Sudden cardiac death in dogs with remodeled hearts is associated with larger beat-to-beat variability of repolarization

Increased proarrhythmia in dogs with chronic AV block (AVB) has been explained by ventricular remodeling causing a decrease in repolarization reserve. Beat-to-beat variability of repolarization (BVR) has been suggested to reflect repolarization reserve, in which high variability represents diminished reserve and larger propensity for repolarization-dependent ventricular arrhythmia. A subset of chronic AVB dogs (10%) suffers sudden cardiac death (SCD). With the assumption that repolarization defects constitute a potentially lethal proarrhythmic substrate, we hypothesized that BVR in SCD dogs are larger than in matched control chronic AVB dogs. From a population of 200 chronic AVB dogs, initially two groups were chosen retrospectively: 8 dogs that died suddenly (SCD) and 8 control dogs. Control dogs had a longer lifespan after AVB (10 to 18 weeks) than SCD dogs (5 to 10 weeks). All dogs had undergone electrophysiological testing under anesthesia where ECG, left and right ventricular endocardial monophasic action potentials (MAP) were recorded. BVR was assessed from 30 consecutive beats, illustrated by Poincare plots and was the only parameter discriminating between SCD and control group. All other electrophysiological parameters (RR, QT and MAP durations) were comparable for the two groups. Extending the number of animals and groups confirmed a larger BVR in the SCD group (SCD: 5.1 +/- 2.7; n = 11 versus control: 2.5 +/- 0.4 ms; n = 61; P < 0.05) and showed reverse-use dependence of BVR. In comparison, dogs with acute AVB had low variability (1.3 +/- 0.3 ms; n = 9; P < 0.05 versus chronic AVB). Cardiac electrical remodeling after AVB is associated with an increase in beat-to-beat variability of repolarization. Chronic AVB dogs displaying further elevated variability of repolarization are prone to arrhythmia-related SCD.

Molecular and electrical characterization of the canine cardiac ventricular septum

Electrophysiological heterogeneity in the ventricular septum (VS) has been poorly addressed. In this study we investigated the electrophysiological and molecular composition of the VS in control sinus rhythm (SR) and chronic, complete atrio-ventricular block (CAVB) dogs. In the latter model, we anticipated that the increased inter-ventricular differences in action potential duration (APD; LV >RV) would accentuate the intrinsic heterogeneous composition of the VS. Steady-state mRNA levels of 10 important cardiac ion channels subunits as well as action potential (AP) characteristics (APD95, phase 1 amplitude (P1A), resting membrane potential) were measured in both sides of the VS excluding a small mid-myocardial strip (right: RVS, left: LVS). In SR, differences in steady-state mRNA between the two septal layers were observed for KChIP2 (approximately fivefold, P <0.01) and KCNQ1 (approximately twofold, P <0.05) with significantly higher levels of steady-state mRNA in the RVS compared to LVS. Correspondingly, shorter APDs and lower P1As (more spike and dome) were found in RVS, although the AP differences were subtle. This transseptal expression of KChIP2 and KCNQ1 corresponded with the observed differential expression levels in the right ventricle (RV) and left ventricular (LV) free wall, respectively. Electrical remodeling due to CAVB was also observed in the VS as was shown by approximately twofold lower levels in KCND3, KCNH2 and KCNQ1 mRNA (P <0.05) in the LVS compared to SR, thereby creating new or eliminating existing transseptal gradients. In parallel to changes in steady-state mRNA, CAVB resulted in a loss of the spike and dome morphology and longer APD95 (P <0.05) in the LVS. It is concluded that similar to other regions in the cardiac ventricles, the canine VS is molecularly and electrically heterogeneous. In the CAVB dog, this septal heterogeneity becomes accentuated as a result of electrical remodeling.

Increased Short-Term Variability of Repolarization Predicts d -Sotalol–Induced Torsades de Pointes in Dogs

Background— Identification of patients at risk for drug-induced torsades de pointes arrhythmia (TdP) is difficult. Increased temporal lability of repolarization has been suggested as being valuable to predict proarrhythmia. The predictive value of different repolarization parameters, including beat-to-beat variability of repolarization (BVR), was compared in this serial investigation in dogs with chronic AV block.

Methods and Results— In anesthetized dogs with electrically remodeled hearts, the dose-dependent difference in drug-induced TdP ( d -sotalol, 2 and 4 mg/kg IV over 5 minutes, 25% and 75% TdP, respectively) could not be accounted for by prolongation of QT c (410±37 to 475±60 versus 415±47 to 484±52 ms, respectively). BVR was quantified by Poincaré plots at baseline and immediately before onset of d -sotalol–induced extrasystolic activity. TdP occurrence was associated with an increase in short-term variability (STV) of the left ventricular monophasic action potential duration (3.5±1.5 to 5.5±1.6 versus 3.0±0.7 to 8.6±3.8 ms, respectively), which was reversible when TdP was abolished by I K,ATP activation. The absence of TdP despite QT c prolongation after chronic amiodarone treatment could also be explained by an unchanged STV. In experiments with isolated ventricular myocytes, STV increased after I Kr block and was highest in cells that subsequently showed early afterdepolarizations.

Conclusions— Proarrhythmia is not related to differences in prolongation of repolarization but corresponds to BVR, here quantified as STV of the left ventricle. STV could be a new parameter to predict drug-induced TdP in patients.

Electrophysiological and proarrhythmic parameters in transmural canine left-ventricular needle biopsies

This study was designed to validate the use of small, transmural, left-ventricular biopsies in the dog for investigations of electrophysiological and proarrhythmic properties of the heart. This technique could facilitate pharmacological in vitro testing in remodelled hearts of both man and animal. Small, transmural, semi-cylindrical, left-ventricular biopsies from dogs with normal sinus rhythm (SR) were characterized electrophysiologically and compared with biopsies from electrically remodelled hearts from dogs with chronic, complete AV-block (CAVB). In at least five biopsy segments recordings were made to determine the action potential duration (APD), the transmural gradient of repolarization, the maximal transmural dispersion (deltatM(max)) and presence of early after-depolarizations (EADs) at different pacing cycle lengths (PCLs) in the absence and presence of a class-III agent, ibutilide (10(-6) M). The biopsies showed stable and normal AP characteristics, a conduction velocity of 0.22 +/- 0.05 m/s and normal frequency dependence of the APD. The location of the longest APD varied, thus creating transmural repolarization gradients with differing morphology. Ibutilide prolonged the APD, accentuated repolarization gradients and induced EADs. CAVB biopsies had significantly longer APDs, a larger dispersion of repolarization and showed more EADs in the presence of ibutilide than SR biopsies. We conclude that this biopsy technique provides coherent and valid transmural electrophysiological data in dogs under various conditions.

Tridimensional activation patterns of acquired torsade-de-pointes-tachycardias in dogs with chronic AV-block

BACKGROUND

Dogs with chronic AV block exposed to type-III antiarrhythmic agents develop polymorphic ventricular tachycardias (PVT). Controversy exists regarding PVT mechanism and underlying pathophysiology.

METHODS AND RESULTS

In dogs with acute (n = 10, AAVB) or chronic AV block (n = 14, CAVB, 62 +/- 5 days after AV-node ablation) 60 pins (12 mm long, 4 bipolar electrodes) were inserted into both ventricles. QT intervals and effective refractory periods (ERP) at 56 +/- 22 randomly selected sites (extrastimulus technique, 800 ms basic cycle length) were determined before and after Almokalant (0.34 micromol/kg). A multiplexer mapping system was used to reconstruct 3D activation patterns. The heart-to-body-weight index (HBWI) was obtained after the experiments. CAVB led to a significant increase in HBWI (11.3 +/- 1.5 vs. 9 +/- 1.2 g/kg BW, p < 0.001), and a significant increase in ERP (280 +/- 28 ms vs. 260 +/- 37 ms, p < 0.05) and QT interval (339 +/- 16 vs. 288 +/-12 ms, p < 0.05). Dispersion (DISP) of ERP was similar for AAVB and CAVB dogs. No AAVB dog, but 9 of 14 CAVB dogs developed PVTs in response to Almokalant. All PVTs originated from an endocardial focus. Consecutive beats continued to reveal centrifugal activation patterns in 8 of 10 episodes. In only 2 episodes was reentrant activation evident.

CONCLUSION

Myocardial hypertrophy associated with CAVB predisposes the canine heart to drug induced PVTs. This seems to be primarily linked to prolonged repolarization. PVTs in this model are not only initiated, but also perpetuated by a centrifugal spread of activation.

