Electrophysiologic substrate of torsade de pointes: dispersion of repolarization or early afterdepolarizations?

Proarrhythmia with class III antiarrhythmic drugs: types, risks, and management

The nature of the proarrhythmic reactions induced by antiarrhythmic drugs is linked to the electrophysiologic effects of these agents. Torsades de pointes is the classic form of proarrhythmia observed during therapy with any drug that prolongs repolarization, for example, the class III agents. Its precise electrophysiologic mechanism is not fully elucidated, although the arrhythmia is generally considered to be due either to early afterdepolarization in the context of prolonged cardiac repolarization or to an increase in spatial or temporal dispersion of repolarization. Among the class III drugs the proarrhythmic risk appears to be lowest for amiodarone, probably due to its complex electrophysiologic profile that may create significant myocardial electrical homogeneity. In the case of d,l-sotalol, the incidence of torsades de pointes increases with dose and the baseline values of the QT interval. Where d-sotalol and other pure class III agents might fall into the varying spectrum of proarrhythmic potential remains unclear. That d-sotalol has been found to increase mortality in postinfarction patients with ventricular dysfunction (the Survival With Oral d-Sotalol [SWORD] trial) is a matter of considerable concern. It raises the possibility that such a phenomenon may be a common property of most, if not all, pure class III compounds. Accordingly, care must be taken to minimize the likelihood of proarrhythmia; in particular, therapy with a class III agent should only be initiated in the presence of a defined indication established on the basis of clinical trials. When class III antiarrhythmic drug-induced proarrhythmia occurs, immediate cessation of therapy with the responsible agent and correction of predisposing factors, such as electrolyte disorders or bradycardia, is mandatory. Intravenous administration of high-dose magnesium sulfate has been demonstrated to be effective in terminating and preventing new episodes of torsades de pointes. Temporary pacing may be necessary.

Sodium channel block with mexiletine is effective in reducing dispersion of repolarization and preventing torsade des pointes in LQT2 and LQT3 models of the long-QT syndrome

BACKGROUND

This study examines the contribution of transmural heterogeneity of transmembrane activity to phenotypic T-wave patterns and the effects of pacing and of sodium channel block under conditions mimicking HERG and SCN5A defects linked to the congenital long-QT syndrome (LQTS).

METHODS AND RESULTS

A transmural ECG and transmembrane action potentials from epicardial, M, and endocardial or Purkinje cells were simultaneously recorded in an arterially perfused wedge of canine left ventricle. d-Sotalol was used to mimic LQT2, whereas ATX-II mimicked LQT3. d-Sotalol caused a preferential prolongation of the M cell action potential duration (APD90, 291+/-14 to 354+/-35 ms), giving rise to broad and sometimes low-amplitude bifurcated T waves and an increased transmural dispersion of repolarization (TDR, 51+/-15 to 72+/-17 ms). QT interval increased from 320+/-13 to 385+/-37 ms. ATX-II produced a preferential prolongation of the M cell APD90 (280+/-25 to 609+/-49 ms) and caused a marked delay in the onset of the T wave and a sharp rise in TDR (40+/-5 to 168+/-40 ms). QT-, APD90-, and dispersion-rate relations were much steeper in the ATX-II than in the d-sotalol model. Mexiletine (2 to 20 micromol/L) dose-dependently abbreviated the QT interval and APD90 of all cell types, more in the ATX-II than in the d-sotalol model, but decreased TDR equally in the two models. Mexiletine 2 to 5 micromol/L totally suppressed spontaneous torsade de pointes (TdP) and reduced the vulnerable window during which single extrastimuli could induce TdP in both models. Higher concentrations of mexiletine (10 to 20 micromol/L) totally suppressed stimulation-induced TdP.

CONCLUSIONS

Our results suggest that although pacing and sodium channel block are very effective in abbreviating the QT interval and TDR in LQT3, these therapeutic approaches may also be valuable in reducing the incidence of arrhythmogenesis in LQT2.

Abnormal QT dispersion in Behçet's disease

Behçet's disease, which was originally described by Hulusi Behçet in 1937, is a generalized chronic inflammatory disease characterized by recurrent oral and genital ulcerations, ocular and dermal manifestations. Cardiac manifestations include pericarditis, myocarditis, conduction system disturbances, coronary arteritis, mitral valve insufficiency, dilated cardiomyopathy, ventricular arrhythmias and sudden cardiac death. There is little knowledge about the mechanism of ventricular arrhythmias in Behçet's disease. In this study, we examined the value of dispersion of ventricular repolarization as a diagnostic tool to assess the risk for ventricular arrhythmias and sudden cardiac death in Behçet's disease. We examined 38 patients (age: 34 +/- 4.6 years, 20F, 18M) with Behçet's disease and 30 age-matched healthy subjects were selected to serve as the control group. Repolarization dispersion parameters were calculated as the difference between maximal and minimal values of QT, QTc, JT and JTC from 12-lead ECG recordings at 25 or 50 mm/s. We found QTd, QTc-d, JTd and JTc-d intervals of 60.65 +/- 16.1, 78.45 +/- 11.4, 71.51 +/- 18.3 and 92.33 +/- 15.4 ms in Behçet's disease patients, these values in control subjects were 40.1 +/- 9.7, 56.36 +/- 7.5, 41.66 +/- 4.3 and 53.92 +/- 9.2 ms respectively (p < 0.001). Striking increases in QT and JT dispersion indicating regional inhomogeneity of ventricular repolarization were noted in patients with Behçet's disease. This new finding suggests a possible explanation for the presence of ventricular arrhythmias in patients with Behçet's disease.

Time- and rate-dependent alterations of the QT interval precede the onset of torsade de pointes in patients with acquired QT prolongation

OBJECTIVES

The purpose of this study was to determine whether the QT interval dynamics that precede torsade de pointes are consistent with the initiation of this arrhythmia by early afterdepolarization-induced triggered activity.

BACKGROUND

Early afterdepolarization-induced triggered activity has been suggested as an electrophysiologic mechanism for torsade de pointes. Consequently, the initiation of torsade de pointes should involve time- and rate-dependent alterations of ventricular repolarization similar to those known to modulate the development of early afterdepolarizations.

METHODS

RR and QT intervals were measured in digitized 24-h ambulatory electrocardiographic recordings obtained from seven patients with acquired prolongation of ventricular repolarization. Each patient had one or more episodes of torsade de pointes. The relation between RR and QT intervals was determined before, during and after multiple episodes of torsade de pointes.

RESULTS

In patients with multiple episodes of ventricular arrhythmias, the onset of the arrhythmias was associated with a critical prolongation of the QT interval. In some episodes, prolongation of the QT interval was associated with sudden prolongation of the sinus cycle length, whereas in other episodes, the QT interval prolonged progressively at a constant cycle length.

CONCLUSIONS

The association between a critically prolonged QT interval and the onset of ventricular arrhythmias suggests that the initial complex of torsade de pointes is an early afterdepolarization-induced triggered response. However, prolongation of the QT interval itself was not sufficient to account for the initiation of torsade de pointes, suggesting that other, as yet unidentified factors are required.

Optical mapping of drug-induced polymorphic arrhythmias and torsade de pointes in the isolated rabbit heart

OBJECTIVES

This study sought to 1) test the hypothesis that in the setting of bradycardia and drug-induced action potential prolongation, multiple foci of early afterdepolarizations (EADs) result in beat to beat changes in the origin and direction of the excitation wave front and are responsible for polymorphic arrhythmias; and 2) determine whether EADs may initiate nonstationary reentry, giving rise to the typical torsade de pointes (TDP) pattern.

BACKGROUND

In the past, it has been difficult to associate EADs or reentry with the undulating electrocardiographic (ECG) patterns of TDP.

METHODS

A voltage-sensitive dye was used for high resolution video imaging of electrical waves on the epicardial and endocardial surface of the Langendorff-perfused rabbit heart. ECG and monophasic action potentials from the right septal region were also recorded. Bradycardia was induced by ablation of the atrioventricular node.

RESULTS

Perfusion of low potassium chloride Tyrode solution plus quinidine led to prolongation of the action potential and the QT interval. Eventually, EADs and triggered activity ensued, giving rise to intermittent episodes of polymorphic arrhythmia. In one experiment, triggered activity was followed by a long episode of vortex-like reentry with an ECG pattern characteristic of TDP. However, in most experiments, focal activity of varying origins and propagation patterns was observed. Triggered responses also showed varying degrees of local block. Similar results were obtained with E-4031. Burst pacing both at control conditions and in the presence of quinidine consistently led to vortex-like reentry whose ECG pattern resembled TDP. However, the cycle length of the arrhythmia with quinidine was longer than that for control ([mean +/- SEM] 194 +/- 12 vs. 132 +/- 8 ms, p < 0.03).

