Sodium Channel Block With Mexiletine Is Effective in Reducing Dispersion of Repolarization and Preventing Torsade de Pointes in LQT2 and LQT3 Models of the Long-QT Syndrome
Background This study examines the contribution of transmural heterogeneity of transmembrane activity to phenotypic T-wave patterns and the effects of pacing and of sodium channel block under conditions mimicking HERG and SCN5A defects linked to the congenital long-QT syndrome (LQTS).
Methods and Results A transmural ECG and transmembrane action potentials from epicardial, M, and endocardial or Purkinje cells were simultaneously recorded in an arterially perfused wedge of canine left ventricle. d-Sotalol was used to mimic LQT2, whereas ATX-II mimicked LQT3. d-Sotalol caused a preferential prolongation of the M cell action potential duration (APD90, 291±14 to 354±35 ms), giving rise to broad and sometimes low-amplitude bifurcated T waves and an increased transmural dispersion of repolarization (TDR, 51±15 to 72±17 ms). QT interval increased from 320±13 to 385±37 ms. ATX-II produced a preferential prolongation of the M cell APD90 (280±25 to 609±49 ms) and caused a marked delay in the onset of the T wave and a sharp rise in TDR (40±5 to 168±40 ms). QT-, APD90-, and dispersion-rate relations were much steeper in the ATX-II than in the d-sotalol model. Mexiletine (2 to 20 μmol/L) dose-dependently abbreviated the QT interval and APD90 of all cell types, more in the ATX-II than in the d-sotalol model, but decreased TDR equally in the two models. Mexiletine 2 to 5 μmol/L totally suppressed spontaneous torsade de pointes (TdP) and reduced the vulnerable window during which single extrastimuli could induce TdP in both models. Higher concentrations of mexiletine (10 to 20 μmol/L) totally suppressed stimulation-induced TdP.
Conclusions Our results suggest that although pacing and sodium channel block are very effective in abbreviating the QT interval and TDR in LQT3, these therapeutic approaches may also be valuable in reducing the incidence of arrhythmogenesis in LQT2.
Recent genetic linkage analysis studies have identified three forms of the congenital LQTS caused by mutations in ion channel genes located on chromosomes 3 (SCN5A), 7 (HERG), and 11 (KvLQT1).1 2 3 Mutations in SCN5A (LQT3 syndrome) result in incomplete inactivation of the sodium channel,1 whereas mutations in HERG (LQT2 syndrome) impair the flow of ions across the potassium channel responsible for IKr.4 A mutation in KvLQT12 3 produces a defect in the subunit that coassembles with the product of minK to form the channel responsible for IKs.5 6
Moss et al7 recently reported that patients with these ion channel defects display different phenotypic T-wave patterns in the ECG. LQT3 patients, those manifesting the sodium channel defect, show distinctive late-appearing T waves, whereas LQT1 or LQT2 patients, manifesting defects of voltage-gated potassium channels, display broad-based prolonged T waves or low-amplitude T waves. Another phenotypic feature of the congenital LQTS, shown by Malfatto et al8 and Lehmann and coworkers9 to be associated with increased risk for sudden death, is the presence of notched or bifurcated T waves. A recent study from Dausse and coworkers10 also indicates that notched or bifurcated T waves are commonly seen in the left precordial leads of LQT2 patients but not of LQT1 patients.
Differences in therapeutic approaches for the genetically distinct forms of LQTS have recently been suggested partly on the basis of studies such as that by Priori et al11 demonstrating the effectiveness of sodium channel blockade to abbreviate prolonged guinea pig myocyte action potentials in in vitro models of LQT3 but not in models of LQT2. Preliminary observations in the clinic have shown that sodium channel block by agents such as mexiletine and increased heart rate are effective in abbreviating the QT interval in LQT3 patients but not in LQT1 or LQT2 patients.12 These findings provide support for a syndrome-specific therapy, although the specificity is not yet defined. The effectiveness of an intervention to abbreviate the QT interval is not necessarily congruent with its efficacy to reduce the incidence of arrhythmogenesis or risk of sudden death.
