Efficacy of procainamide on ventricular tachycardia: relation to prolongation of refractoriness and slowing of conduction

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Effects of Na+ channel blockers on the restitution of refractory period, conduction time, and excitation wavelength in perfused guinea-pig heart

Na⁺ channel blockers flecainide and quinidine can increase propensity to ventricular tachyarrhythmia, whereas lidocaine and mexiletine are recognized as safe antiarrhythmics. Clinically, ventricular fibrillation is often precipitated by transient tachycardia that reduces action potential duration, suggesting that a critical shortening of the excitation wavelength (EW) may contribute to the arrhythmic substrate. This study examined whether different I_Na blockers can produce contrasting effects on the rate adaptation of the EW, which would explain the difference in their safety profile. In perfused guinea-pig hearts, effective refractory periods (ERP), conduction times, and EW values were determined over a wide range of cardiac pacing intervals. All I_Na blockers tested were found to flatten the slope of ERP restitution, indicating antiarrhythmic tendency. However, with flecainide and quinidine, the beneficial changes in ERP were reversed owing to the use-dependent conduction slowing, thereby leading to significantly steepened restitution of the EW. In contrast, lidocaine and mexiletine had no effect on ventricular conduction, and therefore reduced the slope of the EW restitution, as expected from their effect on ERP. These findings suggest that the slope of the EW restitution is an important electrophysiological determinant which can discriminate I_Na blockers with proarrhythmic and antiarrhythmic profile.

Anti-arrhythmic efficacy of nifekalant hydrochloride, a pure class III anti-arrhythmic agent, in patients with healed myocardial infarction and inducible sustained ventricular tachycardia

In recent clinical trials, class III anti-arrhythmic drugs were found to reduce arrhythmic deaths in patients after myocardial infarction. The purpose of this study was to assess the electrophysiologic properties and anti-arrhythmic efficacy for inducible sustained ventricular tachycardias (VTs) of the pure class III agent nifekalant hydrochloride (MS-551) in comparison with those of procainamide. Programmed ventricular stimulation of up to three extra stimuli was performed for induction of VTs. Effective refractory period (ERP) of the ischemic zone and normal zone was also measured before and after nifekalant. Nifekalant and procainamide suppressed sustained VT induction in four of 15 patients and in six of 15 patients, respectively (p = NS). Sinus cycle length, PR interval, and QRS duration were not changed, but QT and QTc intervals were significantly increased with nifekalant (p < 0.01). Ventricular ERP also increased, whereas there were no significant differences in the increase of ERP between the ischemic and normal zones. The suppression of VT induction did not correlate with the changes in QT, QTc, and ERP after nifekalant administration. There were no significant differences in induced VT cycle length at baseline study between responders and nonresponders to nifekalant. Reverse use dependence was not apparent on review of electrophysiologic parameters. Neither proarrhythmic events nor hemodynamic disturbances occurred after nifekalant administration. It was concluded that nifekalant could be used safely and showed comparable effectiveness to procainamide for the suppression of VT induction.

Absence of change of signal-averaged electrocardiogram identifies patients with ventricular arrhythmias who are non-responders to amiodarone

OBJECTIVES

The aim of this study was to assess the ability of a non-invasive study, the signal-averaged ECG (SAECG), to predict the effect of amiodarone at ventricular level.

BACKGROUND

Amiodarone is the main drug drug used in the treatment of ventricular arrhythmias. Standard ECG does not detect any change in QRS complex resulting from amiodarone therapy. SAECG is more sensitive than ECG for detecting changes in QRS complex.

METHODS

The study examined the effects of amiodarone on SAECG in relation to the results of programmed ventricular stimulation in 68 patients with old myocardial infarction, spontaneous and inducible sustained ventricular tachycardia (VT).

RESULTS

Amiodarone prolonged the total QRS duration (dur) (129+/-28 vs. 140+/-30 ms, P<0.05) and low amplitude signal (LAS) dur (45+/-20 vs. 51+/-20 ms, P<0.1), whereas the root-mean-square voltage of the last 40 ms of QRS complex (RMS 40) was significantly reduced (20+/-16 vs. 14+/-9 microV, P<0.05). Changes in SAECG parameters did not differ significantly in patients in whom amiodarone prevented the inducibility of VT (n=15) and those in whom VT remained inducible with amiodarone (n=53), but in baseline QRS duration was significantly shorter in patients in whom amiodarone prevented the VT induction (118+/-26 vs. 133+/-28 ms, P<0.05). In patients in whom amiodarone did not prolong the cycle length of VT (n=15), SAECG did not change significantly (QRS dur 131+/-29 vs. 132+/-27 ms, LAS 42+/-20 vs. 42+/-19 ms, RMS 40 22+/-14 vs. 19+/-11 microV). Comparison of the SAECG data in patients with no inducible VT and those with slowed VT differed significantly (P<0.05) between the control state and the recording with amiodarone.

CONCLUSIONS

The effects of amiodarone on VT inducibility are predicted by a shorter baseline QRS duration and the degree of drug-induced prolongation of filtered QRS duration. Amiodarone prolonged the QRS duration, LAS duration and decreased RMS 40; this effect was more important in patients with no inducible VT and in those with only slowed VT, than in patients with unchanged or accelerated VT. The absence of changes of QRS duration predicted the induction of a more rapid or not slowed VT with amiodarone with a sensitivity of 87% and a specificity of 83%. Therefore, SAECG appears as an useful and simple means to predict the effects of amiodarone in patients with myocardial infarction and VT.

Can antiarrhythmic agents be selected based on mechanism of action?

When selecting an antiarrhythmic agent the clinician needs to be able to accurately predict the probability that a particular drug will serve its intended purpose in a given patient. This is difficult because of the complexity of variables which govern the relationship between drug administration and clinical outcome. The efficacy of a drug may potentially be predicted from its mechanism of action. At least two classifications of antiarrhythmic agents based on mechanism of action have been proposed. The Vaughan Williams classification is based on the predominant electrophysiological effects of a drug on the action potential. In the Sicilian Gambit approach, a number of potential targets ('vulnerable parameters') for drug action are identified and antiarrhythmic drugs or substances that affect cardiac electrophysiology are characterised by their actions on each of these. The usefulness of these classification systems in predicting antiarrhythmic drug efficacy are limited. Furthermore, in the Vaughan Williams classification not all drugs in the same class have identical effects, whereas some drugs in different classes have overlapping actions. The Sicilian Gambit requires in-depth knowledge regarding cellular and molecular targets of antiarrhythmic agents which may make it intimidating or simply impractical for regular clinical use. Surrogate measures such as 24-hour Holter monitoring and programmed electrical stimulation have been used to predict anti-arrhythmic drug efficacy. However, studies such the Cardiac Arrhythmia Suppression Trial (CAST) have shown that suppression of ventricular ectopy on Holter monitoring does not necessarily correlate with improved survival and may in fact be dangerous. Conversely, studies using programmed electrical stimulation to assess drug effect on variables such as tachycardia inducibility, refractory period and ventricular tachycardia cycle length show that suppression of tachycardia inducibility, prolongation of refractory period and prolongation of ventricular tachycardia cycle length, are all associated with reduced recurrence of tachycardia and possibly improved survival. The most practical use of the current classification systems applied to antiarrhythmic agents may be in their ability to predict with reasonable accuracy, the risk and type of proarrhythmia based on the mechanism of action of an agent.

