Epicardial Action Potential Duration Research Articles

Sex-specific repolarization heterogeneity in mouse left ventricle: Optical mapping combined with mathematical modeling predict the contribution of specific ionic currents.

Ventricular repolarization shows notable sex-specificity, with female sex being associated with longer QT-intervals in electrocardiography irrespective of the species studied. From a clinical point of view, women are at a greater risk for drug-induced torsade de pointes and symptomatic long-QT syndrome. Here, we present an optical mapping (OM) approach to reveal sex-specific action potential (AP) heterogeneity in a slice preparation of mouse hearts. Left ventricular epicardial repolarization in female versus male mice shows longer and, interindividually, more variable AP duration (APD), yielding a less prominent transmural APD gradient. By combining OM with mathematical modeling, we suggest a significant role of IKto,f and IKur in AP broadening in females. Other transmembrane currents, including INaL , only marginally affect basal APD. As in many cardiac pathophysiologies, increasing [Ca2+ ]i poses a risk for arrhythmia, the response of AP morphology to enhanced activation of L-type calcium channels (LTCC) was assessed in a sex-selective manner. Both APD and its variation increased significantly more in female versus male mice after pharmacological LTCC activation, which we hypothesize to be due to sex-specific INaL expression based on mathematical modeling. Altogether, we demonstrate a more delayed repolarization of LV epicardium, a leveled LV transmural APD gradient, and a more pronounced epicardial APD response to Ca2+ influx in females versus males. Mathematical modeling quantifies the relative contributions of selected ionic currents to sex-specific AP morphology under normal and pathophysiological conditions.

Levosimendan attenuates electrical alternans and prevents ventricular arrhythmia during therapeutic hypothermia in isolated rabbit hearts

Therapeutic hypothermia (TH) increases the susceptibility to ventricular arrhythmias (VAs) by prolonging action potential duration (APD) and facilitating arrhythmogenic spatially discordant alternans (SDA). Levosimendan, a calcium sensitizer, has been reported to shorten APD by enhancing the adenosine triphosphate (ATP)-sensitive K current. The purpose of this study was to test the hypothesis that, during TH, levosimendan shortens the already prolonged APD, attenuates SDA, and prevents VA. Langendorff-perfused isolated rabbit hearts were subjected to TH (30°C) for 15 minutes, followed by treatment with either levosimendan 0.5 μM (n = 9) or vehicle (n = 8) for an additional 30 minutes under TH. Using an optical mapping system, epicardial APD was evaluated by S1 pacing. SDA threshold was defined as the longest pacing cycle length (PCL) that induces the phenomenon of SDA. Ventricular fibrillation (VF) inducibility was evaluated by burst pacing for 30 seconds at the shortest PCL that achieved 1:1 ventricular capture. During TH, levosimendan shortened ventricular APD (PCL 400 ms; from 259 ± 8 ms to 241 ± 18 ms; P = .036) and decreased SDA threshold (from 327 ± 88 ms to 311 ± 68 ms; P = .011). VF inducibility was lowered from 39% ± 30% to 14% ± 12% with levosimendan (P = .018), whereas APD at PCL 400 ms (P = .161), SDA threshold (P = 1), and VF inducibility (P = .173) were not changed by vehicle. During TH, levosimendan could protect hearts against VA by shortening APD and decreasing SDA threshold. Enhancing ATP-sensitive K current with levosimendan might be a novel approach to preventing VA during TH.

Impact of late sodium current inhibition on cardiac electrophysiology parameters and ventricular arrhythmias in isolated Langendorff perfused rabbit hearts with short QT interval

