Effects of quinidine on atrioventricular nodal reentrant paroxysmal tachycardia

The atrioventricular nodal reentrant tachycardia circuit: A proposal

Several models of the atrioventricular nodal reentrant tachycardia circuit have been proposed. Recently, there has been experimental and clinical electrophysiology evidence that the right and left inferior extensions of the human atriventricular node and the atrionodal inputs they facilitate may provide the anatomic substrate of the slow pathway. Inferior nodal extensions appear to constitute a necessary limb of the tachycardia circuit in all forms of atrioventricular nodal reentrant tachycardia and represent the ablation target for all forms of this arrhythmia. Anatomic variations of multiple atrionodal inputs via atrial transitional cells may create the conditions for tachycardia inducibility and differing patterns of retrograde atrial activation. In the present article, we summarize the available evidence and propose a comprehensive model of the tachycardia circuit for all forms of atrioventricular nodal reentrant tachycardia based on the concept of atrionodal inputs.

Effects of cardioactive medications on retrograde conduction: Continuing relevance for current devices

INTRODUCTION

Retroconduction (ventriculo-atrial conduction) remains a problem for patients with implanted cardiac rhythm devices. Pacemaker algorithms can detect and terminate endless loop tachycardia (ELT), but actual prevention of ELT may require anti-arrhythmic drugs (AADs). Similarly, AADs can affect ICD rhythm discrimination algorithms that depend on atrio-ventricular ratios. There is concern whether these drugs remain effective during stress situations.

METHODS

Electrophysiologic studies that included retroconduction testing using slow ramp pacing were done in 1332 patients. The presence or absence of retroconduction at baseline and with drug was recorded, as was the rate at block. As a stress surrogate, isoproterenol was used to test retroconduction and reversal of drug-induced block.

RESULTS

Procainamide, mexiletine, phenytoin, disopyramide, quinidine, beta-blockers, encainide, and amiodarone caused complete retrograde block or decreased the rate at which block occurred (mean 76% of patients, p < 0.008), whereas digoxin, lidocaine, diltiazem, and verapamil did not. Isoproterenol (in the absence of AADs) increased the rate at block in 82% of 404 patients with retroconduction at baseline (p < 0.005). Of 319 patients without retroconduction at baseline, 134 (42%) developed retroconduction after isoproterenol. Isoproterenol reversed retrograde block in 39% of patients with block on an AAD. Amiodarone, digoxin, and the combination of digoxin plus a beta-blocker were most effective at resisting this reversal of ventriculo-atrial block (80%, 68%, and 75% respectively).

CONCLUSION

Most of the AADs reviewed increase the cycle length at block or abolish retroconduction, while isoproterenol has the opposite effect. Anti-arrhythmic medications, particularly amiodarone, digoxin, and the combination of digoxin plus a beta-blocker may be considered for a patient with multiple ELT episodes or certain ICD detection problems.

Supraventricular tachycardia

Supraventricular tachycardias (SVT) comprise those tachycardias that originate above the bifurcation of the bundle of His. They can be classified broadly as AV node dependent and AV node independent. The mechanism and clinical manifestation of SVTs, which is essential to their correct diagnosis, is reviewed. The therapeutic management of SVTs, including acute and chronic drug therapy and catheter ablation, is discussed also.

Pharmacologic management of supraventricular tachycardias in children. Part 1: Wolff-Parkinson-White and atrioventricular nodal reentry

OBJECTIVE

To review the literature regarding the use of antiarrhythmic agents in the management of Wolff-Parkinson-White (WPW) syndrome and atrioventricular nodal reentry tachycardia (AVNRT) in infants and children, and to discuss the advantages and disadvantages of specific agents in each arrhythmia in an effort to develop treatment guidelines.

DATA SOURCES

A MEDLINE search encompassing the years 1966-1996 was used to identify pertinent literature for discussion. Additional references were found in the articles that were retrieved via MEDLINE.

STUDY SELECTION

Clinical trials that address the use of antiarrhythmic agents for the treatment of the supraventricular tachycardias WPW and AVNRT in children were selected. Literature pertaining to dosage, pharmacokinetics, efficacy and toxicity of antiarrhythmic agents in children were considered for possible inclusion in the review, and information judged to be pertinent by the authors was included in the discussion.