Procainamide induced change of the width of the zone of entrainment and its relation to the inducibility of reentrant ventricular tachycardia

Procainamide depresses conduction velocity and prolongs refractoriness in myocardium responsible for reentrant VT, but the mechanism by which the induction of VT is suppressed after procainamide administration remains to be determined. In the present study, the relationship between electrophysiological parameters and the noninducibility of VT was assessed during procainamide therapy with a special reference to the change of an excitable gap. Clinically documented monomorphic sustained VT was induced in 30 patients and, utilizing the phenomenon of transient entrainment, the zone of entrainment was measured as the difference between the cycle length of VT and the longest paced cycle length interrupting VT (block cycle length) which was determined as the paced cycle length decreased in steps of 10 ms, and used as an index of the excitable gap. The effective refractory period was measured at the pacing site and the paced QRS duration was used as an index of the global conduction time in the ventricle. The cycle length of VT, the block cycle length, and the width of the zone of entrainment were determined and compared between the responders and nonresponders. In 15 patients, these parameters were determined at the intermediate dose and related to subsequent noninducibility at the final dose. At the final doses of procainamide, VT was suppressed in 8 (26.7%) of 30 patients. However, the cycle length of VT, the block cycle length, and the width of the zone of entrainment were unable to predict the drug efficacy, i.e., noninducibility. The change in the effective refractory period at the pacing site or the width of the paced QRS duration was not different between the responders and nonresponders. Among the variables, only the width of the zone of entrainment showed a significant narrowing in the responders at the intermediate dose of procainamide, and it was smaller than that of the nonresponders. The significant narrowing of the width of the zone of entrainment was associated with the subsequent noninducibility of VT at the final dose. The present study showed that the baseline cycle length of VT, the block cycle length, the drug induced change of the effective refractory period, or the paced QRS duration was not a predictor of the noninducibility after procainamide administration. However, a significant narrowing of the width of the zone of entrainment at the intermediate dose was associated with the noninducibility of VT at the final dose.

General principles of antiarrhythmic therapy for ventricular tachyarrhythmias

Sudden cardiac death due to ventricular arrhythmias is a significant cause of mortality in patients with structural heart disease. Over the past several decades, the introduction of new pharmacologic and nonpharmacologic therapy has expanded the treatment options available. This article will focus on the use of antiarrhythmic medication for the treatment of ventricular arrhythmias and will review the following: (1) treatment goals for various clinical populations, (2) the mechanisms of antiarrhythmic and proarrhythmic actions of antiarrhythmic medications, and (3) empiric versus guided pharmacologic therapy.

Electrophysiologic mechanisms of antiarrhythmic efficacy of a sotalol and class Ia drug combination: elimination of reverse use dependence

OBJECTIVES

We sought to determine the electrophysiologic mechanisms explaining the efficacy of combination therapy with DL-sotalol and a type Ia drug in the treatment of ventricular tachycardia (VT).

BACKGROUND

Combination antiarrhythmic drug therapy with low dose DL-sotalol plus a type Ia antiarrhythmic agent has been shown to prevent spontaneous and induced VT. The mechanisms underlying the efficacy of this drug combination have not been fully elucidated.

METHODS

We studied 32 patients with spontaneous sustained VT by using programmed electrical stimulation in the drug-free condition and after treatment with DL-sotalol (average dose [mean +/- SE] 151 +/- 8 mg/day) and a class Ia agent (quinidine, 1,337 +/- 59 mg/day, or procainamide, 2,083 +/- 327 mg/day). Sustained VT was induced in all patients at baseline study, and induction was reattempted during drug therapy. Monophasic action potential duration at 90% repolarization (APD90) and ventricular effective refractory period (ERP) were recorded with use of a contact electrode.

RESULTS

Ventricular ERP increased from 258 +/- 4 ms at baseline to 310 +/- 6 ms at a 600-ms drive cycle length (DCL600) with treatment (p < 0.001). APD90 increased from 288 +/- 6 ms by +10.1% at DCL600 and from 267 +/- 7 ms by +13.3% at a 400-ms drive cycle length (DCL400) (p < 0.001). Paced QRS duration increased from 141 +/- 3 to 158 +/- 6 ms at DCL400 (p < 0.05). At baseline, the shortest achieved coupling interval between successive propagated extrastimuli decreased progressively with respect to the first extrastimulus, following double and triple extrastimuli, at both DCL600 (-14.0% and -20.0%, respectively) and at DCL400 (-16.4% and -22.4%, respectively). This "peeling back" of refractoriness was attenuated on therapy with sotalol plus a class Ia antiarrhythmic agent to -6.7% and -10.5% (DCL600, p < 0.05), and -8.1%, -9.5% (DCL400, p < 0.05), for double and triple extrastimuli, respectively. The absolute prolongation of functional refractory periods by the drug combination increased with successive extrastimuli, from 55 +/- 6 ms for the V1V2 interval to 75 +/- 6 ms for V2V3 and 67 +/- 6 ms for V3V4 at DCL600, and from 51 +/- 5 ms for V1V2 to 69 +/- 6 ms for V2V3 and 74 +/- 7 ms for V3V4 at DCL400 (p < 0.001).

CONCLUSIONS

The combination of low dose sotalol and a class Ia agent greatly prolongs refractoriness. The magnitude of the effect increases at shorter coupling intervals.

Frequency-dependent electrophysiologic effects of d,l-sotalol and quinidine and modulation by beta-adrenergic stimulation

INTRODUCTION

Frequency-dependent electrophysiologic actions of oral quinidine and oral sotalol may be clinically important, but these properties and their modulation by beta-adrenergic sympathetic stimulation have not been determined.