Assessing predictors of drug-induced torsade de pointes

Torsades de pointes (TdP) is a malignant polymorphic ventricular tachyarrhythmia that can be caused by drugs that induce electrophysiological changes. Although the number of drugs known to cause TdP has increased in recent years, there is no cell-based assay, in vitro heart preparation or animal model that predicts the potential of a drug to induce TdP in humans. Nevertheless, certain electrophysiological events are known to be associated with the development of TdP. For example, a drug that prolongs action potential duration, induces early afterdepolarizations and ectopic beats, and increases dispersion of ventricular repolarization is likely to cause TdP. By contrast, a drug that does not induce these changes is unlikely to cause TdP. The exact relationship between these electrophysiological events and the development of TdP has not been defined, but the potential of a drug to elicit these events might predict its pro-arrhythmic risk.

Genetic basis of drug-induced arrhythmias

Drug-induced torsade de pointes arrhythmia (TdP) is frequently seen in patients. This proarrhythmia is not restricted to anti-arrhythmics but includes a variety of drugs. A genetic predisposition is an attractive explanation for this clinical problem. In this review, we: 1) explain the arrhythmogenic mechanisms of TdP, 2) provide data for a genetic cause based upon mutations in the long QT or in cytochrome genes responsible for drug metabolism, and 3) present pathology-based electrical remodeling as an alternative explanation. It can be concluded that the current evidence for a genetic basis for drug-induced TdP is weak.

Electrophysiological safety of sertindole in dogs with normal and remodeled hearts

Inhibition of the potassium current IKr and QT prolongation are associated with drug-induced torsades de pointes arrhythmias (TdP) and sudden cardiac death. We investigated the cardiac electrophysiological effects of sertindole, an antipsychotic drug reported to prolong the QT interval in schizophrenic patients. In cell cultures, sertindole seemed to be a selective blocker of IHERG over other ion currents. For IHERG, the IC50 value was 64 +/- 7 nM, whereas ISCN5A, ICa,L, ICa,T, IKCNQ1/KCNE1, and IKv4.3 were blocked in the micromolar range. In canine ventricular myocytes, the IC50 value for IKr inhibition by sertindole was 107 +/- 21 nM. Action potentials in these cells prolonged in a reverse rate- and concentration-dependent manner at 10 to 300 nM sertindole. In vivo, sertindole was administered to anesthetized dogs at clinically relevant (0.05-0.20 mg/kg) and high doses (1.0-2.0 mg/kg) i.v. At 0.05 to 0.20 mg/kg sertindole (plasma concentrations 30-157 nM), QTc was prolonged by 1 to 5% in normal dogs and by 9 to 20% in dogs with remodeled hearts due to chronic atrioventricular block (CAVB). TdP was not induced at these doses in normal dogs or in CAVB dogs with reproducible induction of TdP by dofetilide in previous experiments. At 1.0 to 2.0 mg/kg sertindole (plasma concentrations 0.5-3.1 microM), QTc prolonged by 6 to 11% in normal dogs and by 22% in dofetilide-sensitive CAVB dogs. TdP occurred in three of five animals in the latter group. Thus, at high i.v. doses sertindole can pose a serious proarrhythmic risk when electrical remodeling of the ventricles is present. At clinically relevant doses, however, sertindole does not cause TdP in anesthetized dogs with normal or remodeled hearts.

Accumulation of slowly activating delayed rectifier potassium current (IKs) in canine ventricular myocytes

In guinea-pig ventricular myocytes, in which the deactivation of slowly activating delayed rectifier potassium current (IKs) is slow, IKs can be increased by rapid pacing as a result of incomplete deactivation and subsequent current accumulation. Whether accumulation of IKs occurs in dogs, in which the deactivation is much faster, is still unclear. In this study the conditions under which accumulation occurs in canine ventricular myocytes were studied with regard to its physiological relevance in controlling action potential duration (APD). At baseline, square pulse voltage clamp experiments revealed that the accumulation of canine IKs could occur, but only at rather short interpulse intervals (< 100 ms). With action potential (AP) clamp commands of constant duration (originally recorded at rate of 2 Hz), an accumulation was only found at interpulse intervals close to 0 ms. Transmembrane potential recordings with high-resistance microelectrodes revealed, however, that at the fastest stimulation rates with normally captured APs (5 Hz) the interpulse interval exceeded 50 ms. This suggested that no IKs accumulation occurs, which was supported by the lack of effect of an IKs blocker, HMR 1556 (500 nM), on APD. In the presence of the beta-adrenergic receptor agonist isoproterenol (isoprenaline, 100 nM) the accumulation with AP clamp commands of constant duration was much more pronounced and a significant accumulating current was found at a relevant interpulse interval of 100 ms. HMR 1556 prolonged APD, but this lengthening was reverse rate dependent. AP clamp experiments in a physiologically relevant setting (short, high rate APs delivered at a corresponding rate) revealed a limited accumulation of IKs in the presence of isoproterenol. In conclusion, a physiologically relevant accumulation of IKs was only observed in the presence of isoproterenol. Block of IKs, however, led to a reverse rate-dependent prolongation of APD indicating that IKs does not have a dominant role at short cycle lengths.

Asynchronous development of electrical remodeling and cardiac hypertrophy in the complete AV block dog

OBJECTIVE

Left ventricular hypertrophy has been associated with the prolongation of QT-time, and an increased risk of ventricular arrhythmias. The renin angiotensin system has been implicated in the development of ventricular hypertrophy. At 5 weeks complete AV block (CAVB) in the dog, there is: (1) biventricular hypertrophy associated with a transient activation of components of the renin angiotensin system, (2) increased APD, more pronounced in the left than in the right ventricle leading to spatial dispersion of repolarization, and (3) enhanced susceptibility to drug-induced torsade de pointes arrhythmias. To investigate whether these remodeling processes develop in parallel, time dependency was assessed in absence or presence of the AT1 receptor-blocker Irbesartan.

METHODS AND RESULTS

Dogs in sinus rhythm, 2 and 5 weeks CAVB were compared to dogs chronically treated with Irbesartan (30 mg/kg BID). Endocardial monophasic APD of left and right ventricle was measured and susceptibility to torsade de pointes was tested by infusing Dofetilide (0.025 mg/kg/5'). Hypertrophy was determined by relating heart-to-body weight at sacrifice. Left ventricular APD had increased more than right ventricular APD at 2 and 5 weeks CAVB, leading to an increase in spatial dispersion. At that time torsade de pointes were evocable in the majority of the dogs. Hypertrophy had only developed completely at 5 weeks CAVB. Irbesartan had no effect on electrical and structural parameters or on arrhythmogenicity.

CONCLUSIONS

In the CAVB dog ventricular hypertrophy is not a prerequisite for electrical remodeling or drug-induced torsade de pointes, and the AT1-receptor has no dominant role in the completion of these remodeling processes.

Probing the contribution of IKs to canine ventricular repolarization: key role for beta-adrenergic receptor stimulation

BACKGROUND

In large mammals and humans, the contribution of IKs to ventricular repolarization is still incompletely understood.

METHODS AND RESULTS

In vivo and cellular electrophysiological experiments were conducted to study IKs in canine ventricular repolarization. In conscious dogs, administration of the selective IKs blocker HMR 1556 (3, 10, or 30 mg/kg PO) caused substantial dose-dependent QT prolongations with broad-based T waves. In isolated ventricular myocytes under baseline conditions, however, IKs block (chromanols HMR 1556 and 293B) did not significantly prolong action potential duration (APD) at fast or slow steady-state pacing rates. This was because of the limited activation of IKs in the voltage and time domains of the AP, although at seconds-long depolarizations, the current was substantial. Isoproterenol increased and accelerated IKs activation to promote APD95 shortening. This shortening was importantly reversed by HMR 1556 and 293B. Quantitatively similar effects were obtained in ventricular-tissue preparations. Finally, when cellular repolarization was impaired by IKr block, IKs block exaggerated repolarization instability with further prolongation of APD.

CONCLUSIONS

Ventricular repolarization in conscious dogs is importantly dependent on IKs. IKs function becomes prominent during beta-adrenergic receptor stimulation, when it promotes AP shortening by increased activation, and during IKr block, when it limits repolarization instability by time-dependent activation. Unstimulated IKs does not contribute to cellular APD at baseline. These data highlight the importance of the synergism between an intact basal IKs and the sympathetic nervous system in vivo.

