Effects of chronic treatment by amiodarone on transmural heterogeneity of canine ventricular repolarization in vivo: interactions with acute sotalol.
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OBJECTIVE: The present study was designed to examine the effects of chronic amiodarone on the different ventricular cell subtypes in situ and to evaluate its interactions with sotalol. METHODS: Three groups of dogs were studied. Group I (n = 8) received no treatment. Group II (n = 7) and group III (n = 8) received, respectively, 100 and 200 mg amiodarone orally twice a day for 6 weeks to 8 months. In vivo studies were performed under halothane anesthesia 14 h after the last administration of amiodarone. Three leads ECG, femoral blood pressure and left ventricular intramural monophasic action potentials (MAP) were continuously recorded. Bradycardia was obtained by clamping the sinus node and beta-blockade and the heart was driven by atrial pacing. Three weeks before the in vivo experiments, the cellular electrophysiologic properties of right ventricular tissues obtained by cardiac biopsy in six treated and six control dogs were studied with standard microelectrodes. RESULTS: Amiodarone produced a dose-dependent decrease in plasma levels of triiodothyronine (T3; 5.9 +/- 0.4 pM in control dogs, 3.1 +/- 0.2 pM in group III, P < 0.001) without affecting thyroxine (T4). Under anesthesia, the QT interval was 14% larger in group III compared to group I at a paced cycle length (PCL) of 1500 ms (P < 0.05). This is consistent with the 10% increase in endocardial MAP duration in group III at the same PCL (P < 0.05). There was no significant increase in transmural dispersion of MAP duration. In group I, sotalol induced a significant reverse use-dependent increase in MAP duration. This effect was reduced in group II and completely suppressed in group III. Amiodarone prevented the sotalol-induced increase in transmural dispersion of ventricular repolarization which was 69 +/- 12 ms in untreated dogs, 41 +/- 8 ms in group II (P < 0.05) and 34 +/- 8 ms (P < 0.05) in group III at PCL = 1500 ms. Amiodarone also prevented the sotalol-induced ventricular tachyarrhythmias. In vitro, the action potential duration was longer in amiodarone-treated dogs that in control ones (208 +/- 5 ms versus 188 +/- 9 ms at PCL = 1000 ms, P < 0.05). The sotalol-induced prolongation of repolarization was reduced in amiodarone-treated dogs. CONCLUSION: Chronic treatment of dogs with amiodarone induced a moderate prolongation of the QT interval and MAP duration without affecting transmural dispersion of repolarization and inhibited the effects of acute sotalol, including the prolongation of repolarization, the increase in transmural dispersion of repolarization and the induction of arrhythmias. (+info)
Antipsychotic agents and QT changes.
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Recently, antipsychotic medications of the novel or atypical classes have received increased attention because of concerns with respect to potential lengthening of the QT interval, yet the currently available and commonly prescribed conventional antipsychotics are significantly more cardiotoxic, particularly agents in the butyrophenone and phenothiazine classes. Lengthening of the QT interval can be associated with a fatal paroxysmal ventricular arrhythmia known as torsades de pointes. The specific duration of the QT interval at which the risk of an adverse cardiac event is greatest, is not established. There is not only significant variation in the applied definition of an abnormal interval, but the maximal QT interval in healthy volunteers is greater than the currently accepted standards. The QT interval is influenced by normal physiological and pathologic factors, but the mechanisms remain unclear. Using recombinant technology, haloperidol and sertindole have been demonstrated to be high-affinity antagonists of a human cardiac potassium channel encoded by the human ether-a-go-go-related gene. Pimozide, however, has been shown to act principally through calcium channel antagonism, and chlorpromazine may affect sodium channels. Nevertheless, it is possible that these effects are significant only in the presence of predisposing factors, either genetic or acquired. Despite proven efficacy in clinical trials and subsequent supervised use in Europe, a number of recently developed antipsychotic medications are not available to patients in North America. Yet, conventional antipsychotic medications that would not be approved by current safety standards continue to be widely used. (+info)
Arrhythmias in the congenital long QT syndrome: how often is torsade de pointes pause dependent?
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OBJECTIVE: To determine the frequency and predictors of pause dependent torsade de pointes among patients with the congenital long QT syndrome and spontaneous ventricular tachyarrhythmias. DESIGN: The literature on the "congenital long QT" was reviewed. Articles with illustrations demonstrating the onset of spontaneous polymorphic ventricular arrhythmias in the absence of arrhythmogenic drugs were included. RESULTS: Illustrations of 62 spontaneous episodes of torsade de pointes among patients with congenital long QT syndrome were found in the literature. The majority (74%) of documented arrhythmias were "pause dependent"; 82% of these pauses were longer than the basic cycle length by > 100 ms. Age and sex correlated with the mode of arrhythmia initiation. Arrhythmias in infants (+info)
D0870, an antifungal agent, induces reverse use-dependent QT prolongation in dogs.
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We previously reported that D0870 induced QT prolongation and sudden death due to torsades de pointes (TdP) in dogs and that catecholamines played an important part in the development of the sudden death. In the present study, we analyzed in detail the ambulatory electrocardiographic recordings obtained from the just-mentioned study to elucidate the mechanism of the onset of TdPs and conducted an in vitro study using isolated canine Purkinje fibers to assess the effect of D0870 on repolarization. The hearts with TdPs observed before the sudden death showed a higher sinus rate for 5 and 10 sec before the onset, a shorter coupling interval, and a higher ventricular tachycardia rate compared with those having the non-sustained TdPs. These findings suggest that D0870-induced fatal TdPs may be provoked by a triggered activity developed from delayed after depolarizations. In contrast, as the pause-dependent, non-sustained TdPs in bradycardia showed a typical "short-long-short" sequence, they may be developed from early afterdepolarization . Moreover, the results of the in vitro study supported our contention that D0870 induced QT prolongation in a reverse use-dependent manner in vivo and suggested that it may inhibit not only rapidly activating delayed rectifier potassium current (Ik(r)) but also L-type Ca current (I(ca-L)). (+info)
CYP2D6 and CYP2C19 genotypes of patients with terodiline cardiotoxicity identified through the yellow card system.
