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

Antiarrhythmic effects of potassium channel openers in rhythm abnormalities related to delayed repolarization

BACKGROUND

Earlier observations have indicated that repolarization-delaying agents may, under certain circumstances, have the propensity to induce polymorphous ventricular tachyarrhythmias (PVTs) (i.e., torsade de pointes). We have studied whether the potassium channel opener pinacidil and two of its pyridylcyanoguanidine analogues (P1075 and P1188) have any antiarrhythmic effects on clofilium-induced PVTs and triggered responses in rabbits in vivo and in vitro.

METHODS AND RESULTS

Anesthetized rabbits were pretreated with propranolol (2 mumol/kg i.v.) and subsequently given a concomitant intravenous infusion of clofilium (63 nmol/kg/min for maximally 15 minutes) and the alpha 1-agonist methoxamine (70 nmol/kg/min). In vehicle-pretreated rabbits (n = 19), clofilium invariably induced PVTs, which closely resembled torsade de pointes and were preceded by a marked prolongation of the QTU interval (27 +/- 2.4%, p less than 0.001). In a separate group of seven rabbits in which monophasic action potentials were recorded from the left ventricular endocardium, the tachyarrhythmia was preceded by deflections consistent with early afterdepolarizations (EADs) of the plateau repolarization phase of the monophasic action potentials. Intravenous administration of the pyridylcyanoguanidines in doses reducing mean arterial blood pressure by 25 or 50 mm Hg, respectively, was associated with a dose-dependent attenuation in the occurrence of clofilium-induced PVTs. In the pinacidil-pretreated rabbits (0.41 mumol/kg or 1.86 mumol/kg i.v.), the occurrence of PVTs was reduced from seven of seven rabbits to five of six and to three of seven rabbits (p = 0.035 versus vehicle-pretreated controls), respectively. In rabbits pretreated with the low dose of P1075 (0.01 mumol/kg i.v.), PVT occurrence was reduced from six of six rabbits to two of six rabbits (p = 0.030), whereas in six rabbits given the high dose of P1075 (0.13 mumol/kg), no PVTs appeared (p = 0.001). When the sulfonylurea glibenclamide (10 mumol/kg i.v.) was administered to rabbits before P1075 (0.13 mumol/kg) was infused, clofilium induced PVTs in five of six rabbits (not significantly different from the incidence in the vehicle-pretreated rabbits). Pretreatment with P1188 (4.36 mumol/kg or 11.88 mumol/kg i.v.) caused a reduction in the occurrence of PVT from six of six rabbits to five of six and to none of six rabbits (p = 0.001), respectively. In the six animals pretreated with the high dose of P1188 in which no clofilium-induced arrhythmias were elicited, glibenclamide (20 mumol/kg i.v.) was injected after the entire dose of clofilium had been administered. In these rabbits, premature ventricular systoles and PVTs appeared within a few minutes in five and four of the animals, respectively. In contrast to the pyridylcyanoguanidines, diltiazem pretreatment (0.9 mumol/kg i.v., decreasing arterial pressure by 50 mm Hg) did not attenuate PVT occurrence (five of six rabbits). Acute administration of P1075 (0.13 mumol/kg) during recurrent attacks of PVTs abruptly regularized the rhythm in 12 of 13 animals and diminished EADs observed in monophasic action potentials recorded from the left ventricular endocardium. In in vitro experiments, action potentials were simultaneously recorded from rabbit Purkinje fibers and ventricular muscle cells. Clofilium markedly prolonged action potential duration in Purkinje fibers but not in ventricular muscle cells, and eventually, bradycardia-dependent EADs and triggered activity were elicited. P1075 completely abolished EADs and triggered activity in all (six of six) experiments. Glibenclamide antagonized the suppressive effect of P1075; hence, EADs and triggered responses reappeared and resembled those present before P1075.