METHODS AND RESULTS

The frequency-dependent effects of oral quinidine (n = 17) and oral d,l-sotalol (n = 17) were determined at: (1) drug-free baseline; (2) during steady-state drug dosing; and (3) during isoproterenol infusion to patients receiving quinidine or d,l-sotalol. The monophasic APD90 and RVERP were prolonged 12% to 17% (P < 0.001) during pharmacologic therapy, and frequency-dependent effects were only observed for the RVERP during sotalol. In both drug groups, isoproterenol significantly reduced the sinus cycle length and reduced the RVERP to a greater extent at longer than at shorter paced cycle lengths. While isoproterenol fully reversed quinidine's effects on the APD90 and RVERP, sotalol-induced APD90 prolongation was reduced by only 2% to 4%, and the RVERP was unaffected. Isoproterenol attenuated the frequency-dependent effects of quinidine on QRS duration by a relatively fixed amount of 7% to 10%. Isoproterenol fully reversed quinidine-induced, but did not affect sotalol-induced prolongation in the sustained VT cycle length.

CONCLUSIONS

(1) Over the range of examined cycle lengths, oral quinidine and d,l-sotalol did not exert frequency-dependent effects on ventricular repolarization. (2) Isoproterenol fully reversed quinidine's effects on refractoriness, repolarization, and prolongation of VT cycle length, whereas d,l-sotalol's effects were largely preserved, despite significant reductions in sinus cycle length. (3) These results suggest that beta-blockade is important in preventing reversal of antiarrhythmic drug effects by augmented sympathetic nervous system tone.

Quinidine pharmacodynamics in patients with arrhythmia: effects of left ventricular function

OBJECTIVES

This study was undertaken to determine whether quinidine pharmacodynamics are altered in the presence of left ventricular dysfunction.

BACKGROUND

Left ventricular function is an independent predictor of antiarrhythmic drug efficacy. However, the effects of left ventricular dysfunction on the pharmacodynamics of antiarrhythmic drugs have not been studied extensively.

METHODS

Signal-averaged electrocardiograms were obtained and quinidine plasma concentrations measured during 24-h quinidine washout in 22 patients.

RESULTS

Linear quinidine concentration-effect relations were observed for QRS and QT intervals corrected for heart rate. The slopes of the concentration-effect relation describing changes in the corrected QT (QTc) interval were significantly higher in the group with left ventricular ejection fraction > or = 0.35 ([mean +/- SD] 29.5 +/- 11.2 ms/micrograms per ml) than in the group with a low left ventricular ejection fraction (15.7 +/- 9.7 ms/micrograms per ml, p = 0.001). The QRS concentration-effect relations were not different in the two groups. A significant linear correlation was observed between the slopes of the concentration-effect relations describing changes in QTc intervals and left ventricular ejection fraction (r = 0.7, p < 0.001). Nineteen patients with inducible ventricular tachycardia underwent serial electrophysiologic studies for evaluation of quinidine efficacy. Ventricular tachycardia could not be induced during quinidine therapy in eight patients. The slopes of the quinidine concentration-effect relations for QTc intervals were significantly higher in quinidine responders than in nonresponders (p < 0.05).

CONCLUSIONS

The effects of quinidine on ventricular repolarization are linearly related to left ventricular ejection fraction. Quinidine concentration-effect relations describing ventricular repolarization are associated with antiarrhythmic efficacy in patients with ventricular tachycardia.

Electrophysiologic effects of antiarrhythmic drug therapy in the prediction of successful suppression of induced ventricular tachycardia

Predictors of a successful outcome of serial electrophysiologic (EP) and drug studies have been identified from among baseline patient characteristics but not from among measures of baseline and drug-related EP effects. Identifying such predictors would be useful in explaining the mechanism of successful drug therapy and in guiding drug development and selection. We prospectively studied EP characteristics in 159 trials in 62 patients with ventricular tachycardia or ventricular fibrillation during antiarrhythmic therapy and compared EP measures between successful (n = 30) and failed trials (n = 129). The average age of the patients was 64 years (range 27 to 78 years); 82% were men and 18% women; and 87% had coronary artery disease. Measurements included R-R, QRS, and QT intervals during intrinsic rhythm and during pacing at cycle lengths of 600 of 400 msec; ventricular effective refractory periods (ERP) during pacing at cycle lengths of 600 and 400 msec; and changes in these measures, comparing treatment with drug-free baseline. Univariate predictors of success (in order of significance) included ERP600/QRS600, sotalol versus other drugs, ERP400/QRS400, delta ERP600, delta R-R, ERP600, QRS400 (negative association), delta ERP400, QRS600 (negative association), ERP400 (all p < 0.1). In two separate multivariate models, one for each drive cycle length, only the ratio ERP600/QRS600 (p = 0.01) in the first model and ERP400/QRS400 (p = 0.01) in the second model were significantly and independently associated with achieving noninducibility with drug therapy. Therefore measures of greater refractoriness and lesser delays in conduction velocity (ie, greater "wavelength") relate to drug success.(ABSTRACT TRUNCATED AT 250 WORDS)

Multicenter trial of sotalol compared with procainamide in the suppression of inducible ventricular tachycardia: a double-blind, randomized parallel evaluation. Sotalol Multicenter Study Group

Sotalol is the prototype class III agent that combines beta-blocking properties with the propensity to prolong the effective refractory period by lengthening the action potential duration. Its precise effect on the prevention of ventricular tachycardia-ventricular fibrillation (VTVF) compared to class I agents has not been evaluated in a blinded study. In a double-blind parallel-design multicenter study, the electrophysiologic and antiarrhythmic effects of intravenous and oral sotalol (n = 55) and procainamide (n = 55) were therefore compared in patients with VTVF inducible by programmed electric stimulation. Sotalol produced a greater effect on lengthening the ventricular effective refractory period (VERP). It prevented the inducibility of VTVF in 30% versus 20% for procainamide, but this was not significantly different. In an alternate therapy group (n = 41) of similar patients previously refractory to or intolerant of procainamide, intravenous sotalol prevented inducibility in 32%. The pooled overall sotalol efficacy rate was 31%. There was a significant relation between the increase in the VERP and the prevention of inducibility of VTVF (n = 56; p < 0.02). VERP of > or = 300 msec was critical for the prevention of VTVF inducibility. Thirteen sotalol and 6 procainamide responders from the randomized group and 30 from the nonrandomized groups completed 1 year of oral sotalol therapy follow-up. Life-table analysis of these patient in each group showed a trend in favor of sotalol; however, statistical analysis was not possible because of the small numbers of patients. Both sotalol and procainamide were well tolerated. In the randomized group there was one case of sudden death during treatment with sotalol and two cases of nonfatal torsades de pointes in the procainamide group and two in the sotalol group; in the nonrandomized alternate therapy group, there were 6 cases of nonfatal torsades de pointes. The data support the emerging role of sotalol in the control of symptomatic ventricular tachycardia and fibrillation.