CONCLUSIONS

Drug-induced polymorphic ventricular arrhythmias may result from beat to beat changes in wave propagation patterns initiated by EADs or EAD-induced nonstationary reentrant activity. In contrast, burst pacing-induced polymorphic tachycardia in the presence or absence of drugs is the result of nonstationary reentrant activity.

Early afterdepolarizationlike activity in patients with class IA induced long QT syndrome and torsades de pointes

Early afterdepolarizations (EADs) have been linked to the mechanism of torsades de pointes in long QT syndrome. The purpose of this study was to investigate the role of EADs in Class IA induced torsades de pointes. We studied nine patients with Class IA induced torsades de pointes at the time this arrhythmia was present (acute period, n = 7) and after Class IA therapy was discontinued (chronic period, n = 6). ECGs and monophasic action potentials were recorded in both periods. In the chronic period, electrophysiological studies were performed before and after disopyramide infusion. In the acute period, QTc interval was markedly prolonged (655 +/- 32 ms1/2), and EAD-like activity was recorded in all patients. QTc interval returned to normal (428 +/- 45 ms1/2) and EAD-like activity disappeared after discontinuation of IA drug. Although, in the chronic period, disopyramide infusion prolonged QTc interval from 428 +/- 48 ms1/2 to 479 +/- 31 ms1/2 and induced EAD in three of six patients, the degree was not as marked as observed in the acute period. EADs may play an important role in the genesis of long QT and torsades de pointes. Disopyramide infusion in the chronic period could not reproduce marked repolarization abnormalities and torsades de pointes.

[Torsade de pointes tachycardia during administration of quinidine and verapamil in atrial fibrillation]

The following case report shows that life threatening arrhythmias with fatal consequences may occur after treatment with Cordichin a combination of 80 mg verapamil and 160 mg quinidine. A 65-year-old woman was treated with Cordichin due to atrial fibrillation lasting for 3 months. After the first day of treatment the patient suddenly collapsed with loss of consciousness. The patient was resuscitated 15 min later. The emergency physician diagnosed a cardiac arrest. After cardiopulmonary resuscitation and intubation stable circulation was restored. When the patient was admitted in our hospital the ECG showed numerous ventricular extrasystoles, marked prolongation of the QT interval (700 ms) (Figures 1a and b) and nonsustained polymorphic ventricular tachycardias (Figure 2). The arrhythmias could be suppressed by right ventricular stimulation after inserting a pacemaker lead. After a period of 12 h a normal QT interval was restored (Figure 3). Unfortunately the patient died 3 days later due to irreversible cerebral damage. The concept of suppressing proarrhythmic effects of quinidine by calcium antagonists is discussed. Despite theoretical advantages of a combination therapy this case report shows that life threatening dysrhythmias cannot be prevented by additional calcium antagonism.

d-Sotalol Induces Marked Action Potential Prolongation and Early Afterdepolarizations in M but Not Empirical or Endocardial Cells of the Canine Ventricle

BACKGROUND: Despite its class III antiarrhythmic actions, experimental and clinical studies have shown that d-sotalol can also be proarrhythmic; a recent clinical trial that evaluated d-sotalol in postmyocardial patients (SWORD) had to be prematurely interrupted because of the excess mortality in the treated group. Previous studies have demonstrated the existence of a marked heterogeneity across the ventricular wall; epicardial, endocardial, and M cells have been shown to display distinct electrophysiologic characteristics and pharmacologic behavior. The present study was designed to test the hypothesis that M cells are the primary target for the class III actions of d-sotalol in canine ventricular myocardium and may contribute to its proarrhythmic effects. METHODS AND RESULTS: We used standard microelectrode techniques to record transmembrane activity from endocardial, epicardial, midmyocardial, and transmural strips, isolated from the canine left ventricle. d-Sotalol (100 µM, 60 minutes of exposure, [K(+)]o = 4 mM) prolongs the action potential in the three cell types, but more so in M than epicardial or endocardial cells, especially at the slower rates. At a basic cycle length of 2000 ms, action potential duration after 90% repolarization increases from 199 +/- 20 to 247.5 +/- 28 ms in epicardium (n = 10), from 212 +/- 26 to 274 +/- 27 ms in endocardium (n = 11), and from 309 +/- 65 to 533 +/- 207 ms in M cells (n = 13). d-Sotalol produces a marked steepening of action potential duration-rate relationships of M cells and an upward shift of restitution of action potential duration curves, more accentuated in M cells. Early afterdepolarizations were observed at slow rates (basic cycle lengths > 1000 ms) in 7 of 13 M cell preparation s(54%) but not in endocardial or epicardial preparations. A sudden acceleration of the rate could also induce a transient prolongation of the action potential and early afterdepolarization activity. CONCLUSION: In canine ventricular tissues, d-sotalol manifests its class III effects preferentially in the M cells, leading to the development of early afterdepolarizations and a marked increase in transmural dispersion of repolarization. The data suggest an important role of M cells in the proarrhythmic effects of the drug.

Highlights in cardiovascular effects of histamine and H1-receptor antagonists

Despite numerous studies, the cardiac actions of histamine are still obscure. Yet, histamine could probably be clinically relevant. It is stored in large amounts in human cardiac tissue, where it is contained in the cytoplasmatic granules of mast cells. Mast cells are present in normal human heart tissue; they are more abundant in diseased human heart tissue where they lie in close proximity to blood vessels and between myocytes. The histamine content of human heart mast cells is comparable to the histamine content of lung parenchymal and skin mast cells. Ultrastructural studies confirmed the presence of mast cells around vessels and between myocytes. Consequently, these cells are easily accessible to circulating antigens, drugs and stimuli that activate the cells to release vasoactive mediators which in turn can exert significant cardiovascular effects. Histamine possesses arrhythmogenic effects and once locally released, may enhance automaticity and induce triggering activity resulting in severe tachyarrhythmias. The major arrhythmogenic effects of histamine consist in increasing sinus rate and ventricular automaticity, and in slowing atrioventricular conduction. In addition, histamine may interfere with depolarization and repolarization through its effects on calcium and potassium currents. These effects are mediated by H2-receptor. Therefore direct activation of histamine receptor can induce cardiac arrhythmias. Consequently, the interference of these histaminergic effects may explain, at least in part, the arrhythmogenic effects described for some second-generation antihistamines, such as terfenadine and astemizole. In this brief review we will discuss the cardiac effects of histamine in experimental animal models and in man, and will review data on the safety of the new second-generation antihistamines, focusing on their cardiotoxic effects.

Relationships between electrocardiographic and echocardiographic findings in systemic sclerosis (scleroderma)

We assessed the prevalence of electrocardiographic abnormalities in patients with systemic sclerosis and evaluated their functional significance through a comparison with echocardiographic findings. Seventy-two patients with systemic sclerosis and 64 controls underwent resting electrocardiogram (ECG) and M-mode, two-dimensional, Doppler and color Doppler echocardiography. Electrocardiographic abnormalities were observed in 48.7% of patients. Conduction disturbances (27.7%) infarction pattern (13.8%), non-specific ST-T wave changes (13.8%) and right ventricular hypertrophy (11.1%) were the most frequent abnormalities. QTc interval was significantly longer in patients with systemic sclerosis than in controls. Significant differences between patients and controls were found in the prevalence of long QTc interval (p = 0.0016) infarction pattern (p = 0.0016), right ventricular hypertrophy (p = 0.007) and non-specific ST-T wave abnormalities (p = 0.0016). All patients with infarction pattern and 90% of patients with prolonged QTc interval had some echocardiographic abnormalities. Electrocardiographic signs of right ventricular hypertrophy were 16% sensitive and 93% specific for pulmonary hypertension; the sensitivity and specificity of the combination of right ventricular hypertrophy, right atrial enlargement and right bundle branch block were 35% and 90%, respectively. Standard ECG is useful to assess cardiac involvement in patients with systemic sclerosis. If infarction pattern, right ventricular hypertrophy or long QTc interval are present, a cardiac involvement is very likely.