DATA EXTRACTION

Although there are numerous reports of antiarrhythmic use in children, very few large studies are designed to evaluate an individual antiarrhythmic agent for a specific arrhythmia. Controlled, comparison trials of antiarrhythmic agents in children are virtually nonexistent. Ideally, controlled clinical trials are used to develop clinical guidelines; however, in this situation, most data and information must be obtained from case series of children treated. Although the results from these type of studies may be useful in developing guidelines for the optimal use of these agents, controlled trials are required for establishing standard treatment guidelines for all patients.

DATA SYNTHESIS

Despite limited scientific evaluation of conventional agents in the treatment of WPW and AVNRT in children, they continue to be used as standard of care. Most information regarding the use of conventional agents in children has been extrapolated from the adult literature. Little justification for the use of agents or dosing in children is available. Controlled trials regarding the use of new antiarrhythmic agents (propafenone, amiodarone, flecainide) are available; however, the variance in dosing schemes, presence of structural heart disease, and patient age make the development of recommendations difficult.

CONCLUSIONS

Because of greater clinical experience with these conventional antiarrhythmic agents, they continue to be first-line therapy in the management of most supraventricular tachycardia (SVT) in children. The management of SVT in children with WPW syndrome should begin with the use of a beta-blocker with the addition of digoxin or procainamide for treatment failures. The use of digoxin monotherapy, although frequently used by many practitioners in infants and children with WPW, cannot be recommended. For failures to conventional agents, flecainide is the preferred agent, while therapy with propafenone, amiodarone, and sotalol remains to be elucidated. The management of AVRNT is similar to that of WPW; however, digoxin is the agent of first choice. Trials of beta-blockers and procainamide should follow for treatment failures with flecainide again being the preferred "newer" antiarrhythmic for use in resistant cases. Additional well-designed, controlled trials are needed to further evaluate the comparative efficacy of antiarrhythmics in the management of WPW and AVNRT in children, as well as to evaluate dosing and toxicity in various age groups.

"AV nodal" reentry: Part II: AV nodal, AV junctional, or atrionodal reentry?

The classical model of "atrioventricular (AV) nodal" reentrant tachycardia suggests that the reentrant circuit is entirely within the compact AV node and that AV nodal tissue is present proximal and distal to the circuit. Recent evidence from mapping studies and from examination of the effects of curative procedures, however, suggests that the upper end of the circuit uses perinodal atrial or transitional tissue. Moreover, the anatomical substrate of dual "AV nodal" pathways is likely to be the multiple connections between compact AV node and atrium rather than discrete intranodal pathways. The antegrade slow pathway appears to be situated at the posteroinferior approaches to the AV node in the region between the coronary sinus orifice and the tricuspid annulus. The retrograde fast pathway appears to be situated in the anterior atrionodal connections at the apex of Koch's triangle, close to the His bundle. The lower turnaround point of the circuit is likely to be within the AV node.

Pirmenol in the termination of paroxysmal supraventricular tachycardia

The effects of pirmenol in terminating paroxysmal supraventricular tachycardia were studied in 25 patients. Pirmenol was administered as 1 or 2 injections of 50 mg to 17 patients during a spontaneous attack, or as a 50-mg bolus followed by steady infusion of 2.5 mg/min in 8 patients during a tachycardia that was induced electrophysiologically. Termination was successful in 11 of 17 patients who had a spontaneous attack and in 3 of 8 patients who had induced tachycardia. Pirmenol was effective in 3 of 5 patients with atrioventricular nodal reentrant mechanism, but in none of 3 patients with a reentrant tachycardia with a retrogradely conducting atrioventricular bypass tract. Conversion to sinus rhythm was achieved in 14 of 25 patients (56%). No hemodynamic adverse effects occurred. Pirmenol increased the atrial effective refractory period, but had little effect on conduction in the atrioventricular node and His-Purkinje system. Reentry was abolished through a block in the retrograde part of the dual atrioventricular nodal pathway, which is typical of class I antiarrhythmic agents.