We recently developed an arterially perfused preparation consisting of a wedge of canine left ventricle in which we are able to simultaneously record transmembrane activity from epicardial, M, and endocardial or subendocardial Purkinje sites along the transmural surface of the ventricular wall using floating glass microelectrodes. A pseudo-ECG recorded concurrently along the same vector permits correlation of transmembrane and ECG activity.13 The wedge is capable of developing and sustaining a variety of arrhythmias, including TdP. In the present study, we use this preparation to (1) assess the contribution of transmural differences in action potential morphology to the phenotypic manifestation of the electrocardiographic T wave under conditions of “acquired” long QT that mimic two genetic defects linked to LQT2 and LQT31 4 and (2) examine the effects of rapid pacing and sodium channel block to abbreviate QT and prevent TdP in these models of LQT2 and LQT3.
Arterially Perfused Wedge of Canine Left Ventricle
Dogs weighing 20 to 25 kg were anticoagulated with heparin and anesthetized with pentobarbital (30 to 35 mg/kg IV). The chest was opened via a left thoracotomy, and the heart was excised, placed in a cardioplegic solution consisting of cold (4°C) or room-temperature Tyrode’s solution containing 8.5 mmol/L [K+]o, and transported to a dissection tray. Transmural wedges with dimensions of ≈2×1.5×0.9 cm to 3×2×1.5 cm were dissected from the left ventricle (Fig 1⇓). The tissue was cannulated via a small (diameter ≈100 μm) native branch of the left descending coronary artery and perfused with cardioplegic solution. The total period of time from excision of the heart to cannulation and perfusion of the artery was <4 minutes in all experiments. Unperfused tissue, readily identified by its maintained red appearance (erythrocytes not washed away), was carefully removed with a razor blade. The preparation was then placed in a small tissue bath and arterially perfused with Tyrode’s solution of the following composition (mmol/L): NaCl 129, KCl 4, NaH2PO4 0.9, NaHCO3 20, CaCl2 1.8, MgSO4 0.5, and glucose 5.5, buffered with 95% O2/5% CO2 (37±1°C). The perfusate was delivered to the artery by a roller pump (Cole Parmer Instrument Co). Perfusion pressure was monitored with a pressure transducer (World Precision Instruments, Inc) and maintained between 40 and 50 mm Hg by adjustment of the perfusion flow rate. The preparations remained immersed in the arterial perfusate, which was allowed to rise to a level 2 to 3 mm above the tissue surface when possible. It was often difficult to maintain the floating microelectrode impalements with the bath solution above the height of the tissue. In such cases, the level was brought to just below the top of the preparation, and the chamber was covered to the extent possible in order to avoid a temperature gradient between the top and lower segments of the preparation. Preparations displaying significant ST-segment elevation or depression or in which temperature gradients were evident along the length of the preparation were excluded from the study.
Recordings of a Transmural ECG and Transmembrane Action Potentials
The ventricular wedges were allowed to equilibrate in the tissue bath until electrically stable, usually 1 hour. The preparations were stimulated at BCLs ranging from 300 to 2000 ms with bipolar silver electrodes insulated except at the tips and applied to the endocardial surface.
A transmural ECG was recorded with electrodes consisting of AgCl half-cells attached to Tyrode’s solution–filled tapered polyethylene electrodes. The electrodes were placed in the Tyrode’s solution bathing the preparation, 1.0 to 1.5 cm from the epicardial and endocardial surfaces of the preparation, along the same vector as the transmembrane recordings (epicardial, + pole). The electrical field of the preparation as a whole was measured by this technique; in previous experiments, we have demonstrated that the morphology of the ECG is not altered other than in amplitude when the distance of the electrodes from the preparation is increased beyond 1 cm. Thus, the ECG registration represents a pseudo-ECG of that part of the left ventricle. To differentiate it from local electrogram activity, we refer to it as an ECG in the remainder of the text.
Transmembrane action potentials were simultaneously recorded from the epicardial, M, and endocardial or subendocardial Purkinje sites with three to four separate intracellular floating microelectrodes (DC resistance, 10 to 20 MΩ) filled with 2.7 mol/L KCl and connected to a high-input impedance amplifier. Impalements were obtained from the cut surface as well as epicardial and endocardial surfaces of the preparation at positions approximating the transmural axis of the ECG recording.
To ensure that transmembrane activity recorded at the cut surface of the preparation was representative of the activity in the intramural layers, in some experiments we used unipolar electrodes to measure the ARI in the deeper layers of the wedge. The ARI values were then compared with values for APD recorded at the surface.