Objective: To determine the electrophysiological effects and related mechanisms of late sodium current inhibitors on hearts with short QT intervals. Methods: The electrophysiological study was performed on isolated Langendorff perfused rabbit hearts. A total of 80 New Zealand White rabbits were used and 34 hearts without drug treatment were defined as control group A, these hearts were then treated with IKATP opener pinacidil, defined as pinacidil group A. Then, 27 hearts from pinacidil group A were selected to receive combined perfusion with sodium channel inhibitors or quinidine, a traditional drug used to treat short QT syndrome, including ranolazine combined group (n=9), mexiletine combined group (n=9), and quinidine combined group (n=9). Nineteen out of the remaining 46 New Zealand rabbits were selected as control group B (no drug treatments, n=19), and then treated with pinacidil, defined as pinacidil group B (n=19). The remaining 27 rabbits were treated with sodium inhibitors or quinidine alone, including ranolazine alone group (n=9), mexiletine alone group (n=9), and quinidine alone group (n=9). Electrocardiogram (ECG) physiological parameters of control group A and pinacidil group A were collected. In control group B and pinacidil group B, programmed electrical stimulation was used to induce ventricular arrhythmias and ECG was collected. ECG physiological parameters and ventricular arrhythmia status of various groups were analyzed. The concentrations of pinacidil, ranolazine, mexiletine and quinidine used in this study were 30, 10, 30 and 1 μmol/L, respectively. Results: Compared with control group A, the QT interval, 90% of the repolarization in epicardial and endocardial monophasic action potential duration (MAPD90-Epi, MAPD90-Endo) was shortened, the transmural dispersion of repolarization (TDR) was increased, and the effective refractor period (ERP) and post-repolarization refractoriness (PRR) were reduced in pinacidil group A (all P<0.05). Compared with the pinacidil group A, MAPD90-Epi, MAPD90-Endo, QT interval changes were reversed in quinidine combined group and mexiletine combined group (all P<0.05), but not in ranolazine combined group. All these three drugs reversed the pinacidil-induced increases of TDR and the decreases of ERP and PRR. The induced ventricular arrhythmia rate was 0 in control group B, and increased to 10/19 (χ2=13.6, P<0.05) in pinacidil group B during programmed electrical stimulation. Compared with the pinacidil group B, incidences of ventricular arrhythmia decreased to 11% (1/9), 11% (1/9) and 0 (0/9) (χ2=4.5, 4.5, 7.4, P<0.05) respectively in ranolazine group, mexiletine group and quinidine group. Conclusions: Inhibition of late sodium current does not increase but even decreases the risk of malignant arrhythmia in hearts with a shortened QT interval. The antiarrhythmic mechanism might be associated with the reversal of the increase of TDR and the decrease of refractoriness (including both ERP and PRR) of hearts with shortened QT interval.

Multimodal imaging shows fibrosis architecture and action potential dispersion are predictors of arrhythmic risk in spontaneous hypertensive rats.

Hypertensive heart disease (HHD) increases risk of ventricular tachycardia (VT) and ventricular fibrillation (VF). The roles of structural vs. electrophysiological remodelling and age vs. disease progression are not fully understood. This cross‐sectional study of cardiac alterations through HHD investigates mechanistic contributions to VT/VF risk. Risk was electrically assessed in Langendorff‐perfused, spontaneously hypertensive rat hearts at 6, 12 and 18 months, and paced optical membrane voltage maps were acquired from the left ventricular (LV) free wall epicardium. Distributions of LV patchy fibrosis and 3D cellular architecture in representative anterior LV mid‐wall regions were quantified from macroscopic and microscopic fluorescence images of optically cleared tissue. Imaging showed increased fibrosis from 6 months, particularly in the inner LV free wall. Myocyte cross‐section increased at 12 months, while inter‐myocyte connections reduced markedly with fibrosis. Conduction velocity decreased from 12 months, especially transverse to the myofibre direction, with rate‐dependent anisotropy at 12 and 18 months, but not earlier. Action potential duration (APD) increased when clustered by age, as did APD dispersion at 12 and 18 months. Among 10 structural, functional and age variables, the most reliably linked were VT/VF risk, general LV fibrosis, a measure quantifying patchy fibrosis, and non‐age clustered APD dispersion. VT/VF risk related to a quantified measure of patchy fibrosis, but age did not factor strongly. The findings are consistent with the notion that VT/VF risk is associated with rate‐dependent repolarization heterogeneity caused by structural remodelling and reduced lateral electrical coupling between LV myocytes, providing a substrate for heterogeneous intramural activation as HHD progresses. Key points There is heightened arrhythmic risk with progression of hypertensive heart disease.Risk is related to increasing left ventricular fibrosis, but the nature of this relationship has not been quantified.This study is a novel systematic characterization of changes in active electrical properties and fibrotic remodelling during progression of hypertensive heart disease in a well‐established animal disease model.Arrhythmic risk is predicted by several left ventricular measures, in particular fibrosis quantity and structure, and epicardial action potential duration dispersion.Age alone is not a good predictor of risk.An improved understanding of links between arrhythmic risk and fibrotic architectures in progressive hypertensive heart disease aids better interpretation of late gadolinium‐enhanced cardiac magnetic resonance imaging and electrical mapping signals.