Absence of reverse electrical remodeling during regression of volume overload hypertrophy in canine ventricles

OBJECTIVE

Ventricular hypertrophy predisposes for cardiac arrhythmias, presumably due to prolongation of repolarization (electrical remodeling). The temporal relation between the development of hypertrophy and electrical remodeling, as well as their reversibility upon restoration of normal load, however, are poorly understood. This was investigated in the present study using volume overload hypertrophy induced by atrio-ventricular (AV) block and normalization of load by pacing.

METHODS

Dogs were subjected to either 16 weeks of AV-block (CAVB group, n=9) or 8 weeks of AV-block followed by 8 weeks of right ventricular (RV) pacing at physiological heart rate (CAVB+PACE group, n=9).

RESULTS

Left ventricular (LV) mass (2D-echocardiography) increased after 8 weeks of AV-block to approximately 30% above baseline and returned to 10+/-14% after 8 weeks of pacing. QT-time (surface ECG) also increased after AV-block. However, 8 weeks of pacing did not decrease QT and QTc-time (c=corrected for heart rate), neither during physiological pacing nor during temporary pacing at 100 beats/min. Lack of reverse electrical remodeling was confirmed by the absence of changes in LV and RV action potential duration (monophasic action potentials) at week 8 and 16.

CONCLUSIONS

In volume overload hypertrophy due to AV-block, structural and electrical remodeling develop in parallel but restoration of physiological heart rate causes dissociation between reverse structural remodeling and reverse electrical remodeling.

A novel mutation L619F in the cardiac Na+ channel SCN5A associated with long-QT syndrome (LQT3): a role for the I-II linker in inactivation gating

Congenital long QT syndrome type 3 (LQT3) is caused by mutations in the gene SCN5A encoding the alpha-subunit of the cardiac Na(+) channel (Nav1.5). Functional studies of SCN5A mutations in the linker between domains III and IV, and more recently the C-terminus, have been shown to alter inactivation gating. Here we report a novel LQT3 mutation, L619F (LF), located in the domain I-II linker. In an infant with prolonged QTc intervals, mutational analysis identified a heterozygous missense mutation (L619F) in the domain I-II linker of the cardiac Na(+) channel. Wild-type (WT) and mutant channels were studied by whole-cell patch-clamp analysis in transiently expressed HEK cells. LF channels increase maintained Na(+) current (0.79 pA/pF for LF; 0.26 pA/pF for WT) during prolonged depolarization. We found a +5.8mV shift in steady state inactivation in LF channels compared to WT (WT, V(1/2)=-64.0 mV; LF, V(1/2)=-58.2 mV). The positive shift of inactivation, without a corresponding shift in activation, increases the overlap window current in LF relative to WT (1.09 vs. 0.58 pA/pF), as measured using a positive voltage ramp protocol (-100 to +50 mV in 2s). The increase in window current, combined with an increase in non-inactivating Na(+) current, may act to prolong the AP plateau and is consistent with the disease phenotype observed in patients. Moreover, the defective inactivation imposed by the L619F mutation implies a role for the I-II linker in the Na(+) channel inactivation process.

Should class III drugs be initiated in hospital to prevent drug-induced torsade de pointes arrhythmias?

OBJECTIVES

In the US, the FDA requires in-hospital institution of class III drugs. This study retrospectively assessed whether these criteria, which differ markedly from the Dutch exclusion criteria, could predict sotalol-induced torsade de pointes arrhythmias (TdP).

METHOD

Oral sotalol effect in a control group (50 patients, 62±12 years, 23 men, 27 women) was compared with five patients developing TdP (75±5years, all women), using known and new (JTU area measured in lead V2) risk parameters. Paroxysmal atrial fibrillation was the most common indication for sotalol treatment.

RESULTS

At baseline the strict US regulations would have identified four of five TdP patients on the basis of individual K⁺ levels, creatinine clearance and QT_c. However, 7 of 49 controls would also have been excluded, although they did not develop documented TdP in the >2 years follow-up. Sotalol slightly increased QT_c (361±34 to 387±33ms) in controls, due to heart rate reduction. In the TdP group, sotalol dramatically increased QT_c (467±33 to 626±52 ms, +35%, p<0.05) accompanied by deep negative TU waves and an increased JTU area and all could be identified as risk patients.

CONCLUSION

Patients developing TdP on oral sotalol can be identified using the FDA risk criteria and hospitalisation may therefore be appropriate. A European prospective study is required to investigate the costs, sensitivity and specificity of this strategy.

Coordinated down-regulation of KCNQ1 and KCNE1 expression contributes to reduction of I(Ks) in canine hypertrophied hearts

OBJECTIVE

In animal models of hypertrophy, electrical remodeling giving rise to QT prolongation occurs rapidly and is associated with the development of torsade de pointes (TdP) arrhythmias and sudden death. Chronic AV block (CAVB)-induced hypertrophy in dogs has been associated with a reduction in the slow component (I(Ks)) of the delayed rectifier potassium current (I(K)), which contributes to a prolongation of ventricular repolarization, the development of an acquired form of long QT, and the substrate for triggered activity and TdP. The present study was designed to probe the molecular basis for the decrease in I(Ks) by studying the characteristics of KCNE1 and KCNQ1, the putative genes responsible for formation of the channel.

METHODS AND RESULTS

Using a combination of Northern blot, competitive multiplex PCR and immunoblot assays, we found that CAVB reduces KCNE1 and KCNQ1 RNA in the canine ventricles by 70 and 80%, respectively. Protein levels of KCNE1 and KCNQ1 were reduced by 60 and 50%, respectively. We also demonstrate at the molecular level the basis for inter-ventricular difference in I(Ks) density previously reported in hearts of normal dogs and show the basis for reduction of this difference in the CAVB dog.

CONCLUSIONS

Our results indicate that the CAVB-induced reduction in I(Ks) is due to a down-regulation of KCNE1 and KCNQ1 transcription. The data suggest that electrical remodeling of the cardiac ventricle during hypertrophy involves regulation of the gene expression through modulation of transcriptional and translational regulatory pathways. The reduction in KCNE1 and KCNQ1 expression increases the dependence of ventricular repolarization on the rapid component of I(K) and may potentiate the action of Class III antiarrhythmic agents.

Intracellular Na+ and altered Na+ transport mechanisms in cardiac hypertrophy and failure

Altered intracellular Na(+) ([Na(+)](i)) is a potentially important factor in the functional adaptation of the hypertrophied and failing heart. We review the currently reported changes in [Na(+)](i) and Na(+) transport in different models of cardiac hypertrophy and heart failure. Direct measurements are limited, but most of these indicate that there is a rise in [Na(+)](i), in particular in hypertrophy. In addition to these direct measurements, several studies report a rise in Na(+) influx or an upregulation of Na(+) influx transporters. The most extensive literature on Na(+) regulating pathways concerns the Na/K-ATPase. Total Na/K-ATPase activity decreases in most models of cardiac hypertrophy and failure, though few measurements were actually performed in intact cells. This decrease can been related to a selective reduction of high-affinity (for cardiac glycosides) Na/K pump alpha-isoforms, across many species and models, including human heart failure. We have used these data to predict changes of [Na(+)](i) in a simulation model, varying the contribution of total Na/K pump capacity and expression of isoforms with different Na(+)(i) affinities, and varying Na(+) influx. A rise in Na(+) in cardiac hypertrophy and failure may improve systolic contractile function, though at the cost of worsening of diastolic function and increased risk of ventricular arrhythmias. The benefit of further increasing [Na(+)](i,) e.g. with cardiac glycosides, is thus compromised. Future therapies may include selective isoform blockers, which could raise [Na(+)](i) in restricted subcellular compartments, drug associations that reduce the arrhythmic risk, or even drugs that lower [Na(+)](i) and thus interfere with the remodelling pathways.

Novel insights in the congenital long QT syndrome

BACKGROUND

The congenital long QT syndrome is a potentially fatal, inherited cardiac syndrome. Early diagnosis and preventive treatment are instrumental to prevent sudden cardiac death in patients with the congenital long QT syndrome.

PURPOSE

To review new insights in genetics and cellular electrophysiology, as well as the current understanding of the clinical diagnosis and treatment of the congenital long QT syndrome.

DATA SOURCES

Authors' personal databases and search of PubMed database from 1966 to 2001.

STUDY SELECTION

Experimental and clinical studies on the congenital long QT syndrome.

DATA EXTRACTION

Data from peer-reviewed studies were manually extracted, classified, and summarized.