Frequency-dependent electrophysiologic properties of ventricular repolarization in patients with congenital long QT syndrome

OBJECTIVES

This study was performed to evaluate the frequency dependency of ventricular repolarization and the effect of epinephrine in patients with congenital long QT syndrome (LQTS).

BACKGROUND

The efficacy of pacemakers in addition to antiadrenergic therapy in the treatment of congenital LQTS has been reported.

METHODS

Monophasic action potentials were recorded from right and left ventricular endocardium during atrial pacing at heart rates from 70 to 140 beats/min at baseline and from 100 to 140 beats/min during epinephrine infusion (0.1 microgram/kg body weight per min) in 11 patients with congenital LQTS and 10 control patients. The response of monophasic action potential duration at 90% repolarization (MAPD90) and the dispersion of MAPD90 were examined.

RESULTS

At baseline, both the MAPD90 and the dispersion of MAPD90 were significantly (p < 0.001) longer in the congenital LQTS group than the control group. The differences in these variables between the two groups significantly decreased (MAPD90: from 105 to 31 ms; dispersion of MAPD90: from 55 to 13 ms, p < 0.001) at heart rate was increased. Epinephrine prolonged the MAPD90 and increased the dispersion of MAPD90 significantly (p < 0.001) at all paced heart rates in the congenital LQTS group without frequency dependency but did not change in the control group. Thus, epinephrine increased the differences in these variables between the two groups.

CONCLUSIONS

The repolarization abnormalities in congenital LQTS were attenuated by increasing the heart rate, which supported the efficacy of pacemaker therapy. However, during sympathetic stimulation, the effects of increased heart rate on these repolarization abnormalities were limited.

The electrophysiological mechanism of ventricular arrhythmias in the long QT syndrome. Tridimensional mapping of activation and recovery patterns

We have previously developed a canine in vivo model of the long QT syndrome (LQTS) using the neurotoxin anthopleurin A (AP-A), which acts by slowing sodium channel inactivation. The recent discovery of a genetic mutation in the cardiac sodium channel in some patients with the congenital LQTS, resulting in abnormal gating behavior similar to sodium channels exposed to AP-A, provides a strong endorsement of this animal model as a valid surrogate to the clinical syndrome of LQTS. In the present study, we conducted high-resolution tridimensional isochronal mapping of both activation and repolarization patterns in puppies exposed to AP-A that developed LQTS and polymorphic ventricular tachyarrhythmias (VTs). To map repolarization, we measured activation-recovery intervals (ARIs) using multiple unipolar extracellular electrograms. We demonstrated, for the first time in vivo, the existence of spatial dispersion of repolarization in the ventricular wall and differences in regional recovery in response to cycle-length changes that were markedly exaggerated after AP-A administration. Analysis of tridimensional activation patterns showed that the initial beat of polymorphic VT consistently arose as focal activity from a subendocardial site, whereas subsequent beats were due to successive subendocardial focal activity, reentrant excitation, or a combination of both mechanisms. Reentrant excitation was due to infringement of a focal activity on the spatial dispersion of repolarization, resulting in functional conduction block and circulating wave fronts. The polymorphic QRS configuration of VT in the LQTS was due to either changing the site of origin of focal activity, resulting in varying activation patterns, or varying orientations of circulating wave fronts.

Computer-assisted study of ECG indices of the dispersion of ventricular repolarization

A new computer-assisted method for the quantitative assessment of the dispersion of ventricular repolarization (DVR) has been developed. Through interactive editing of an averaged QRS-T cycle from a 15-lead electrocardiographic (ECG) record (12-lead ECG + XYZ leads), five ECG indices of DVR are automatically computed: they represent the maximal interlead difference of QT and the intervals from the J point to the T wave end, from the J point to the T wave apex, and from the T wave apex to the T wave end. The standard limits of these indices were then established in six clinical groups, including normal subjects and patients with left ventricular hypertrophy, with myocardial infarction, and with intraventricular conduction defect, all subjects being without ventricular arrhythmias and without interacting drugs. The mean values and percentile ranges of all DVR indices were lower in the normal group than in all pathologic groups. The 97.5th percentiles of the QT end dispersion and the JT end dispersion were, respectively, 65 and 76 ms in normal subjects, 84 and 86 ms in patients with inferior MI; 89 and 100 ms in those with anterior MI; 90 and 98 ms in those with left ventricular hypertrophy; and 94 and 99 ms in those with intraventricular conduction defects. This suggests that increased DVR is associated with the varieties of heart disease represented in this study, even in the absence of ventricular arrhythmias, and also that individual measurements of DVR used as predictors of future arrhythmic events should be referred to the standard range of their own clinical group.

Significance of QT dispersion on ventricular arrhythmias in mitral valve prolapse

The present study was designed to detect the arrhythmogenic effect of mitral valve prolapse, and the relationship between QT, QT dispersion and ventricular arrhythmias in subjects with mitral valve prolapse. Sixty-four mitral valve prolapse subjects (24 men and 40 women, mean age 27 +/- 6), and 80 healthy control subjects (32 men and 48 women, mean age 28 +/- 7) were studied. The frequency of ventricular arrhythmias by means of 24-h ambulatory electrocardiographic (ECG) monitoring was investigated. Grade > or = 3 ventricular arrhythmias according to modified Lown and Wolf classification were accepted as complex arrhythmias. The QT intervals were measured from the beginning of depolarization of the QRS complex to the end of the T wave. Using the Bazett formula, QT interval was corrected (QTc) for heart rate. QT dispersion was calculated as the difference between the maximum and minimum QT intervals on any of 12 leads. Premature ventricular complexes seemed to develop in 56 of 64 (87.5%) subjects with mitral valve prolapse. Thirty-eight of the mitral valve prolapse subjects had complex premature ventricular complexes during 24-h ECG and the prevalence of premature ventricular complexes was found to be higher than the control subjects (P < 0.001). We found QT and QTc intervals of 388 +/- 27 and 406 +/- 33 ms in mitral valve prolapse subjects, these values in control subjects were 382 +/- 18 and 402 +/- 11 ms respectively (P > 0.05). QT dispersion and QTc dispersion intervals were 71 +/- 17 and 78 +/- 19 ms in mitral valve prolapse subjects and these values were 43 +/- 16 and 52 +/- 22 ms in control subjects, respectively (P < 0.001). No correlation was found between complex premature ventricular complexes and QT, but there was a correlation between complex premature ventricular complexes and QT dispersion in the mitral valve prolapse subjects. It was concluded that QT dispersion might be a useful marker of cardiovascular morbidity and mortality due to complex ventricular arrhythmias.

Cisapride-induced prolongation of cardiac action potential and early afterdepolarizations in rabbit Purkinje fibres

Cisapride, a gastrointestinal prokinetic agent, has been associated with cases of Torsades de Pointes but its effects on the cardiac action potential have not been described. We investigated its electrophysiological effects on rabbit isolated Purkinje fibres. The results demonstrated that cisapride (0.01-10 microM) lengthened concentration-dependently the action potential duration without modifying other parameters and induced early after depolarizations and subsequent triggered activity. This typical class III antiarrhythmic effect, that showed "reverse" rate-dependence and was reduced by increasing external K concentration, can account for clinical arrhythmogenesis.

The long QT syndrome. A review of recent molecular genetic and physiologic discoveries

There is great reason for optimism in the field of research into the long QT syndrome (LQT). We have made considerable progress, but there is much more to be done. We used molecular genetics to identify genes responsible for 2 forms of LQT (cardiac potassium and sodium channel genes HERG and SCN5A, respectively). These discoveries have led to improved mechanistic undertaking of the disorder and created the possibility for genetic testing. We are working to develop genetic tests for autosomal dominant LQT, but this will require identification of additional LQT genes. Specialized research laboratories like ours can provide genetic testing for many families, but these tests are not yet generally available. These tests may be particularly useful for families with LQT, since asymptomatic LQT gene carriers are still at risk for sudden death. Finally, molecular genetic and physiologic studies offer the possibility of new strategies for treatment and prevention of this cardiovascular disease.

