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.     

The Zone of Vulnerability to T Wave Shocks in Humans

Vulnerability to VF in Humans. Introduction: Shocks during the vulnerable period of the cardiac cycle induce ventricular fibrillation (VF) if their strength is above the VF threshold (VFT) and less than the upper limit of vulnerability (ULV). However, the range of shock strengths that constitutes the vulnerable zone and the corresponding range of coupling intervals have not been defined in humans. The ULV has been proposed as a measure of defibrillation because it correlates with the defibrillation threshold (DFT), but the optimal coupling interval for identifying it is unknown. Methods and Results: We studied 14 patients at implants of transvenous cardioverter defibrillators. The DFT was defined as the weakest shock that defibrillated after 10 seconds of VF. The ULV was defined as the weakest shock that did not induce VF when given at 0, 20, and 40 msec before the peak of the T wave or 20 msec after the peak in ventricular paced rhythm at a cycle length of 500 msec. The VFT was defined as the weakest shock that induced VF at any of the same four intervals. To identify the upper and lower boundaries of the vulnerable zone, we determined the shock strengths required to induce VF at all four intervals for weak shocks near the VFT and strong shocks near the ULV. The VFT was 72 ± 42 V, and the ULV was 411 ± 88 V. In all patients, a shock strength of 200 V exceeded the VFT and was less than the ULV. The coupling interval at the ULV was 19 ± 11 msec shorter than the coupling interval at the VFT (P < 0.001). The vulnerable zone showed a sharp peak at the ULV and a less distinct nadir at the VFT. A 20-msec error in the interval at which the ULV was measured could have resulted in underestimating it by a maximum of 95 ± 31 V. The weakest shock that did not induce VF was greater for the shortest interval tested than for the longest interval at both the upper boundary (356 ± 108 V vs 280 ± 78 V; P < 0.01) and lower boundary (136 ± 68 msec vs 100 ± 65 msec; P < 0.05). Conclusions: The human vulnerable zone is not symmetric with respect to a single coupling interval, but slants from the upper left to lower right. Small differences in the coupling interval at which the ULV is determined or use of the coupling interval at the VFT to determine the ULV may result in significant variations in its measured value. An efficient strategy for inducing VF would begin by delivering a 200-V shock at a coupling interval 10 msec before the peak of the T wave.     

Implantable Cardioverter Defibrillator Oversensing During Periods of Rate-Related Bundle Branch Block

ICD Oversensing During Rate-Related BBB. A patient with an implantable cardioverter defibrillator (ICD) and a dual chamber pacemaker experienced inappropriate ICD therapies only during periods of rate-dependent right bundle branch block. Analysis of both stored and real-time ICD electrograms was critical to correctly diagnosing the problem and offering a solution.     

Effect of Shock Waveform on Relationship Between Upper Limit of Vulnerability and Defibrillation Threshold

ULV-DFT Waveform. Introduction: The upper limit of vulnerability (ULV) correlates with the defibrillation threshold (DFT). The ULV can he determined with a single episode of ventricular fibrillation and is more reproducible than the single-point DFT. The critical-point hypothesis of defibrillation predicts that the relation between the ULV and the DFT is independent of shock waveform. The principal goal of this study was to test this prediction. Methods and Results: We studied 45 patients at implants of pectoral cardioverter defibrillators. In the monophasic-biphasic group (n = 15), DFT and ULV were determined for monophasic and biphasic pulses from a 120-μF capacitor. In the 60- to 110-μF group (n = 30), DFT and ULV were compared for a clinically used 110-μF waveform and a novel 60-μF waveform with 70% phase 1 tilt and 7-msec phase 2 duration. In the monophasic-biphasic group, all measures of ULV and DFT were greater for monophasic than biphasic waveforms (P < 0.0001). In the 60- to 110-/tF group, the current and voltage at the ULV and DFT were higher for the 60-μF waveform (P < 0.0001), hut stored energy was lower (ULV 17%, P < 0.0001; DFT 19%, P = 0.03). There was a close correlation between ULV and DFT for both the monophasic-biphasic group (monophasic r²= 0.75, P < 0.001; hiphasic r²= 0.82, P < 0.001) and the 60- to 110-μF group (60 μF r²= 0.81 P < 0.001; 110 μF r²= 0.75, P < 0.001). The ratio of ULV to DFT was not significantly different for monophasic versus biphasic pulses (1.17 ± 0.12 vs 1.14 ± 0.19, P = 0.19) or 60-μF versus 110-μF pulses (1.15 ± 0.16 vs 1.11 ± 0.14, P = 0.82). The slopes of the ULV versus DFT regression lines also were not significantly different (monophasic vs biphasic pulses, P = 0.46; 60-μF vs UO-μF pulses, P = 0.99). The sample sizes required to detect the observed differences between experimental conditions (P < 0.05) were 4 for ULV versus 6 for DFT in the monophasic-biphasic group (95% power) and 11 for ULV versus 31 for DFT in the 60- to 110-μF group (75% power). Conclusion: The relation between ULV and DFT is independent of shock waveform. Fewer patients are required to detect a moderate difference in efficacy of defibrillation waveforms by ULV than by DFT. A small-capacitor biphasic waveform with a long second phase defibrillates with lower stored energy than a clinically used waveform.     