Plunge electrodes consisting of silver wire (120 μm in diameter), Teflon insulated except at the tip, were introduced to various depths within the preparation with a 21-gauge or larger needle, which was immediately withdrawn. Each electrode was referenced to the bath ground (AgCl electrode). Caution was exercised to ensure that the position of the bath ground did not influence the morphology of the unipolar electrogram. Each unipolar recording was differentiated, and the ARI approximating the APD at each site was measured as the interval between the Vmin of the QRS deflection and the Vmax of the T wave. ARI was compared with either APD90 or the interval between Vmax and Vmin of the differentiated action potential trace. Validation of the use of this technique for the approximation of APD at transmural sites within canine ventricular myocardium was recently provided by El-Sherif and coworkers.14
All amplified signals were digitized, stored on magnetic media and WORM-CD, and analyzed with Spike 2 (Cambridge Electronic Design).
The IKr blocker d-sotalol (100 μmol/L) was used to create a model of acquired long QT as well as a model that mimics the defect in IKr or HERG believed to underlie the congenital LQT2 syndrome. ATX-II (20 nmol/L), an agent that slows the inactivation of the sodium channel, was used to mimic the LQT3 syndrome. The validity of these pharmacological models as surrogates for the congenital syndromes has been demonstrated in previous studies11 14 and is further substantiated in this one.
In both models, we examined (1) the contribution of transmural heterogeneity of action potential to phenotypic ECG patterns; (2) the rate dependence of the QT interval, of APD, and of TDR across the ventricular wall at BCLs ranging from 300 to 2000 ms; and (3) the dose-dependent effect of the sodium channel blocker mexiletine (2, 5, 10, and 20 μmol/L) to reverse the effects of d-sotalol or ATX-II.
Control measurements were generally obtained after 1 hour of equilibration. The ATX-II and d-sotalol data were collected for a period of up to 1 hour starting 1 hour after addition of the respective drug, and mexiletine data were recorded after 30 minutes of exposure to each concentration of drug.
APD was measured at 90% repolarization (APD90). TDR was defined as the difference between the longest and the shortest repolarization times (activation time+APD90) of transmembrane action potentials recorded across the wall. The QT interval was defined as the time between QRS onset and the point at which the line of maximal downslope of the T wave crossed the baseline.
The ability of d-sotalol and ATX-II to spontaneously induce polymorphic VT displaying characteristics of TdP was assessed in both models. In the absence of spontaneously induced TdP, PES using single extrastimulation was applied to the epicardial surface of the wedge to induce TdP. The dose-dependent effect of mexiletine to suppress spontaneous TdP induced by d-sotalol and ATX-II was examined. The relative effectiveness of mexiletine to suppress PES-induced TdP was also evaluated.
Statistical analysis of the data was performed with Student’s t test for paired data or ANOVA coupled with Scheffé’s test, as appropriate. Data are expressed as mean±SD except for those shown in the figures, which are expressed as mean±SEM. Significance was defined as a value of P<.05.
Contribution of Transmural Differences in Action Potential Morphology to ECG Patterns
Transmembrane activity recorded from epicardial, M, and endocardial regions of the arterially perfused left ventricular wedge preparations is illustrated in Fig 2⇓. Fig 2A⇓ through 2D shows activity recorded in the d-sotalol model perfused with normal or low-potassium Tyrode’s solution (BCL, 2000 ms). Under control conditions, the peak of the T wave in the ECG was coincident with the repolarization of the epicardial cell, whereas the end of the T wave was coincident with the repolarization of the M region (deep subendocardium) (Fig 2A⇓ and 2C⇓). Repolarization of the endocardial action potential was intermediate between that of the M cell and epicardial cell. Dispersion of repolarization across the ventricular wall, defined as the difference in the repolarization time between the M cell (longest action potential) and epicardial cell (shortest action potential), is denoted by an asterisk. d-Sotalol 100 μmol/L significantly prolonged the QT interval from 320±13 to 385±37 ms (n=8; P<.0005) at a BCL of 2000 ms. The change in QT was paralleled by an increase in APD90 of the M cell (291±14 to 354±35 ms; n=8; P<.0005). d-Sotalol increased the TDR from 51±15 to 72±17 ms (n=8; P<.0005) as a result of a greater prolongation of the APD90 of the M cell than of the epicardial cell (224±17 to 268±41 ms; n=8; P<.0005). d-Sotalol always caused a widening of the T wave (Fig 2B⇓), in some cases giving rise to low-amplitude T waves with a notched or bifurcated appearance (Fig 2D⇓). The latter were more prevalent in the presence of low potassium (2 to 3 mmol/L), which in combination with d-sotalol produced a very significant slowing of phase 3. In the presence of d-sotalol as in control, repolarization of the epicardial cell defined the peak of the T wave, whereas repolarization of the M cell defined the end of the T wave.