A model study of propagation of early afterdepolarizations

Early afterdepolarizations (EAD's) are irregularities of the cardiac action potential that interrupt or retard repolarization. EAD's have been linked to the development of specific types of cardiac arrhythmias, however, the mechanism underlying the development of these arrhythmias remains unclear. We implemented a two-element kinetic model of the ventricular action potential to investigate a potentially arrhythmogenic form of triggered activity. By approximating EAD's by a sinusoidal driving force, we were able to study the effects of interelement coupling resistivity and sinusoidal frequency and amplitude on the triggering of action potentials. We demonstrated EAD's in a ventricular action potential model by altering the potassium and calcium channels to simulate experimental conditions under which EAD's occur. We also found that triggered activity depends critically on the frequency and amplitude of the driving force and also on the degree of cellular uncoupling between the elements. Our results suggest that triggered activity (due to EAD's) may be suppressed by drugs that improve coupling in unhealthy tissue, or ones that prevent EAD formation by inhibiting calcium channels.

Prevention of class III-induced proarrhythmias by flecainide in an animal model of the acquired long QT syndrome

The influence of the class Ic agent, flecainide, on the incidence of class III-induced torsades de pointes was examined in an animal model of the acquired long QT syndrome. Twenty-four chloralose-anaesthetized rabbits were pretreated at random with flecainide or vehicle and subsequently given a concomitant infusion of methoxamine and the class III antiarrhythmic agent almokalant. In seven out of eight vehicle-treated rabbits, almokalant induced torsades de pointes which was preceded by a significant lengthening of the JTU interval (used as an indirect measure of ventricular repolarization time) by 55 +/- 9.2 msec. Flecainide dose-dependently reduced the incidence of almokalant-induced torsades de pointes. Hence, in a group of rabbits given a low dose (0.14 mumol/kg + 1.4 mumol/kg/hr) of flecainide, four out of eight animals experienced torsades de pointes (P = 0.1538 versus vehicle) whereas no case (n = 8) was observed after a higher dose (4.8 mumol/kg + 4.8 mumol/kg/hr, P = 0.0007). In the former group almokalant induced a maximal increase in the JTU interval not differing from that seen in the vehicle-treated group (58 + 12.1 msec, P > 0.05). Pretreatment with the high dose of flecainide, however, caused a significant attenuation of the almokalant-induced lengthening of the JTU interval (18 +/- 6.5 msec. P < 0.05). It is concluded that flecainide reduces the risk of proarrhythmia in the setting of delayed repolarization partially by attenuating the primary electrophysiological effect of class III agents.

Efficacy and proarrhythmic hazards of pharmacologic cardioversion of atrial fibrillation: prospective comparison of sotalol versus quinidine

OBJECTIVES

This study compared the efficacy and safety of sotalol and quinidine for conversion and prevention of recurrent atrial fibrillation.

BACKGROUND

Atrial fibrillation is the most common arrhythmia. Pharmacologic therapy has been advocated for both immediate restoration of sinus rhythm and prevention of recurrent atrial fibrillation. Quinidine is the therapeutic mainstay for both purposes, but its safety has recently been questioned. Although sotalol has been used successfully to maintain sinus rhythm after direct current cardioversion, its efficacy in pharmacologically reverting atrial fibrillation has not been examined.

METHODS

Fifty consecutive patients with persistent atrial fibrillation were randomized to receive quinidine or sotalol for up to 7 days to restore sinus rhythm. Patients were followed up for 6 months.

RESULTS

Quinidine was more effective than sotalol in terminating atrial fibrillation (60% vs. 20%, p = 0.009). When nonresponders to drug therapy underwent subsequent direct current cardioversion, total conversion rates in the quinidine and sotalol groups were comparable (88% vs. 68%, p = 0.17), as was the efficacy of the two drugs in preventing recurrent atrial fibrillation. Side effects necessitating drug discontinuation were more often observed with quinidine. No patient receiving sotalol but four patients receiving quinidine had drug-associated arrhythmia (torsade de pointes in three patients, sustained ventricular tachycardia in one patient). Precordial QT dispersion determined on the surface electrocardiogram (ECG) increased with quinidine (mean +/- SD 34 +/- 9 vs. 44 +/- 16 ms, p = 0.02), indicating enhanced inhomogeneity in ventricular repolarization. There was no change in QT dispersion in patients receiving sotalol (36 +/- 18 vs. 40 +/- 17 ms, p = 0.44).

CONCLUSIONS

Quinidine is more effective than sotalol in terminating atrial fibrillation but is associated with more side effects. The proarrhythmic risk may be related to quinidine's propensity to increase disparity in ventricular repolarization. This risk warrants careful ECG monitoring during the 1st 4 to 7 days of therapy. Because most proarrhythmic effects occurred shortly after restoration of sinus rhythm, observation should continue > or = 2 to 3 days after sinus rhythm is reestablished.

Spontaneous sequences of onset of torsade de pointes in patients with acquired prolonged repolarization: quantitative analysis of Holter recordings

OBJECTIVES

This study investigated the cycle length changes preceding the spontaneous onset of torsade de pointes in patients with acquired prolonged ventricular repolarization.

BACKGROUND

Torsade de pointes is a polymorphic ventricular tachycardia generally associated with prolonged ventricular repolarization. Because torsade de pointes is not inducible by programmed electrical stimulation, quantitative analysis of Holter recordings of spontaneous episodes may clarify the mechanisms favoring the onset of torsade de pointes in actual clinical conditions.

METHODS

The digitized Holter recordings of 12 patients were analyzed by a computerized Holter system (ATREC). All arrhythmias were grouped according to three classes: 1) isolated premature ventricular beats (n = 47,147, mean/patient [+/- SD] 3,929 +/- 11,571); 2) salvos of 2 to 4 consecutive beats (n = 2,003, mean/patient 167 +/- 359); 3) torsade de pointes > or = 5 beats (n = 105, mean/patient 9 +/- 11). For each patient and class of arrhythmias, six variables were computed from the 10 min and the 10 cycles preceding the event onset.

RESULTS

A significant heart rate increase in the last minute (p < 0.01) and typical oscillatory short-long-short cycle length sequences preceded the onset of arrhythmias, with greater oscillation preceding torsade de pointes than salvos and premature ventricular beats. The cycle lengths preceding the onset were highly correlated with the class of arrhythmias (r = 0.65, p < 0.005) and allowed the correct classification of 69% of events by discriminant analysis (p < 0.0001). A significant negative correlation was observed between the duration of torsade de pointes and the mean length of the initial cycles (r = -0.62, p < 0.001), indicating that longer torsade de pointes had a faster rate than that at onset.

CONCLUSIONS

In patients with acquired prolonged repolarization, the spontaneous onset of ventricular arrhythmias was preceded by an increasing heart rate in the last minute and escalating oscillatory "short-long-short" cycle length patterns, with greater oscillations preceding torsade de pointes than salvos and isolated ventricular beats. These findings suggest that adrenergic- and pause-dependent mechanisms (possibly inducing afterdepolarizations and triggered activity) may have a synergetic role in the genesis of complex ventricular arrhythmias associated with delayed ventricular repolarization.

Torsade de pointes caused by high-dose intravenous haloperidol in cardiac patients

Intravenous haloperidol is the agent of choice for controlling severe agitated delirium in seriously ill cardiac patients in many institutions. Prior reports have proposed that high-dose intravenous haloperidol may be without untoward effects in these patients. Recently, however, a few reports of significant QTc prolongation and torsade de pointes as complications of high-dose intravenous haloperidol therapy have appeared. The present report describes three patients with definite haloperidol-induced QTc prolongation and torsade. In each case, QTc prolongation preceded the arrhythmia and disappeared following the discontinuation of haloperidol. Neither electrolyte imbalance, therapy with other cardiac drugs, bradycardia, ischemia, left ventricular dysfunction, nor other known cause of torsade was present in these patients. It is hypothesized that QTc prolongation and torsade likely are idiosyncratic, unpredictable reactions to high-dose haloperidol in select patients. Careful serial electrocardiographic monitoring and prompt discontinuation of the drug should suffice to prevent this relatively uncommon, life-threatening complication of high-dose intravenous haloperidol.

Electrophysiologic mechanisms of the long QT interval syndromes and torsade de pointes

PURPOSE

To review the current understanding of the mechanisms and treatment of the long QT interval syndromes and torsade de pointes.

DATA SOURCES

Personal databases of the authors and a search of the MEDLINE database from 1966 to 1994.

STUDY SELECTION

Experimental and clinical studies and topical reviews on the electrophysiologic mechanisms and treatment of torsade de pointes were analyzed.

