Ventricular arrhythmias: why is it so difficult to find a pharmacologic cure?

Combined effect of disopyramide and erythromycin on ventricular repolarization in dogs with complete atrioventricular block

The combined effects of disopyramide (DP) and erythromycin (EM) on ventricular repolarization and the inciden-ces of ventricular premature contractions (VPCs) and torsades de pointes (TdP) were investigated in 12 anesthetized dogs with complete atrioventricular block. Monophasic action potentials (MAPs) were measured from the left and right ventricular (LV and RV) endocardium. The right or left ventricle was paced at a cycle length of 750-1000 msec. Dogs were divided into 2 groups and given either intravenous DP at 3 mg/kg and then intravenous EM at 50 mg/kg (group 1, n = 8), or intravenous EM at 50 mg/kg and then intravenous DP at 3 mg/kg (group 2, n = 4). MAP duration at 90% repolarization (MAPD(90)) was measured before drug administration (baseline) and again after administration of each drug. RV MAPD(90) and LV MAPD(90) increased significantly (P < 0.02) after administration of each drug in group 1 (RV MAPD(90): from 247.0 ± 36.3 [baseline] to 283.5 ± 38.3 to 321.8 ± 56.7; LV MAPD(90): from 262.6 ± 49.1 (baseline) to 296.1 ± 58.8 to 351.0 ± 80.6). Early afterdepolarizations developed in 2 group 1 dogs after administration of DP and in 4 additional dogs after administration of EM. Frequent VPCs occurred in 1 dog after administration of DP and in 2 additional dogs after administration of EM, and TdP and ventricular tachycardias developed in 2 of the 3 dogs after administration of EM. Similar trends occurred in group 2. These results indicate a potentially fatal interaction between DP and EM administered in clinically relevant doses.

Effect of the ATP-sensitive K⁺ channel opener nicorandil in a canine model of proarrhythmia

Increased action potential duration (APD) induces early afterdepolarization (EAD) in vitro and torsade de pointes in vivo, and ATP-sensitive K(+) channel openers decrease APD in cardiac tissue. We tested whether the ATP-sensitive K(+) channel opener nicorandil has antiarrhythmic effects on class III antiarrhythmic drug-induced ventricular arrhythmia. In 10 anesthetized dogs with chronic atrioventricular block, we recorded monophasic action potentials (MAPs) from the left and right ventricular (LV and RV) endocardium. The class III antiarrhythmic drug nifekalant (1 mg/kg, IV) was administered at 5 minute intervals (total doses; 2-6 mg/kg) until the appearance of EADs, premature ventricular contractions (PVCs), or polymorphic ventricular tachycardias (PVTs). Five minutes after the end of nifekalant administration, nicorandil (1.0 mg/kg) was administered over 5 minutes. Nifekalant decreased the ventricular escape rate from 75 ± 5 beats/minute to 45 ± 10 beats/minute and increased RV-MAP duration (MAPD) from 217 ± 32 msec to 308 ± 2 msec (P < 0.01) and LV-MAPD from 232 ± 32 msec to 353 ± 82 msec (P < 0.01). EADs were recorded in 9 dogs, frequent premature ventricular contractions (PVCs) developed in 10 dogs, incessant PVTs developed in 3 dogs, and monomorphic ventricular tachycardia developed in 3 dogs after nifekalant administration. Nicorandil decreased RV-MAPD to 267 ± 57 msec and LV-MAPD to 279 ± 44 msec. It suppressed EADs, decreased the incidence of PVCs, and abolished PVT. Nicorandil may be clinically useful for treatment of PVCs and PVTs accompanying acquired long QT syndrome.

Endocardial arrhythmogenic mechanisms of torsades de pointes in patients with the congenital long QT syndrome

We injected acetylcholine (Ach) into the coronary artery to ascertain whether coronary vasospasm contributed to the syncopal events or chest oppression suffered by 3 patients with long QT syndrome (LQTS). During the test, a quadripolar electrode catheter was placed in the right ventricle and the activation-recovery interval was reanalyzed from the stored data. Intracoronary Ach transiently prolonged the QT intervals in all 3 patients without inducing coronary vasospasm. The Ach-induced QT prolongation was associated with enhanced spatial and temporal dispersion of intra-ventricular repolarization. The electrophysiological abnormalities were consistent with the putative arrhythmogenic mechanisms identified in experimental studies of LQTS.

Effects of sympathetic stimulation on various repolarization indices in the congenital long QT syndrome

Sympathetic stimulation or catecholamines modulate ventricular repolarization and provoke ventricular tachyarrhythmias in a variety of heart diseases and conditions. Among those, the congenital form of long QT syndrome (LQTS) has long been known to be a Rosetta stone for sympathetic-related ventricular tachyarrhythmias. Recent experimental studies employing arterially-perfused ventricular wedge preparations as well as some clinical studies have greatly advanced our knowledge of the cellular mechanism of the T wave and the various repolarization indices in the ECG, as well as the effect of sympathetic stimulation on these repolarization indices under normal and long QT conditions. Differences in the time course of repolarization of the three predominant cell types, the epicardial, midmyocardial (M), and endocardial cells, across the ventricular wall give rise to voltage gradients responsible for the inscription of normal T waves as well as the manifestation of abnormal T waves in the congenital LQTS. The data from the wedge experiments suggest that the repolarization time of the longest M cell action potential determines the Q-Tend interval, while that of the epicardial action potential determines the Q-Tpeak interval. Therefore, Tpeak-end interval in the ECG may provide an index of transmural dispersion of repolarization (TDR). In this review article, sympathetic stimulation with isoproterenol or epinephrine infusion is demonstrated to modulate differentially these repolarization indices in the ECG as well as the action potentials of the three cells between the LQT1, LQT2, and LQT3 syndromes both experimentally and clinically, explaining the differences in the sensitivity of genotypes of congenital LQTS to sympathetic stimulation.