Fig 2E⇑ and 2F⇑ show activity recorded in the ATX-II model. ATX-II 20 nmol/L markedly prolonged the QT interval from 308±26 to 646±58 ms (n=8; P<.0005) at a BCL of 2000 ms, widened the T wave, and caused a sharp rise in the dispersion of repolarization across the wall from 40±5 to 168±40 ms (n=8; P<.0005). As in the d-sotalol model, the latter changes appeared to be the result of a greater prolongation of the APD90 in the M region (280±25 to 609±49 ms; n=8; P<.0005) than in the epicardium (229±24 to 431±76 ms; n=8; P<.0005). ATX-II also produced a marked delay in onset of the T wave due to relatively large effects of the drug to prolong epicardial and endocardial action potentials (Fig 2F⇑), consistent with the late-appearing T-wave pattern observed in patients with the LQT3 syndrome. Concordant with their different ionic mechanisms of action, d-sotalol and ATX-II produce very different alterations in the morphology of the action potential in all three cell types. Whereas d-sotalol slows repolarization of both phase 2 and phase 3, ATX-II prolongs the action potential by slowing only phase 2, leaving phase 3 largely unaffected.
Correspondence Between the Electrical Activity of Surface and Intramural Layers
Transmembrane activity recorded from the cut surface of the wedge can be influenced by a variety of factors, including the coupling of the surface layers to deeper layers and juxtaposition to injured cells. It is of some importance to determine whether the electrical heterogeneity recorded at the surface is representative of the rest of the preparation. This is particularly important in attempts to correlate transmembrane and ECG data. To address this issue, we monitored the ARI using ultrathin unipolar plunge electrodes (120 μm) inserted into regions subtending the area mapped with the transmembrane electrodes. Fig 3⇓ compares the ARI measurements made in the intramural layers with APD measurements of the transmembrane action potential recorded at the surface under control conditions and after the addition of ATX-II (10 nmol/L) to amplify the TDR. Composite data of four experiments in which recordings were obtained before and after ATX-II (10 or 20 nmol/L) show reasonable correspondence between the transmembrane APD recorded at the cut surface and ARI recorded from subtending intramural sites (Fig 3⇓ and Table⇓). The relatively small discrepancy between the epicardial APD and ARI when transmural dispersion is amplified (ATX-II) appears to be due to the wider field of view of the unipolar electrode. These data clearly indicate that activity at the cut transmural surface is representative of the subtending intramural layers with respect to repolarization time and that activation of the surface layers is normal as well.
Rate Dependence of QT Interval, APD, and Dispersion of Repolarization
The rate-dependent changes in the QT interval were closely approximated by changes in the repolarization time of the M cell in both models, as illustrated in Fig 4⇓. Each panel shows superimposed action potentials recorded simultaneously from endocardial, M, and epicardial regions together with a transmural ECG at BCLs ranging from 300 to 2000 ms. d-Sotalol and ATX-II produced a significant rate-dependent prolongation of the APD90 of the three cell types and of the QT interval. These changes were more striking in the ATX-II model (Fig 4C⇓ and 4D⇓) than in the d-sotalol model (Fig 4A⇓ and 4B⇓). As a consequence, APD90 and QT-rate relations were much steeper in the ATX-II (LQT3) model, although both were more pronounced than under control conditions (Fig 5A⇓, 5B⇓, and 5C⇓). Moreover, both d-sotalol and ATX-II produced a preferential prolongation of the M cell action potential. TDR increased in a rate-dependent manner (Fig 5D⇓). In both models, rapid pacing (BCL of 300 ms) abbreviated the APD90, the QT interval, and TDR to nearly control values.