RESULTS

The long QT interval syndromes have been classified into acquired and hereditary forms, both of which are associated with a characteristic type of life-threatening polymorphic ventricular tachycardia called torsade de pointes. The acquired form is caused by various agents and conditions that reduce the magnitude of outward repolarizing K+ currents, enhance inward depolarizing Na+ or Ca2+ currents, or both, thereby triggering the development of early afterdepolarizations that initiate the tachyarrhythmia. The hereditary form appears to result from an abnormal response to adrenergic or sympathetic nervous system stimulation. At least some cases of the hereditary long QT interval syndromes may result from a single gene defect that alters the intracellular regulatory proteins responsible for the modulation of K+ channel function. Treatment of the acquired form is primarily directed at identifying and withdrawing the offending agent, although emergent therapy using maneuvers and agents that favorably modulate transmembrane ion currents can be lifesaving. In torsade de pointes associated with the hereditary long QT interval syndromes, early diagnosis leading to treatments designed to both shorten the QT interval and block the beta-adrenergic-induced instability of the QT interval is essential.

CONCLUSIONS

The long QT interval syndromes and torsade de pointes are potentially life-threatening conditions caused by various agents, conditions, and genetic defects. The mechanisms responsible for these conditions and available treatment options for them are reviewed.

A mechanistic link between an inherited and an acquired cardiac arrhythmia: HERG encodes the IKr potassium channel

Mutations in HERG cause an inherited cardiac arrhythmia, long QT syndrome (LQT). To define the function of HERG, we expressed the protein in Xenopus oocytes. The biophysical properties of expressed HERG are nearly identical to the rapidly activating delayed rectifier K+ current (IKr) in cardiac myocytes. HERG current is K+ selective, declines with depolarizations above 0 mV, is activated by extracellular K+, and is blocked by lanthanum. Interestingly, HERG current is not blocked by drugs that specifically block IKr in cardiac myocytes. These data indicate that HERG proteins form IKr channels, but that an additional subunit may be required for drug sensitivity. Since block of IKr is a known mechanism for drug-induced cardiac arrhythmias, the finding that HERG encodes IKr channels provides a mechanistic link between certain forms of inherited and acquired LQT.

A molecular basis for cardiac arrhythmia: HERG mutations cause long QT syndrome

To identify genes involved in cardiac arrhythmia, we investigated patients with long QT syndrome (LQT), an inherited disorder causing sudden death from a ventricular tachyarrythmia, torsade de pointes. We previously mapped LQT loci on chromosomes 11 (LQT1), 7 (LQT2), and 3 (LQT3). Here, linkage and physical mapping place LQT2 and a putative potassium channel gene, HERG, on chromosome 7q35-36. Single strand conformation polymorphism and DNA sequence analyses reveal HERG mutations in six LQT families, including two intragenic deletions, one splice-donor mutation, and three missense mutations. In one kindred, the mutation arose de novo. Northern blot analyses show that HERG is strongly expressed in the heart. These data indicate that HERG is LQT2 and suggest a likely cellular mechanism for torsade de pointes.

Cardiac electrophysiological actions of the histamine H1-receptor antagonists astemizole and terfenadine compared with chlorpheniramine and pyrilamine

We compared the cardiac electrophysiological actions of two types of H1-receptor antagonists--the piperidines, astemizole and terfenadine, and the nonpiperidines, chlorpheniramine and pyrilamine-in vitro in guinea pig ventricular myocytes and in vivo in chloralose-anesthetized dogs. Astemizole and terfenadine significantly increased action potential duration of guinea pig myocytes. This concentration-dependent prolongation of action potential duration was reverse frequency dependent and led to development of early afterdepolarizations, which occurred more frequently at higher concentrations and slower pacing frequencies. Astemizole and terfenadine potently blocked the rapidly activating component of the delayed rectifier, IKr, with IC50 values of 1.5 and 50 nmol/L, respectively. At 10 mumol/L, terfenadine but not astemizole blocked the slowly activating component of the delayed rectifier, IKs (58.4 +/- 3.1%), and the inward rectifier, IK1 (20.5 +/- 3.4%). Chlorpheniramine and pyrilamine blocked IKr relatively weakly (IC50 = 1.6 and 1.1 mumol/L, respectively) and IKs and IK1 less than 20% at 10 mumol/L. Astemizole and terfenadine (1.0 to 3.0 mg/kg IV) significantly prolonged the QTc interval and ventricular effective refractory period in vivo. Chlorpheniramine and pyrilamine (< or = 3.0 mg/kg) did not significantly affect these parameters. Block of repolarizing K+ currents, particularly IK1, by astemizole and terfenadine produces reverse rate-dependent prolongation of action potential duration and development of early afterdepolarizations, delays ventricular repolarization, and may underlie the development of torsade de pointes ventricular arrhythmias observed with the use and abuse of these agents.

Dispersion of repolarization. Relation to heart rate and repolarization duration

Repolarization duration is highly dependent on heart rate. A major concern when evaluating dispersion of repolarization is the possible influence of heart rate on the magnitude of dispersion. Another consideration relates to a potential relationship between overall duration of repolarization and the magnitude of dispersion, that is, whether patients with longer repolarization duration present with increased or decreased dispersion of repolarization. Therefore, the following relationships were studied in 380 normal subjects, 68 coronary artery disease (CAD) patients, and 41 long QT syndrome (LQTS) patients: the magnitude of dispersion (JTd) versus cycle length (R-R) and dispersion versus repolarization duration (QTc interval). Dispersion of repolarization (JTd), measured as the maximal difference in JT interval duration between precordial leads, was significantly higher in LQTS patients than in normal subjects or CAD patients (120 +/- 72 vs 53 +/- 42 and 48 +/- 22 ms, respectively). In neither normal subjects, CAD patients, or LQTS patients were there significant relationships between the magnitude of dispersion and the R-R interval (r = .094, .158, and .233, respectively; not significant) and between the magnitude of dispersion and QTc duration (r = .0443, -.094, and .126, respectively; not significant). In normal subjects, CAD patients, and LQTS patients, the magnitude of dispersion is not significantly related to heart rate, indicating that there is no need for heart rate adjustment of dispersion parameters. In addition, there is no significant association between the magnitude of dispersion and duration of repolarization.

Diagnostic value of recovery time measured by body surface mapping in patients with congenital long QT syndrome

The QT interval of the resting 12-lead electrocardiogram is normal or borderline in some patients with congenital long QT syndrome (LQTS). Recently, several in vivo experimental studies have shown that the time of maximum dV/dt in the ST-T segment is correlated with the time of local ventricular recovery. The purpose of this study was to examine the value of the body surface recovery time measured by 87-lead body surface mapping for detecting LQTS. Body surface mapping and 12-lead electrocardiography were performed simultaneously in 18 patients with LQTS and 40 controls of similar age and sex. The recovery time (RT), that is, the interval between QRS onset and the time of maximum dV/dt in the ST-T segment, was measured automatically by computer from each of the 87 mapping leads, and the corrected RT (RTc) was calculated by Bazett's method. The QT interval was measured from each of the 12 standard electrocardiographic leads, and the corrected QT (QTc) interval was also calculated. The maximum RT and RTc, the minimum RT and RTc, and the RT and RTc dispersions (difference between maximum and minimum RT and RTc in each patient) were significantly longer in the LQTS group than in the control group. In addition, a maximum RT of 390 msec, a maximum RTc of 430 msec 1/2, an RT dispersion of 170 msec, and an RTc dispersion of 170 msec1/2 separated the 2 groups completely (i.e., no overlap). The maximum QT and QTc, the minimum QT and QTc, and the QT and QTc dispersions (difference between maximum and minimum QT and QTc in each patient) were also significantly longer in the LQTS group than in the control group. However, the maximum QTc was normal (< or = 440 msec1/2) or borderline (< or = 460 msec1/2) in 5 of the 18 LQTS patients, and none of these parameters clearly separated the 2 groups. These results suggest that measurement of RT by 87-lead body surface mapping is useful for diagnosing latent or borderline LQTS.