Antiarrhythmic versus antifibrillatory actions: inference from experimental studies

Pathophysiology of the coronary circulation is a major contributor to altering the myocardial substrate, rendering the heart susceptible to the onset of arrhythmias associated with sudden cardiac death. Antiarrhythmic drug therapy for the prevention of sudden cardiac death has been provided primarily on the basis of trial and error and in some instances based on ill-suited preclinical evaluations. The findings of the Cardiac Arrhythmia Suppression Trial (CAST) requires a reexamination of the manner in which antiarrhythmic drugs are developed before entering into clinical testing. The major deficiency in this area of experimental investigation has been the lack of animal models that would permit preclinical studies to identify potentially useful or deleterious therapeutic agents. Further, CAST has emphasized the need to distinguish between pharmacologic interventions that suppresses nonlethal disturbances of cardiac rhythm as opposed to those agents capable of preventing lethal ventricular tachycardia or ventricular fibrillation. Preclinical models for the testing of antifibrillatory agents must consider the fact that the superimposition of transient ischemic events on an underlying pathophysiologic substrate makes the heart susceptible to lethal arrhythmias. Proarrhythmic events, not observed in the normal heart, may become manifest only when the myocardial substrate has been altered. We describe a model of sudden cardiac death that may more closely simulate the clinical state in humans who are at risk. The experimental results show a good correlation with clinical data regarding agents known to reduce the incidence of lethal arrhythmias as well as those showing proarrhythmic actions.

Choice and chance in drug therapy of cardiac arrhythmias: technique versus drug-specific responses in evaluation of efficacy

Numerous recent advances in pharmacotherapy for arrhythmia have necessitated a reorientation in terms of choice of specific agents, techniques for predicting drug effects, and the endpoints for judging therapeutic efficacy. For the management of ventricular arrhythmias and preventing mortality, several trends are becoming clear. It is unlikely that sodium channel blockers will continue to play a major role, except in patients with structurally normal hearts. Emphasis is shifting way from class I agents to those that act by prolonging repolarization without effect on conduction. These latter agents have been termed pure class III agents and have been developed because of the clinical experience with sotalol and amiodarone. On the other hand, there is compelling evidence that sympathetic inhibition per se (as exemplified by beta blockers) or as an integral component of more complex molecules (e.g., sotalol, amiodarone) is a critical feature of desirable antifibrillatory agents. Thus, compared with D,L-sotalol or amiodarone, pure class III agents are likely to be much less effective and may need to be used in combination with antiadrenergic compounds. Compared with amiodarone, they are likely to induce a higher incidence of torsades de pointes, especially in the case of concomitant diuretic therapy. Therapy guided by programmed electrical stimulation or Holter monitoring is likely to play a diminishing role in the development of antiarrhythmic drug regimens, and thus an antiarrhythmic agent's effectiveness may need to be evaluated against the background of implantable cardioverter-defibrillators or against amiodarone therapy. There is increasing evidence that "guided" therapy may simply identify responders from nonresponders and objective endpoints of therapy may be influenced more by drug-specific responses than by the techniques used for their selection. The data raise the issue whether in the future, therapy for ventricular tachycardia or fibrillation might be chosen empirically but from a limited range of compounds, such as beta blockers, amiodarone, sotalol, and possibly certain pure class III agents that are presently under development. Although it is reasonably certain that there is a need to shift from delaying conduction as a means for treating arrhythmias to one that entails prolongation of repolarization, it remains to be determined what might be the characteristics of an ideal antifibrillatory compound. The greatest promise is the area of complex molecules with a diversity of electrophysiologic actions, as exemplified by amiodarone and similar compounds that have the property of blunting sympathetic excitation. The complexity of their electrophysiologic and pharmacodynamic properties might provide a more favorable match with the vulnerable substrate for reducing electrical instability, thereby preventing ventricular fibrillation.

Frequency-dependent electrophysiologic effects of amiodarone in humans

BACKGROUND

In general, antiarrhythmic agents that prolong the action potential duration (APD) have attenuated effects on repolarization at short cycle lengths (reverse frequency dependence), and this may limit their efficacy for controlling ventricular arrhythmias. The frequency-dependent effects of amiodarone on repolarization may differ from those of other antiarrhythmic agents and have not been determined in humans.

METHODS AND RESULTS

The frequency-dependent effects of amiodarone on repolarization and conduction were determined during electrophysiologic study in 19 patients at drug-free baseline and after 11 days of amiodarone loading (1621 +/- 162 mg/d, group A) and in 15 additional patients after > or = 1 year of chronic amiodarone therapy (380 +/- 56 mg/d, group B). The two groups were similar in all clinical characteristics. The ventricular APD at 90% repolarization (APD90), right ventricular effective refractory period (VERP), and QRS duration were determined at paced cycle lengths of 300 to 600 milliseconds. In group A, amiodarone significantly (10% to 13%, P < .001) increased the APD90 at all paced cycle lengths by approximately 30 milliseconds compared with baseline. Similarly, there were no frequency-dependent effects on the percent increase in VERP. However, there was greater amiodarone-induced prolongation of the VERP magnitude at longer paced cycle lengths than at shorter cycle lengths (P = .04), although the VERP remained significantly prolonged at the shortest paced cycle length (300 milliseconds) by 33 +/- 22 milliseconds (16.9% increase from baseline, P < .001). Amiodarone significantly (P < .01) increased the QRS duration at paced cycle lengths < or = 500 milliseconds by a maximum of 28% compared with baseline measurements. The increase in ventricular conduction time was frequency dependent (P < .01), consistent with significant sodium channel blockade. The VERP/APD90 ratio (determined at twice diastolic threshold) was significantly prolonged by amiodarone (as compared with baseline) at cycle lengths > or = 400 milliseconds, indicative of both time- and voltage-dependent effects on refractoriness. The increase in induced sustained ventricular tachycardia cycle length in group A patients after amiodarone loading was significantly correlated with the increase in VERP (r = .68, P = .044) but not with increases in QRS duration or APD90. In addition, there were no significant differences in frequency-dependent effects of amiodarone between groups A and B.