Effect of Mexiletine on QT Interval, APD, and Dispersion of Repolarization
Figs 6⇓ and 7⇓ show the effect of mexiletine on transmembrane and ECG activity in the ATX-II and d-sotalol models, respectively. Shown are superimposed action potentials recorded simultaneously from the M and epicardial regions together with a transmural ECG. ATX-II 20 nmol/L dramatically prolonged APD90 and the QT interval. In the continued presence of ATX-II, 2 to 20 μmol/L of mexiletine dose-dependently abbreviated APD90 of the three cell types as well as the QT interval; 20 μmol/L mexiletine totally reversed the effects of ATX-II on APD90, QT interval, and TDR (Figs 6⇓, 8C⇓, and 8D⇓).
d-Sotalol (100 μmol/L) increased APD90, QT interval, and TDR because of a greater prolongation of the M cell action potential. In the continued presence of d-sotalol, 2 to 20 μmol/L of mexiletine dose-dependently abbreviated APD90 of the three cell types and the QT interval. Unlike its effects in the ATX-II model, 20 μmol/L mexiletine failed to totally reverse the actions of d-sotalol (Figs 7⇑ and 8A⇑). However, because mexiletine abbreviated the M cell action potential more than that of the epicardial cell (Figs 7⇑ and 8A⇑), the TDR decreased to control values even in the d-sotalol model (Fig 8B⇑).
This point is illustrated more clearly in Fig 9⇓, in which the effects of d-sotalol and ATX-II are normalized so that the 100% value represents the maximum prolongation produced by these agents. Each panel shows the dose-response relationship (semilog scale) for the effect of mexiletine to reverse the effect of d-sotalol and of ATX-II on APD90, the QT interval, and TDR. In both models, mexiletine abbreviated APD90 of the M and epicardial cells, the QT interval, and the dispersion of repolarization in a dose-dependent fashion. In the ATX-II model, 20 μmol/L of mexiletine totally reversed the effect of ATX-II (Fig 9⇓, open triangles). In contrast, in the d-sotalol model, 20 μmol/L of mexiletine reversed 70% of the effect of d-sotalol to prolong the APD90 of the M cell (Fig 9A⇓, solid circles) and the QT interval (Fig 9C⇓, solid circles) but only 40% of the effect of d-sotalol to prolong the epicardial action potential (Fig 9B⇓, solid circles). As a consequence, 20 μmol/L of mexiletine totally reversed the effect of d-sotalol to increase the TDR (Fig 9D⇓, solid circles). Thus, at all concentrations tested, the effect of mexiletine to reduce dispersion of repolarization was similar in the two models (Fig 9D⇓).
Effect of Mexiletine on TdP Induced by d-Sotalol and ATX-II
Under control conditions, no ventricular arrhythmias were observed in either model other than an occasional premature beat. d-Sotalol (100 μmol/L) induced repeated spontaneous episodes of nonsustained (<30 seconds) polymorphic VT displaying characteristics of TdP in two of eight preparations (Fig 10A⇓ and 10B⇓), whereas ATX-II induced repeated episodes of spontaneous TdP in four of eight preparations (Fig 11A⇓). In both models, a bigeminal rhythm, couplets, and triplets generally preceded the occurrence of TdP (Fig 10A⇓ and 10B⇓). The first spontaneous premature beat did not appear to arise from any particular site consistently. There were no episodes of sustained polymorphic VT that had to be terminated by DC cardioversion in either model. The mean cycle lengths of TdP spontaneously induced by ATX-II (LQT3 model) were significantly longer than those induced by d-sotalol (LQT2 model) (242±46 versus 135±26 ms; P<.005). The average number of beats per episode of TdP in the ATX-II model was significantly smaller than in the d-sotalol model (6±2 versus 20±9 beats; P<.001). Relatively low concentrations of mexiletine (2 to 5 μmol/L) totally suppressed spontaneous TdP in both models (Figs 10C⇓ and 11B⇓). In preparations in which TdP did not appear spontaneously, single premature stimuli applied to the epicardial surface induced TdP in four of six preparations in the d-sotalol model and in three of four preparations in the ATX-II model. Low concentrations of mexiletine (2 to 5 μmol/L) reduced the vulnerable window (range of S1S2 intervals at which single extrastimuli succeeded in inducing TdP). Mexiletine 5 μmol/L reduced the vulnerable window from 34±11 to 18±8 ms in four d-sotalol–treated preparations (P<.05) and from 95±44 to 25±6 ms in three ATX-II–treated preparations (P<.05). At the highest concentration of mexiletine (10 to 20 μmol/L), PES failed to induce TdP in all (three of three) ATX-II–treated preparations in which PES was previously successful and three of four d-sotalol–treated wedge preparations.