Dispersion of ventricular repolarization and arrhythmic cardiac death in coronary artery disease

In a recent prospective study of myocardial ischemia, arrhythmic cardiac death occurred in 17 of 936 patients (2%) during a 2-year follow-up after acute myocardial infarction or unstable angina. Dispersion of ventricular repolarization was evaluated on the 12-lead electrocardiogram at enrollment in 17 patients who subsequently died of cardiac arrhythmia and in 51 matched survivors. The aim of this study was to evaluate the relation between various measurements of dispersion of repolarization and subsequent arrhythmic cardiac death, and to determine if dispersion of repolarization makes an independent contribution to the risk of arrhythmic cardiac death. Ventricular depolarization quantitated in terms of mean QRS (QRS-m) duration, and ventricular repolarization quantitated in terms of mean (m), maximal-minimal dispersion (d), standard deviation (s), and coefficient of variation (cv) of QT and JT intervals, were determined. Univariate analyses revealed that 2 standard electrocardiographic parameters, QRS-m and QT-m, and 3 dispersion variables, JT-d, JT-s, and JTc-d, were associated with arrhythmic cardiac death (p < 0.01). Multivariate analyses revealed that the combination of the dispersion parameter (JT-d, JT-s, or JTc-d) and QRS-m made an independent contribution to the risk of arrhythmic cardiac death. The findings highlight the importance of both delayed depolarization and heterogenous repolarization as risk factors for arrhythmic cardiac death. Thus, increased dispersion of repolarization is associated with an elevated likelihood of arrhythmic cardiac death. Prolonged QRS duration and increased dispersion of repolarization make independent contributions to the risk of arrhythmic cardiac death in patients with coronary artery disease.

QT dispersion: problems of methodology and clinical significance

QT dispersion is defined as the difference in QT interval between the different leads of the surface 12-lead ECG. This may provide an indirect measure of the underlying inhomogeneity of myocardial repolarization, which is believed to be important in arrhythmogenesis. Methodology for determining QT dispersion varies significantly between studies, and the results of these studies need to be interpreted in light of the methodology used. Although QT dispersion is developing into an important research tool, as yet it has no established role in clinical practice. Once standardization of methodology is achieved a clinical role may emerge, particularly in the assessment of patients before and after intervention aimed at reduction of arrhythmia risk.

T wave alternans in idiopathic long QT syndrome

OBJECTIVES

The study evaluates the association between T wave alternans and the risk of cardiac events (syncope, aborted cardiac arrest or cardiac death) in a large population of patients with idiopathic long QT syndrome.

BACKGROUND

T wave alternans is an infrequently recorded electrocardiographic (ECG) finding in patients with delayed repolarization, and its clinical significance is not clear.

METHODS

A total of 4,656 ECG recordings in 2,442 patients enrolled in the International Long QT Syndrome Registry were reviewed for episodes of T wave alternans. To determine the risk associated with T wave alternans, independent of corrected QT interval (QTc) duration, patients with T wave alternans were matched for QTc value (every 0.025 s1/2) to patients with long QT syndrome without T wave alternans.

RESULTS

T wave alternans was identified in 30 patients (25 of whom had a QTc interval > 0.50 s1/2). A strong association between QTc prolongation and T wave alternans was observed (odds ratio 1.23 per 0.01-s1/2 unit increase in QTc, p < 0.0001). Conditional logistic regression analyses with adjustment for age, gender, status and QTc value revealed that T wave alternans did not make a significant independent contribution to the risk of cardiac events. The risk of experiencing a major cardiac event was primarily related to length of QTc.

CONCLUSIONS

T wave alternans, a marker of electrical instability and regional heterogeneity of repolarization, identifies a high risk subset of patients with prolonged repolarization. Patients with T wave alternans have an increased risk of cardiac events, but this risk is primarily related to the magnitude of repolarization delay (QTc prolongation). T wave alternans does not make an independent contribution to the risk of cardiac events after adjustment for QTc length.

Clinical relevance of cardiac arrhythmias generated by afterdepolarizations. Role of M cells in the generation of U waves, triggered activity and torsade de pointes

Recent findings point to an important heterogeneity in the electrical behavior of cells spanning the ventricular wall as well as important differences in the response of the various cell types to cardioactive drugs and pathophysiologic states. These observations have permitted a fine tuning and, in some cases, a reevaluation of basic concepts of arrhythmia mechanisms. This brief review examines the implications of some of these new findings within the scope of what is already known about early and delayed afterdepolarizations and triggered activity and discusses the possible relevance of these mechanisms to clinical arrhythmias.

Automatic detection of spatial and dynamic heterogeneity of repolarization

Heterogeneity of ventricular repolarization is associated with the development of life-threatening ventricular arrhythmias. Temporal heterogeneity of repolarization may be manifest in an individual beat (spatial heterogeneity) or in a sequence of beats (dynamic heterogeneity). Spatial inhomogeneity of repolarization throughout the myocardium may be expressed electrocardiographically as dispersion of repolarization durations computed in simultaneously recorded leads. The beat-to-beat changes in the repolarization pattern (duration and/or amplitude) may account for a dynamic (time-dependent) dimension of heterogeneity, occasionally seen as T-wave alternans. A visual detection of heterogeneous repolarization is a time-consuming, observer-dependent, and frequently inaccurate process. Therefore, we developed computer algorithms designed to detect automatically (1) dispersion of repolarization and (2) nonvisible T-wave alternans from digitally recorded (1,000 Hz) X, Y, and Z electrocardiogram leads. This automatic approach was subsequently tested in 10 patients with idiopathic long QT syndrome and in 10 age-matched normal subjects. Long QT syndrome patients presented with significantly higher indices of heterogeneity in comparison with the control subjects; the dispersion of repolarization was 44 +/- 11 and 13 +/- 6 ms, respectively (P < .01), and T-wave alternans index was 0.40 +/- 0.37 and 0.03 +/- 0.06, respectively (P < .01). Simultaneous evaluation of spatial (dispersion of repolarization) and dynamic (T-wave alternans) aspects of repolarization provides new insight into heterogeneity of electrical recovery after myocardial depolarization. The automatic detection of repolarization dispersion and T-wave alternans in digital electrocardiogram recordings provides a practical method to evaluate heterogeneity of repolarization and may be useful for stratifying patients at risk of ventricular arrhythmias.

Short-coupled variant of torsade de pointes. A new electrocardiographic entity in the spectrum of idiopathic ventricular tachyarrhythmias

BACKGROUND

Torsade de pointes is characterized not only by its particular ECG pattern but by its context of congenital or acquired long QT syndrome and the long coupling interval of the initial premature beat.

METHODS AND RESULTS

We observed 14 patients aged 34.6 +/- 10 years (mean +/- SD) with no structural heart disease who presented with syncope related to a typical ECG aspect of torsade de pointes. However, there was no evidence of long QT syndrome, and the torsade had the unusual particularity of an extremely short coupling interval of the first beat or of the isolated premature beats (245 +/- 28 milliseconds). In 10 cases they deteriorated into ventricular fibrillation. Four patients had a familial history of sudden death. Only 2 patients had a tachyarrhythmia inducible by programmed stimulation. At Holter recordings the heart rate variability was globally and significantly depressed, the vagal limb of the autonomic nervous system being predominantly affected. During a mean follow-up of 7 years there were 5 deaths (4 sudden). Nine patients are alive, 3 with implanted defibrillators and 6 treated with verapamil alone. Unlike the other types of antiarrhythmic agents including beta-blockers and amiodarone, verapamil is in our experience the only drug apparently active on the arrhythmias; however, it does not prevent sudden death.

CONCLUSIONS

The short-coupled variant of torsade de pointes should be identified because of their ECG pattern and the risk of sudden death in young adults with no structural heart disease.

Primary hyperparathyroidism presenting as torsades de pointes

Images

Two unusual complications after surgical interruption of an accessory pathway

In a patient with the Wolff-Parkinson-White syndrome, Ebstein's anomaly of the tricuspid valve, a right atrial Chiari net and a patent foramen ovale two unusual complications developed after surgical epicardial dissection combined with cryoablation of the anomalous pathway. The first complication was that ablation of the right atrial wall led to changes in interatrial pressure gradients and the development of a right to left shunt necessitating surgical closure of the atrial septal defect. The second complication was the development of a thrombotic mass in the Chiari net simulating on intracavity tumour, which also had to be removed surgically.

Use-dependent block and use-dependent unblock of the delayed rectifier K+ current by almokalant in rabbit ventricular myocytes

A voltage-clamp analysis of the effect of almokalant on the delayed rectifier K+ current (IK) was made in rabbit ventricular myocytes. The two-suction pipette method was used, and appropriate voltage-clamp protocols were used to study more specifically use dependence, block development, and recovery from block. Almokalant interacted with the IK in two ways: it shifted the activation curve in the hyperpolarizing direction (stimulatory effect) and blocked the open IK channel in a use-dependent way (inhibitory effect). For 2-second voltage clamps to +20 mV, half-maximum block was obtained at 5 x 10(-8) mol/L, with a Hill coefficient of 1.76. Use-dependent block was related to an open-channel block that occurred at 0 mV with a time constant of 1.07 second and a rather slow recovery from block: at -50 mV, recovery time constant was approximately 10 seconds; at -75 mV, recovery was practically absent. The absence of an important recovery at negative membrane potentials is consistent with the hypothesis of the drug being trapped in the channel. A limited frequency-dependent block could be demonstrated. Use-dependent unblock was demonstrated by a rapid recovery from block during stimulation following complete washout of the drug. It is concluded that almokalant shifts the activation curve of IK in the hyperpolarizing direction, blocks the open channel, and is trapped by the closure of the activation gate.