CONCLUSIONS

The frequency-dependent response of the electrophysiologic effects of amiodarone are similar after 11 days of loading or > or = 1 year of chronic therapy. Amiodarone does not exert frequency-dependent effects on ventricular repolarization; it prolongs refractoriness by both time- and voltage-dependent mechanisms and exerts frequency-dependent effects on ventricular conduction. The absence of amiodarone-induced reverse frequency-dependent effects on repolarization, together with its time-dependent effects on refractoriness may account in part for the high efficacy of the drug and its low propensity to cause torsade de pointes.

Antiarrhythmic effects of selective prolongation of refractoriness. Electrophysiologic actions of sematilide HCl in humans

BACKGROUND

Recent data have suggested that antiarrhythmic agents that act largely by delaying conduction may not be as effective in controlling ventricular arrhythmias as those that prolong repolarization. Recently, numerous "pure" class III agents have been developed.

METHODS AND RESULTS

The antiarrhythmic and electrophysiologic profiles of sematilide, a "pure" class III agent, were determined in 27 patients with clinical ventricular arrhythmias and inducible sustained ventricular tachycardia during electrophysiologic study. After treatment with oral sematilide (mean dose, 133 +/- 29 mg every 8 hours), the patients underwent repeat 24-hour ambulatory ECG monitoring and electrophysiologic study. The baseline sinus cycle length and QT, QTc, JT, and JTc intervals were significantly increased 8 to 17% by sematilide (P = .001 to .029). There were no changes in the PR or QRS intervals. Sematilide (at a paced cycle length of 600 ms) significantly increased the atrial effective refractory period (238 +/- 32 to 264 +/- 32 ms; 11 +/- 16% increase from baseline; P = .013), atrioventricular nodal effective refractory period (296 +/- 74 to 354 +/- 71 ms; 20 +/- 19%; P = .029), and right ventricular effective refractory period (252 +/- 25 to 281 +/- 30 ms; 12 +/- 8%; P < .001) but did not significantly change the PA or HV intervals, the corrected sinus node recovery time, or the Wenckebach cycle length. Determination of the frequency-dependent effects of sematilide (n = 10) on the right ventricular monophasic action potential duration (APD90) during ventricular pacing at cycle lengths of 600 to 300 ms revealed that the APD90 was significantly prolonged by sematilide during ventricular pacing at 600 to 350 ms (APD90 increase of 40 +/- 17, 27 +/- 21, 18 +/- 18, and 14 +/- 15 ms, respectively) but not at 300 ms (APD increase of 13 +/- 19 ms). Sematilide significantly prolonged the APD90 to a greater degree at longer than at shorter cycle lengths (P = .02). The ventricular effective refractory period had a similar reverse frequency-dependent relation as the APD90. Sematilide had no effect on the ventricular effective refractory period-to-APD90 ratio or on ventricular conduction. Sematilide suppressed the induction of sustained ventricular tachycardia in 41% of all patients exposed to sematilide. Prolongation of ventricular refractoriness was correlated with ventricular tachycardia suppression. The right ventricular effective refractory period (at 600 ms) increased by 38 +/- 14 ms in patients whose sustained ventricular tachycardia was suppressed by sematilide and by 19 +/- 18 ms in patients not suppressed (P = .015). One patient developed short runs of pause-dependent nonsustained ventricular tachycardia. Eight patients were placed on long-term sematilide therapy, and during a mean follow-up period of 7.0 +/- 7.5 months, two patients developed sudden cardiac death, and one additional patient had recurrent sustained ventricular tachycardia.

CONCLUSIONS

The electrophysiologic profile of sematilide is consistent with selective block of outward potassium currents and associated isolated lengthening of the ventricular effective refractory period and APD; sematilide demonstrates a significant degree of reverse frequency-dependence of the ventricular APD and effective refractory period; and suppression of ventricular tachycardia inducibility by sematilide appears to be correlated with increases in the right ventricular effective refractory period.

Noninvasive prediction of efficacy of type IA antiarrhythmic drugs by the signal-averaged electrocardiogram in patients with coronary artery disease and sustained ventricular tachycardia

This study attempted to determine if specific changes on the signal-averaged electrocardiogram (ECG) after type IA antiarrhythmic therapy are predictive of efficacy in the treatment of ventricular tachycardia (VT). Scalar and signal-averaged ECGs were obtained at baseline and after type IA drug therapy in 15 patients with coronary artery disease and inducible VT at baseline electrophysiologic testing. Signal-averaged QRS duration, root-mean-square amplitude in the last 40 ms of signal-averaged QRS, and the duration under 40 mu v of the signal-averaged QRS (low-amplitude signal), as well as ventricular effective refractory period at electrophysiologic study, and QTc on the scalar ECG were compared. At drug study, 6 patients (group A) had persistent but slower VT, whereas 9 (group B) had VT rendered noninducible. The baseline signal-averaged QRS duration was longer in group A than in B (136 +/- 10 vs 115 +/- 13 ms; p < 0.05), as was the scalar QRS (115 +/- 19 vs 98 +/- 11 ms; p < 0.05). After antiarrhythmic therapy, group A had a greater prolongation of both signal-averaged QRS (24 +/- 10 vs 8 +/- 3 ms; p < 0.05) and low-amplitude signal (31 +/- 13 vs 3 +/- 7 ms; p < 0.05), whereas group B had a greater increase in ventricular effective refractory period (49 +/- 20 vs 20 +/- 13 ms; p < 0.05) and corrected QT interval (100 +/- 39 vs 43 +/- 23 ms; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Sotalol and type IA drugs in combination prevent recurrence of sustained ventricular tachycardia

OBJECTIVES

This study assessed the efficacy of the combination of sotalol and either quinidine or procainamide in preventing sustained ventricular tachycardia inducibility and recurrence and prospectively evaluated the ability of the drug combination to prevent ventricular tachycardia recurrence when the arrhythmia remained inducible but was modified.

BACKGROUND

Individual antiarrhythmic drugs are often ineffective in preventing the induction and recurrence of sustained ventricular tachycardia. Beta-adrenergic blockade and prolongation of refractoriness may be important components of successful antiarrhythmic therapy in patients with ventricular tachycardia. We reasoned that the combination of sotalol, which has beta-adrenergic blocking properties and prolonged ventricular refractoriness, and quinidine or procainamide, two agents that slow conduction and prolong refractory periods, would be effective therapy in such patients.