DATA SYNTHESIS

The congenital long QT syndrome is characterized by abnormally prolonged ventricular repolarization, which predisposes patients to syncope, ventricular arrhythmias, and sudden cardiac death. The recent discovery of mutations in genes encoding ion channels has improved our understanding of the cellular origin of this condition. The congenital long QT syndrome may result from inherited defects in cardiac K+ and Na+ channels, which both result in prolongation of the ventricular action potential. The diagnosis is based on electrocardiographic and clinical criteria. Genetic screening of symptomatic patients or asymptomatic family members may identify patients at risk for life-threatening ventricular arrhythmias. beta-Blocking agents are the mainstay of treatment. Certain patients may also benefit from a pacemaker or implantable cardioverter defibrillator. Recent studies suggest that genotype-specific treatment of the congenital long QT syndrome will be feasible in the near future.

CONCLUSIONS

The congenital long QT syndrome is a potentially life-threatening condition caused by mutations in genes encoding cardiac ion channels. Better understanding of the mechanisms responsible for this condition will guide genotype-specific therapy in the near future.

Role of the Na/Ca exchanger in arrhythmias in compensated hypertrophy

Sudden, presumably arrhythmic, death is common in heart failure patients. Although total mortality is highest in end-stage failure, the fraction of sudden death in total mortality is higher in the early stages. In each of these stages various, not necessarily identical, ionic mechanisms may contribute to arrhythmogenesis. Dogs with chronic complete atrioventricular block (6-8 weeks) have an increased risk for arrhythmias and sudden death and have compensated biventricular hypertrophy. In this animal model, Ca(2+) release from the sarcoplasmic reticulum (SR) is not reduced. For low frequencies of stimulation, the SR Ca(2+) content is increased, related to a higher activity of the Na/Ca exchanger. Spontaneous Ca(2+) release induces inward Na/Ca exchange current, which can lead to delayed afterdepolarizations (DADs) triggering a new action potential. Such arrhythmogenic DADs and ectopic beats also can be observed in vivo during monophasic action potential recording. They appear after pacing protocols, and/or administration of ouabain, which result in contractile potentiation, suggestive of a enhanced sarcoplasmic reticulum Ca(2+) content. Other arrhythmogenic mechanisms related to increased dispersion of repolarization also can be identified in vivo. Downregulation of delayed K(+) currents is an important factor in prolongation of action potentials. In conclusion, in this animal model of compensated hypertrophy, Ca(2+) handling is different from end-stage heart failure. It is possible that arrhythmogenic mechanisms related to a higher Ca(2+) load contribute to the high incidence of sudden death in stages of compensated hypertrophy before overt heart failure. However, more than one ionic remodeling process is likely to be present, and different cellular mechanisms of arrhythmias can coexist.

The JT-area indicates dispersion of repolarization in dogs with atrioventricular block

Heterogeneity in cardiac repolarization (Delta APD) is known to be arrhythmic. In the dog model of chronic complete AV-block and acquired long QT syndrome, an increase in Delta MAPD (defined as left ventricular monophasic action potential duration (MAPD) minus right ventricular MAPD) is often associated with changes in T-wave morphology. The purpose of this study was to correlate known changes in Delta MAPD with the planimetric total area of the T-wave on the surface ECG (integral of J-T, mVx ms).

METHODS

The relationship between Delta MAPD and total area of the T-wave (i.e., JT-area) was assessed in four different protocols with different types of dispersion: (1) class III drugs followed by levcromakalim (n= 7), (2) LAD coronary artery occlusion and reperfusion (n = 6), (3) dronedarone i.v., an amiodarone like agent (n = 5) and (4) steady state pacing at cycle lengths of 1000 ms and 500 ms (n = 5).

RESULTS

Class III drugs increased Delta MAPD (55 +/- 40 ms to 120 +/- 50 ms(#), P<0.05), which was correlated (r = 0.74, P < 0.001) with JT-area (50 +/- 40 mV. ms to 95 +/- 35 mV x ms(#)). Ischemia increased both Delta MAPD (30 +/- 25 ms to 90 +/- 40 ms(#)) and JT-area (60 +/- 55 mV x ms to 75 +/- 50 mV x ms(#)). Both levcromakalim and reperfusion reversed these conditions. Dronedarone had no effect on Delta MAPD or on JT-area while a faster frequency reduced both Delta MAPD and JT-area.

CONCLUSION

Changes in dispersion of ventricular repolarization are reflected by alterations in JT-area. This non-invasive parameter may therefore be used to indicate changes in heterogeneity in ventricular repolarization.

Altered Na/Ca exchange activity in cardiac hypertrophy and heart failure: a new target for therapy?

Increased Na/Ca exchange (NCX) expression may be part of the genetic reprogramming in cardiac remodeling. In this review we address the following questions: (1) Is increased NCX activity a general feature of cardiac remodeling in hypertrophy and heart failure? (2) How does this contribute to the contractile and electrical phenotype of hypertrophy and heart failure? (3) Should be consider NCX a potential therapeutic target? From a review of the literature we found that NCX activity can be increased, unchanged, or even downregulated during cardiac remodeling. When NCX activity is increased, it can be considered compensatory for contractile function, but with negative side-effects, including an increased risk of arrhythmias. Changes in activity do not necessarily reflect changes in gene expression. Altered NCX activity can also be a consequence of changes in other Ca(2+) fluxes or in [Na(+)](i) homeostasis. The role of NCX in contractile alterations and arrhythmogenesis varies with the different stimuli or stages of cardiac remodeling. Pharmacological block of NCX in heart failure or hypertrophy may thus be useful, but most likely only in specific conditions, perhaps as part of a combined approach. Development of drugs that target only a specific mode of the exchanger may offer a further advantage.

Chronic amiodarone evokes no torsade de pointes arrhythmias despite QT lengthening in an animal model of acquired long-QT syndrome

BACKGROUND

Amiodarone is an effective antiarrhythmic drug rarely associated with torsade de pointes arrhythmias (TdP). The noniodinated compound dronedarone could resemble amiodarone and be devoid of the adverse effects. In the dog with chronic complete atrioventricular (AV) block (CAVB) and acquired long-QT syndrome, the electrophysiological and proarrhythmic properties of the drugs were compared after 4 weeks of oral treatment.

METHODS AND RESULTS

Amiodarone (n=7, 40 mg. kg(-1). d(-1)) and dronedarone (n=8, 20 mg/kg BID) were started at 6 weeks of CAVB (baseline). Six dogs served as controls. Surface ECGs and endocardially placed monophasic action potential catheters in the left (LV) and right (RV) ventricles were recorded to assess QTc time, action potential duration (APD), interventricular dispersion (DeltaAPD=LV APD minus RV APD), early afterdepolarizations (EADs), ectopic beats, and TdP. Both amiodarone (+21%) and dronedarone (+31%) increased QTc time. Amiodarone showed no increase in DeltaAPD in 4 of 7 dogs, whereas dronedarone augmented DeltaAPD in 7 of 8 animals. After dronedarone, TdP occurred in 4 of 8 dogs with the highest DeltaAPD (105+/-20 ms). TdP was never seen with amiodarone, not even in the dogs that had DeltaAPD values comparable to those with dronedarone. Furthermore, a difference existed in EADs and ectopic activity incidence (dronedarone 3 of 8; amiodarone 0 of 7), which was also seen during an epinephrine challenge.

CONCLUSIONS

In the CAVB dog model, both amiodarone and dronedarone prolong QT time (class III effect). The absence of TdP with amiodarone seems to be related to homogeneous APD lengthening in the majority of dogs and the lack of EADs and/or ventricular ectopic beats in all.

Electrophysiologic parameters and predisposing factors in the generation of drug-induced Torsade de Pointes arrhythmias

When a new (cardiovascular) drug shows signs of QT interval prolongation on the ECG (delay in repolarization time), the regulatory agencies demand screening of its possible proarrhythmic potential before approving it for clinical practice. In this review, identified predisposing factors have been related to specific electrophysiological parameters, allowing quantification of their contribution to Torsade de Pointes arrhythmias. In addition, arrhythmogenic mechanisms involved in the initiation and perpetuation of drug-induced Torsade de Pointes are discussed.