Acceleration-Induced Action Potential Prolongation and Early Afterdepolarizations

Acceleration-Induced Early Afterdepolarizations. Introduction: Precipitation of torsades de pointes (TdP) has been shown to he associated with acceleration of heart rate in both experimental and clinical studies. To gain insight into the cellular mechanism(s) responsible for the initiation of acceleration-induced TdP, we studied the effect of acceleration of pacing rate in canine left ventricular epicardial, M region, endocardial, and Purkinje fiber preparations pretreated with E-4031, an I_Kr blocker known to induce the long QT syndrome and TdP. Methods and Results: Standard microelectrode techniques were used. E-4031 (1 to 2 μM) induced early afterdepolarization (EAD) activity in 31 of 36 M cell, 0 of 10 epicardial, 0 of 10 endocardial, and 9 of 12 Purkinje fiber preparations at basic cycle lengths (BCLs) ≥ 800 msec. In 30 of 36 M cells, sudden acceleration from a BCL range of 900 to 4,000 msec to a range of 500 to 1,500 msec induced transient EAD activity if none existed before or increased the amplitude of EADs if already present. Acceleration-induced augmentation of EAD activity was far less impressive and less readily demonstrable in Purkinje fibers (4/12). In M cells, appearance of EAD activity during acceleration usually was accompanied by an abbreviation of action potential duration (APD). Within discrete ranges of rates in the physiologic range, acceleration caused a transient prolongation of APD in 38% of M cells, whether or not a distinct EAD was generated. Acceleration produced still more dramatic APD prolongation and EADs in M cells after the BCL was returned to the original slow rate. Epicardium and endocardium APD showed little change immediately after acceleration. A decrease of BCL as small as 10% and, in some cases, a single premature heat could promote EAD activity and APD prolongation in some M cells. Ryanodine (1 μM, 10/10), flunarizine (10 μM, 3/6), and low Na (97 vs 129 mM, 5/5) abolished the acceleration-induced EAD activity and APD prolongation as well as the EAD activity observed at slow rates in M cells pretreated with E-4031. Conclusion: Our results suggest that acceleration from an initially slow rate or a single premature beat can induce or facilitate transient EAD activity and APD prolongation in canine ventricular M cell preparations pretreated with an I_Kr blocker via a mechanism linked to intracellular calcium loading. Our data provide evidence in support of an important contribution of electrogenic Na/Ca exchange current to this process. These acceleration-induced changes can result in the development of triggered activity as well as a marked dispersion of repolarization in ventricular myocardium and, thus, may contribute to the precipitation of TdP in patients with the congenital (HERG defect) and acquired (drug-induced) long QT syndrome.     

Left Ventricular Scar Burden Specifies the Potential for Ventricular Arrhythmogenesis: An LGE‐CMR Study

Late Gadolinium Enhancement and Arrhythmias . Introduction : The extent of left ventricular (LV) scar, characterized by late gadolinium enhancement cardiac MRI (LGE‐CMR), has been shown to predict the occurrence of ventricular arrhythmias in implantable cardioverter defibrillator (ICD) recipients. However, the specificity of LGE‐CMR for sudden cardiac death (SCD) versus non‐SCD is unclear. The aim of this retrospective, observational study was to evaluate this relationship in a cohort of ICD recipients. Methods and Results : We included consecutive patients who had undergone LGE‐CMR before ICD implantation over a 4‐year period (2006–2009). Scar (defined as myocardium with a signal intensity ≥50% of the maximum in scar tissue) was characterized in terms of percent scar and number of transmural LV scar segments in a 17‐segment model. The endpoints were appropriate ICD therapy and all‐cause mortality. Sixty‐four patients (average age 66 ± 11 years, 51 male, median LVEF 30%) were included. During 42 ± 13 months follow‐up, appropriate ICD therapy occurred in 28 patients (44%), and 14 patients (22%) died. Number of transmural scar segments (P = 0.005) and percentage LV scar (P = 0.03) were both significantly associated with appropriate ICD therapy. However, neither number of transmural scar segments (P = 0.32) or percent LV scar (P = 0.59) was significantly associated with all‐cause mortality. Conclusion : In this observational study, in medium‐term follow‐up, the extent of LV scar characterized by LGE‐CMR was strongly associated with the occurrence of spontaneous ventricular arrhythmias but not all‐cause mortality. We hypothesize that scar quantification by LGE‐CMR may be more specific for SCD than non‐SCD, and may prove a valuable tool for the selection of patients for ICD therapy. (J Cardiovasc Electrophysiol, Vol. 24, pp. 430‐436, April 2013)     