Abstract P2091: Age-dependent Remodeling Of The Perinexus Modulates The Phenotype And Arrhythmic Risk Of Long Qt Syndrome Type 3

Background: The Long-QT Syndrome Type 3 (LQT3) is characterized by action potential duration (APD) prolongation due to cardiac voltage-gated sodium channel (Nav1.5) gain of function. Arrhythmia risk increases with age in LQT3 patients, but it is unknown whether this is due to progressive APD prolongation. Recent studies have shown that the LQT3 phenotype can be exacerbated by widening the perinexus, an intercalated disc nanodomain adjacent to gap junctions. However, it remains unclear if the perinexus mediates the LQT3 phenotype during aging. Objective: To investigate the role of the perinexus during aging in LQT3. Methods: Ventricular epicardial APD was obtained by optically mapping Langendorff-perfused adult (3-month) and aged (15-month) guinea pig hearts. LQT3 phenotype was induced with anemone toxin (ATXII), and the perinexus was widened with an intercalated disc adhesion inhibitor (βadp1). Perinexal width (Wp) was quantified by transmission electron microscopy. Susceptibility to arrhythmia was assessed with burst pacing in the absence and presence of βadp1 and evaluated from electrocardiograms (ECG). Results: APD was not different between adult and aged hearts at baseline, with ATXII, or ATXII+βadp1. Wp was significantly wider in adult LQT3 hearts either without or with βadp1 relative to aged hearts (without βadp1: 29.6±2.5 vs 22.9±3.8 nm; with βadp1: 38.3±1.7 vs 29.9±1.2 nm). These data suggest the perinexus narrows with aging and attenuates previously reported cellular APD prolongation such that there is no difference in whole-heart, epicardial APD between adult and aged LQT3 hearts. However, ECG Tpeak-Tend interval was not prolonged in adult, but significantly prolonged in aged hearts after treatment with ATXII+βadp1 (18.7±5.6 vs 35.5±11.5 ms), suggesting that βadp1 increases transmural dispersion of repolarization during aging in LQT3. Furthermore, ATXII+βadp1 did not induce arrhythmias in adult (0/6) but was arrhythmogenic in aged hearts (3/5). Conclusions: Perinexal narrowing associated with aging attenuates APD prolongation and reduces arrhythmic risk in aged LQT3 hearts. Loss of perinexal adhesion in aged LQT3 hearts results in significant and heterogeneous APD prolongation and increased risk of arrhythmia.

B-PO04-012 ACUTE TESTOSTERONE ADMINISTRATION DOES NOT SHORTEN ACTION POTENTIAL DURATION IN LANGENDORFF PERFUSED RABBIT VENTRICLES

A previous study showed that testosterone (T) rapidly shortens the action potential duration (APD) with EC50=2.1 to 8.7 nmol/L in guinea pig ventricular myocytes due to non-transcriptional enhancement of IKs and suppression of ICa,L. These findings have not been reproduced in intact rabbit ventricles. To test the hypothesis that T and dihydroxytestosterone (D) acutely shorten epicardial APD in rabbit ventricles. We performed optical mapping studies in 6 female and 7 male Langendorff-perfused rabbit hearts. Blebbistatin (10-20 μM) was used to suppress motion. Right atrium was paced during mapping. After baseline recording, mapping was done with T (N=10) or D (N=3). In T group, either high (3 μM) or low (1 to 100 nM) doses were applied to 5 and 4 hearts respectively, and 1 received both low and high doses. In D group, 1 received only low (3 to 100 nM), and 2 received low and then high dose (1 μM). In one heart we determined the QT interval recorded with widely spaced bipolar electrodes by using Tyrode’s solution only, without optical dye (RH237) or blebbistatin. At pacing cycle length (PCL) 350 ms, 300 ms, and 250 ms, there were no significant differences of APD (ms) at baseline vs high dose T (182 ± 15 vs 186 ± 5, p=0.73; 160 ± 24 vs 159 ± 20, p=0.95, 152 ± 23 vs 149 ± 15 ms, p=0.71, respectively). High-dose D tends to lengthen APD (ms) in 2 hearts at PCL 350 ms (161 vs 179 and 143 vs 153), 300 ms (152 vs 167 and 139 vs 148), and 250 ms (139 vs 158 and 129 vs 137). APDs (ms) at baseline vs low-dose T were (144 ± 9 vs 147 ± 8 at 1nM, p=0.23 and 150 ± 8 at 10 nM, p=0.05). APDs at baseline vs low dose D were 146 ± 9 vs 153 ± 10 at 3 nM (p=0.34), 162 ± 15 at 10 nM (p=0.34), and 160 ± 11 at 100 nM (p=0.19) at PCL 300 ms. APD significantly lengthened after 100 nM of T (154 ± 12 ms at 100 nM, p=0.01). In one rabbit heart without RH237 or blebbistatin, QT interval (ms) at PCL 280 ms did not shorten after T (170 at baseline, 172 at 10 nM, and 176 at 100 nM). When separately analyzed, neither male nor female hearts showed APD shortening with T or D. T and D do not shorten and may lengthen ventricular repolarization in normal male or female rabbit hearts. These findings do not support non-transcriptional regulation of cardiac ion currents by T or D to shorten APD.