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AIMS: Terodiline has concentration dependent QT prolonging effects and thus the potential for cardiotoxicity. Pharmacogenetic variation in terodiline metabolism could be responsible for cardiotoxicity. We sought to determine whether CYP2D6 (debrisoquine hydroxylase) or CYP2C19 (S-mephenytoin hydroxylase) status is a risk factor for terodiline cardiotoxicity. METHODS: Using the UK Yellow Card scheme to identify patients, blood samples were obtained from eight patients who survived ventricular tachycardia or torsades de pointes suspected to be due to terodiline, for determination of CYP2D6 and CYP2C19 genotypes. Genotype prevalence was compared with that in published general population groups. RESULTS: One patient was a CYP2D6 poor metaboliser (CYP2D6*4 homozygous) and a second was heterozygous for CYP2D6*4, a slightly lower frequency for these genotypes compared with the general population (P = 0.31). In the case of CYP2C19, one patient was a poor metaboliser and four were heterozygous for the variant CYP2C19*2 allele, compared with general population frequencies of 2% and 23%, respectively (P = 0.035). CONCLUSIONS: These findings suggest that debrisoquine poor metaboliser status is not primarily responsible for terodiline cardiotoxicity. However, possession of the CYP2C19*2 allele appears to contribute to adverse cardiac reactions to terodiline. The present study demonstrates the feasibility of using spontaneous adverse drug reaction reporting schemes to determine the contribution of genotype for metabolizing enzymes to uncommon adverse drug reactions. (+info)
Drug-induced torsade de pointes: from molecular biology to bedside.
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A progressively increasing number of cardiac and noncardiac drugs prolong the ventricular action potential duration (QT interval of the electrocardiogram) and cause a distinctive polymorphic ventricular tachycardia termed torsades de pointes (TdP) that can degenerate into ventricular fibrillation and sudden cardiac death. Drugs prolong the QT interval and cause TdP by blocking cardiac K+ channels in general and selectively blocking the rapidly activating delayed rectifier channel IKr. Coassembly of HERG (human-ether-a-go-go-related gene) alpha-subunits and MiRP1 (MinK-related peptide 1) beta-subunits recapitulate the behavior of native human IKr and mutations of HERG and MiRP1 decrease the repolarizing current, delay ventricular repolarization and prolong the QT. Thus, drug-induced QT prolongation and TdP might represent an iatrogenic reproduction of the congenital LQTS. In patients with silent forms of the congenital LQTS associated with mutations in IKr, arrhythmic symptoms developed almost exclusively after exposure to QT-prolonging drugs. This review centers on the possible cellular mechanisms underlying drug-induced QT prolongation and TdP, the description of specific drugs and risk factors facilitating the development of TdP, and the recommendations for preventing and treating this potentially fatal arrhythmia. (+info)
Effects of a K(+) channel opener to reduce transmural dispersion of repolarization and prevent torsade de pointes in LQT1, LQT2, and LQT3 models of the long-QT syndrome.
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BACKGROUND: This study examines the effects of nicorandil, a K(+) channel opener, on transmural dispersion of repolarization (TDR) and induction of torsade de pointes (TdP) under conditions mimicking the LQT1, LQT2, and LQT3 forms of the congenital long-QT syndrome (LQTS). METHODS AND RESULTS: Transmembrane action potentials of epicardial, M, and endocardial cells were recorded simultaneously from an arterially perfused wedge of canine left ventricle together with a transmural ECG. Chromanol 293B (30 micromol/L) was used to block I(Ks) (LQT1 model). Isoproterenol (50 to 100 nmol/L) was used to mimic an increase in beta-adrenergic tone, d-sotalol (100 micromol/L) to block I(Kr) (LQT2 model), and ATX-II (20 nmol/L) to augment late I(Na) (LQT3 model). Isoproterenol+chromanol 293B, d-sotalol, and ATX-II produced preferential prolongation of the action potential duration at 90% repolarization (APD(90)) of the M cell, an increase of TDR, and spontaneous as well as stimulation-induced TdP (LQT1, 3/6; LQT2, 3/6; LQT3, 5/6). Nicorandil (2 to 20 micromol/L) abbreviated the QT interval and APD(90) of the 3 cell types in the 3 models. High concentrations (10 to 20 micromol/L) completely reversed the effects of 293B+/-isoproterenol and those of d-sotalol to increase APD(90) and TDR and to induce TdP in LQT1 and LQT2 models. Nicorandil 20 micromol/L reversed only 50% of the effect of ATX-II and failed to completely suppress TdP in the LQT3 model (5/6 to 3/6). CONCLUSIONS: Our data suggest that K(+) channel openers may be capable of abbreviating the long QT interval, reducing TDR, and preventing spontaneous and stimulation-induced TdP when congenital or acquired LQTS is secondary to reduced I(Kr) or I(Ks) but less so when it is due to augmented late I(Na). (+info)
Clarithromycin associated with torsades de pointes.
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Two cases of QT prolongation and torsades de pointes (TdP) are presented. The patients had been taking clarithromycin (400 mg/day) for respiratory disease. Although erythromycin is reportedly associated with TdP, this is the first report of clarithromycin associated with TdP in the absence of other drugs already known to produce QT prolongation. (+info)