Sotalol. An updated review of its pharmacological properties and therapeutic use in cardiac arrhythmias

Sotalol is a nonselective beta-adrenoceptor antagonist which prolongs cardiac repolarisation independently of its antiadrenergic action (class III antiarrhythmic properties). The antiarrhythmic action of sotalol appears to arise predominantly from its class III properties, and the drug exhibits a broader antiarrhythmic profile than the conventional beta-blockers. Sotalol is effective in controlling paroxysmal supraventricular tachycardias and the ventricular response to atrial fibrillation/flutter in Wolff-Parkinson-White syndrome, in maintaining sinus rhythm after cardioversion of atrial fibrillation/flutter, and in preventing initiation of supraventricular tachyarrhythmias following coronary artery bypass surgery. Sotalol shows promise in the control of nonmalignant and life-threatening ventricular arrhythmias, particularly those associated with ischaemic heart disease. It is effective in suppressing complex forms of ventricular ectopy, displaying superior antiectopic activity to propranolol and metoprolol. The acute efficacy of sotalol in preventing reinduction of sustained ventricular tachyarrhythmias and suppressing spontaneous episodes of these arrhythmias on Holter monitoring is translated into long term prophylactic efficacy against arrhythmia recurrence in approximately 55 to 85% of patients with refractory life-threatening ventricular arrhythmias. In addition, sotalol offers the advantage over the class I agents of reducing cardiac and all-cause mortality in the high risk population with life-threatening ventricular arrhythmias. The adverse effects of sotalol are primarily related to its beta-blocking activity and its class III property of prolonging cardiac repolarisation. Sotalol is devoid of overt cardiodepressant activity in patients with mild or moderate left ventricular dysfunction. The overall arrhythmogenic potential is moderately low, but torsade de pointes may develop in conjunction with excessive prolongation of the QT interval due to bradycardia, hypokalaemia or high plasma concentrations of the drug. In summary, sotalol displays a broad spectrum of antiarrhythmic activity, is haemodynamically well tolerated, and confers a relatively low proarrhythmic risk. It is likely to prove particularly appropriate in the treatment and prophylaxis of life-threatening ventricular tachyarrhythmias.

Use-dependent block of the delayed K+ current in rabbit ventricular myocytes

Block of the delayed K+ current, iK, and the concomitant increase in refractoriness is considered an alternative to a decrease of conduction in the treatment of reentry arrhythmias. Ideally an agent should selectively prolong the action potential at high frequencies. A minimum requirement is use-dependent block. A number of drugs were tested for the existence of use dependence by applying a train of depolarizing clamps to single cardiac myocytes of the rabbit ventricle. Development of block during a long depolarizing clamp and recovery from block were also measured. Five of the nine drugs tested, i.e., disopyramide, encainide, quinidine, sotalol, and tedisamil, did not show use dependence. When a train of depolarizing clamps was applied, block was already present for the first depolarization and did not increase with repetition of the pulse. This result suggests block of the channel in the rested state or a very fast block of the open channel. Almokalant and amiodarone, and to a lesser extent dofetilide and E4031, showed use-dependent block, i.e., block increased during the train of depolarizing clamps. The time constant for the open channel block was 1.07 seconds for almokalant and 0.67 seconds for amiodarone. Recovery from block for almokalant and amiodarone was very slow: time constants measured at -50 mV were 13.9 and 12.7 seconds, respectively. For dofetilide it was in the order of minutes. The existence of this slow recovery explains why frequency-dependent changes in block were negligible or absent for frequencies above 0.5 Hz. Future research should be aimed to select drugs with a slower onset of active state block and faster recovery from block.

T wave alternans associated with amiodarone

T wave alternans is an intriguing phenomenon widely observed in some experimental conditions, but rare in clinics. The authors describe a case of T wave alternans associated with amiodarone treatment. This electrocardiographic finding spontaneously reverted and was not associated with ventricular arrhythmias or hemodynamic depression. Mechanisms that may be responsible for alternation of the T wave are discussed.

Monophasic action potentials in a patient with multiform ventricular tachycardia without QT prolongation

A 41 year old woman had multiform ventricular tachycardia without QT prolongation. Monophasic action potentials were recorded from the right ventricle during the attacks of multiform ventricular tachycardia and effective refractory periods were examined at the same sites. There was no abnormal hump to suggest early afterdepolarisation in the monophasic action potentials, but there was dispersion of the effective refractory period in the right ventricle (80 ms). Stimulation from the right ventricular apex, where the effective refractory period was shortest, reproducibly induced multiform ventricular tachycardia. Two weeks after admission, when her condition was stable, multiform ventricular tachycardia could not be induced and the dispersion of the effective refractory period in the right ventricle was 20 ms.

Experimental models of torsades de pointes

Torsades de pointes is the most typical ventricular tachycardia involving QT-interval prolongation. It is a rather unusual but potentially lethal ventricular tachycardia with a distinctive morphology favored by bradycardia, antiarrhythmic drugs and hypokalemia and requires specific treatment. Torsades de pointes has been shown to be related to bradycardia-dependent early afterdepolarizations (EAD) and/or increased dispersion of repolarization. However, although EAD can be obtained relatively easily in vitro with quinidine or sotalol, torsades de pointes are very difficult to reproduce in animal models. The models of torsades de pointes which have been proposed can be categorized as morphological, EAD-related or pharmacological models. The purpose of the 'morphological' models was to reproduce the twisting of QRS axis typical of torsades de pointes, with no consideration of other aspects such as long QT or bradycardia. These models were produced by epicardial electrical or chemical (aconitine) stimulation at two distant ventricular sites or by overdosing of quinidine in dogs with acute myocardial infarction. The second type of model focused on the conditions producing EAD in vitro. Ventricular tachycardias were obtained in anesthetized dogs using toxics such as cesium or anthopleurine, both producing EAD in vitro. These ventricular tachycardias were shown to be sensitive to magnesium, heart rate and autonomic tone, but torsades de pointes remained rare, at least after cesium injections. The pharmacological models that could be used to study the QT-dependent proarrhythmic effects of drugs are the anesthetized rabbit with alpha-adrenergic stimulation, and the conscious dog model with chronic AV-block and diuretic-induced hypokalemia. Methoxamine-treated anesthetized rabbits develop ventricular tachycardias during clofilium infusions. These ventricular tachycardias, although appearing at very high heart rates, have typical torsades de pointes aspects and are often associated with giant QT waves. The specificity of the model remains to be tested. In our conscious bradycardic and hypokalemic dogs, quinidine and sotalol but not flecainide, propranolol or lidocaine induced QT-dependent arrhythmogenic effects and torsades de pointes. Efficacy of high rate stimulations and magnesium were repeatedly observed. This demanding model, especially designed for qualitative drug comparisons, is also well suited to studies on the mechanisms of initiation of torsades de pointes. The pertinence of these models for estimating the risk of QT-dependent proarrhythmias associated with non-antiarrhythmic agents remains to be tested.

K+ channels and control of ventricular repolarization in the heart

K+ channels form a large family, in which voltage-operated and ligand-operated channels can be distinguished. Under physiological conditions, four K+ currents contribute to the repolarization process and their role is discussed: i) the transient outward current (ito) is responsible for the rapid initial repolarization process from the crest of the action potential to the plateau level; ii) the delayed K+ current (iK) is involved in the overall repolarization process during the plateau; iii) the inward rectifier (iK1) is responsible for the final rapid repolarization and the maintenance of the resting potential; iv) a ligand-operated channel activated by acetylcholine and adenosine participates in the repolarization process and the maintenance of the resting potential in nodal, atrial and Purkinje cells. In the context of antiarrhythmic interventions, block of outward K+ current and prolongation of refractoriness is currently considered as an alternative to block of the Na+ current and reduction of conduction velocity. Although some of these drugs show use-dependent block, the frequency-dependent changes in current and action potential duration are not ideal.