Phenotypic ECG Patterns Caused by Transmural Electrical Heterogeneity
Recent studies have highlighted the importance of transmural heterogeneity within the heart (for reviews, see References 15 through 1715 16 17 ), demonstrating regional differences in electrical and pharmacological distinctions between endocardium and epicardium of the canine, feline, rabbit, rat, and human heart,13 18 19 as well as M cells located in the deep structures of the canine, guinea pig, and human ventricles.16 19 20 21 22 23 24 25 26 27 28
M cells differ from epicardial and endocardial cells in that they display a much greater APD at slow rates.16 17 20 28 A weaker IKs25 and larger late sodium current29 contribute to the longer APD of the M cell in the dog. The weaker net outward current active during the plateau phase also contributes to the greater response of M cells to agents that prolong APD. M cells show a preferential response to agents that inhibit IK (eg, quinidine, erythromycin, sotalol, E-4031), augment ICa (eg, BAY K 8644), or slow the inactivation of INa (eg, ATX-II).
Genetic linkage analyses performed in patients with congenital LQTS have identified three gene defects located on chromosomes 3 (SCN5A), 7 (HERG), and 11 (KvLQT1).1 2 3 The consequences of these three mutations are (1) a reduced IKs (LQT1)5 6 or IKr (LQT2)4 and (2) a larger late sodium current (LQT3).1 Both result in a weaker net outward current and a prolongation of the APD, especially in Purkinje fibers and M cells.
The full extent to which these genetic mutations display different phenotypic features is not known, nor is the relative risk for development of life-threatening arrhythmias, such as TdP. Genotypic-phenotypic correlation is an important first step to addressing these issues. In the present study, we used pharmacological agents to mimic two genetic defects (LQT2 and LQT3 syndrome). The perfused-wedge preparation provided us with the capability to assess the phenotypic expression of changes in ion channel activity on transmembrane activity and on the ECG. Our data indicate that the IKr blocker d-sotalol produces a much greater prolongation of the APD of the M cell than of epicardial and endocardial cells in the perfused-wedge preparation. As a result, d-sotalol increased the QT interval and the TDR and caused a widening of the T wave. In the presence of low potassium, d-sotalol often gave rise to low-amplitude T waves with a notched or bifurcated appearance commonly seen in patients with the LQT2 syndrome.8 9 10 This phenotypic ECG change appears to be related to a very significant slowing of repolarization due to more potent d-sotalol–induced inhibition of IKr and smaller IK1 at the lower [K+]o.30 Similarly, ATX-II, an agent that slows the inactivation of the sodium channel, markedly prolonged the QT interval, widened the T wave, and caused a sharp rise in the dispersion of repolarization as a result of a greater prolongation of the APD in the M cell. In contrast to the d-sotalol (LQT2) model, the greater response of the M cell to ATX-II may be due to a larger late sodium current in the M cell.29 ATX-II also produced a marked delay in onset of the T wave due to relatively large effects of the drug on epicardial and endocardial APD, consistent with the late-appearing T-wave pattern observed in patients with the LQT3 syndrome.7 These results point to an important contribution of transmural electrical heterogeneity to the phenotypic T-wave morphology seen in LQTS. The concordance of these results and those discussed below with the phenotypic manifestation of congenital LQTS observed in the clinic further validate the use of these pharmacological models11 14 as well as of the perfused wedge13 in studying the effects of drugs and other interventions on ECG manifestations and arrhythmogenesis related to LQT2 and LQT3.
Effect of Rapid Pacing on QT Interval, APD, and Dispersion of Repolarization
The value of pacemaker therapy in patients with congenital LQTS resistant to antiadrenergic therapy (eg, β-blocking agents, left cervicothoracic sympathetic ganglionectomy) has long been appreciated.31 Several clinical studies have suggested that repolarization abnormalities are attenuated at faster rates in patients with congenital LQTS.32 33 More recent data indicate that increases in heart rate recorded during exercise testing or on Holter recording are effective in significantly abbreviating the QT interval in LQT3 but not LQT2 patients.12 Using guinea pig ventricular myocytes, Priori et al11 also demonstrated greater pacing-induced abbreviation of APD in the LQT3 than in the LQT2 model.