ECG markers of local but not global increase in dispersion of ventricular repolarization (simulation study)

BackgroundAn increase in local dispersion of repolarization (DOR) may contribute more to arrhythmogenesis as compared to changes of global DOR. The aim of this simulation study was to find ECG markers of local increase in DOR in conditions where global DOR remains normal. MethodsIn the framework of van Oosterom and Oostendorp ECGSIM model, the local DOR was increased in 10 different ventricular locations by (1) action potential duration (APD) shortening/lengthening both on epi- and endocardium, (2) epicardial APD shortening, and (3) endocardial APD shortening. The simulation cases where the increase in local DOR was accompanied by increase in global DOR were excluded from consideration. T-wave parameters were analyzed in the simulated precordial and anatomically ordered limb leads. ResultsThe increase in local DOR resulted in increased lead-to‑lead differences in Tpeak and Tend instants in 28 out of 32 simulated scenarios, and in an increased dispersion of Tpeak-Tend interval throughout 12 standard leads in 8 out of 32 simulated scenarios. In all simulations, the global DOR measured as a difference between earliest and latest repolarization times and standard APD deviation was the same. ConclusionsThe local increase in DOR was expressed in increased lead-to‑lead differences in Tpeak and Tend instants between adjacent anatomically ordered standard leads (aVL, I, aVR(−), II, aVF, III, and V1-V6), even if global DOR, Tpeak-Tend interval and Tpeak-Tend dispersion were within a normal range.

P2564Levosimendan shortens action potential duration, decreases alternans threshold and prevents ventricular arrhythmia during therapeutic hypothermia in isolated rabbit hearts

Abstract Background Therapeutic hypothermia (TH) increases the susceptibility to ventricular arrhythmias (VA) by prolonging action potential duration (APD) and facilitating arrhythmogenic spatially discordant alternans (SDA). The calcium sensitizer levosimendan has been reported to shorten APD by enhancing ATP-sensitive K current. We hypothesize that levosimendan might shorten the already prolonged APD during TH, decreasing SDA threshold, and prevent the occurrence of VA. Methods Langendorff-perfused isolated rabbit hearts were subjected to 15-min TH (30°C) followed by 30-min treatment with levosimendan (0.5 μM, n=9) or vehicle (n=8). Using an optical mapping system, epicardial APD was evaluated by S1 pacing. SDA threshold was defined as the longest pacing cycle length (PCL) that induced SDA phenomenon. Ventricular fibrillation (VF) inducibility was evaluated by burst pacing for 30 s using the shortest PCL that achieved 1:1 ventricular capture. Results Levosimendan shortened the ventricular APD (at PCL 300 ms, from 229±9 ms to 211±18 ms, p=0.02) and decrease the SDA threshold (from 327±88 ms to 311±68 ms, p=0.001) during TH. The VF inducibility was decreased by levosimendan from 39±30% at 30°C to 14±12% after levosimendan infusion. In control hearts, the APD (p=0.75), SDA threshold (p=ns) and VF inducibility (p=0.12) were not changed by vehicle during TH. Conclusions Levosimendan protects the hearts against VA during TH by shortening APD and decreasing SDA threshold. Enhancing ATP-sensitive K current with levosimendan might be a novel approach to prevent VA during TH.

Effects of antiarrhythmics on the electrical restitution in perfused guinea-pig heart are critically determined by the applied cardiac pacing protocol.

What is the central question of this study? Are modifications in the restitution of ventricular action potential duration induced by antiarrhythmic drugs the same when assessed with premature extrastimulus application at variable coupling intervals (the standard stimulation protocol) and with steady state pacing at variable rates (the dynamic stimulation protocol)? What is the main finding and its importance? With class I and class III antiarrhythmics, the effects on electrical restitution determined with the standard stimulation protocol dissociate from those obtained during dynamic pacing. These findings indicate a limited value of the electrical restitution assessments based on extrasystolic stimulations alone, as performed in the clinical studies, in estimating the outcomes of antiarrhythmic drug therapies. A steep slope of the ventricular action potential duration (APD) to diastolic interval (DI) relationships (the electrical restitution) can precipitate tachyarrhythmia, whereas a flattened slope is antiarrhythmic. The derangements in APD restitution responsible for transition of tachycardia to ventricular fibrillation can be assessed with cardiac pacing at progressively increasing rates (the dynamic stimulation protocol). Nevertheless, this method is not used clinically owing to the risk of inducing myocardial ischaemia. Instead, the restitution kinetics is determined with a premature extrastimulus application at variable coupling intervals (the standard stimulation protocol). Whether the two protocols are equivalent in estimating antiarrhythmic drug effects is uncertain. In this study, dofetilide and quinidine, the agents blocking repolarizing K+ currents, increased epicardial APD in perfused guinea-pig hearts, with effects being greater at long vs. short DIs. These changes were more pronounced during dynamic pacing compared to premature extrastimulations. Accordingly, although both agents markedly steepened the dynamic restitution, there was only a marginal increase in the standard restitution slope with dofetilide, and no effect with quinidine. Lidocaine and mexiletine, selective Na+ channel blockers, prolonged the effective refractory period without changing APD, and increased the minimum DI that enabled ventricular capture during extrastimulations. No change in the minimum DI was noted during dynamic pacing. Consequently, although lidocaine and mexiletine reduced the standard restitution slope, they failed to flatten the dynamic restitution. Overall, these findings imply a limited value of the electrical restitution assessments with premature extrastimulations alone in discriminating arrhythmic vs. antiarrhythmic changes during drug therapies.