Effects of Intravenous Magnesium in a Prolonged QT Interval Model of Polymorphic Ventricular Tachycardia Focus on Transmural Ventricular Repolarization

BACKGROUND

This study was performed to clarify the antiarrhythmic effects of magnesium sulfate (Mg(++)) in a prolonged QT interval canine model of polymorphic ventricular tachyarrhythmia (VTA).

METHODS

In six experiments in a canine model of prolonged QT by anthopleurin-A, Mg(++) was administered in boluses of 0.2 mL/kg during repetitive episodes of self-terminating polymorphic VTA or frequent premature ventricular complexes (PVCs). The distribution of ventricular repolarization across the left ventricular(LV) wall and dispersion of transmural repolarization were analyzed before, and 30 and 120 seconds after Mg(++) administration, during ventricular pacing at 100 bpm. Transmural unipolar electrograms were recorded from multipolar needle electrodes, and local activation-recovery intervals (ARI) were measured.

RESULTS

Mg(++) rapidly eliminated self-terminating polymorphic VTA and all isolated PVCs. During ventricular pacing at 100 bpm, Mg(++) caused modest shortening of ARI at all recording sites. Since the magnitude of ARI shortening was greater at mid-myocardial sites than at other ventricular sites, mean transmural ARI dispersion decreased from 80 +/- 22 to 45 +/- 18 ms within 30 seconds after Mg(++) injection. However, this effect was transient, and, at 120 seconds after Mg(++) administration, ARI had increased all sites and transmural ARI dispersion lengthened to 65 +/- 18 ms. Besides suppression of triggered premature activity, homogenization of transmural ventricular repolarization was associated with the antiarrhythmic effects of intravenous Mg(++) in this model.

CONCLUSION

Since these effects were transient, a continuous intravenous infusion of Mg(++) is preferred to prevent recurrences of VTA.

Chronic amiodarone effects on epicardial conduction and repolarization in the isolated porcine heart

Amiodarone is a potent antiarrhythmic agent with complex chronic effects, notably on repolarization and conduction, that are not fully understood. Its low arrhythmogenic potential has been related to a lack of increase in repolarization dispersion. Since its effects are not documented in pigs we conducted a mapping study of activation and repolarization in isolated perfused porcine hearts. Amio20 female pigs (n = 7) received amiodarone 20 mg/kg per day over 4 weeks while Amio50 female pigs (n = 7) received 50 mg/kg per day over 4 weeks. Concentrations of the drug encompassed values found in clinical studies. Then, activation patterns and activation-to-recovery intervals (ARI) were mapped epicardially from 128 unipolar electrograms in isolated perfused hearts in corroboration of epicardial action potential recordings. Mean ARI was longer in Amio20 experiments compared to the seven control hearts (325 +/- 11 ms vs 288 +/- 5 ms at 1,000 ms), whereas ARI dispersion was not different, being comprised between 7 and 11 ms and generating smooth gradients. In Amio50 experiments, mean ARI was further prolonged (390 +/- 10 ms at 1,500 ms) with an exaggerated reverse rate dependence concomitant with a depressant effect on the plateau of the action potential. Again, ARI dispersion did not differ from controls. Finally, the drug depressed the maximal rate of depolarization (Vmax) and slowed conduction in a rate dependent and concentration dependent fashion. In conclusion, chronic amiodarone induces Class I and Class III antiarrhythmic effects in ventricular porcine epicardium that are concentration dependent but does not affect dispersion of repolarization. This may partly explain its low arrhythmogenic potential.

Transmural heterogeneity of ventricular repolarization under baseline and long QT conditions in the canine heart in vivo: torsades de pointes develops with halothane but not pentobarbital anesthesia

INTRODUCTION

In vitro studies have provided evidence for the existence of M cells. The present study examines the contribution of the M cell to transmural dispersion of repolarization (TDR) and to the development of torsades de pointes (TdP) in the canine heart in vivo in animals anesthetized with either pentobarbital or halothane.

METHODS AND RESULTS

Monophasic action potentials (MAPs) were recorded from 4 to 7 transmural sites, before and after d-sotalol. Cells displaying the longest MAP duration (MAPD) generally were localized to the deep subendocardium to mid-myocardium (M region) in the anterior wall of the left ventricle. d-Sotalol preferentially prolonged the MAPD of the M region, increasing TDR significantly more (P < 0.05) in animals anesthetized with halothane (31+/-5 to 88+/-17 msec) than in those receiving pentobarbital (24+/-9 to 53+/-7 msec; basic cycle length 1,500 msec). In halothane-anesthetized dogs, a remarkable transient increase in M cell MAPD followed interpolation of one or more extrasystole(s), leading to a transient increase in TDR and TdP. TdP was never observed with pentobarbital anesthesia.

CONCLUSION

Our results demonstrate that transmural heterogeneity of repolarization is amplified under acquired long QT conditions and that the increase in TDR underlies the development of TdP in halothane- but not pentobarbital-anesthetized dogs. The data support an important contribution of M cells to TDR and to the development of TdP in the canine heart in vivo. Our data also highlight the importance of acceleration-induced prolongation of MAPD (a phenomena observed principally in M cells) in the development of TdP.

Effects of 17beta-estradiol on tachycardia-induced changes of atrial refractoriness and cisapride-induced ventricular arrhythmia

INTRODUCTION

Gender difference is known to be associated with the occurrence of arrhythmia. However, the effects of female sex hormone on atrial electrophysiology, and on the occurrence of torsades de pointes (TdP) induced by cisapride have been unclear.