In the present study, APD-, QT-, and dispersion-rate relations were clearly much steeper in the ATX-II model than in the d-sotalol model. This may be related to the very slow kinetics of reactivation of sodium current increased by ATX-II. These rate relations were more pronounced in the d-sotalol model than under control conditions. With acceleration to BCLs of 1000 and 800 ms, the dispersion of repolarization decreased significantly in both models, approaching control values in both models at a BCL of 300 ms (Fig 5D⇑).
Although not addressed in this study, recent investigations have shown that calcium loading secondary to fluctuation in rate can transiently exaggerate the effects of d-sotalol to prolong the APD of the M cell but not that of the epicardial or endocardial cells, leading to marked increases in TDR and the development of EADs.34 The arrhythmogenic consequences of long pauses, short-long-short sequences, and other perturbations of rate in LQTS are well documented in both experimental and clinical studies.35 36 37 38 Constancy in the rate of ventricular activation is therefore desirable in LQT2. These results suggest that although pacing therapy is likely to be very effective in the treatment of LQT3, its usefulness in LQT2 should not be discounted.
Effect of Mexiletine on TDR
Schwartz and coworkers12 reported that mexiletine significantly shortens the QT interval in LQT3 patients (QTc decreased from 535±32 to 445±31 ms, P<.005) but not in LQT2 patients (QTc decreased from 530±79 to 503±60 ms, P=NS). Mexiletine is a class IB antiarrhythmic agent that, like lidocaine, shows rapid dissociation kinetics from the sodium channel. At relatively slow rates, its effects to block the late sodium current at the level of the action potential plateau are significant, although its effects on fast sodium current and on normal conduction are negligible at the lower concentrations.
In the present study, the effect of mexiletine to abbreviate the QT interval was greater in the ATX-II than the d-sotalol model, although its effect to reduce dispersion of repolarization was equal in the two models. The latter was due to the relatively large effect of mexiletine to abbreviate the APD of the M cell and very small effect to abbreviate the APD of the epicardial cell in the d-sotalol model.
Our results showing a moderate effect of mexiletine to abbreviate the APD of the M cell in the presence of d-sotalol are in contrast to those of Priori and coworkers,11 in which 10 to 100 μmol/L mexiletine failed to abbreviate the APD of guinea pig myocytes pretreated with the IKr blocker dofetilide. This discrepancy may be because their dissociation procedure may have excluded M cells. The response of their dofetilide-treated guinea pig cells to mexiletine was similar to that of our d-sotalol–treated canine epicardial and endocardial cells but not M cells.
The effectiveness of mexiletine to reduce TDR in both models is consistent with the ability of the drug to prevent the development and induction of TdP. Low concentrations of mexiletine completely abolished spontaneous episodes of TdP, whereas higher concentrations were needed to prevent TdP induction with PES. Like the clinical observations of Schwartz et al,12 we observed a much greater effect of mexiletine on QT interval in the ATX-II (LQT3) than the d-sotalol (LQT2) model. Although the clinical study did not quantify the effect of mexiletine on transmural dispersion or on relative risk for development of TdP in the two forms of LQTS, our experimental models are able to evaluate these parameters. Our data suggest that sodium channel block with mexiletine may be effective in abbreviating the QT interval, reducing the TDR, and preventing TdP in LQT3 but may also be of great value in reducing dispersion of repolarization and arrhythmogenesis in the LQT2 syndrome.
Although the effectiveness of sodium channel blockers in suppressing TdP has been reported in patients with acquired LQTS,39 there have been no reports regarding its effect on TdP in patients with congenital LQTS.
Mechanisms of TdP and of the Effect of Mexiletine
TdP is an atypical polymorphic VT most often associated with prolongation of the QT interval in both congenital and acquired LQTS. TdP has been commonly observed in patients on quinidine who also develop hypokalemia and present with slow heart rates or long pauses. These conditions are similar to those under which quinidine and other agents induce EADs and triggered activity in isolated Purkinje fibers and M cells.40 Moreover, Shimizu and coworkers,32 39 41 42 using monophasic action potential recording techniques, demonstrated EAD-like activity in both congenital and acquired LQTS. These experimental and clinical observations suggested a role for EAD-induced triggered activity in the genesis of TdP. However, the role of EADs and triggered activity in the genesis and maintenance of TdP is not well defined. Some investigators have suggested that TdP at times may be initiated and maintained by triggered activity simultaneously originating at two independent foci, whereas others have suggested that TdP may be initiated by a triggered beat but maintained by a circus movement reentry mechanism.16 32 41 El-Sherif et al14 recently developed a canine in vivo model of LQTS using anthopleurin A, an agent that slows the inactivation of the sodium channel like ATX-II. Nonsustained polymorphic VT with characteristics of TdP developed spontaneously in all nine puppies studied, and sustained polymorphic VT, which required DC cardioversion to be terminated, developed in seven of the nine animals. Using high-resolution tridimensional isochronal maps of activation and repolarization patterns, they showed that the initial beat of the polymorphic VT appeared to arise from a focal subendocardial site, whereas subsequent beats were due to successive subendocardial focal activity, reentrant excitation, or a combination of the two mechanisms.