CONCLUSIONS

These results suggest that ATP-sensitive potassium channel activat

BACKGROUND

Earlier observations have indicated that repolarization-delaying agents may, under certain circumstances, have the propensity to induce polymorp

Sotalol-induced torsade de pointes: management with magnesium infusion

A 69 year old woman was treated with sotalol (320 mg daily) for intermittent atrial fibrillation. Sotalol was initially well tolerated and reversion to sinus rhythm with sinus bradycardia occurred 4 weeks after initiation of therapy. Shortly thereafter, the patient developed recurrent syncope due to torsade de pointes. This was treated successfully with intravenous magnesium infusion and withdrawal of sotalol. Subsequently, the atrial fibrillation was adequately managed using amiodarone, with no recurrence of torsade de pointes. Development of bradycardia associated with reversion to sinus rhythm represents a potential cause of 'late' pro-arrhythmic effects of sotalol.

Late potentials in a bradycardia-dependent long QT syndrome associated with sudden death during sleep

The purpose of this study was to determine the incidence of late potentials and their relation to QT prolongation in a family with a high incidence of sudden death during sleep at a young age and bradycardia-dependent QT prolongation (n = 9) and to compare the findings with those in consanguineous family members without QT prolongation (n = 13). Six (67%) of the 9 family members with QT prolongation had late potentials on the signal-averaged electrocardiogram (ECG) compared with 1 of the 13 normal subjects (p less than 0.007). Positive predictive accuracy of the signal-averaged ECG for the detection of subjects with QT prolongation was 86%; negative predictive accuracy was 80%. During exercise testing, the QT interval normalized, whereas late potentials did not change significantly. Exercise testing did not reveal the presence of coronary artery disease as a possible cause of late potentials. It is concluded that 1) compared with family members with a normal QT interval, patients with this type of bradycardia-dependent QT prolongation have a high incidence of late potentials; 2) late potentials persist despite normalization of the QT interval at high heart rates, indicating that there is no direct relation between late potentials and QT prolongation; and 3) late potentials are not caused by coronary artery disease in these subjects. Therefore, the detection of late potentials might be a new aid in the detection and risk stratification of patients with the long QT syndrome. Late potentials possibly indicate a substrate for ventricular tachyarrhythmias in this type of bradycardia-dependent QT prolongation.

Assessment of QT dispersion in symptomatic patients with congenital long QT syndromes

It has been suggested that QT dispersion recorded on the surface electrocardiogram may be a predictor of arrhythmic events in patients with congenital QT prolongation. To evaluate this, 9 patients (6 female, mean age 17.6 years) with congenital long QT syndromes, all of whom had syncope and documented torsades de pointes, were studied. Patients were studied off treatment and during therapy with beta-blocking agents. Three patients were also studied after left stellate ganglionectomy. An age-matched control group was also studied. Good quality 12-lead electrocardiograms were recorded from all patients. For each lead, QT and RR intervals were measured, and QTc value was calculated. QT and QTc dispersions were calculated for each patient. Patients had a significantly longer mean QT interval compared with that of the control group (450 +/- 100 vs 359 +/- 63 ms; p = 0.015) at similar mean RR intervals (736 +/- 231 vs 783 +/- 289 ms), with a longer mean QTc value (0.53 +/- 0.08 vs 0.41 +/- 0.02 s1/2; p = 0.004). Patients also had longer QT and QTc dispersions compared with those of the control group (110 +/- 45 vs 43 +/- 12 ms [p = 0.004], and 0.108 +/- 0.03 vs 0.05 +/- 0.02 s1/2 [p = 0.002], respectively). QT and QTc dispersions on and off beta-blocking agents were not significantly different. Comparing patients with frequent and those with infrequent symptoms, there was no difference in QT or QTc dispersion either off treatment or during therapy with beta-blocking agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of relatively refractory canine myocardium to monophasic and biphasic shocks

BACKGROUND

Certain biphasic waveforms defibrillate at lower energies than monophasic waveforms, although the mechanism is unknown.