Abrupt rate accelerations or premature beats cause life-threatening arrhythmias in mice with long-QT3 syndrome

Deletion of amino-acid residues 1505-1507 (KPQ) in the cardiac SCN5A Na(+) channel causes autosomal dominant prolongation of the electrocardiographic QT interval (long-QT syndrome type 3 or LQT3). Excessive prolongation of the action potential at low heart rates predisposes individuals with LQT3 to fatal arrhythmias, typically at rest or during sleep. Here we report that mice heterozygous for a knock-in KPQ-deletion (SCN5A(Delta/+)) show the essential LQT3 features and spontaneously develop life-threatening polymorphous ventricular arrhythmias. Unexpectedly, sudden accelerations in heart rate or premature beats caused lengthening of the action potential with early afterdepolarization and triggered arrhythmias in Scn5a(Delta/+) mice. Adrenergic agonists normalized the response to rate acceleration in vitro and suppressed arrhythmias upon premature stimulation in vivo. These results show the possible risk of sudden heart-rate accelerations. The Scn5a(Delta/+) mouse with its predisposition for pacing-induced arrhythmia might be useful for the development of new treatments for the LQT3 syndrome.

Time course of structural adaptations in chronic AV block dogs: evidence for differential ventricular remodeling

To determine the nature and time course of biventricular hypertrophy and concomitant electrical and mechanical changes after creation of complete atrioventricular block (CAVB), six adult dogs (22–30 kg) were subjected to serial magnetic resonance imaging (MRI) and electrocardiography. After 6 days of CAVB, left ventricular (LV) mass, ejection fraction (EF), and Q-T time at a paced rhythm of 60 beats/min were already significantly increased. Maximal values were reached within 14–21 days of CAVB: LV mass, from 116 ± 11 to 143 ± 12 g; right ventricular (RV) mass, from 40 ± 3 to 55 ± 6 g; EF, from 68 ± 6% to 86 ± 5%; and Q-T time, from 285 ± 25 to 330 ± 35 ms, all P < 0.05. Cardiac output returned to baseline at day 14. End-diastolic wall thickness increased only in the RV, in which angiotensin type 1 (AT1) receptor mRNA expression was significantly greater. The autopsy correlated well with the MRI results ( r = 0.98, P≤ 0.01). In conclusion, electrophysiological, mechanical, and structural adaptation processes after bradycardia-induced volume overload develop rapidly and are completed within 3 wk. The degree of hypertrophy was greater in the RV, which was associated with an increase in AT1receptor mRNA.

Electrophysiological parameters indicative of sudden cardiac death in the dog with chronic complete AV-block

BACKGROUND

The dog model of chronic complete AV-block (CAVB) demonstrates a considerable incidence of (witnessed) sudden death (16/117 dogs). In this study we tried to: (1) elucidate the mechanisms of sudden death using an ECG telemetry device and (2) identify retrospectively the risk parameters indicative of this arrhythmogenic death.

METHODS

Between 1994 and 1998, 78 anesthetized dogs underwent an extensive electrophysiological study including: (1) left- (LV) and right ventricular (RV) monophasic action potential (MAP) recordings to assess Delta MAPD (LV APD minus RV APD) and (2) pacing protocols (PES) to induce torsade de pointes arrhythmias (TdP) at 4--6 weeks CAVB. Eight animals experienced sudden cardiac death (SCD) during the follow-up period (mean 7+/-3 weeks CAVB). Since the response of the CAVB dog to class III drugs is not uniform we also made comparisons among the SCD group, TdP drug responders and non-responders. For this purpose we selected all animals which (1) received almokalant (n=15, 0.12 mg/kg/5 min) or ibutilide (n=9, 0.025 mg/kg/5 min) as an additional challenge to induce TdP and (2) had a follow-up period of at least 4 weeks.

RESULTS

Six out of eight SCD dogs showed inducible TdP at baseline. Two of eight dogs had telemetric ECG surveillance and both revealed polymorphic VT as the cause of SCD. Baseline Delta MAPD of the SCD (90+/-15 ms) was significantly higher than the non-SCD group (n=70, 60+/-30 ms). Of the 24 dogs which received class III drugs, 12 belonged to the TdP responder group. Delta MAPD of the TdP responder group (80+/-15 ms) was similar to the SCD group and significantly higher compared to the non-responder group (n=12, 40+/-25 ms). QT-time and cycle length of idioventricular rhythm were not different.

CONCLUSION

In the CAVB dog model, SCD is (1) most probably related to TdP while (2) inducible TdP and the measure of Delta MAPD at baseline indicate susceptibility to SCD.

Myocardial repolarization and drugs. Impossibility to predict the dominance of anti-arrhythmic over pro-arrhythmic effects of drugs due to differential and ventricular electrical remodeling

It is known that application of anti-arrhythmic drugs for the acute treatment of arrhythmias can not only result in successful termination or prevention, but also can lead to unwanted pro-arrhythmic effects. On the basis of two arrhythmias, atrial fibrillation and Torsade de Pointes arrhythmias, we will highlight the relevance of differential atrial and ventricular electrical remodeling to explain the delicate and dynamic balance between anti-arrhythmic efficacy and pro-arrhythmogenic consequences of class III anti-arrhythmic drugs.

Azimilide and dofetilide produce similar electrophysiological and proarrhythmic effects in a canine model of Torsade de Pointes arrhythmias

Torsade de Pointes arrhythmias are a feared proarrhythmic effect of (antiarrhythmic) drugs. In dogs with chronic complete AV-block bradycardia-induced volume overload leads to electrical remodeling, which includes increased susceptibility to drug-induced Torsade de Pointes arrhythmias. The IKr channel blocker, dofetilide (Tikosyn, 0.025 mg/kg/5 min), and the less specific ion channel blocker, azimilide (5 mg/kg/5 min), were compared in nine anesthetized dogs at 4 and 6 weeks of AV-block in a randomized cross-over design. Dosages were based on our own dose-dependence studies and on anti-arrhythmic dosages reported in the literature. Monophasic action potential catheters were placed endocardially in both the left and right ventricle to measure action potential duration, visualize early afterdepolarizations, and to assess interventricular dispersion of repolarization (i.e. left ventricular monophasic action potential duration (at 100%) minus right ventricular monophasic action potential duration (at 100%). Cycle length of idioventricular rhythm, QT-time and the occurrence of drug-induced Torsade de Pointes arrhythmias were determined using the surface electrocardiogram (ECG). Before drug administration, the electrophysiological parameters were identical at 4 and 6 weeks. Both azimilide and dofetilide increased monophasic action potential duration, cycle length of idioventricular rhythm, and QT-time. Dissimilar lengthening of left ventricular and right ventricular monophasic action potential duration increased the interventricular dispersion significantly from 55 to 110 ms for both drugs. All dogs had early afterdepolarizations, while, in the majority, ectopic ventricular beats developed (dofetilide 8/9 and azimilide 7/9). Torsade de Pointes arrhythmias incidence was comparable for dofetilide (6/9) and azimilide (5/9). In conclusion, azimilide and dofetilide show similar electrophysiological and proarrhythmic effects in our canine model with a high incidence of Torsade de Pointes arrhythmias.

Observations on the onset of torsade de pointes arrhythmias in the acquired long QT syndrome

OBJECTIVE

Premature ectopic beats may create a specific sequence of events (e.g. short-long-short) preceding Torsade de Pointes arrhythmias (TdP) in the long QT syndrome. The relevance of this sequence for the initiation of TdP is not clear. In our dog model of TdP, interventricular dispersion (DeltaAPD=left-right ventricular monophasic action potential duration: APD) is associated with TdP, therefore we tested the hypothesis that the ectopic beats contributes to DeltaAPD.

METHODS

In 17 anaesthetized dogs with chronic AV-block, which showed spontaneous TdP after class III medication, APD was analyzed to 1. quantitate the alterations due to (multiple) ectopic beats on the left and right APD (measured with endocardial catheters) and 2. compare the DeltaAPD prior to the occurrence of premature beats (steady state) in dogs with non-sudden onset of TdP (n=10) and sudden onset TdP (n=7). Three phases were distinguished: phase 1: steady state beats prior to ectopic beats, phase II: the beat(s) belonging to the dynamic phase, and phase III: the beat causing TdP. Because the coupling interval of premature beats in this condition often falls within the APD, the DeltaAPD(50) was validated as an alternative for the previously applied DeltaAPD(100) (r=0.51, P<0.01).

RESULTS

In steady state (phase I) DeltaAPD(50) is longer in the sudden onset TdP (130+/-35 ms) as in the non-sudden onset TdP (65+/-40 ms). In the non-sudden TdP group the dynamic phase II contribute to the heterogeneity in APD, i.e. LV-APD increases more than RV-APD leading to a DeltaAPD(50) increase to 130+/-100 ms (P<0.01) just preceding TdP (phase III).

CONCLUSION

The synergism between ectopic beats (short-long-short sequence) and DeltaAPD create the circumstances for TdP initiation.