METHODS AND RESULTS

Two experiments were included in this study. In experiment 1, effective refractory periods (ERPs) from five epicardial atrial sites were measured before and after rapid atrial pacing at 800 beats/min for 30 minutes in dogs with pretreatment of verapamil (n = 10), 17beta-estradiol (n = 10), or without pretreatment (n = 10, control group). In experiment 2, limb-lead ECG and monophasic action potentials in the left and right ventricles were recorded before and after each dose of cisapride (2 to 6 mg/kg) during different ventricular rates in dogs with (n = 9) and without (n = 14) concomitant administration of 17beta-estradiol (0.3 microg/kg). After 17beta-estradiol administration, there were greater atrial ERPs in the study dogs than in the control group. The atrial ERPs were shortened significantly after rapid atrial pacing, but the degree was greater in the control group than in the dogs pretreated with verapamil or 17beta-estradiol. Moreover, the recovery of atrial ERPs was faster in dogs pretreated with verapamil or 17beta-estradiol than in the control group. In experiment 2, cisapride prolonged the QT interval and biventricular APD90 and induced early afterdepolarizations (EADs) in a dose-dependent manner. However, dogs receiving cisapride combined with 17beta-estradiol had a greater increase of ventricular repolarization and a higher incidence of EADs than those receiving cisapride only. Moreover, dogs receiving cisapride combined with 17beta-estradiol (3/9, 33%) had a greater incidence of TdP than those receiving cisapride only (0/14, 0%, P < 0.05).

CONCLUSIONS

17beta-estradiol has a significant effect on atrial electrophysiology, which may be related to the prevention of atrial fibrillation. However, the high incidence of TdP in dogs receiving cisapride combined with 17beta-estradiol suggests that the female sex hormone is an important risk factor of cisapride-induced proarrhythmia.

Chronic amiodarone reduces transmural dispersion of repolarization in the canine heart

INTRODUCTION

Amiodarone is a potent antiarrhythmic agent used in the management of both atrial and ventricular arrhythmias. In addition to its beta-blocking properties, amiodarone is known to block the sodium, potassium, and calcium channels in the heart. Its complex electropharmacology notwithstanding, the reasons for the high efficacy of the drug remain unclear. Also not well understood is the basis for the low incidence of proarrhythmia seen with amiodarone relative to other agents with Class III actions. The present study was designed to examine the effects of chronic amiodarone in epicardial, endocardial, and M cells of the canine left ventricle.

METHODS AND RESULTS

We used standard microelectrode techniques to record transmembrane activity from endocardial, epicardial, mid-myocardial, and transmural strips isolated from the canine left ventricle. Tissues were obtained from mongrel dogs receiving amiodarone orally (30 to 40 mg/kg per day) for 30 to 45 days or from untreated controls. Chronic amiodarone produced a greater prolongation of action potential duration in epicardium and endocardium, but less of an increase, or even a decrease at slow rates, in the M region, thereby reducing transmural dispersion of repolarization. In addition, chronic amiodarone therapy suppressed the ability of the IKr blocker, d-sotalol, to induce a marked dispersion of repolarization or early afterdepolarization activity.

CONCLUSION

Our data demonstrate for the first time a direct effect of chronic amiodarone treatment to differentially alter the cellular electrophysiology of ventricular myocardium so as to produce an important decrease in transmural dispersion of repolarization, especially under conditions in which dispersion is exaggerated. These results may contribute to our understanding of the effectiveness of amiodarone in the treatment of life-threatening arrhythmias as well as to our understanding of the low incidence of proarrhythmia attending therapy with chronic amiodarone in comparison with other Class III agents.

Differential effects of D-sotalol, quinidine, and amiodarone on dispersion of ventricular repolarization in the isolated rabbit heart

INTRODUCTION

Increased dispersion of ventricular repolarization has been suggested as a cause of proarrhythmic effects of Class IA or III antiarrhythmic drugs, such as d-sotalol, quinidine, and amiodarone.

METHODS AND RESULTS

The influence of d-sotalol, quinidine, and amiodarone on the dispersion of monophasic action potential (MAP) durations was studied in 55 isolated Langendorff-perfused rabbit hearts at different pacing cycle lengths (CLs). MAP duration measured at 90% repolarization (APD90) was determined from 6 to 8 endocardial and epicardial MAP recordings with dispersion of ventricular repolarization defined as the range of APD90. The protocol was repeated 60 minutes after initiation of a perfusate containing increasing concentrations of d-sotalol (n = 12, 10[-6] M, 10[-5] M, and 5 x 10[-5] M) and quinidine (n = 8, 10[-6] M and 10[-5] M). Seventeen rabbits were fed with an aqueous solution of amiodarone (50 mg/kg per day over 4 weeks). The data of these experiments (n = 17) were compared with a series of 18 untreated control rabbits. Dispersion of ventricular repolarization was unchanged with the low concentration of d-sotalol (10[-6] M) but was increased-particularly at long CLs-with higher d-sotalol concentrations. With both concentrations of quinidine, dispersion of ventricular repolarization was increased in a rate-independent manner. Amiodarone did not affect dispersion of ventricular repolarization.

CONCLUSIONS

Rate-dependent and concentration-dependent increases in dispersion of ventricular repolarization by d-sotalol and quinidine in this isolated rabbit heart model may help explain their proarrhythmic effects while the absence of an increase in dispersion of ventricular repolarization with amiodarone correlates with its clinically observed lower incidence of proarrhythmia.

Electrophysiologic features of torsades de pointes: insights from a new isolated rabbit heart model

INTRODUCTION

The exact electrophysiologic mechanism of torsades de pointes (TdP) is under intense investigation. No isolated animal heart model of this particular arrhythmia exists.