Precordial QT interval dispersion as a marker of torsade de pointes. Disparate effects of class Ia antiarrhythmic drugs and amiodarone

BACKGROUND

Patients with a history of class Ia drug-induced torsade de pointes have been treated with chronic amiodarone without recurrence of torsade de pointes despite comparable prolongation of the QT interval. We hypothesized that in such patients, class Ia drugs cause nonhomogeneous prolongation of cardiac repolarization times, whereas amiodarone causes homogeneous prolongation of cardiac repolarization times.

METHODS AND RESULTS

Thirty-eight consecutive patients who received both class Ia drug therapy and chronic amiodarone therapy were evaluated. Standard 12-lead ECGs at baseline and during each therapy were used to calculate precordial QT interval dispersion (maximum QT in leads V1 through V6 minus minimum QT leads V1 through V6) as a measure of regional variabilities in ventricular repolarization times. Nine of these patients had torsade de pointes during class Ia drug therapy. In these nine patients, class Ia drug therapy and amiodarone significantly prolonged the maximum QT interval to comparable extents. However, class Ia drug therapy but not amiodarone therapy significantly increased precordial QT interval dispersion (101 +/- 37 versus 49 +/- 26 msec; baseline, 44 +/- 12 msec; p = 0.002). In the 29 patients without class Ia drug-induced torsade de pointes, neither class Ia drug therapy nor amiodarone therapy significantly increased QT interval dispersion (50 +/- 6 versus 69 +/- 7 msec; baseline, 54 +/- 5 msec). None of the patients with class Ia drug-induced torsade de pointes had recurrent torsade de pointes during chronic amiodarone therapy.

CONCLUSIONS

An increase in regional QT interval dispersion during class Ia antiarrhythmic drug therapy is associated with torsade de pointes. Chronic amiodarone therapy in patients with a history of class Ia drug-induced torsade de pointes produces comparable maximum QT interval prolongation but does not increase QT interval dispersion. This characteristic may explain its apparent safe use in patients with a history of class Ia drug-induced torsade de pointes.

Torsade de pointes complicating acute myocardial infarction: the importance of autonomic dysfunction as assessed by heart rate variability

Torsade de pointes is a polymorphic ventricular tachycardia associated with QT-interval prolongation rarely reported to occur in the setting of an acute myocardial infarction. Autonomic dysfunction has been implicated as a major stimulus for the development of this dysrhythmia. We describe the case of an 80-year-old woman who presented with an acute myocardial infarction and progressive QT-interval lengthening. An 89-beat run of torsade de pointes occurred during the time of the peak creatine phosphokinase (CPK) without electrolyte abnormalities or antiarrhythmic therapy. Assessment of autonomic tone using power spectral analysis of two consecutive 24-h Holter recordings was performed indicating that a transient decrease in heart rate variability and increase in sympathetic tone preceded the tachyarrhythmia. This case shows the potential usefulness of heart rate variability analysis as a marker for autonomic dysfunction and arrhythmogenesis, particularly during myocardial ischemia.

Direct effect of dobutamine on action potential duration in ischemic compared with normal areas in the human ventricle

BACKGROUND AND OBJECTIVES

The arrhythmogenic effect of beta-adrenoceptor stimulation is complex and may differ in ischemic and normal myocardium. In this study we examined the differential effect of beta-adrenergic stimulation on ventricular action potential duration and, hence, dispersion of repolarization in potentially ischemic versus nonischemic human ventricular myocardium.

METHODS

Simultaneous biventricular monophasic action potentials were recorded in 14 patients (28 recording sites) during infusion of dobutamine in incremental doses (low dose 5 micrograms/kg per min, high dose 10 to 15 micrograms/kg per min) during atrial pacing. Perfusion at the action potential recording site was assessed by incorporating myocardial perfusion scintigraphy with injection of technetium-99m hexakis-2-methoxy-2-methylpropyl-isonitrile during the recording at peak doses of dobutamine. Action potential duration during dobutamine infusion was compared with that during atrial pacing to identical rates in the absence of dobutamine.

RESULTS

In 21 normal zone recordings, dobutamine produced a variable effect over that produced by atrial pacing to identical heart rates, either lengthening or shortening the action potential duration. The mean (+/- SEM) value for the additional effect of dobutamine was 0.9 +/- 2.5 ms with low doses and -4 +/- 2.6 ms with high doses (p = NS). In seven recordings from potentially ischemic zones, low dose dobutamine had a similar effect (mean change -3.4 +/- 6.5 ms; p = NS vs. normal zone values). However, the high dose dobutamine invariably shortened the action potential duration by a mean of -22.9 +/- 2.9 ms. (p less than 0.05 vs. low dose in ischemic areas, p less than 0.01 vs. normal zone recordings). Pacing alone or the addition of dobutamine had no significant effect on the normal dispersion of action potential duration between two nonischemic recording sites. In recordings in a normal and an abnormally perfused site, high dose dobutamine significantly altered the dispersion of action potential duration.

CONCLUSIONS

These results suggest a different effect of beta adrenergic stimulation in potentially ischemic compared with nonischemic human ventricular myocardium. The abnormal dispersion of repolarization thus created may well be important in beta-receptor-mediated arrhythmogenesis during myocardial ischemia.

Hypothesis for the molecular physiology of the Romano-Ward long QT syndrome

OBJECTIVES

The aim of this review was to develop a hypothesis for the molecular pathophysiology of the inherited long QT syndrome.

BACKGROUND

The pathophysiology of the long QT syndrome is unknown. An abnormality of the sympathetic nervous system has been suspected because of the slow heart rates observed and the common precipitation of syncope by adrenergic stimulation (exercise or fright). The characteristic QT prolongation and torsade de pointes arrhythmias suggest a potassium ion (K+) abnormality. Recent findings from molecular biology and genetic linkage analysis studies provide a basis for a new hypothesis that unifies these clinical manifestations.

METHODS

Several recent studies regarding ras proteins were evaluated and correlated. Associations between ras proteins, G protein function and the known features of the long QT syndrome were identified. Based on these associations, a hypothesis for the molecular pathophysiology was developed.

RESULTS

The Romano-Ward long QT phenotype is linked to the Harvey ras-1 gene on chromosome 11 in many, but not all, families. Ras genes exhibit G protein properties, acting as intermediaries in transmembrane signaling pathways including K+ and beta-adrenergic channels. Mutation of ras p21 protein significantly reduces the G protein function of the ras protein, thereby interfering with signal transduction.

CONCLUSIONS

These various studies suggest that a mutation of the disease gene (not necessarily H-ras-1) alters the G protein function of the gene and interferes with delayed rectifier K+ and beta-receptor channel signaling. This hypothesis appears to explain and unify the several characteristic features of the long QT syndrome.

Cardiac alternans: diverse mechanisms and clinical manifestations

OBJECTIVES

The purpose of this review is to assemble the widely dispersed information about cardiac alternans and to categorize the types and mechanisms of alternans, their clinical manifestations and possible therapeutic implications.

BACKGROUND

The phenomena of mechanical and electrical alternans have been of continuing interest to both physiologists and clinicians. Recent studies have enhanced this interest because of the reported association of alternans with experimental myocardial ischemia and cardiac arrhythmias.

METHODS

The review formulates concepts based on extensive review of published studies and personal observations.

RESULTS

Cardiac alternans has been subdivided into the following four categories: 1) mechanical, 2) electrical, 3) in association with myocardial ischemia, and 4) in association with cardiac motion. Mechanical alternans can be explained by hemodynamic or inotropic alterations, or both. Mechanical alternans in the ventricular muscle is accompanied by alternans of action potential shape. In the Purkinje fibers, action potential duration alternates without change in shape and is determined by the duration of the preceding diastolic interval. However, in ventricular muscle fiber, alternans can occur in the presence of constant diastolic intervals. T wave alternans reflects changes in action potential duration and is frequently associated with a long QT interval. Electrocardiographic manifestations of conduction alternans occur at many different sites within the conducting system and myocardium. During myocardial ischemia, additional mechanisms of repolarization alternans have been proposed. Alternans occurring in the presence of a large pericardial effusion is attributed to swinging motion of the heart maintaining two-beat periodicity.

CONCLUSIONS

Since its origin as "pulsus alternans" described by Traube in 1872, the definition of alternans has evolved into a term encompassing multiple physiologic and pathologic phenomena that, although united by the term cardiac alternans, diverge widely with respect to etiology, mechanism and clinical significance.