Role of the Purkinje-Muscle Junction on the Ventricular Repolarization Heterogeneity in the Healthy and Ischemic Ovine Ventricular Myocardium.

Alteration of action potential duration (APD) heterogeneity contributes to arrhythmogenesis. Purkinje-muscle junctions (PMJs) present differential electrophysiological properties including longer APD. The goal of this study was to determine if Purkinje-related or myocardial focal activation modulates ventricular repolarization differentially in healthy and ischemic myocardium. Simultaneous epicardial (EPI) and endocardial (ENDO) optical mapping was performed on sheep left ventricular (LV) wedges with intact free-running Purkinje network (N = 7). Preparations were paced on either ENDO or EPI surfaces, or the free-running Purkinje fibers (PFs), mimicking normal activation. EPI and ENDO APDs were assessed for each pacing configuration, before and after (7 min) of the onset of no-flow ischemia. Experiments were supported by simulations. In control conditions, maximal APD was found at endocardial PMJ sites. We observed a significant transmural APD gradient for PF pacing with PMJ APD = 347 ± 41 ms and EPI APD = 273 ± 36 ms (p < 0.001). A similar transmural gradient was observed when pacing ENDO (49 ± 31 ms; p = 0.005). However, the gradient was reduced when pacing EPI (37 ± 20 ms; p = 0.005). Global dispersion of repolarization was the most pronounced for EPI pacing. In ischemia, both ENDO and EPI APD were reduced (p = 0.005) and the transmural APD gradient (109 ± 55 ms) was increased when pacing ENDO compared to control condition or when pacing EPI (p < 0.05). APD maxima remained localized at functional PMJs during ischemia. Local repolarization dispersion was significantly higher at the PMJ than at other sites. The results were consistent with simulations. We found that the activation sequence modulates repolarization heterogeneity in the ischemic sheep LV. PMJs remain active following ischemia and exert significant influence on local repolarization patterns.

In Silico QT and APD Prolongation Assay for Early Screening of Drug-Induced Proarrhythmic Risk.

Drug-induced proarrhythmicity is a major concern for regulators and pharmaceutical companies. For novel drug candidates, the standard assessment involves the evaluation of the potassium hERG channels block and the in vivo prolongation of the QT interval. However, this method is known to be too restrictive and to stop the development of potentially valuable therapeutic drugs. The aim of this work is to create an in silico tool for early detection of drug-induced proarrhythmic risk. The system is based on simulations of how different compounds affect the action potential duration (APD) of isolated endocardial, midmyocardial, and epicardial cells as well as the QT prolongation in a virtual tissue. Multiple channel-drug interactions and state-of-the-art human ventricular action potential models ( O'Hara , T. , PLos Comput. Biol. 2011 , 7 , e1002061 ) were used in our simulations. Specifically, 206.766 cellular and 7072 tissue simulations were performed by blocking the slow and the fast components of the delayed rectifier current ( IKs and IKr, respectively) and the L-type calcium current ( ICaL) at different levels. The performance of our system was validated by classifying the proarrhythmic risk of 84 compounds, 40 of which present torsadogenic properties. On the basis of these results, we propose the use of a new index (Tx) for discriminating torsadogenic compounds, defined as the ratio of the drug concentrations producing 10% prolongation of the cellular endocardial, midmyocardial, and epicardial APDs and the QT interval, over the maximum effective free therapeutic plasma concentration (EFTPC). Our results show that the Tx index outperforms standard methods for early identification of torsadogenic compounds. Indeed, for the analyzed compounds, the Tx tests accuracy was in the range of 87-88% compared with a 73% accuracy of the hERG IC50 based test.