METHODS AND RESULTS

In isolated rabbit hearts, TdP was induced by means of bradycardia in the presence of a high concentration of d-sotalol (10(-4) M) and shortly after lowering the concentration of potassium and magnesium in the perfusate. Multiple simultaneous epicardial and endocardial monophasic action potentials (MAPs) and volume-conducted 12-lead ECGs were recorded. d-Sotalol prolonged repolarization and increased dispersion of ventricular repolarization compared to baseline recordings. With the onset of low potassium and magnesium concentrations, repolarization was further prolonged and dispersion of repolarization was further increased followed by the occurrence of early afterdepolarizations (EADs) in the majority of MAP recordings, i.e., at both endocardial and epicardial locations of both ventricles. Upon increase of EAD amplitude, triggered arrhythmias with TdP of up to 42 beats ensued in 10 of 11 hearts studied. MAP duration at 90% repolarization (APD90), dispersion of APD90, and the incidence of EADs as well as dispersion of the QT interval and T wave area were significantly higher in beats triggering bigemini, couplets, or runs of TdP.

CONCLUSION

TdP observed in this new isolated heart model was associated with markedly increased dispersion of ventricular repolarization and the occurrence of EADs in multiple locations of the heart. TdP is initiated when the amplitude of an EAD reaches threshold for initiation of the first beat of an episode.

Influence of epinephrine, propranolol, and atrial pacing on spatial distribution of recovery time measured by body surface mapping in congenital long QT syndrome

INTRODUCTION

Sympathetic stimulation plays an important role in the genesis of QT(U) prolongation and ventricular arrhythmias in congenital long QT syndrome (LQTS). Permanent pacemaker as well as beta blockers are reported to be effective in the management of this syndrome. The purpose of this study was to examine influence of epinephrine (alpha- and beta-adrenergic stimulation), propranolol (beta blocker), and atrial pacing on the spatial distribution of repolarization using body surface recovery time (RT) in congenital LQTS.

METHODS AND RESULTS

Body surface mapping was recorded in 16 patients with congenital LQTS and 20 control patients before and after epinephrine infusion (0.1 microg/kg per min), oral propranolol (1 to 2 mg/kg per day), addition of epinephrine during oral propranolol, atrial pacing at a cycle length of 600 or 750 msec, and addition of epinephrine during atrial pacing. The RT, that is, the interval between the QRS onset and the maximum dV/dt point in the ST-T segment, was measured automatically by a computer from each of the 87 mapping leads, and the corrected RT (RTc) was calculated using Bazett's method. In patients with congenital LQTS, epinephrine markedly changed the T(U) wave morphology and spatial distribution of RT, especially the distribution of maximum RT of the left anterior chest and back. Epinephrine prolonged the maximum RTc and the minimum RTc in 87 leads and increased the RTc dispersion (difference between maximum and minimum RTc in each patient). Neither propranolol nor atrial pacing changed the T(U) wave morphology, spatial distribution of RT, or any RTc parameters at rest. Propranolol prevented the influences of epinephrine on the T(U) wave morphology, spatial distribution of RT, and RTc parameters, whereas atrial pacing did not. In control patients, marked changes of the T(U) wave morphology and RTc parameters were not recognized during the entire protocol.

CONCLUSIONS

Our results indicate that epinephrine markedly changes the spatial distribution of repolarization and increases the dispersion of repolarization, which probably are linked to arrhythmogenesis in congenital LQTS. The data suggest that propranolol but not atrial pacing are effective to suppress repolarization abnormalities during sympathetic stimulation.

Reproducible induction of "atypical" torsades de pointes by programmed electrical stimulation: a novel form of sotalol-induced proarrhythmia?

We present a patient with sotalol-induced polymorphic ventricular tachycardia that was seen only with programmed ventricular stimulation. Electrophysiologic studies performed prior to initiation of sotalol therapy revealed inducible monomorphic ventricular tachycardia. Possible underlying electrophysiologic mechanisms are discussed.

T wave alternans in idiopathic long-QT syndrome: insight from body surface mapping

Body surface mapping was recorded during T wave alternans in two patients with idiopathic long QT syndrome. Eighty-seven lead ECGs of two consecutive beats during T wave alternans demonstrated that alternating changes in the morphology and polarity of the T wave existed mainly in the left frontal chest. The QRST isointegral maps of the same two consecutive beats showed alternation of a large negative area appearing mainly in the left frontal chest. These results indicate that T wave alternans in the two patients were related to alternate heterogeneous prolongation of the action potential duration in this region.

Phenylephrine-induced ventricular arrhythmias in dogs with inherited sudden death

INTRODUCTION

Dogs with an inherited predisposition to sudden death display ventricular arrhythmias having certain characteristics, such as pause dependence, that are suggestive of early afterdepolarization-induced triggered activity. We hypothesized that alpha-adrenergic stimulation may facilitate the development of these arrhythmias by inducing a reflex bradycardia and by exerting a direct myocardial effect.

METHODS AND RESULTS

Twenty affected dogs and 7 unaffected dogs were studied. The incidence and severity of ventricular arrhythmias were determined after administration of phenylephrine (0.01 mg/kg IV), with or without pretreatment with propranolol (0.1 to 0.3 mg/kg IV), atropine (0.04 mg/kg IV), or prazosin (0.5 mg/kg IV). Third-degree heart block was induced by AV nodal ablation in 4 affected dogs. Phenylephrine increased ventricular arrhythmias in affected dogs, with or without pretreatment with propranolol, but did not induce ventricular arrhythmias in unaffected dogs. In dogs with intact AV nodal conduction, atropine increased sinus rate, which suppressed baseline and phenylephrine-induced arrhythmias. In dogs with heart block, arrhythmias were increased during baseline and after phenylephrine, with or without pretreatment with atropine. Prazosin and overdrive ventricular pacing suppressed phenylephrine-induced arrhythmias.

CONCLUSION

Phenylephrine increases ventricular arrhythmias in dogs with inherited sudden death via both an induction of reflex bradycardia and a direct myocardial effect. Superimposition of heightened alpha-adrenergic and vagal tone may facilitate the development of sudden death in these animals.

Increased propensity of women to develop torsades de pointes during complete heart block

INTRODUCTION

To determine whether an increased female gender susceptibility to torsades de pointes (TdP) may exist in a clinical model of bradycardia-induced long QT syndrome, we investigated reported cases of TdP associated with acquired complete heart block.