Atrial fibrillation in the goat induces changes in monophasic action potential and mRNA expression of ion channels involved in repolarization

INTRODUCTION

Sustained atrial fibrillation (AF) is characterized by a marked shortening of the atrial effective refractory period (AERP) and a decrease or reversal of its physiologic adaptation to heart rate. The aim of the present study was to investigate whether the AF-induced changes in AERP in the goat are associated with changes in the atrial monophasic action potential (MAP) and whether an abnormal expression of specific ion channels underlies such changes.

METHODS AND RESULTS

Following thoracotomy, MAPs were recorded from the free wall of the right atrium both before induction of AF (control) and after cardioversion of sustained AF (>2 months) in chronically instrumented goats. In control goats, MAP duration at 80% repolarization (MAPD80) shortened (P < 0.01) from 132+/-4 msec during slow pacing (400-msec interval) to 86+/-10 msec during fast pacing (180 msec). After cardioversion of sustained AF, the MAPD80 during slow pacing was as short as 67+/-5 msec (electrical remodeling). Increasing the pacing rate resulted in prolongation (P = 0.02) of the MAPD80 to 91+/-6 msec. Also, MAPD20 (20% repolarization) shortened (P = 0.05) from 32+/-4 msec (400 msec) to 14+/-7 msec (180 msec) in the control goats, whereas it prolonged (P = 0.03) from 20+/-3 msec (400 msec) to 33+/-5 msec (180 msec) in sustained AF. mRNA expression of the L-type Ca2+ channel alpha1c gene and Kv1.5 potassium channel gene, which underlie ICa and IKur, respectively, was reduced in sustained AF compared with sinus rhythm by 32% (P = 0.01) and 45% (P < 0.01), respectively. No significant changes were found in the mRNA levels of the rapid Na+ channel, the Na+/Ca2+ exchanger, or the Kv4.2/4.3 channels responsible for Ito.

CONCLUSION

AF-induced electrical remodeling in the goat comprises shortening of MAPD and reversal of its physiologic rate adaptation. Changes in the time course of repolarization of the action potential are associated with changes in mRNA expression of the alpha subunit genes of the L-type Ca2+ channel and the Kv1.5 potassium channel.

Enhanced Ca(2+) release and Na/Ca exchange activity in hypertrophied canine ventricular myocytes: potential link between contractile adaptation and arrhythmogenesis

BACKGROUND

Ventricular arrhythmias are a major cause of sudden death in patients with heart failure and hypertrophy. The dog with chronic complete atrioventricular block (CAVB) has biventricular hypertrophy and ventricular arrhythmias and is a useful model to study underlying cellular mechanisms. We investigated whether changes in Ca(2+) homeostasis are part of the contractile adaptation to CAVB and might contribute to arrhythmogenesis.

METHODS AND RESULTS

In enzymatically isolated myocytes, cell shortening, Ca(2+) release from the sarcoplasmic reticulum (SR), and SR Ca(2+) content were enhanced at low stimulation frequencies. Ca(2+) influx through L-type Ca(2+) channels was unchanged, but Ca(2+) influx via the Na/Ca exchanger was increased and contributed to Ca(2+) loading of the SR. Inward Na/Ca exchange currents were also larger. Changes in Ca(2+) fluxes were less pronounced in the right versus left ventricle.

CONCLUSIONS

Enhanced Na/Ca exchange activity may improve contractile adaptation to CAVB but at the same time facilitate arrhythmias by (1) increasing the propensity to Ca(2+) overload, (2) providing more inward current leading to (nonhomogeneous) action potential prolongation, and (3) enhancing (arrhythmogenic) currents during spontaneous Ca(2+) release.

Contractile adaptations preserving cardiac output predispose the hypertrophied canine heart to delayed afterdepolarization-dependent ventricular arrhythmias

BACKGROUND

In dogs, chronic complete atrioventricular block (CAVB) results in structural (biventricular hypertrophy) and electrical (delayed repolarization) remodeling, which predisposes the heart to torsade de pointes arrhythmias. We assessed the contractile alterations in the CAVB dog and tested the hypothesis that these adaptations increase delayed afterdepolarization (DAD)-dependent triggered arrhythmias.

METHODS AND RESULTS

Steady-state and dynamic (fast pacing: 1 to 68 stimuli) left and right ventricular systolic and diastolic parameters were determined by positive and negative inotropic interventions at acute AVB and CAVB. Concomitantly, left and right ventricular endocardial monophasic action potentials were registered. In CAVB, all systolic contractile parameters were markedly increased, resulting in preserved cardiac output. The increase was most pronounced at low heart rates, altering the force-frequency response. At both acute AVB and CAVB, the degree of potentiation of cardiac function with pacing was dependent on the number of stimuli and showed a maximum at 8 to 13 stimuli. With CAVB, this potentiation curve was shifted upward, and it was only then that pacing resulted in DADs (in 8 of 10 dogs) and ectopic beats (EBs, in 6 of 10 dogs). The incidence of EBs in relation to the number of stimuli also had a maximum at 8 to 13 stimuli. Ouabain increased the incidence of DADs and EBs, whereas the negative inotropic interventions prevented them completely.

CONCLUSIONS

The alterations responsible for improvement in systolic contractile function in CAVB dogs predispose the hypertrophied heart to DAD-dependent triggered arrhythmias during positive inotropic interventions.

High uniformity of left and right ventricular repolarization dynamics induced by an abrupt decrease in pacing cycle length in a dog is not affected by left ventricular ischemia

INTRODUCTION

After an abrupt increase in heart rate, action potential duration (APD) will shorten. To assess the effect of ischemia on APD shortening dynamics, we compared right ventricular (RV) and left ventricular (LV) APD shortening induced by an abrupt decrease in pacing cycle length (PCL) during control and LV ischemia.

METHODS AND RESULTS

In eight anesthetized AV block dogs, endocardial LV and RV APD were determined simultaneously after an abrupt PCL decrease from 800 to 350 msec. Measurements were repeated during left anterior descending coronary artery (LAD) occlusion. During control, LV and RV APD shortened 97 +/- 27 and 71 +/- 14 msec, respectively (P < 0.05). Shortening was pronounced in a short initial phase and gradual in the longer secondary phase. Linear regression analysis revealed very high uniformity of LV and RV APD shortening dynamics (r2 = 0.96 +/- 0.01). During repeated LAD occlusion, ischemia induced a gradual LV APD shortening from 314 +/- 25 msec to a new steady-state value of 251 +/- 23 msec, whereas RV APD remained stable at 289 +/- 28 msec. The additional PCL decrease resulted in LV and RV APD shortening of 72 +/- 8 and 68 +/- 15 msec, respectively, with the same high uniformity of shortening dynamics as seen during control (r2 = 0.94 +/- 0.03).

CONCLUSION

There is a pronounced difference in APD shortening dynamics induced by an abrupt decrease in PCL compared with ischemia. LV shortening dynamics induced by a decrease in PCL are not affected by LV ischemia, preserving a high interventricular uniformity of repolarization dynamics.

Downregulation of delayed rectifier K(+) currents in dogs with chronic complete atrioventricular block and acquired torsades de pointes

BACKGROUND

Acquired QT prolongation enhances the susceptibility to torsades de pointes (TdP). Clinical and experimental studies indicate ventricular action potential prolongation, increased regional dispersion of repolarization, and early afterdepolarizations as underlying factors. We examined whether K(+)-current alterations contribute to these proarrhythmic responses in an animal model of TdP: the dog with chronic complete atrioventricular block (AVB) and biventricular hypertrophy.

METHODS AND RESULTS

The whole-cell K(+) currents I(TO1), I(K1), I(Kr), and I(Ks) were recorded in left (LV) and right (RV) ventricular midmyocardial cells from dogs with 9+/-1 weeks of AVB and controls with sinus rhythm. I(TO1) density and kinetics and I(K1) outward current were not different between chronic AVB and control cells. I(Kr) had a similar voltage dependence of activation and time course of deactivation in chronic AVB and control. I(Kr) density was similar in LV myocytes but smaller in RV myocytes (-45%) of chronic AVB versus control. For I(Ks), voltage-dependence of activation and time course of deactivation were similar in chronic AVB and control. However, I(Ks) densities of LV (-50%) and RV (-55%) cells were significantly lower in chronic AVB than control.

CONCLUSIONS

Significant downregulation of delayed rectifier K(+) current occurs in both ventricles of the dog with chronic AVB. Acquired TdP in this animal model with biventricular hypertrophy is thus related to intrinsic repolarization defects.