Anti-arrhythmic effects of hypercalcemia in hyperkalemic, Langendorff-perfused mouse hearts.

The present study examined the ventricular arrhythmic and electrophysiological properties during hyperkalemia (6.3 mM [K+] vs. 4 mM in normokalemia) and anti-arrhythmic effects of hypercalcemia (2.2 mM [Ca2+]) in Langendorff-perfused mouse hearts. Monophasic action potential recordings were obtained from the left ventricle during right ventricular pacing. Hyperkalemia increased the proportion of hearts showing provoked ventricular tachycardia (VT) from 0 to 6 of 7 hearts during programmed electrical stimulation (Fisher's exact test, P<0.05). It shortened the epicardial action potential durations (APDx) at 90, 70, 50 and 30% repolarization and ventricular effective refractory periods (VERPs) (analysis of variance, P<0.05) without altering activation latencies. Endocardial APDx and VERPs were unaltered. Consequently, ∆APDx (endocardial APDx-epicardial APDx) was increased, VERP/latency ratio was decreased and critical intervals for reexcitation (APD90-VERP) were unchanged. Hypercalcemia treatment exerted anti-arrhythmic effects during hyperkalemia, reducing the proportion of hearts showing VT to 1 of 7 hearts. It increased epicardial VERPs without further altering the remaining parameters, returning VERP/latency ratio to normokalemic values and also decreased the critical intervals. In conclusion, hyperkalemia exerted pro-arrhythmic effects by shortening APDs and VERPs. Hypercalcemia exerted anti-arrhythmic effects by reversing VERP changes, which scaled the VERP/latency ratio and critical intervals.

The Role of Transmural Repolarization Gradient in the Inversion of Cardiac Electric Field: Model Study of ECG in Hypothermia.

The changes in ventricular repolarization gradients lead to significant alterations of the electrocardiographic body surface T waves up to the T wave inversion. However, the contribution of a specific gradient remains to be elucidated. The objective of the present investigation was to study the role of the transmural repolarization gradient in the inversion of the body surface T wave with a mathematical model of the hypothermia-induced changes of ventricular repolarization. By means of mathematical simulation, we set the hypothermic action potential duration (APD) distribution on the rabbit ventricular epicardium as it was previously experimentally documented. Then the parameters of the body surface potential distribution were tested with the introduction of different scenarios of the endocardial and epicardial APD behavior in hypothermia resulting in the unchanged, reversed or enlarged transmural repolarization gradient. The reversal of epicardial repolarization gradients (apicobasal, anterior-posterior and interventricular) caused the inversion of the T waves regardless of the direction of the transmural repolarization gradient. However, the most realistic body surface potentials were obtained when the endocardial APDs were not changed under hypothermia while the epicardial APDs prolonged. This produced the reversed and increased transmural repolarization gradient in absolute magnitude. The body surface potentials simulated under the unchanged transmural gradient were reduced in comparison to those simulated under the reversed transmural gradient. The simulations demonstrated that the transmural repolarization gradient did not play a crucial role in the cardiac electric field inversion under hypothermia, but its magnitude and direction contribute to the T wave amplitude.

Flecainide attenuates rate adaptation of ventricular repolarization in guinea-pig heart

Objectives: Flecainide is class Ic antiarrhythmic agent that was found to increase the risk of sudden cardiac death. Arrhythmic responses to flecainide could be precipitated by exercise, suggesting a role played by inappropriate rate adaptation of ventricular repolarization. This study therefore examined flecainide effect on adaptation of the QT interval and ventricular action potential duration (APD) to abrupt reductions of the cardiac cycle length. Design: ECG and ventricular epicardial and endocardial monophasic APD were recorded in isolated, perfused guinea-pig heart preparations upon a sustained cardiac acceleration (rapid pacing for 30 s), and following a single perturbation of the cycle length evoked by extrasystolic stimulation. Results: Sustained increase in heart rate was associated with progressive bi-exponential shortening of the QT interval and APD. Flecainide prolonged ventricular repolarization, delayed its rate adaptation, and decreased the amplitude of QT interval and APD shortening upon rapid cardiac pacing. During extrasystolic stimulation, flecainide attenuated APD shortening in premature ventricular beats, with effect being greater upon using a longer basic drive cycle length (S1-S1=550 ms versus S1-S1=300 ms). Conclusions: Flecainide-induced arrhythmia may be partly accounted for by attenuated adaptation of ventricular repolarization to sudden changes in cardiac cycle length provoked by transient tachycardia or ectopic beats.