METHODS AND RESULTS

Seventy-two cases reported in the medical literature dating from 1941 through 1993 were identified, all describing TdP or "transient ventricular tachycardia/fibrillation" (to include those cases reported prior to the use of TdP terminology) in the setting of acquired complete heart block unassociated with QT prolonging drugs. Expected female prevalence in complete heart block was estimated at 52%, based on projections derived from 206,016 hospital discharges in the National Inpatient Profile (Commission on Professional and Hospital Activities, Ann Arbor, MI), over the years 1985 through 1992. During complete heart block, mean heart rate was 37 beats/min in both sexes (combined n = 43), and absolute QT interval ranged from 0.52 to 0.88 seconds, with a mean of 0.68 seconds (n = 25). Female prevalence among patients with TdP during complete heart block was greater than expected: 72% for all studied cases (P < 0.001); 70% (P < 0.04) and 74% (P < 0.02) among those reported prior to (n = 35) and during or after (n = 37) 1980, respectively; 73% (P < 0.03) among those with documented normokalemia (n = 26); and 68% (P = 0.2) among those with a prolonged QT interval and known polymorphic VT (i.e., unequivocal TdP; n = 25).

CONCLUSION

Despite inherent limitations of this retrospective study, the data are consistent in suggesting a greater than expected female prevalence among patients with TdP related to complete heart block. This finding lends support to a broadening concept of increased susceptibility of women to the development of TdP in various settings of QT prolongation.

Proarrhythmia with class III antiarrhythmic drugs: definition, electrophysiologic mechanisms, incidence, predisposing factors, and clinical implications

Antiarrhythmic drugs can and do induce unexpected and sometimes fatal reactions by either producing new symptomatic arrhythmias or by aggravating existing arrhythmias. The definition of proarrhythmia has changed since controlled clinical studies showed a dichotomy between arrhythmia suppression and mortality. The nature of proarrhythmic reactions is linked to the electrophysiologic effects of various antiarrhythmic drugs. Whereas Class I agents without accompanying effects on repolarization generally produce ventricular tachycardia (often incessant) or fibrillation, Class III agents typically produce torsades de pointes that may deteriorate into ventricular fibrillation. The precise mechanism of torsades de pointes is not fully elucidated, although early after-depolarization and increases in spatial or temporal dispersion of repolarization are likely possibilities. Proarrhythmic risk is lowest for amiodarone and is probably related to the drug's complex electrophysiologic profile. The incidence of torsades with sotalol increases with dose and the baseline values of the QT interval; the incidence with d-sotalol and other pure Class III agents remains unclear. Prospective, randomized, placebo-controlled studies to evaluate this are under way. The fact that d-sotalol increases mortality in postinfarction patients suggests that it may possibly be a common property of most, if not all, pure Class III compounds. The ongoing clinical trials with various Class III agents are likely to provide the critical information on this important therapeutic issue.

Basic concepts in cellular cardiac electrophysiology: Part II: Block of ion channels by antiarrhythmic drugs

Antiarrhythmic drugs have relative specificity for blocking each of the major classes of ion channels that control the action potential. The kinetics of block is determined by the state of the channel. Those channel states occupied at depolarized potentials generally have greater affinity for the blocking drugs. The kinetics of the drug-channel interaction is important in determining the blocking profile observed clinically. The increased mortality resulting from drug treatment in CAST and several atrial fibrillation trials has resulted in a shift in antiarrhythmic drug development from the Na+ channel blocking (Class I) drugs to the K+ channel blocking (Class III) drugs. While both Classes of drugs have a proarrhythmic potential, this may be less for the Class III agents. Their lack of negative inotropy also make them more attractive. It is important that the potential advantages of these agents be evaluated in controlled clinical trials. In several laboratories, the techniques of molecular biology and biophysics are being combined to determine the block site of available drugs. This information will aid in the future development of agents with greater specificity, and hopefully greater efficacy and safety than those currently in clinical use.

The effect of flunarizine and ryanodine on acquired torsades de pointes arrhythmias in the intact canine heart

INTRODUCTION

Ryanodine, a specific blocker of the Ca2+ release channel of the sarcoplasmic reticulum, and flunarizine, a [Ca2+]i overload blocker, possess antiarrhythmic effects against delayed afterdepolarizations (DADs) and DAD-dependent arrhythmias. In vitro controversy exists about their effect on early after-depolarizations (EADs): no effect was reported on cesium-induced EADs, while ryanodine did prevent EADs induced by isoproterenol. To study the possible role of intracellular Ca2+ overload in acquired EAD-dependent torsades de pointes (TdP) arrhythmias, we tested the effects of flunarizine and ryanodine in our animal model of TdP.

METHODS AND RESULTS

Anaesthetized dogs with chronic AV block received d-sotalol or almokalant followed by pacing. A subset of dogs with reproducible TdP (> or = 3 times) were selected to receive flunarizine (2 mg/kg per 2 min) or ryanodine (10 micrograms/kg per 10 min). After d-sotalol, TdP was induced at a mean cycle length of the idioventricular rhythm (CL-IVR) of 2070 +/- 635 msec and a QT(U) interval of 535 +/- 65 msec. Induction of TdP was prevented by flunarizine in all experiments (8/8): electrophysiologically this was associated with a decrease in CL-IVR, QT(U), and QTc interval (390 +/- 100 to 320 +/- 45, P < 0.05). Ryanodine prevented TdP induction in 4 of 5 experiments and decreased the CL-IVR, QT(U), and the QTc interval from 385 +/- 75 to 320 +/- 20 msec (P < 0.05). Both drugs also suppressed the almokalant-induced EADs and related ectopic activity. This antiarrhythmic action corresponded with the inability to reinduce TdP by pacing.