Rate dependent effects of procainamide on the threshold current for pacing in the setting of postrepolarization refractoriness in dogs

Normally, ventricular APD exceeds the VERP. However, under specific circumstances this relation may change and can become inverse. This phenomenon of postrepolarization refractoriness may be caused by a decrease in excitability. The threshold current (TC) for pacing has never been quantified as a possible explanation for these observations. Using a MAP pacing catheter in the right ventricular apex, the rate dependent behavior of TC, VERP, and APD before and after procainamide (dose 20 mg/kg in 10 min + 5 mg/min infusion) was determined in 17 dogs with chronic complete AV block. Initially, TC was determined with 0.1 mA accuracy. Using a pacing current of at least twice TC, VERP and APD showed a similar, rate dependent shortening for PCLs 800, 575, and 350 ms. Procainamide treatment led to an equal, rate independent VERP and APD increase: no post repolarization refractoriness. Subsequently, accuracy for TC determination was increased to 0.01 mA. Comparing PCLs 800 and 250 ms, TC doubled from 0.05 +/- 0.01 to 0.10 +/- 0.09 mA during control and almost tripled from 0.06 +/- 0.02 to 0.17 +/- 0.10 mA (P < 0.05) after procainamide. Using a fixed pacing current of exactly twice TC found at 800 ms PCL during control, VERP exceeded APD after procainamide treatment at 300 and 250 ms PCL: postrepolarization refractoriness. Increasing the pacing current to twice the rate dependent TC, the relation between VERP and APD normalized: no postrepolarization refractoriness. We conclude that after procainamide, rate dependent TC increase is of major importance for the phenomenon of postrepolarization refractoriness.

Evaluation of the acute electrophysiologic effects of intravenous dronedarone, an amiodarone-like agent, with special emphasis on ventricular repolarization and acquired torsade de pointes arrhythmias

In the anesthetized dog with complete chronic AV block (CAVB), we evaluated and compared the acute electrophysiologic effects of dronedarone i.v. (Dron, 2 times 2.5 mg/kg/10 min) and amiodarone i.v. (Amio, 2 times 5 mg/kg/10 min). This canine model with a high sensitivity for acquired torsade de pointes (TdP) provides an ideal substrate to evaluate ventricular repolarization abnormalities. Six ECG leads and two endocardial monophasic action potential (MAP) recordings in the left and right ventricle (LV and RV) were simultaneously recorded to measure QT time, action-potential duration (APD), interventricular dispersion (deltaAPD = LV(APD) - RV(APD)), early afterdepolarizations (EADs), ectopic beats (EBs), and TdP. Measurements were made at the spontaneous idioventricular rhythm (IVR) and 1,000-ms steady-state pacing. To investigate its short-term, antiarrhythmic properties, Dron was given after almokalant (0.12 mg/kg)-induced TdP. Furthermore, in another set of experiments, oral Dron (20 mg/kg, b.i.d) was given for 3 weeks to conscious CAVB dogs. Dron, i.v., shortened ventricular repolarization (QT, 435 +/- 60 to 360 +/- 55; LV(APD) 395 +/- 75 to 335 +/- 60 ms; p < 0.05), whereas IVR and ventricular effective refractory period (VERP, 225 +/- 30 to 230 +/- 30 ms) remained similar. Therefore the VERP/QT ratio increased (0.55 +/- 0.04 to 0.61 +/- 0.03; p < 0.05). Similar results were obtained with Amio, i.v.. Almokalant-induced TdP was characterized by an increased repolarization duration, deltaAPD, and EADs. Dron, i.v., suppressed the EADs, EBs, and TdP by a reduction and homogenization of repolarization (LV(APD), 505 +/- 110 to 455 +/- 80 ms, and deltaAPD, 110 +/- 55 to 65 +/- 40 ms). Long-term oral Dron increased the PP interval, CL-IVR, and QT(c) time. In contrast to oral treatment, Dron i.v. shortens ventricular repolarization parameters, resulting in suppression of EAD-dependent acquired TdP. The increased VERP/QT ratio after Dron i.v. may indicate an important second antiarrhythmic property.

Repolarizing K+ currents ITO1 and IKs are larger in right than left canine ventricular midmyocardium

BACKGROUND

The ventricular action potential exhibits regional heterogeneity in configuration and duration (APD). Across the left ventricular (LV) free wall, this is explained by differences in repolarizing K+ currents. However, the ionic basis of electrical nonuniformity in the right ventricle (RV) versus the LV is poorly investigated. We examined transient outward (ITO1), delayed (IKs and IKr), and inward rectifier K+ currents (IK1) in relation to action potential characteristics of RV and LV midmyocardial (M) cells of the same adult canine hearts.

METHODS AND RESULTS

Single RV and LV M cells were used for microelectrode recordings and whole-cell voltage clamping. Action potentials showed deeper notches, shorter APDs at 50% and 95% of repolarization, and less prolongation on slowing of the pacing rate in RV than LV. ITO1 density was significantly larger in RV than LV, whereas steady-state inactivation and rate of recovery were similar. IKs tail currents, measured at -25 mV and insensitive to almokalant (2 micromol/L), were considerably larger in RV than LV. IKr, measured as almokalant-sensitive tail currents at -50 mV, and IK1 were not different in the 2 ventricles.

CONCLUSIONS

Differences in K+ currents may well explain the interventricular heterogeneity of action potentials in M layers of the canine heart. These results contribute to a further phenotyping of the ventricular action potential under physiological conditions.

The dynamic behavior of the diastolic slope of monophasic action potential can be related to the occurrence and maintenance of delayed afterdepolarization dependent arrhythmias

We have described the value of the diastolic slope of the MAP recording at the end of a pacing train as a qualifying marker for the induction of delayed afterdepolarization (DAD) dependent arrhythmias. In the present study (1) the behavior of the slope at different time points during a pacing train was quantified and related to the arrhythmogenic outcome (group A) and (2) termination of DAD dependent VT was related to changes in the slope steepness (group B). In dogs with chronic complete AV block, a MAP was recorded during (1) ventricular pacing, before and after ouabain administration (group A) and (2) 6 spontaneous and 6 lidocaine induced VT terminations (group B). During control (group A), the slope at the end of pacing train was 5 +/- 3 m V/s (mean +/- SD), independent of the pacing duration. During ouabain, this increased to 20 +/- 15 mV/s (P < 0.05), varying with the duration of pacing. The slope was steeper after pacing for 4 seconds, compared to 20 seconds (26 +/- 12 mV/s vs 16 +/- 13 mV/s, P < 0.05) which corresponded with more frequent VT induction. In spontaneously terminating VTs (group B), CL increased from 353 +/- 54 ms at the start to 434 +/- 78 ms (P < 0.05) before VT termination. This corresponded with a decreasing steepness of the slope from 19 +/- 10 mV/s to 6 +/- 5 mV/s (P < 0.05). In lidocaine induced VT termination, the CL and the steepness of the slope showed an identical behavior. There is a dynamic variation in the steepness of the diastolic slope during pacing, which depends on the duration of pacing and predicts arrhythmogenic outcome. Furthermore, a decrease in steepness of the slope during DAD dependent VT can be used to predict VT termination.

Cellular basis of biventricular hypertrophy and arrhythmogenesis in dogs with chronic complete atrioventricular block and acquired torsade de pointes

BACKGROUND

In the dog with chronic complete atrioventricular block (AVB), torsade de pointes arrhythmias (TdP) can be induced reproducibly by class III antiarrhythmic agents. In vivo studies reveal important electrophysiological alterations of the heart at 5 weeks of AVB, resulting in increased proarrhythmia. Autopsy studies indicate the presence of biventricular hypertrophy. In this study, the cellular basis of proarrhythmia and hypertrophy in chronic AVB was investigated.

METHODS AND RESULTS

From chronic-AVB dogs with increased heart weights and TdP, left midmyocardial and right ventricular myocytes were isolated by enzymatic dispersion. These myocytes were significantly larger than sinus rhythm (SR) controls. In chronic AVB, the action potential spike-and-dome configuration was preserved. However, the action potential duration (APD) at 95% and 50% of repolarization of the left midmyocardium was significantly larger in chronic AVB than in SR, with little change in the right ventricle, causing enhanced interventricular dispersion of repolarization at slow pacing rates. Treatment with the class III agent almokalant increased the APD to a much larger extent in chronic-AVB than in SR myocytes and resulted in a higher incidence of early afterdepolarizations (EADs). EADs had their takeoff potential between -35 and 0 mV. There was no evidence that spontaneous sarcoplasmic reticulum Ca2+ release underlies these EADs.