Drug-Induced Afterdepolarizations and Triggered Activity Occur in a Discrete Subpopulation of Ventricular Muscle Cells (M Cells) in the Canine Heart:

EADs and DADs in M Cells. Introduction: Oscillations of membrane potential that attend or follow the cardiac action potential and depend on preceding Tran membrane activity for their manifestation are known as aflerdepolarizations. Early aflerdepolarizations (EADs) interrupt or retard repolarization of the cardiac action potential, whereas delayed afterdepolarizations (DADs) arise after full repolarization. EADs and DADs can give rise to spontaneous action potentials or triggered activity believed to be responsible for a variety of cardiac arrhythmias. Recent studies from our laboratory have highlighted differences in the electrophysiology and pharmacology of three functionally distinct myocardial cell types found in the canine ventricle. Epicardial, M region, and endocardia tissues and cells show distinct, sometimes opposite, responses to a variety of drugs, including those capable of inducing EADs and DADs. Methods and Results: In the present study, we used standard microelectrode techniques to examine the pharmacologic response of these cellular subtypes to therapeutic levels of quinidine and toxic levels of digitalis. Quinidine readily produced prominent EADs and EAD induced triggered activity in tissue preparations from the M region (deep subepicardium), but not in those from epicardium. endocardium, or deep subendocardium of the canine ventricle. Acetylstrophanthidin produced prominent DADs in M cell preparations and subendocardiat Purkinje fibers but only minute DADs, if any, in epicardium, endocardium, or deep subendocardium. DAD-induced triggered activity was observed to arise only in Purkinje and M cells and never in myocardial tissues from the epicardial, endocardial, or deep subendocardial regions of the ventricular wall. Conclusion: We conclude that EADs, DADs, and triggered activity caused by therapeutic levels of quinidine and toxic levels of digitalis are limited to or much more readily induced in a select population of cells in the deep subepicardial (M cell) region of the canine ventricle in addition to the Purkinje system of the heart.     

Effect of macrophage infection by Leishmania on the proliferation of an antigen-specific T-cell line, TPB1, to a non-parasite antigen

The ability of Leishmania mexicana amazonensis to inhibit antigen specific T-cell proliferation against a non-parasite polypeptide antigen, poly(LTyr, LGlu)-poly(DLAla)–poly(LLys), was examined. Infection of mouse peritoneal macrophages by promastigotes blocked the proliferation of the T-cell line, TPB1. This effect was correlated with the level of parasite infection, and the timing of macrophage infection and antigen addition. Peritoneal macrophages from both BALB/b and C57BL/6 mice showed reduced ability to serve as antigen presenting cells.     

Evaluation of the Anemia of Rheumatoid Arthritis

Thirty-six patients with active rheumatoid arthritis in whom anemia wasslight were investigated, in part with radioiron and radiochromium, for abnormalities which might lead to anemia.

The primary defect appeared to be a pronounced reduction of abilityto absorb iron from the intestinal tract. There was an equally marked reduction in the serum concentration of iron which could not be attributedto the slight reduction in the iron-binding capacity of the serum. Despitethe reduction level of iron in the serum, the apparent turnover of iron wasso rapid that the total clearance per day was normal or moderately elevated.However, the fraction of the iron cleared from the blood and appearing inthe circulation in newly synthesized erythrocytes was slighfly depressed.Erythrocyte survival was normal or very slightly shortened; hemolysis seemsto play no important role. The fecal excretion of radioiron, not associatedwith erythrocytes, was greatly increased in two patients in whom there wasno evidence of intestinal bleeding. A moderate increase in plasma vohime wascommon.

Submitted on August 3, 1962 Accepted on November 3, 1962