检测到HERG蛋白和HERG通道电流的表达。结论成功构建HERG基因真核表达重组质粒pcDNA3-HERG,转染心肌细胞后可表达有功能的离子通道,为今后研究突变型HERG的功能提供了可行的技术平台。