Use of calcium antagonists for cardiac arrhythmias

Calcium antagonists have emerged as a new class of antiarrhythmic agents for the control of certain supraventricular and ventricular arrhythmias. Electrophysiologically, these agents are heterogeneous but their main action is mediated through a depressant effect on the slow calcium channel in cardiac muscle, most readily demonstrated in isolated tissue preparations. In vivo, their actions are modulated by their reflex actions and by their interaction with the autonomic nervous system due to the noncompetitive adrenergic-blocking actions that some of the compounds exhibit. The major agents exerting antiarrhythmic actions are verapamil, diltiazem, gallopamil, tiapamil and bepridil; the dihydropyridines are devoid of electrophysiologic actions in vivo. Calcium antagonists prolong intranodal conduction time, lengthen the effective and functional refractory periods in the atrioventricular node but exert little or no effect on atrial, ventricular, His-Purkinje or bypass tract conduction or refractoriness (except in the case of bepridil, which has additional electrophysiologic properties). These effects form the basis of the clinical antiarrhythmic effects of this class of agents. The most striking action is the predictable and prompt termination of the reentrant supraventricular tachycardia by intravenous verapamil and diltiazem and the slowing of the ventricular response in atrial flutter and fibrillation. These agents may also be of value in the long-term control of ventricular response in atrial flutter and fibrillation; their role in multifocal atrial tachycardia and other ectopic tachycardias is less well defined. Calcium antagonists reverse ischemic ventricular arrhythmias caused by coronary artery spasm but exert little or no action in the usual forms of sustained ventricular tachyarrhythmias associated with severe structural heart disease. They are poor suppressants of ventricular premature complexes. Recent data have established their role in exercise-induced tachycardia occurring in the context of ischemic heart disease; they are also of value in ventricular tachycardia occurring in young patients who develop tachycardia with a right bundle branch block and left axis deviation morphology, an arrhythmia thought to be due to triggered automaticity.

Pirmenol in termination of paroxysmal supraventricular tachycardia

An assessment was made of the effect of pirmenol in the termination of paroxysmal supraventricular tachycardia (SVT). Sinus rhythm was restored by intravenous administration in 11 of 17 patients during a spontaneous attack. Another 8 patients were studied electrophysiologically. Pirmenol terminated an induced SVT in 3 of 5 patients having an atrioventricular (AV) intranodal re-entry mechanism but in none of 3 patients having an atrioventricular bypass tract as one re-entrant limb. The overall success in restoring sinus rhythm was 14 of 25 patients (56%). The drug was hemodynamically well tolerated even in cases of continued SVT. Pirmenol increased the atrial effective refractory period and had no obvious effect on AH and HV intervals. The functional refractory period of the AV node was decreased, probably by an anticholinergic effect. The effective and functional refractory periods of retrograde atrioventricular conduction via the AV node and bypass tract were increased in some patients. The mechanism terminating the AV intranodal SVT was a block in the retrograde part of the dual AV nodal pathway, a typical antiarrhythmic Class I effect.

Curative surgery for atrioventricular junctional ("AV nodal") reentrant tachycardia

A new surgical approach was studied prospectively in 10 consecutive patients with atrioventricular (AV) junctional reentrant tachycardia. The aim was to abolish tachycardia yet preserve normal AV conduction. On the basis of electrophysiologic study before operation, patients were classified as type A (ventriculoatrial [VA] intervals during tachycardia less than or equal to 40 ms) (seven patients) or type B (VA intervals greater than 40 ms) (three patients). Dual AV junctional pathways were demonstrable with single extrastimulus testing in seven patients before operation. Endocardial mapping during tachycardia at surgery revealed earliest atrial activation anteromedial to the AV node in type A patients and posterior to the node in the type B patients. The perinodal atrium in the region of earliest atrial activation during tachycardia was carefully disconnected from the AV node. After operation, AV junctional reentrant tachycardia was not inducible at comprehensive electrophysiologic study in any patient, and no clinical recurrences have occurred during a follow-up period of 2 to 14 months (mean 8 +/- 4). Normal AV conduction was preserved in all cases. Anterograde slow AV junctional pathway conduction was abolished in five of seven cases. Retrograde His to atrium conduction time was prolonged in type A patients but the capacity for retrograde VA conduction remained excellent. Retrograde His to atrium conduction was interrupted or severely compromised in the type B patients. These data show that there are at least two types of AV junctional reentry. Perinodal atrium appears to be part of the reentrant circuit in human AV junctional reentry. Although the most consistent effect of surgery was on the retrograde limb of the circuit, anterograde slow pathway conduction was also modified. AV junctional reentry is surgically curable with a high success rate.