METHODS AND RESULTS

The relative ability of monophasic and biphasic shocks to stimulate partially refractory myocardium was compared because defibrillation is thought to involve stimulating relatively refractory myocardial tissue. Shocks of 25-125 V were given during regularly paced rhythm in 11 open-chest dogs. Computerized recordings of shock potentials, and of activations before and after the shocks, were made at 117 epicardial sites. To quantify the shock field strength, the shock potential gradients were calculated at the electrode sites. Monophasic action potential (MAP) electrode recordings, obtained in five dogs, confirmed direct myocardial excitation by the shock, that is, activations beginning during the shock. Tissue was directly excited up to 4 cm from the shocking electrode, and the area directly excited increased as the shock was made stronger or given less prematurely. In six dogs, strength-interval curves for direct excitation were determined from plots of potential gradient versus refractoriness at each electrode site. The biphasic curves were located to the right of the monophasic curves by 8 +/- 4 msec, indicating a lesser ability to excite refractory myocardium. When the gradient at the directly excited border was at least 3.8 +/- 1 V/cm, conduction failed to propagate away from the directly excited zone after the shock, and MAP recordings made near the border showed a shock-induced graded response. This graded response, which prolonged repolarization, may have been responsible for the failure of conduction from the directly excited zone. Although better for defibrillating, the biphasic waveform was thus less effective than the monophasic one in exciting relatively refractory myocardium.

CONCLUSIONS

These results indicated that waveform selection for defibrillation should not be guided solely by the ability of the waveform to stimulate tissue, as these two properties can be discordant.

Dynamics of ventricular repolarisation in the congenital long QT syndromes

Patients with congenital QT interval prolongation are at risk of ventricular arrhythmias and sudden death. It has been suggested that the susceptibility to arrhythmias in these syndromes may be related to the abnormal dynamics of ventricular repolarisation. The dynamics of ventricular repolarisation, including assessment of the effect of changing heart rate on the QT interval and the duration of the right ventricular monophasic action potential, were studied in eight patients with congenital long QT syndromes. The effects of altered sympathetic tone on these dynamics were investigated with isoprenaline, propranolol, and left stellate ganglion block. The rate adaptation of the QT interval was abnormal in only a few patients and in some patients this feature may be related to the severity of the condition. These abnormalities may be exaggerated by isoprenaline and lessened by propranolol and left stellate ganglion block. Monophasic action potential dynamics were normal in all patients. The hypothesis that impaired QT rate adaptation may play a role in the genesis of ventricular arrhythmias in these syndromes is not, in general, supported by the present data. However, in patients with impaired adaptation the normalisation of QT dynamics after beta blockade and left stellate ganglion block was consistent with the efficacy of these forms of treatment.

Clinical correlates of the molecular and cellular actions of magnesium on the cardiovascular system

Magnesium is a ubiquitous element that participates in metabolic processes essential for life. Magnesium acts as a metallic cofactor in more than 300 enzymatic reactions; notably it is essential for all reactions requiring ATP. Magnesium also functions as a transmembrane and intracellular modulator of other ions. Altered magnesium homeostasis, particularly a deficiency, can cause alterations in metabolic functions that result in clinically recognizable events. Recognition of magnesium deficiency is problematic, since there is no test that will reliably and consistently detect this condition. A high index of suspicion for magnesium deficiency is necessary and treatment should be given when indicated. This article reviews the molecular and cellular actions of magnesium and correlates these basic scientific findings with clinically recognized cardiovascular events in humans. In addition, management guidelines are delineated.

Torsades de pointes and early afterdepolarizations

It is suggested that torsades de pointes may be only one of a group of arrhythmias that are characterized by being pause induced or bradycardia induced. A distinction is made between the cause of the "twisting of the points" and the cause of the action potentials that initiate and sustain the tachycardia. It is pointed out that torsades de pointes and other pause-induced arrhythmias share many features with rhythmic activity arising from early afterdepolarizations. Both are seen after pauses or at low rates, both are seen in quinidine intoxication, and both are seen in hypokalemia. The short-long-short sequence that is seen in torsades de pointes and certain other pause- or bradycardia-induced arrhythmias can be fully explained by the behavior of rhythmic activity initiated and sustained by early afterdepolarizations, as can the abrupt onset and termination of pause-induced arrhythmias and their tendency to show initial warming up and terminal slowing down.