0244: Pro-arrhythmic ventricular remodeling in a porcine model of repaired tetralogy of Fallot

Ventricular arrhythmias are frequent in patients with repaired tetralogy of Fallot but their underlying mechanisms remain unclear. In this study, ventricular electrical and structural remodelling was assessed in an animal model that mimics postoperative tetralogy of Fallot. Piglets underwent a tetralogy of Fallot repair-like surgery (rTOF N=6) or were sham-operated (Sham N=5). Following cardiac function assessment in vivo by MRI 3-4 months after surgery, pigs were euthanized and their hearts rapidly excised. Electrophysiological properties of right (RV) and left ventricles (LV) were obtained by optical mapping. Fibrosis was assessed histologically. RV dysfunction was evident while LV function remained unaltered in rTOF pigs. LV action potential duration (APD) was significantly longer on the epicardium (Sham 280±50ms; rTOF 390±76ms) and endocardium (Sham 301±20ms; rTOF 403±34ms) of rTOF animals (P<0.05). RV epicardial and endocardial APD were not different between rTOF and Sham RVs. LV conduction velocity (CV) was significantly reduced in the longitudinal direction in rTOF pigs (65.37 ±6.76 cm/s vs 51.27±0.57 cm/s; P<0.05) but remained unchanged in the transverse direction compared to Sham animals (27.34±3.08 cm/s vs 30.19±1.97 cm/s). In the RV both longitudinal and transverse CVs were significantly in rTOF compared to Sham (P<0.05). An elevated collagen content was found in both the LV and RV of rTOF pigs (P<0.05). A trend for a lateralization of Connexin43 was found in the LV of rTOFs which, together with the increased fibrosis, may account for the reduced longitudinal conduction velocity in this ventricle. Altogether, these findings highlight the presence of an arrhytmic substrate in the ventricles of rTOF pigs. Interestingly, the remodeling in LV is reminiscent to that observed in LV failure, despite no functional alterations, and is likely to contribute to ventricular arrhythmias in patients with rTOF.

Oscillatory behavior of ventricular action potential duration in heart failure patients at respiratory rate and low frequency.

Oscillations of arterial pressure occur spontaneously at a frequency of approximately 0.1 Hz coupled with synchronous oscillations of sympathetic nerve activity (“Mayer waves”). This study investigated the extent to which corresponding oscillations may occur in ventricular action potential duration (APD). Fourteen ambulatory (outpatient) heart failure patients with biventricular pacing devices were studied while seated upright watching movie clips to maintain arousal. Activation recovery intervals (ARI) as a measure of ventricular APD were obtained from unipolar electrograms recorded from the LV epicardial pacing lead during steady state RV pacing from the device. Arterial blood pressure was measured non-invasively (Finapress) and respiration monitored. Oscillations were quantified using time frequency and coherence analysis. Oscillatory behavior of ARI at the respiratory frequency was observed in all subjects. The magnitude of the ARI variation ranged from 2.2 to 6.9 ms (mean 5.0 ms). Coherence analysis showed a correlation with respiratory oscillation for an average of 43% of the recording time at a significance level of p < 0.05. Oscillations in systolic blood pressure in the Mayer wave frequency range were observed in all subjects for whom blood pressure was recorded (n = 13). ARI oscillation in the Mayer wave frequency range was observed in 6/13 subjects (46%) over a range of 2.9 to 9.2 ms. Coherence with Mayer waves at the p < 0.05 significance level was present for an average of 29% of the recording time. In ambulatory patients with heart failure during enhanced mental arousal, left ventricular epicardial APD (ARI) oscillated at the respiratory frequency (approximately 0.25 Hz). In 6 patients (46%) APD oscillated at the slower Mayer wave frequency (approximately 0.1 Hz). These findings may be important in understanding sympathetic activity-related arrhythmogenesis.

0328: Electrotonic modulation of the transmural action potential duration heterogeneity in sheep ventricles

Transmural APD heterogeneity is thought to play an important role in homogeneous repolarization of the ventricles. Abnormal heterogeneity of action potential duration (APD) in ventricles is known to contribute to arrhythmogenesis. It has been shown to be modulated by electrotonic influences in small species, yet this finding remains to be confirmed in large mammalians. The goal of the present study was to investigate APD differences between epi- and endocardium depending on the pacing location and after a reduction in intercellular coupling in sheep ventricles. Optical mapping experiments were performed in coronary-perfused wedge preparations from sheep left (LV; N=8) or right ventricles (RV; N=3). The wedges were paced at 2Hz on either the endo- or epicardial surface. We also investigated the effect of carbenoxolone (50 μM) in the RV experiments. In the LV experiments, we observed a significant APD difference between endo- (306.45±18.29ms) and epicardium (276.05±9.87ms) when pacing the endocardium (P<0.01). However, this transmural heterogeneity was lost when pacing the epicardium, with endocardial APDs of 284.10±25.67 ms vs 289.8±36.24 ms at the epicardium. A similar observation was made in the RV with a significant increase in epicardial APD when pacing the epicardium (230.17±11.32 ms) vs the endocardium (217.73±19.50 ms) (P<0.01). Upon perfusion with carbenoxolone epicardial APDs significantly increased by 10% (P<0.01) and the difference in epicardial APD between epicardial and endocardial pacing was no longer significant. In conclusion, we find that the epi- and endocardial APDs are modulated by the activation sequence. Specifically, we observe a significant increase in epicardial vs endocardial APD when pacing the epicardium in both the RV and LV and an associated decrease in transmural APD heterogeneity. This effect was abolished upon perfusion with carbenoxolone indicating a role for electronic currents in modulating transmural APD heterogeneity.