CONCLUSIONS

Blockade of the Ca2+ release channel of the sarcoplasmic reticulum by ryanodine or the reduction of [Ca2+]i overload by flunarizine prevents induction of EAD-dependent acquired TdP arrhythmias, suggesting a role for [Ca2+]i overload in acquired TdP.

Primary infantile hypomagnesaemia: report of two cases

OBJECTIVE

To present case reports of two siblings with primary hypomagnesaemia both presenting with seizures, and one also with a cardiac arrhythmia. To briefly review the pathophysiology, clinical features, diagnosis, management and genetics of this disorder.

METHODOLOGY

Published literature reports of primary hypomagnesaemia and studies of hypomagnesaemia in humans. Reports of the clinical features and inheritance of primary hypomagnesaemia.

RESULTS

The information is descriptive of the pathophysiology, clinical features, diagnostic criteria, and management. Considered modes of inheritance are presented. Two cases of primary hypomagnesaemia in brothers of consanguineous parents are described. Cardiac arrhythmia at presentation has not previously been reported. Diagnosis and adequate magnesium supplementation controls the biochemical disorder and the neurological development is normal.

CONCLUSIONS

Primary hypomagnesaemia should be considered in infants with seizures, as failure to identify this metabolic disorder can result in death. Subsequent siblings, particularly male, should be closely monitored.

Epinephrine-induced ventricular premature complexes due to early afterdepolarizations and effects of verapamil and propranolol in a patient with congenital long QT syndrome

We report a patient with congenital long QT syndrome in whom early afterdepolarizations (EADs) were demonstrated on monophasic action potential (MAP) recordings in the left ventricular mid-base inferior wall. Epinephrine infusion at 5 micrograms/min increased the amplitude of the EADs and the late component of the T(U) wave. Epinephrine also induced ventricular premature complexes (VPCs) with right bundle branch block morphology and left-axis deviation that occurred from the peak of the EADs. Verapamil injection (5 mg) during continuous epinephrine infusion abolished all VPCs with a slight reduction in the amplitude of the EADs. Propranolol injection (5 mg) in addition to verapamil further reduced the amplitude of the EADs and the late component of the T(U) wave. These findings suggest that the epinephrine-induced VPCs were closely related to triggered rhythm arising from the EADs, and that both verapamil and propranolol were effective for the suppression of VPCs and EADs.

Empiric use of amiodarone and sotalol

Several studies have demonstrated that class I antiarrhythmic drugs do not reduce, and may increase, sudden cardiac death mortality and total cardiac mortality. Because of this, alternative drug choices for antiarrhythmic therapy are necessary. Amiodarone has been demonstrated to be an important and effective antiarrhythmic agent, as has sotalol. The purpose of this article is to review the various indications and possible benefits of the empiric use of these 2 antiarrhythmic agents.

Drug therapy for torsade de pointes

Torsade de pointes is an uncommon and unique type of ventricular tachycardia. It differs from other forms of ventricular tachycardia by its morphological features, underlying mechanism, and modes of therapy. Recognizing torsade de pointes is of major clinical importance, as standard antiarrhythmic regimens might not only be ineffective in abolishing this life-threatening arrhythmia but may aggravate it. Torsade de pointes is most commonly precipitated by QT prolonging drugs, mainly type IA antiarrhythmic therapy such as quinidine and disopyramide, and other antiarrhythmic agents are reported to cause torsade de pointes as well. Predisposing factors known to increase the likelihood of developing torsade de pointes are: electrolyte imbalance (hypokalemia, hypomagnesemia, or both) and slow heart rate induced either by sinus bradycardia or heart block. Treatment of torsade de pointes is aimed at shortening the QT interval. By acceleration of the heart rate, the QT interval is shortened, thus preventing the recurrence of the arrhythmia. Treatment of torsade de pointes includes: isoproterenol infusion, cardiac pacing, and intravenous atropine. Intravenous magnesium sulfate, a relatively new mode of therapy for torsade de pointes, was proven to be extremely effective and is now regarded as the treatment of choice for this arrhythmia.

Effect of high-current stimulation in patients with sustained ventricular tachycardia rendered noninducible by antiarrhythmic drugs

Successful antiarrhythmic drug therapy for sustained ventricular tachycardia (VT) is presumed to be related to effects on myocardium within the re-entrant circuit. To test the hypothesis that prevention of VT induction may be related to effects on myocardium other than that directly involved in the tachycardia circuit, high-current stimulation was used to achieve shorter coupling intervals in 22 patients with sustained uniform VT that was rendered noninducible by antiarrhythmic agents during stimulation at twice threshold. Sustained uniform VT was induced in 10 patients in response to high-current stimulation (group 1), including 4 tachycardias with the same morphology observed in the baseline study. There were no inducible arrhythmias in 12 patients (group 2). Patients were receiving several different antiarrhythmic regimens, but there was no particular drug associated with the induction of VT using high-current stimulation. There was no statistically significant difference between groups 1 and 2 in baseline VT cycle length (247 +/- 41 vs 253 +/- 44 ms), drug-induced increase in effective refractory period (20 +/- 15 vs 16 +/- 7%), QRS duration (25 +/- 10 vs 20 +/- 17%) or maximal current strength delivered (10.9 +/- 5.3 vs 9.3 +/- 4.0 mA). There was no significant difference in local activation with high-current stimulation between groups 1 and 2. In conclusion, sustained uniform VT was induced in 45% (10 of 22) of patients whose arrhythmias were rendered noninducible by antiarrhythmic agents during programmed stimulation at twice threshold.(ABSTRACT TRUNCATED AT 250 WORDS)

TU alternans, long QTU, and torsade de pointes: clinical and experimental observations