Torsades de pointes: arrhythmia, syndrome, or chimera? A perspective in the light of the Lambeth Conventions

What is torsades de pointes? Is it an arrhythmia or a syndrome? The distinction is critical. In this article I have attempted to explain why this is so. Both from the clinical and nonclinical standpoint, it is of overriding importance that torsades de pointes be amenable to measurement and quantification. This is the fundamental prerequisite for any variable to be of value as an endpoint in an investigation. Measurement and quantification require that a variable has an objective definition that is both inclusive and exclusive. In his seminal work, Dessertenne coined the term torsades de pointes to describe an arrhythmia with unusual features. However, torsades de pointes has been reinterpreted and redefined by Dessertenne's successors. It was originally described as occurring in certain settings (e.g., hypokalemia). However, this has been reinterpreted to mean that a specific set of antecedent conditions (such as hypokalemia) are part of the definition of torsades de pointes. If this is the case, then torsades de pointes is a syndrome, not an arrhythmia. For those more concerned with arrhythmias than with syndromes, the key issue to be determined is what to call the arrhythmia that is part of the syndrome. I have put forward some suggestions with the objective of answering this question, using the Lambeth Conventions as a guide. I believe that there is strong case for ventricular tachyarrhythmias to be classified simply as tachycardia or fibrillation, with the optional use of the term delayed repolarization syndrome in cases where a long QT interval is present. The latter term should be used as one might use the term acute myocardial ischemia, i.e., to denote an underlying condition; it should not be used to define the arrhythmia itself. Twenty-four years after its introduction, the term torsades de pointes has now become a chimera and is best abandoned.

Torsades de pointes: arrhythmia, syndrome, or chimera?

Although torsades de pointes is not a tightly demarcated entity, the diagnosis has important clinical implications. The concept should be retained.

Electrophysiologic mechanisms involved in the development of torsades de pointes

Torsades de pointes (TDP) is a polymorphic ventricular tachycardia with a peculiar electrocardiographic pattern of continuously changing morphology of the QRS complex twisting around an imaginary baseline. The clinical setting under which TDP develops covers many clinico-pathologic conditions, including the long QT syndrome (LQTS). In the present review, we analyze the evolution of the hypotheses for the mechanisms underlying TDP and we discuss some of the experimental models used and their related clinico-pathologic counterparts. Together with the hypothesis that TDP represents a form of reentrant arrhythmia, recent evidence has suggested the possibility that triggered activity may indeed be responsible for TDP. Data collected in vitro are presented that demonstrate a role for catecholamines in the development of afterpotentials in ventricular tissue. Whether adrenergic-mediated afterdepolarizations are the mechanism responsible for TDP in the clinical setting of LQTS has not yet been proven and remains an important area of investigation.

Antiarrhythmic properties of tedisamil (KC8857), a putative transient outward K+ current blocker

1. Rats were used to evaluate the antiarrhythmic properties of tedisamil, a novel agent with the electrophysiological properties of a Class III antiarrhythmic drug. Tedisamil was tested against coronary artery occlusion-induced arrhythmias in conscious animals. 2. The actions of tedisamil on the ECG, as well as responses to electrical stimulation, were compared with those on the configuration of epicardial intracellular action potentials recorded in vivo. 3. Tedisamil (1-4 mg kg-1, i.v.) caused bradycardia, elevated blood pressure and dose-dependently reduced ventricular fibrillation (VF) induced by occlusion of the left anterior descending coronary artery. Other ischaemia-associated arrhythmias were not so well suppressed. Antiarrhythmic activity was greatest when the tedisamil-induced bradycardia was prevented by electrically-pacing the left ventricle. 4. Tedisamil dose-dependently lengthened the effective refractory period and prevented electrically-induced VF. In vivo, tedisamil (0.5-4 mg kg-1, i.v.) prolonged the duration of epicardial intracellular action potentials by up to 400%. 5. Results showed that tedisamil possessed antifibrillatory actions in rats that were related to Class III electrophysiological actions as revealed by electrical stimulation and electrophysiological analyses.