0354: Pro-arrhythmic ventricular remodeling in a porcine model of repaired tetralogy of Fallot

Ventricular arrhythmias are frequent in patients with repaired tetralogy of Fallot but their underlying mechanisms remain unclear. In this study, ventricular electrical and structural remodelling was assessed in an animal model that mimics postoperative tetralogy of Fallot. Piglets underwent a tetralogy of Fallot repair-like surgery (rTOF N=6) or were sham-operated (Sham N=5). Following cardiac function assessment in vivo by MRI 3–4 months after surgery, pigs were euthanized and their hearts rapidly excised. Electrophysiological properties of right (RV) and left ventricles (LV) were obtained by optical mapping. Fibrosis was assessed histologically. RV dysfunction was evident while LV function remained unaltered in rTOF pigs. LV action potential duration (APD) was significantly longer on the epicardium (Sham 280±50 ms; rTOF 390±76 ms) and endocardium (Sham 301±20 ms; rTOF 403±34 ms) of rTOF animals (P<0.05). RV epicardial and endocardial APD were not different between rTOF and Sham RVs. LV conduction velocity (CV) was significantly reduced in the longitudinal direction in rTOF pigs (65.37±6.76 cm/s vs 51.27±0.57 cm/s; P<0.05) but remained unchanged in the transverse direction compared to Sham animals (27.34±3.08 cm/s vs 30.19±1.97 cm/s). In the RV both longitudinal and transverse CVs were significantly decreased in rTOF compared to Sham (P<0.05). An elevated collagen content was found in both the LV and RV of rTOF pigs (P<0.05). A trend for a lateralization of Connexin43 was found in the LV of rTOFs which, together with the increased fibrosis, may account for the reduced longitudinal conduction velocity in this ventricle. Altogether, these findings highlight the presence of an arrhytmic substrate in the ventricles of rTOF pigs. Interestingly, the remodeling in LV is reminiscent to that observed in LV failure, despite no functional alterations, and is likely to contribute to ventricular arrhythmias in patients with rTOF.

急性心筋虚血時の心室不整脈の発生機序：心筋間質K&lt;SUP&gt;+&lt;/SUP&gt;濃度，心室内伝導時間，心室筋活動電位持続時間よりの検討

急性心筋虚血時の心室不整脈の発生機序：心筋間質K&lt;SUP&gt;+&lt;/SUP&gt;濃度，心室内伝導時間，心室筋活動電位持続時間よりの検討

Procainamide and lidocaine produce dissimilar changes in ventricular repolarization and arrhythmogenicity in guinea‐pig

Procainamide is class Ia Na(+) channel blocker that may prolong ventricular repolarization secondary to inhibition of IK r , the rapid component of the delayed rectifier K(+) current. In contrast to selective IN a blockers such as lidocaine, procainamide was shown to produce arrhythmogenic effects in the clinical setting. This study examined whether pro-arrhythmic responses to procainamide may be accounted for by drug-induced repolarization abnormalities including impaired electrical restitution kinetics, spatial gradients in action potential duration (APD), and activation-to-repolarization coupling. In perfused guinea-pig hearts, procainamide was found to prolong the QT interval on ECG and left ventricular (LV) epicardial monophasic APD, increased the maximum slope of electrical restitution, enhanced transepicardial APD variability, and eliminated the inverse correlation between the local APD and activation time values determined at distinct epicardial recording sites prior to drug infusion. In contrast, lidocaine had no effect on electrical restitution, the degree of transepicardial repolarization heterogeneities, and activation-to-repolarization coupling. Spontaneous episodes of monomorphic ventricular tachycardia were observed in 57% of procainamide-treated heart preparations. No arrhythmia was induced by lidocaine. In summary, this study suggests that abnormal changes in repolarization may contribute to pro-arrhythmic effects of procainamide.