CONCLUSIONS

In the dog, chronic AVB leads to hypertrophy of both right and left ventricular myocytes. The repolarization abnormalities predisposing for class III-dependent TdP in vivo are the results of cellular electrophysiological remodeling.

Enhanced susceptibility for acquired torsade de pointes arrhythmias in the dog with chronic, complete AV block is related to cardiac hypertrophy and electrical remodeling

BACKGROUND

Chronic, complete AV block (CAVB) in the dog leads to ventricular hypertrophy, which has been described as an independent risk factor for arrhythmias. In this model, we examined (1) whether the short- and long-term electrical adaptations predispose to acquired torsade de pointes arrhythmias (TdP) and (2) the nature of the structural and functional adaptations involved.

METHODS AND RESULTS

We determined (1) endocardial right (RV) and left (LV) ventricular APD, DeltaAPD (LV APD-RV APD), presence of EADs at 0 weeks (acute: AAVB), and CAVB (6 weeks) and inducibility of TdP by pacing and d-sotalol (n=10); (2) steady-state and dynamic LV hemodynamics at 0 and 6 weeks (n=6); (3) plasma neurohumoral levels in time (n=7); (4) structural parameters of the LV and RV of CAVB dogs (n=6) compared with sinus rhythm (SR) dogs (n=6); and (5) expression of ventricular mRNA atrial natriuretic factor (ANF) in CAVB (n=4) and SR (n=4) dogs. Compared with AAVB, CAVB led to nonhomogeneous prolongation of LV and RV APD and different sensitivity for d-sotalol, leading to EADs (4 of 14 versus 9 of 18, P<0.05), increased DeltaAPD (45+/-30 versus 125+/-60 ms, P<0.05), and induction of TdP in most dogs (0% versus 60%, P<0.05). CAVB led to biventricular hypertrophy, whereas LV function was similar in AAVB and CAVB. The neurohumoral levels were transiently elevated. The LV and RV collagen and the capillary/fiber ratio remained normal, whereas ventricular ANF mRNA was not detectable.

CONCLUSIONS

The electrical remodeling occurring after CAVB predisposes the heart to acquired TdP, whereas the structural changes (hypertrophy) are successfully aimed at maintaining cardiac function.

Steady-state and dynamic behavior of ventricular repolarization and refractoriness in the dog: the effect of multiple cycle length changes and d-sotalol administration

In anesthetized dogs with chronic, complete AV block we studied the characteristics of ventricular repolarization and refractoriness. Therefore, we determined: (1) steady-state values of ventricular effective refractory period (VERP), action potential duration (APD), and stimulus T interval (STI) before and after d-sotalol treatment at various pacing cycle lengths (PCLs); and (2) the dynamics of VERP, APD, and STI before and after d-sotalol treatment after the abrupt PCL decreases. VERP, APD, and STI showed a normal frequency dependency. All three parameters increased significantly after d-sotalol administration. During steady-state and dynamic measurements, STI was always longer than APD and APD was always longer than VERP in an individual animal, irrespective of PCL and conditions. Standard deviations of steady-state and dynamic values indicated a considerable interindividual variation. However, the dynamics of VERP, APD, and STI after an abrupt decrease in PCL were highly correlated (linear regression analysis: r2 > or = 0.93). The best mathematical model to describe these dynamics was a bi-exponential model (r2 > or = 0.98) with a very short first and a much longer second time constant. We found that there was a very consistent relation between VERP, APD, and STI, not only during steady-state but also in the dynamic situation after various abrupt PCL decreases. This relation does not change after the administration of d-sotalol. Therefore, STI could be used to predict steady-state and dynamic values of VERP and APD. Since STI can be made available online in implantable pacing systems this could lead to the development of new features in these devices.

Superiority of ibutilide (a new class III agent) over DL-sotalol in converting atrial flutter and atrial fibrillation. The Ibutilide/Sotalol Comparator Study Group

OBJECTIVE

To compare the efficacy and safety of a single dose of ibutilide, a new class III antiarrhythmic drug, with that of DL-sotalol in terminating chronic atrial fibrillation or flutter in haemodynamically stable patients.

DESIGN

Double blind, randomised study.

SETTING

43 European hospitals.

PATIENTS

308 patients (mean age 60 years, 70% men, 48% with heart disease) with sustained atrial fibrillation (n = 251) or atrial flutter (n = 57) (duration three hours to 45 days) were randomised to three groups to receive a 10 minute infusion of 1 mg ibutilide (n = 99), 2 mg ibutilide (n = 106), or 1.5 mg/kg DL-sotalol (n = 103). Infusion was discontinued at termination of the arrhythmia.

MAIN OUTCOME MEASURE

Successful conversion of atrial fibrillation or flutter, defined as termination of arrhythmia within one hour of treatment.

RESULTS

Both drugs were more effective against atrial flutter than against atrial fibrillation. Ibutilide was superior to DL-sotalol for treating atrial flutter (70% and 56% v 19%), while the high dose of ibutilide was more effective for treating atrial fibrillation than DL-sotalol (44% v 11%) and the lower dose of ibutilide (44% v 20%, p < 0.01). The mean (SD) time to arrhythmia termination was 13 (7) minutes with 2 mg ibutilide, 19 (15) minutes with 1 mg ibutilide, and 25 (17) minutes with DL-sotalol. In all patients, the duration of arrhythmia before treatment was a predictor of arrhythmia termination, although this was less obvious in the group that received 2 mg ibutilide. This dose converted almost 48% of atrial fibrillation that was present for more than 30 days. Concomitant use of digitalis or nifedipine and prolongation of the QTc interval were not predictive of arrhythmia termination. Bradycardia (6.5%) and hypotension (3.7%) were more common side effects with DL-sotalol. Of 211 patients given ibutilide, two (0.9%) who received the higher dose developed polymorphic ventricular tachycardia, one of whom required direct current cardioversion.

CONCLUSION

Ibutilide (given in 1 or 2 mg doses over 10 minutes) is highly effective for rapidly terminating persistent atrial fibrillation or atrial flutter. This new class III drug, under monitored conditions, is a potential alternative to currently available cardioversion options.

Further observations to elucidate the role of interventricular dispersion of repolarization and early afterdepolarizations in the genesis of acquired torsade de pointes arrhythmias: a comparison between almokalant and d-sotalol using the dog as its own control

OBJECTIVES

We sought to further elucidate the role of early afterdepolarizations (EADs) and interventricular dispersion of repolarization (deltaAPD) in the genesis of acquired torsade de pointes (TdP) arrhythmias.

BACKGROUND

Administration of class III agents can be associated with TdP. We developed a dog model in which TdP can be reproducibly induced by pacing after d-sotalol. This model shows reproducible results over weeks.

METHODS

In 14 anesthetized dogs with chronic complete atrioventricular block, two separate experiments were performed in which d-sotalol (2 mg/kg body weight) or almokalant (0.12 mg/kg) was administered. Monophasic action potentials were simultaneously recorded from the endocardium of the right and left ventricle to register EADs and to measure the action potential duration (APD). DeltaAPD was defined as the APD of the left ventricle minus that of the right ventricle.

RESULTS

Baseline conditions were identical in the serially performed experiments. The cycle length and QT time increased by 16% and 26% after d-sotalol and by 15% and 31% after almokalant, respectively. After both drugs the action potential of the left ventricle prolonged more than that of the right ventricle, thereby increasing deltaAPD (almokalant [mean +/- SD]: 110 +/- 60 ms; d-sotalol: 80 +/- 45 ms, p < 0.05). The incidence of EADs (18 of 22 vs. 11 of 24, p < 0.05) and single ectopic beats (EBs) (1.5 +/- 2 vs. 24 +/- 32, p < 0.01) was more frequently observed after almokalant than after d-sotalol. Moreover, multiple EBs only occurred after almokalant. These beats interfered with the basic rhythm, leading to dynamic changes in left ventricular APD and to additional increases in deltaAPD. Spontaneous TdP was observed in 9 of 14 dogs after almokalant and could be increased to 12 of 14 with programmed electrical stimulation. After d-sotalol, TdP could only be induced by programmed electrical stimulation (5 of 14, p < 0.05).

CONCLUSIONS

In the same dog, almokalant induced more delay in repolarization, more EADs, multiple EBs and more ventricular inhomogeneity in APD than d-sotalol. These changes were related to a higher incidence of TdP and thereby confirm a strong association of the occurrence of EADs, multiple EBs and deltaAPD in the genesis of TdP. These findings also show the possible value of our model for evaluating the proarrhythmic potential of different drugs.