METHODS

We administered low dose sotalol (205 +/- 84 mg/day) plus quinidine sulfate (1,278 +/- 479 mg/day) or procainamide (2,393 +/- 1,423 mg/day) to 50 patients with spontaneous sustained ventricular tachycardia or fibrillation and inducible ventricular tachycardia.

RESULTS

In 21 (46%) of 46 patients, ventricular tachycardia was rendered noninducible at electrophysiologic study (group I), and in 17 patients (37%), inducible tachycardia was modified according to prospectively identified criteria (group II), for a combined 83% response rate. Ventricular refractory periods increased from 252 +/- 24 to 316 +/- 28 ms and from 265 +/- 33 to 316 +/- 24 ms in groups I and II, respectively (p < 0.001), but from 234 +/- 19 to only 286 +/- 13 ms in the group of patients with unmodified ventricular tachycardia inducibility (n = 8, group III, p < 0.001). Cycle length of induced ventricular tachycardia slowed from 324 +/- 62 to 432 +/- 70 ms in group II patients (p < 0.001), whereas it slowed less in group III patients (279 +/- 73 to 314 +/- 63 ms, p = NS). Forty-two of the 50 patients (including all patients in groups I and II) were discharged on treatment with the drug combination. After 25 +/- 19 months of follow-up, the actuarial recurrence rate of ventricular tachycardia was 6%, 6% and 11% at 1, 2 and 3 years, respectively. Among patients in whom this drug combination was unsuccessful at electrophysiologic study (group III) and in those who received alternative therapy after combination therapy was discontinued because of side effects, actuarial recurrence rates were 9%, 14% and 32% at 1, 2 and 3 years, respectively.

CONCLUSIONS

The combination of sotalol plus quinidine or procainamide markedly prolongs ventricular refractoriness and slows induced ventricular tachycardia in a high proportion of patients. Patients with modified or noninducible tachycardia have a low rate of arrhythmia recurrence in follow-up. This drug combination deserves further evaluation.

Effects of procainamide on the signal-averaged electrocardiogram in relation to the results of programmed ventricular stimulation in patients with sustained monomorphic ventricular tachycardia

OBJECTIVES

The aim of this study was to assess the ability of the signal-averaged electrocardiogram (ECG) to predict the efficacy of procainamide.

BACKGROUND

The main role of the signal-averaged ECG has been the identification of postinfarction patients at risk of sudden death. Prediction of the efficacy of antiarrhythmic drugs represents another potential clinical application of this technique.

METHODS

The study examined the effects of procainamide on the time domain and spectral temporal analysis of the signal-averaged ECG in relation to the results of programmed ventricular stimulation studies in 31 patients with inducible sustained monomorphic ventricular tachycardia.

RESULTS

Procainamide significantly prolonged the total and the initial QRS complex and low amplitude signal durations (mean +/- SD 135 +/- 30 vs. 161 +/- 46 ms, p < 0.0001; 87 +/- 16 vs. 98 +/- 20 ms, p < 0.0001, and 48 +/- 23 vs. 63 +/- 36 ms, p < 0.001, respectively) whereas the root-mean-square voltage of the total QRS complex and of the last 40 ms of the QRS complex was significantly reduced (mean +/- SD 112 +/- 36 vs. 87 +/- 36 microV, p < 0.0001; 21 +/- 19 vs. 13 +/- 12 microV, p < 0.002, respectively). The results of spectral temporal mapping of the signal-averaged ECG were similar before and after procainamide administration. Procainamide prevented the inducibility of sustained ventricular tachycardia or prolonged the cycle length of ventricular tachycardia by > or = 100 ms in 16 patients (52%) (responders). The fractional prolongation of the total QRS duration was significantly greater in responders (26 +/- 15%) than in nonresponders (10 +/- 10%) (p < 0.002) and, when this prolongation was > or = 15%, identified responders with a sensitivity of 94%, a specificity of 87% and an overall predictive accuracy of 90%.

CONCLUSIONS

The effects of procainamide on inducibility of ventricular tachycardia during programmed ventricular stimulation can be predicted by the degree of drug-induced prolongation of the signal-averaged QRS complex.

Rate-dependence of class III actions in the heart

The pharmacodynamics of many antiarrhythmic drugs are altered by heart rate. The ability of sodium channel blockers to decrease conduction velocity (class I action) is more pronounced with rapid heart rates. Drugs with class III action increase action potential duration and refractoriness in the heart. Most drugs with class III actions, currently being developed, produce their action by blocking one or several of the potassium channels responsible for repolarization. In vitro and in vivo studies have shown that their ability to increase repolarization time is less pronounced, or even disappears, at rapid pacing or heart rates. This so called 'inverse' rate-dependence of class III action is a characteristic of all drugs currently used in man except amiodarone, for which prolongation of repolarization time persists to a limited extent with rapid heart rates. It has been suggested that one possible mechanism of the inverse rate-dependence of class III action is related to the preferential binding of drugs to the potassium channels in the closed, polarized state. An inverse rate-dependence of class III action has also been found on prolongation of refractoriness. However, preliminary studies suggest that the positive inotropism of class III drugs not only persists but may increase with rapid heart rates. The clinical consequences of this phenomenon remain unclear, especially in view of the fact that the rate-dependence of class III action on dispersion of repolarization has not been specifically studied and that class III actions tend to decrease in ischemic tissues. However, the increase of action prolongation at slow heart rates may contribute to the bradycardia-dependent development of torsades de pointes arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Prediction of antiarrhythmic efficacy of class I and III agents in patients with ventricular tachycardia by signal-averaged ECG analysis

The effects of procainamide and dofetilide (pure Class III antiarrhythmic agent) on the signal-averaged ECG (SAECG) were examined in relation to the results of programmed ventricular stimulation studies in 25 patients with inducible sustained monomorphic ventricular tachycardia. Procainamide prolonged significantly the total QRS and low amplitude signal durations (140 +/- 31 msec vs 166 +/- 48 msec, P < 0.0001; 50 +/- 25 msec vs 65 +/- 38 msec, P < 0.002, respectively) whereas the root mean square voltage of the last 40 msec of the QRS complex was significantly reduced (22 +/- 21 microV vs 13 +/- 12 microV, P < 0.006). Procainamide was effective (prevention of the inducibility of sustained ventricular tachycardia or prolongation of the cycle length of ventricular tachycardia by > 100 msec) in 15 of 27 drug trials. Of the procainamide induced SAECG changes, the fractional prolongation of the total QRS duration was the best parameter that identified effectively treated patients (24% +/- 16% in responders vs 10% +/- 11% in nonresponders, P < 0.014). A fractional prolongation of the total QRS duration by > 15% identified effectively treated patients with a sensitivity of 87%, specificity of 81%, and an overall predictive accuracy of 84%. Dofetilide did not change the SAECG, and no SAECG parameter predicted the results of programmed ventricular stimulation. The effects of both drugs on the spectral analysis (area ratios) and on the spectral temporal mapping (the values of normality factor) of the SAECG were not consistent. In conclusion, antiarrhythmic efficacy of procainamide can be predicted by the degree of drug induced prolongation of the signal-averaged QRS complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of sotalol on the signal-averaged electrocardiogram in patients with sustained ventricular tachycardia: relation to suppression of inducibility and changes in tachycardia cycle length