Cardiac electrophysiologic testing: its role in the selection of antiarrhythmic drug regimens for supraventricular and ventricular arrhythmias

Cardiac electrophysiology studies use intracardiac recording and programmed stimulation to define the mechanisms and most appropriate therapy for supraventricular and ventricular arrhythmias. Using these techniques, the majority of clinical tachycardias can be reproducibly initiated and terminated in the electrophysiology laboratory, thereby allowing the most appropriate therapy to be selected. With this approach, antiarrhythmic agents can be tested in a systematic, serialized fashion for efficacy, safety and patient tolerance. With both supraventricular and ventricular tachycardias, suppression of arrhythmia induction predicts freedom from recurrence, whereas inducibility carries a poor prognosis in clinical follow-up. Electrophysiology studies provide a safe and effective approach to the treatment of selected patients with cardiac arrhythmias.

Ethmozin. II. Effects of intravenous drug administration on atrioventricular nodal reentrant tachycardia

Electrophysiologic studies were performed in 11 patients with atrioventricular (AV) nodal reentrant tachycardia (SVT) before and after intravenous administration of 1.5 to 2 mg/kg ethmozin. Initially, 9 of 11 patients had induction of sustained SVT, and two remaining patients had nonsustained SVT and atrial echoes, respectively. Ethmozin terminated induced SVT in six of nine patients. In six of nine patients ethmozin prevented the development of sustained SVT, indicating that ethmozin depressed retrograde fast pathway AV nodal conduction. In four of these patients atrial echoes were abolished. In the two remaining cases ethmozin prevented the induction of nonsustained SVT. In only three of these nine patients was sustained SVT induced. Anterograde fast and slow pathway properties did not significantly change with ethmozin administration. Effective refractory period (ERP) of the ventriculoatrial (VA) conduction system and ventricular paced cycle length producing VA block was 305 +/- 40 (mean +/- SEM) and 347 +/- 38 msec before and 424 +/- 105 and 475 +/- 71 msec after ethmozin administration, respectively (p less than 0.01, n = 8), suggesting depression of retrograde pathway with ethmozin administration. Ethmozin significantly (p less than 0.05) lengthened PA, AH, HV, and PR intervals (36 +/- 11 to 45 +/- 14 msec, 84 +/- 21 to 93 +/- 17 msec, 42 +/- 8 to 50 +/- 7 msec, and 163 +/- 23 to 190 +/- 31 msec, respectively). No significant change was observed in sinus rate, QRS and QT intervals, or ERP of atrium and ventricle. Thus, a single intravenous dose of ethmozin terminated induced SVT and prevented induction of sustained SVT in most patients, reflecting depression of retrograde fast pathway conduction.

Electrophysiologic effects of flecainide acetate on sinus node function, anomalous atrioventricular connections, and pacemaker thresholds

The acute electrophysiologic effects of i.v. flecainide acetate (2 mg/kg body weight) were assessed in 71 patients undergoing electrophysiologic study. Ten patients underwent investigation for sinus node dysfunction. Sinus cycle length shortened slightly, from 980 +/- 292 to 931 +/- 276 ms (p less than 0.01). Uncorrected or corrected sinus node recovery times or sinoatrial conduction time (according to the methods of Strauss and Narula) did not change in 6 patients with normal sinus node function and in 3 of 4 patients with abnormal sinus node function at rest. In the remaining patient maximal sinus node recovery time increased from a value at rest of 5,185 ms to 23,460 ms after flecainide. In the same patient sinoatrial conduction times at rest increased from 159 ms (Strauss method) and 143 ms (Narula method) to 1,398 and 1,455 ms, respectively, after flecainide. Thirty-three patients underwent electrophysiologic evaluation of anomalous atrioventricular (AV) pathways and reentrant tachycardias. Flecainide significantly prolonged accessory AV pathway anterograde and retrograde refractoriness. Anterograde accessory pathway block occurred in 33% of patients and retrograde accessory pathway block in 44%. Flecainide was successful in the acute termination of 86% of orthodromic atrioventricular reentrant tachycardias. In 15 patients with dual AV nodal pathways, only retrograde "fast" AH pathway refractoriness was significantly increased by flecainide, which was successful in the acute termination of 88% of intra-AV nodal reentrant tachycardias. In 28 patients who underwent endocardial pacing threshold assessment before and after i.v. flecainide, the acute threshold rose by a maximum of 117%, whereas the chronic threshold rose by a maximum of 83%.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiologic effects and mechanisms of termination of supraventricular tachycardia by intravenous amiodarone