T or U wave alternans in association with long QTU and torsade de pointes (TdP) is uncommon and its mechanism(s) is unknown. We studied three patients with TU alternans, long QTU, and TdP: patient 1 was a newborn with congenital long QTU; patient 2 had marked hypokalemia and hypomagnesemia; and patient 3 was receiving procainamide. In the three patients, TU alternans was tachycardia dependent and preceded the onset of TdP. In the patient on procainamide, TU alternans and TdP occurred at long cardiac cycles. In this patient, endocardial monophasic action potential (MAP) recordings showed that TU alternans was associated with alternation of the duration of the plateau. A deflection consistent with early afterdepolarization (EAD) arose at a constant time interval from phase 0 but alternated from high and low levels of phase 3. The first ectopic beat of TdP arose on the descending limb of the EAD. TU alternans was investigated by MAP recordings in six normal dogs, following the administration of anthopleurin-A (AP-A), a drug shown to delay sodium inactivation and to induce bradycardia dependent long QTU, EADs, and TdP. In two dogs TU alternans was associated with 2:1 recordings of EAD and nearly constant plateau duration. In three dogs, TU alternans was associated with EAD that occurred in consecutive beats at constant time intervals from phase 0, but alternated from high and low phase 3 because of alternation of the duration of the plateau. In one dog, alternation of EAD and plateau duration occurred. In 36 separate episodes of TdP that were analyzed in the six dogs, 32 were bradycardia dependent but four developed on abrupt shortening of the cardiac cycle associated with alternation of action potential duration. Our results suggest: (1) TU alternans may be due to 2:1 propagation of an EAD or to alternation of the recovery kinetics of a repolarization current; (2) The constant occurrence of EAD in relation to phase 0 in spite of alternation of plateau duration suggests an ionic mechanism synchronized to depolarization; (3) Tachycardia dependent TdP in clinical and experimental examples of long QTU seems to be characteristically associated with TU alternans. Dispersion of repolarization may underlie the increased ventricular electrical instability in these cases.

Sudden death in patients with congestive heart failure: future directions

Sudden, unexpected cardiac death continues to be a major clinical problem in patients with congestive heat failure. This review summarizes the current state of knowledge regarding the identification and management of these patients. The roles of ambulatory ECG monitoring, electrophysiological testing, signal-averaged ECG, and other methods of predicting increased risk of sudden death are discussed. The modes of sudden cardiac death and the potential mechanisms of ventricular arrhythmias in congestive heart failure are reviewed. Current therapeutic options including antiarrhythmic drugs, neurohormonal blockade, and automatic implantable cardioverter defibrillators are discussed. Finally, future directions and ongoing clinical investigations of the management of these complex patients are considered.

Effect of the combination of low-dose mexiletine and metoprolol on ventricular arrhythmia

Antiarrhythmic drug therapy is often ineffective or poorly tolerated. Combining antiarrhythmic agents with different electrophysiologic properties may have a synergistic antiarrhythmic effect when compared with each drug alone. If a lower dose of each drug can be used, combination therapy may also result in lower incidence of side effects. The goal of our study was to assess the complementary effect of low-dose mexiletine and metoprolol, when compared with either drug alone. Ten patients with frequent ventricular arrhythmias including 7 patients with nonsustained ventricular tachycardia were evaluated in an open-label sequential study. The response to drug therapy was evaluated by 24-h continuous EKG monitoring, exercise stress testing, and echocardiogram after each treatment. Combination therapy effectively reduced ventricular arrhythmias in 8 patients (80%) in contrast to only 1 patient (10%) on metoprolol alone and 4 patients (40%) on mexiletine alone. In 5 patients (71%) ventricular tachycardia was abolished. The number of couplets was reduced from 51 +/- 39 to 1.9 +/- 2.4 (p less than 0.01) and total premature ventricular beats from 7790 +/- 9047 to 597 +/- 515 (p = 0.06). Combination therapy was well tolerated without proarrhythmia or precipitation of congestive heart failure. It is concluded that low-dose mexiletine combined with metoprolol is effective in suppressing ventricular arrhythmias in selected patients, and enhances the antiarrhythmic effect of either drug alone without significant side effects.

Remodeling of ventricular conduction pathways in healed canine infarct border zones

Remodeling of myocyte interconnections may be an important determinant of ventricular tachycardia in regions bordering healed infarcts. We used quantitative electron microscopy to characterize the distribution of gap junctions in 10 canine left ventricles 3-10 wk after coronary occlusion. In three normal canine left ventricles analyzed ultrastructurally, myocardial gap junctions were distributed anisotropically; gap junction profile length was significantly greater in the transverse than in longitudinal planes of section. In infarct border zone tissues, the normal anisotropic distribution was completely abolished and fewer gap junctions per unit intercalated disk length were observed. Analysis of individual gap junction profile length distributions revealed selective disruption of the largest gap junctions that collectively comprised only 9.6% of total junction profiles, but encompassed nearly 40% of aggregate gap junction length in the transverse plane of section. Three-dimensional reconstructions of myocyte interconnections by high resolution quantitative light microscopy of serial sections demonstrated a reduction in the number of cells connected by intercalated disks to a single myocyte from 11.2 +/- 1.0 in normal myocardium to 6.5 +/- 1.3 in border zone tissues (P less than 0.001). Connections of cells in primarily side-to-side apposition were reduced by 75%, whereas primarily end-to-end connections were reduced by only 22% (P less than 0.05). These alterations would disproportionately enhance axial resistivity in the transverse direction, potentially contributing to development of reentrant arrhythmias.