OBJECTIVES

This study examines the effects of sotalol on the signal-averaged electrocardiogram (ECG) in patients with spontaneous and inducible sustained ventricular tachycardia and correlates these findings with the effect of sotalol on tachycardia inducibility and tachycardia rate.

BACKGROUND

Standard electrocardiography generally does not detect any change in the duration of the QRS complex resulting from sotalol therapy. However, the signal-averaged ECG is more sensitive than the standard ECG for detecting changes in QRS duration induced by antiarrhythmic drugs and can also detect changes in late potential duration.

METHODS

Signal-averaged electrocardiography was performed before therapy in 30 patients with spontaneous and inducible ventricular tachycardia, and both electrophysiologic study and a signal-averaged ECG were repeated during therapy with d,l-sotalol.

RESULTS

During sotalol therapy the signal-averaged QRS duration decreased by 2.6 +/- 6.6 ms in the 11 patients with no inducible tachycardia during therapy, whereas it increased by 3.8 +/- 5.8 ms (p = 0.01) in the 19 patients with inducible tachycardia during therapy. In the latter group there was a significant positive correlation between prolongation of tachycardia cycle length and prolongation of late potential duration by sotalol (r = 0.56, p = 0.01).

CONCLUSIONS

Sotalol can alter QRS and late potential duration as measured by the signal-averaged ECG. Prolongation of QRS duration or late potential duration may reflect a slowing of conduction by sotalol that may interfere with this agent's antiarrhythmic efficacy and slow ventricular tachycardia.

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)

Antiarrhythmic drug exacerbation of ventricular tachycardia inducibility during electrophysiologic study

Most studies examining antiarrhythmic drug exacerbation of ventricular arrhythmias have been performed in patients in whom clinical proarrhythmia developed. The clinical significance and predictors of antiarrhythmic drug exacerbation of inducible ventricular arrhythmias during electrophysiologic study have received less attention. Accordingly, a consecutive number of patients undergoing electrophysiologic study for evaluation of ventricular arrhythmias (but who had no history of clinical proarrhythmia) were prospectively examined. Drug-induced exacerbation was defined as no inducible ventricular tachycardia in the baseline drug-free state that increased to inducible nonsustained or sustained ventricular tachycardia, or inducible nonsustained ventricular tachycardia at baseline that increased to inducible sustained ventricular tachycardia. After administration of primarily type IA antiarrhythmic agents (procainamide and quinidine in 97% of the patients), patients were considered drug test negative (n = 80) when they had no increase in inducible ventricular tachycardia, and patients were considered drug test positive (n = 16) when they had exacerbation of inducible arrhythmias. The drug test-positive group's clinical characteristics differed markedly from those of the drug test-negative group. Compared with the drug test-negative group, the drug test-positive group had reduced (less than 40%) left ventricular ejection fractions (80% vs 39%, p = 0.005) and higher prevalences of myocardial infarctions (81% vs 35%, p = 0.027), left ventricular aneurysms (27% vs 5%, p = 0.026), and bundle branch blocks (53% vs 16%, p = 0.005). Thus exacerbation of ventricular tachycardia induction after antiarrhythmic agent administration was most common in patients with significant organic heart disease. The drug test-positive group was more frequently treated with antiarrhythmic therapy than was the drug test-negative group.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy and electrophysiologic effects of oral sotalol in patients with sustained ventricular tachycardia caused by coronary artery disease

The efficacy of oral sotalol in preventing sustained ventricular tachycardia induction by invasive electrophysiological testing was assessed in 22 patients (60 +/- 9 years) with prior myocardial infarction. Programmed stimulation consisted of two basic drives followed by up to three extrastimuli at two right ventricular sites. At baseline, sustained monomorphic ventricular tachycardia was inducible in all patients. With sotalol (360 +/- 172 mg/day), it was no longer inducible in 10 patients; in 12 others, it remained inducible and its cycle length was only minimally prolonged (322 +/- 42 to 345 +/- 44 msec, p less than 0.05). Sotalol markedly prolonged sinus cycle length, uncorrected QT interval, and right ventricular effective and functional refractory periods, but had little effect on ventricular conduction time either in sinus rhythm or with right ventricular pacing. There was no significant difference in drug dose or in electrophysiologic effect of drug that related to efficacy, nor was there any correlation between drug-induced prolongation of ventricular tachycardia cycle length and its effects. Six patients received oral sotalol over the long term without spontaneous recurrence of ventricular tachycardia (follow-up: 23 +/- 18 months). These results demonstrate that sotalol is effective (45%) against sustained ventricular tachycardia induction at moderate doses and is well tolerated over a long term in the setting of remote myocardial infarction. However, its electrophysiologic effects as measured at invasive testing are not predictive of efficacy against ventricular tachycardia induction.

Effects of antiarrhythmic drugs on canine atrial flutter due to reentry: role of prolongation of refractory period and depression of conduction to excitable gap

Antiarrhythmic drugs prolong the effective refractory period and depress conduction. To determine the exact role played by these two electrophysiologic effects in the termination of reentry, the effects of disopyramide, flecainide, propafenone and E-4031, a new class III drug, were examined in a canine model of atrial flutter (cycle length 120 +/- 4 to 131 +/- 3 ms) caused by reentry. Atrial flutter was induced in 32 anesthetized open chest dogs after placement of an intercaval crush. The excitable gap ranged from 9 +/- 2% to 11 +/- 4% of the basic flutter cycle length. The effective refractory period in the reentrant circuit during atrial flutter was estimated by subtracting the excitable gap from the basic flutter cycle length. Prolongation of flutter cycle length by the test drugs was proportional to the interatrial conduction time (r = 0.87, p less than 0.001). Atrial flutter was terminated by each test drug in all dogs except for flecainide and propafenone in one dog each. E-4031 prolonged the refractory period during atrial flutter to 129 +/- 6 ms, which did not differ significantly from the flutter cycle length immediately before termination (134 +/- 4 ms). The refractory period during atrial flutter after injection of the other drugs was shorter than the flutter cycle length before termination of atrial flutter (for example, flecainide 126 +/- 5 vs. 179 +/- 11 ms, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Drug response at electropharmacologic study in patients with ventricular tachyarrhythmias: the importance of ventricular refractoriness