Electrophysiologic studies were performed in nine patients with reentrant paroxysmal supraventricular tachycardia (PSVT) during a control period and following 5 mg/kg body weight of intravenous amiodarone (Cordarone, Labaz) administered as a slow continuous infusion over 15 to 20 minutes. All nine patients had induction of sustained PSVT during control studies. In seven of nine patients (group 1) the tachycardia was due to atrioventricular (AV) nodal reentry, and in two of nine patients (group 2) a concealed retrograde bypass tract was incorporated in the reentrant process. In group 1, following amiodarone, all seven patients lost the ability to sustain PSVT with either absence of atrial echoes (one patient) or induction of less than or equal to 3 echo beats (six patients) with termination of PSVT in the antegrade pathway (three patients) or retrograde pathway (two patients) or both (one patient). In group 2, following amiodarone, both patients lost the ability to sustain PSVT with absence of atrial echoes (one patient) on induction of a single echo beat (one patient) with block in the retrograde pathway (i.e., the concealed retrograde bypass tract). Amiodarone significantly increased (1) atrial cycle length for AV nodal Wenckebach block, (2) antegrade functional refractory period of the AV node, (3) antegrade effective refractory period of the AV node, (4) ventricular paced cycle length for ventricular atrial block, and (5) the retrograde functional refractory period of the ventricular-atrial conducting system.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of oral diltiazem in paroxysmal supraventricular tachycardia

Electrophysiologic studies were performed before and 2 hours after the oral administration of 270 mg of diltiazem in 3 divided doses at 8-hour intervals in 36 patients with paroxysmal supraventricular tachycardia (SVT). Before diltiazem, all 36 patients had induction of sustained SVT: 24 with atrioventricular (AV) reentrance incorporating an accessory pathway (Group 1) and 12 with AV nodal reentrance (Group 2). After diltiazem, 20 patients in Group 1 lost the ability to induce or sustain SVT because of increased anterograde normal pathway refractoriness in 19 patients and increased retrograde accessory pathway refractoriness in 1. Eight patients in Group 2 could no longer induce or sustain SVT because of increased anterograde slow pathway refractoriness in 2 patients and increased retrograde fast pathway refractoriness in 6. Diltiazem concentration in the blood, measured in 29 patients, was 156 +/- 75 ng/ml (mean +/- standard deviation). Fifteen patients, 2 with and 13 without induction of sustained SVT after diltiazem, were discharged on the same dosage of diltiazem and followed up 5 +/- 3 months. The former 2 patients had attacks of sustained SVT, whereas the latter 13 have been free of sustained SVT. In conclusion, oral diltiazem prevents induction and sustenance of paroxysmal SVT in most patients and may be used as an alternative agent for the prophylaxis of SVT.

Selective blockade of retrograde fast pathway by intravenous disopyramide in paroxysmal supraventricular tachycardia mediated by dual atrioventricular nodal pathways

Electrophysiological effects of 2 to 2.5 mg/kg iv disopyramide were studied in 10 patients with dual nodal pathways who used a slow pathway for anterograde and a fast pathway for retrograde conduction during paroxysmal supraventricular tachycardia (mean cycle length 308.5 +/- 37 ms; range 260-370 ms). Disopyramide terminated the tachycardia in six cases by production of ventriculoatrial block in five and by sinus overdrive in one. In the remaining four patients cycle length of the paroxysmal supraventricular tachycardia increased significantly from 270 +/- 8 ms to 377.5 +/- 28 ms. In all 10 patients disopyramide depressed retrograde fast pathway conduction manifest by an increase in mean ventricular paced cycle length producing ventriculoatrial block from less than or equal to 296.5 +/- 25 ms to 358 +/- 60 ms, and increase in retrograde fast pathway effective refractory period from less than or equal to 246 +/- 34 ms to 325 +/- 36 ms; the drug abolished ventriculoatrial conduction in two cases. Anterograde slow pathway and fast pathway conduction properties were unchanged after disopyramide (atrial paced cycle length producing AH block 292 +/- 30 to 306.5 +/- 30 ms; effective refractory period of anterograde fast pathway less than or equal to 274 +/- 56 to 284 +/- 44 ms, before and after the drug, respectively) suggesting that anterograde conduction was not crucial either for sustainment or for failure to initiate paroxysmal supraventricular tachycardia after the drug. Paroxysmal supraventricular tachycardia could not be reinduced in six cases after disopyramide. In the other four the ventricular paced cycle lengths producing ventriculoatrial block (318 +/- 41 ms) and effective refractory period of retrograde fast pathway (320 +/- 28 ms) were shorter than the cycle length of reinduced paroxysmal supraventricular tachycardia (367.5 +/- 35 ms) allowing perpetuation of the tachycardia. We conclude that disopyramide breaks atrioventricular nodal re-entrant tachycardia by specific blockade of the retrograde fast pathway though the effect on anterograde atrioventricular nodal conduction is variable.