In our study, repeated episodes of nonsustained polymorphic VT displaying characteristics of TdP developed spontaneously in two of eight preparations in the d-sotalol model and in four of eight preparations in the ATX-II model. The relatively low incidence of spontaneous TdP in our experiments may be due to the relatively small size of the wedge preparation (dimensions of ≈2×1.5×0.9 cm to 3×2×1.5 cm).43 In the case of a circus movement, the path length for reentry in the perfused wedge might often be expected to exceed the wavelength, defined as the product of the effective refractory period and conduction velocity. Although a relatively short path length may be responsible for the low incidence of TdP in the two models, the additional limitation imposed by a relatively long wavelength is most likely responsible for the slower and briefer VT in the ATX-II model.
There is general agreement that the initiating event in TdP is an EAD-induced triggered response,14 but still in question is the origin of the triggered beats, because both Purkinje fibers44 and M cells23 34 can generate EADs under similar conditions. Our results are compatible with either site as the source of the premature beat, which is likely to be due to EAD-induced triggered activity.
Among those who concede that reentry is responsible for the maintenance of TdP, there is no clear consensus as to the substrate responsible for the development of the arrhythmia.15 16 45 46 47 48 The recent elegant study by El-Sherif et al14 suggested that reentry proceeds around the right and left ventricular chambers. In contrast, Antzelevitch et al43 demonstrated erythromycin-induced TdP in relatively small, isolated, arterially perfused canine left ventricular wedge preparations. The induction of TdP was critically dependent on the size of the wedge, suggesting intramural reentry as the basis for the maintenance of the arrhythmia.
The effect of mexiletine in preventing TdP may be due to (1) suppression of the triggered activity responsible for the spontaneous premature beats that precipitate TdP or (2) elimination of the substrate for reentry via a reduction of TDR. Our results suggest that low concentrations of mexiletine suppress the triggered ventricular premature beats as well as narrowing the vulnerable window. Higher concentrations further narrow the vulnerable window and prevent induction of the arrhythmia with PES.
In summary, our results highlight the contribution of transmural electrical heterogeneity to the distinctive phenotypic appearance of the T wave in the LQT2 and LQT3 syndromes. Sodium block is shown to be effective in decreasing TDR and in suppressing spontaneous as well as PES-induced TdP in both the d-sotalol and ATX-II models, suggesting that this therapeutic approach may be of value in LQT2 as well as LQT3 and acquired (drug-induced) forms of the LQTS.
Selected Abbreviations and Acronyms
|APD||=||action potential duration|
|APD90||=||APD measured at 90% repolarization|
|BCL||=||basic cycle length|
|I Kr||=||rapidly activating component of delayed rectifier potassium current|
|I Ks||=||slowly activating component of delayed rectifier potassium current|
|PES||=||programmed electrical stimulation|
|TdP||=||torsade de pointes|
|TDR||=||transmural dispersion of repolarization|
|Vmax||=||maximum of first derivative|
|Vmin||=||minimum of first derivative|
This study was supported by grant HL-47678 from the National Institutes of Health and grants from Medtronic Japan and the Sixth and Seventh Manhattan Masonic Districts and New York State and Florida Grand Lodges F&AM. Dr Shimizu was awarded first prize at the NASPE Young Investigators Award Competition on the basis of this work. We gratefully acknowledge the expert technical assistance of Judy Hefferon and Di Hou. Our thanks to Drs S. Sicouri, J.M. Di Diego, and V.V. Nesterenko for assistance with the experimental protocols involving unipolar electrograms and helpful suggestions.
- Received December 31, 1996.
- Revision received April 16, 1997.
- Accepted April 28, 1997.
- Copyright © 1997 by American Heart Association
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