Suppression of incessant polymorphic ventricular tachycardia by selective intracoronary ethanol infusion

Two weeks after an extensive anterior myocardial infarction, a 68-year-old man developed incessant polymorphic ventricular tachycardia (PMVT), unresponsive to all conventional treatment modalities. After requiring greater than 40 direct current cardioversions in less than 3 hours, he underwent attempted intracoronary chemical ablation of his arrhythmia as a treatment of last resort. An infusion catheter was positioned selectively in the subtotally occluded left anterior descending (LAD) coronary artery, the putative "tachycardia-related vessel." Fifty percent ethanol was delivered to the anterior wall through this catheter by slow, constant infusion. Following selective intracoronary ethanol infusion, spontaneous, unprovoked episodes of PMVT ceased, despite discontinuation of all antiarrhythmic drugs. The LAD remained patent. Several days later, the patient underwent coronary artery bypass surgery and implantation of an implantable defibrillator, succumbing in the early postoperative period from recrudescent intractable ventricular fibrillation and cardiogenic shock. Slow intracoronary infusion of 50% ethanol does not cause abrupt vessel occlusion such as occurs after rapid injection of higher concentrations of ethanol. Selective intracoronary infusion of 50% ethanol may provide temporary lifesaving suppression of otherwise intractable polymorphic ventricular tachycardia.

Quantitative analysis of ventricular late potentials in healthy subjects

Signal averaging is a technique that improves the signal-to-noise ratio. Obscuring random noise, it allows the detection of low-amplitude wave forms in the terminal portion of the QRS complex, also known as ventricular late potentials. A higher incidence of arrhythmic events has been found in patients with abnormal ventricular late potentials after an acute myocardial infarction. Few studies have been conducted in healthy subjects to assess normal values. Sixty-one healthy subjects were enrolled in our study (33 men and 28 women). The results (mean +/- standard deviation) are as follows: duration of the filtered QRS (QRS duration) was 95 +/- 10 ms; duration of the low-amplitude signals in the terminal portion of QRS less than 40 microV (LAS less than 40) was 32 +/- 8 ms; and root-mean-square voltage in the last 40 ms (RMS - 40) was 33 +/- 16 microV. A significant difference was noted in QRS duration between men and women (98 +/- 11 vs 92 +/- 6 ms, p = 0.006); no difference was found in LAS less than 40 (31 +/- 8 vs 34 +/- 8 ms) and in RMS-40 (36 +/- 17 vs 30 +/- 13 microV). QRS duration confidence limits of 95% were less than or equal to 114 ms for the total group, less than or equal to 120 ms for men and less than or equal to 104 ms for women. Normalization of QRS duration for height (normal value less than 66 ms/m) eliminated any difference between men and women.(ABSTRACT TRUNCATED AT 250 WORDS)

Sotalol in patients with life-threatening ventricular tachyarrhythmias

To assess the antiarrhythmic efficacy of oral d,l-sotalol, 68 patients with sustained monomorphic ventricular tachycardia (SMVT) (n = 62) or ventricular fibrillation (VF) (n = 6) were studied by programmed ventricular stimulation (PVS). Fifty-one patients had coronary artery disease with a previous myocardial infarction and there were 17 patients without coronary disease: 11 patients had right and/or left ventricular dysplasia, one patient an aortic-valve replacement, and five patients had no visible heart disease. Prior to sotalol patients were treated with a mean of 3.6 +/- 1.3 antiarrhythmic class I drugs. None of these drugs prevented SMVT or VF. During control PVS (PVS 1), VF was induced in 8 patients (12%), SMVT in 47 patients (69%), and nonsustained ventricular tachycardia (NSVT) in 13 patients (19%). After loading with oral d,l-sotalol (320 mg/day), PVS (PVS 2) was repeated 4.2 +/- 3.3 weeks after PVS 1. In one of the patients (1%) VF was inducible, in 15 patients (22%) SMVT was induced, and in 18 patients (26%) NSVT was induced. In 34 patients (50%) either no or a short ventricular response was inducible. Our data show that oral d,l-sotalol is an effective antiarrhythmic agent in patients with SMVT or VF.

Are antiarrhythmic drugs safe?

The indications for antiarrhythmic therapy are far from clearly defined and the choice of treatment is usually based on empiric strategies. Antiarrhythmic agents can have serious side effects. Systemic adverse effects are usually use-related and reversible with withdrawal of the drug. Impairment of left ventricular function is considerable in patients with heart failure. The most important, life-threatening side effect of antiarrhythmic drugs is their proarrhythmic tendency which gives rise to certain concern about their clinical use. Aggravation of arrhythmia often occurs without symptoms, goes unrecognized by the patient, and is exposed only by monitoring, exercise testing, or invasive electrophysiological testing. Patient monitoring with electrolyte measurement, Holter recording, and electrophysiological reassessment can reveal or reduce the proarrhythmic risk but cannot eliminate the problem completely. The institution of antiarrhythmic therapy should be considered in highly symptomatic or life-threatening arrhythmias after careful consideration of the benefit-risk ratio.

Magnesium therapy in ventricular arrhythmias

Magnesium (Mg) is the known activator of 300 enzymes which govern energy utilization, cell permeability, and ionic membrane currents in the cardiac conducting cells. This may explain the antiarrhythmic efficacy of Mg in specific clinical settings, despite its only modest electrophysiological effects. This review summarizes the effect of Mg administration in four clinical conditions: in digitalis toxicity; in drug-induced torsade de pointes; in patients with chronic diuretic therapy; and in acute myocardial infarction. Mg effectively abolished ventricular tachyarrhythmias associated with digitalis intoxication. This effect of Mg is related to the activation of sodium-potassium ATP-ase, which is inhibited by digitalis. Drug-induced torsade de pointes was promptly abolished by Mg sulfate in the clinical setting. Experimental studies showed that Mg suppresses the early afterdepolarizations and the triggered activity responsible for occurrence of the arrhythmia. In diuretic-treated hypertensives, potassium depletion has been associated with increased ventricular ectopy and sudden death. Mg has been found to be an important adjuvant for intracellular repletion of potassium in these patients. Several randomized, double-blind studies in patients with acute infarction showed that Mg administered on admission improved survival or reduced the incidence of complex ventricular arrhythmias. Thus, Mg should be employed as first-line therapy in digitalis intoxication and drug-related torsade de pointes, and should be considered an important adjuvant therapy in hypertensives treated with diuretics and patients with acute myocardial infarction.