Cardiac electrophysiologic effects of flecainide acetate for paroxysmal reentrant junctional tachycardias

Intravenous flecainide acetate was administered to 33 patients undergoing routine electrophysiologic study: 18 patients had a direct accessory atrioventricular (AV) pathway and 15 patients had functional longitudinal A-H dissociation (dual A-H pathways). Flecainide was given to 14 patients during sustained AV reentrant tachycardia and to 9 patients during sustained intra-AV nodal reentrant tachycardia. AV reentrant tachycardia was successfully terminated in 12 of 14 patients. Tachycardia termination was due to retrograde accessory pathway block in 11 patients and AV nodal block in 1. During flecainide administration, tachycardia cycle lengths increased (327 +/- 55 to 426 +/- 84 ms) principally because of retrograde conduction delay in the accessory pathway (127 +/- 34 to 197 +/- 67 ms). After flecainide administration, tachycardia reinitiation was not possible in 6 patients. In all 18 patients with accessory AV pathway conduction, flecainide significantly increased both anterograde and retrograde accessory pathway effective refractory periods, with anterograde accessory pathway block in 3 patients and retrograde accessory pathway block in 8. Intra-AV nodal reentrant tachycardia was successfully terminated in 8 of 9 patients. Tachycardia termination was due to retrograde "fast" A-H pathway block in 7 patients and anterograde "slow" A-H pathway block in 1 patient. During flecainide administration, tachycardia cycle lengths increased (326 +/- 50 to 433 +/- 64 ms) due to both anterograde, A-H and H-V (AV 242 +/- 97 to 343 +/- 75 ms), and retrograde, earliest ventricular to earliest atrial (51 +/- 14 to 70 +/- 23 ms) conduction delay. After flecainide administration, reinitiation of intra-AV nodal reentrant tachycardia was not possible in 4 patients. In all 15 patients with dual A-H pathways, flecainide selectively prolonged the retrograde effective refractory period of the fast A-H pathway, having little effect on anterograde fast A-H pathway refractoriness or on anterograde and retrograde slow A-H pathway refractoriness. Anterograde fast A-H pathway block occurred in 1 patient and retrograde fast A-H pathway block occurred in 6 patients. No serious adverse effects were encountered during the study. Flecainide acetate is an effective agent for the acute termination of both orthodromic AV and intra-AV nodal reentrant tachycardias. This antiarrhythmic action appears to be mediated through a predominant effect on either accessory AV pathway or retrograde fast A-H pathway refractoriness.

Pharmacologic treatment of cardiac arrhythmias: 25 years of progress

This review of practical and theoretical advances in antiarrhythmic drug therapy consists of four parts. Part 1, on clinical applications, compares the approaches to treatment 25 years ago with those of today, examines the current status of antiarrhythmic drugs used 25 years ago, reports on drugs approved for clinical use during the past 25 years, reviews new experimental drugs and suggests an approach to classification of antiarrhythmic drugs. Part 2 summarizes the contributions of cellular electrophysiology to the understanding of drug action, with emphasis on the drug-induced block of the voltage- and time-dependent properties of the rapid sodium channel. The subsequent section contains a brief discussion of the impact made by the new knowledge and the new diagnostic technology on the contemporary practices. The main conclusions are 1) that the more rational approach to treatment has benefited proportionately more patients with supraventricular than with ventricular arrhythmias, and 2) that new advances have made it possible to design successful treatments for certain patients with problems that could not be resolved in the past.