The clinical and electrophysiologic predictors of successful antiarrhythmic drug therapy for patients with inducible ventricular tachycardia were evaluated in 59 consecutive patients undergoing serial electropharmacologic trials. Structural heart disease was less frequently present in patients for whom effective therapy was found (p less than 0.05). The presence of coronary artery disease and a history of prior myocardial infarction were significantly more frequently present in patients for whom antiarrhythmic drug therapy could not be found (p less than 0.05). The corrected QT interval and ventricular effective refractory period measured at a pacing cycle length of 400 ms were significantly shorter in responders compared with nonresponders (QT interval 428 +/- 52 versus 460 +/- 59 ms; ventricular effective refractory period 237 +/- 28 versus 254 +/- 24 ms; (p less than 0.05). In addition, the interelectrogram coupling interval of the ventricular extrastimulus initiating ventricular tachycardia was significantly shorter in responders compared with nonresponders (223 +/- 37 versus 251 +/- 33 ms; p = 0.003). Logistic regression analysis identified a short ventricular interelectrogram coupling interval (p less than 0.01) and absence of prior myocardial infarction (p less than 0.05) as the only independent predictors of antiarrhythmic drug suppression of the induction of ventricular tachycardia. Greater drug-induced increments in the ventricular effective and functional refractory periods were observed in responders than in nonresponders as was the shortest ventricular interelectrogram coupling interval. Thus, baseline electrophysiologic measurements identify patients with inducible ventricular tachycardia who are likely to respond to antiarrhythmic drug therapy. Furthermore, these patients demonstrate greater drug-induced electrophysiologic changes.

Prolongation of ventricular refractoriness by class Ia antiarrhythmic drugs in the prevention of ventricular tachycardia induction

The effects of class la antiarrhythmic drugs (procainamide, quinidine) on the right ventricular effective refractory period (VERP) and intraventricular conduction time were assessed during serial invasive electrophysiologic studies for sustained monomorphic ventricular tachycardia (VT). In 47 patients with remote myocardial infarction, sustained VT was inducible by up to two extrastimuli after the basic drive at one of two basic cycle lengths at the right ventricular apex. With oral drug administration, sustained VT was no longer inducible (group I) in 27 patients but remained inducible (group II) in 20 with the same protocol. Class la drugs prolonged the VERP in both groups, but there was greater lengthening when drugs were effective (e.g., +32 +/- 14 msec in group I vs +12 +/- 19 msec in group II; p less than 0.005, basic cycle length 600 to 700 msec). Prolongation of the VERP by greater than 30 msec had an 88% positive predictive value for prevention of sustained VT induction. In all except one patient in group I, drugs prolonged the VERP such that the coupling intervals that had resulted in sustained VT induction under control conditions were no longer attainable. In contrast, conduction time through the ventricle (surface QRS duration) in sinus rhythm and during right ventricular pacing was prolonged similarly regardless of efficacy (e.g., +33 +/- 21 msec vs +27 +/- 27 msec at a cycle length of 400 msec). The presence of similar plasma levels of drug did not imply equivalent prolongation of the VERP in the two groups. These results suggest that greater prolongation of the VERP by oral procainamide or quinidine correlates with drug efficacy against VT induction and is a better predictor of drug effect than achievement of a "therapeutic plasma level."

Predictive value of electrophysiologic studies during treatment of ventricular tachycardia with the beta-blocking agent nadolol. The Working Group on Arrhythmias of the French Society of Cardiology

Sixty patients with recurrent inducible sustained ventricular tachycardia were prospectively treated with nadolol (40 or 80 mg/day). Old myocardial infarction was present in 43 patients and dilated cardiomyopathy in 12. In group I (n = 36), nadolol was given alone, whereas in group II (n = 24), previously ineffective treatment with amiodarone was continued in combination with nadolol. Left ventricular ejection fraction was higher in patients in group I (0.40 +/- 0.12) than in group II (0.30 +/- 0.10, p less than 0.01) patients. Electrophysiologic study was repeated after short-term treatment with nadolol, which was continued regardless of the results of this test, according to the scheme of the parallel approach. Recurrence of spontaneous tachycardia or sudden death occurred in 21 patients after 10 +/- 9.2 months; sustained tachycardia was inducible in 19 on nadolol therapy. The remaining 39 patients (of whom 21 had inducible tachycardia while taking the drug) have had no recurrence of tachycardia after 27.8 +/- 9.3 months of follow-up study. Sensitivity, specificity and predictive value of a positive and negative test were 90.5%, 46%, 47.5% and 90%, respectively. The results differ between group I and group II patients, the latter having a high percent of false positive responses. This difference is even more obvious with respect to left ventricular ejection fraction: the predictive value of a positive test was 86% when ejection fraction was greater than 0.40 and 39% when it was less than 0.40.(ABSTRACT TRUNCATED AT 250 WORDS)

Block of delayed rectifier potassium current, IK, by flecainide and E-4031 in cat ventricular myocytes

Block of the delayed rectifier potassium current, IK, by the class IC antiarrhythmic agent, flecainide, and by the novel selective class III antiarrhythmic agent, E-4031, were compared in isolated cat ventricular myocytes using the single suction-pipette, voltage-clamp technique. Flecainide (10 microM) markedly reduced IK elicited on depolarization steps to plateau voltages (+10 mV) and nearly completely blocked the "tail currents" elicited on repolarization to -40 mV (93 +/- 4% block at +40 mV, n = 3). E-4031 (1 microM) produced similar effects (96 +/- 3% block at +40 mV, n = 3). Slow voltage ramps from -100 to +40 mV confirmed inward rectifying properties of IK and showed that flecainide and E-4031 have no effects on the background potassium current, IK1. Thus, the results demonstrate that block of IK is a common feature of flecainide and E-4031. IK block by E-4031 most likely underlies the drug's potent class III antiarrhythmic properties. On the other hand, flecainide block of IK during an action potential would tend to prolong repolarization, but this effect may be obscured by concomitant block of plateau Na+ channels to produce little or no change in action potential duration, consistent with its class IC classification.