Spontaneous termination of reciprocating tachycardia owing to interaction of dual atrioventricular nodal pathways in patients with an accessory pathway

Retrospective analysis of the electrophysiologic recordings from 125 consecutive patients with reciprocating tachycardia involving an accessory atrioventricular (AV) pathway suggested, by the mode of spontaneous termination of reciprocating tachycardia, the coexistence of dual AV nodal pathways in 7 patients. Three different modes of spontaneous tachycardia termination were observed. In 2 patients with antidromic tachycardia, termination was a result of AV nodal reentry, preceded by a decrease in retrograde AV nodal conduction. In 3 other patients with antidromic tachycardia, termination occurred after a sudden switch from a slow to a fast AV nodal pathway, leading to conduction block in either the accessory pathway or the His-Purkinje system. In 2 patients with orthodromic tachycardia, termination was caused by a sudden change of anterograde conduction from a fast to a slow AV nodal pathway, eliciting an AV nodal echo beat. The interaction of dual AV nodal pathways within the reentry circuit incorporating the accessory pathway always inhibited sustained reciprocating tachycardia.     

Wolff-Parkinson-White syndrome in children: electrophysiologic and pharmacologic characteristics.

Intracardiac electrophysiologic studies were performed on 28 infants and children, ages 1 month to 18 years, with the Wolff-Parkinson-White syndrome to try to determine 1) the electrophysiologic characteristics of the accessory connection and 2) the mechanisms of associated supraventricular dysrhythmias. Although the antegrade refractory periods of the normal conduction system were shorter than those found in adults, those of the accessory connection were slightly longer. Reciprocating supraventricular tachycardia (SVT), which had been a clinical problem in 26 of 28, could be induced in the laboratory in all 26 subjects. The mechanism involved reentry with antegrade conduction through the atrioventricular (AV) node and retrograde through the accessory connection in 22. Eleven of these 22 had a wide QRS during tachycardia due to a bundle branch block. Three other subjects had wide QRS tachycardia, but the mechanism involved antegrade conduction through the accessory connection and retrograde through the AV node. The other patient had AV node reentry tachycardia. Two patients did not have clinical SVT, and in these two, SVT could not be induced. Neither patient had retrograde conduction through the accessory connection. The site of the accessory connection could be identified in 26 subjects by the sequence of retrograde activation of the atrium during SVT or ventricular pacing. Digitalis shortened the refractory period of the accessory connection in five of the eight patients studied.     

Quintuple pathways participating in three distinct types of atrioventricular reciprocating tachycardia in a patient with Wolff-Parkinson-White syndrome

Electrophysiologic studies were performed in a patient with recurrent supraventricular tachyarrhythmias. Sinus and paced atrial beats had QRS complexes characteristic of atrioventricular (A-V) conduction through a manifest left lateral accessory pathway (Wolff-Parkinson-White syndrome, type A). Three distinct types of A-V reciprocating tachycardia and three different modes of retrograde atrial activation were demonstrated. Type 1 tachycardia involved the slow A-V nodal pathway and a second (left lateral or left paraseptal) accessory A-V pathway capable of retrograde conduction only. Type 2 tachycardia was of the slow-fast A-V nodal pathway type. Type 3 tachycardia involved a heretofore undescribed circuit in that retrograde conduction occurred through an accessory A-V pathway with long retrograde conduction times and anterograde conduction through both the manifest left lateral accessory A-V pathway and fast A-V nodal pathway. Premature ventricular beats delivered late in the cycle of this tachycardia advanced (but did not change) the retrograde atrial activity without affecting the timing of the corresponding anterograde H deflection. In summary, this patient had five (three accessory and two intranodal) pathways participating in three different types of A-V reciprocating tachycardia; the recurrence of these were prevented with oral amiodarone therapy.     

Radiofrequency catheter ablation of two accessory pathways with different unidirectional conduction properties

Simultaneous occurrence of narrow and broad QRS complex tachycardias in patients with WPW syndrome usually indicates a macroreentry in an orthodromic atrioventricular reentry-tachycardia using the AV node as antegrade and the accessory pathway as retrograde conduction and vice versa in an antidromic circuit. We report on a 32-year-old woman with WPW syndrome presenting with both a narrow and a broad QRS complex tachycardia using two accessory pathways with different unidirectional conduction properties in combination of an exclusively antegrade conducting AV node. This case report describes conventional mapping techniques and ablation of this unusual entity of a WPW syndrome.     

Preexcited reciprocating tachycardia in patients with Wolff-Parkinson-White syndrome: incidence and mechanisms

During the electrophysiologic study of 435 patients referred for evaluation of Wolff-Parkinson-White syndrome, 42 (10%) had preexcited reciprocating tachycardia (defined as a macro-reentrant tachycardia that used an accessory atrioventricular [AV] pathway for antegrade conduction). The ages of the patients ranged from 9 to 67 years (27 +/- 14). Thirty-three were male patients, nine female, and eight had Ebstein's anomaly. Preexcited reciprocating tachycardia cycle length was 220 to 430 msec (294 +/- 42). Significant hemodynamic compromise in the laboratory directly related to preexcitated reciprocating tachycardia occurred in only one patient. However, in 10 patients a transformation to atrial fibrillation was seen after a spontaneously occurring premature atrial contraction. Only 17 of the 42 patients with preexcited reciprocating tachycardia during electrophysiologic study had the same tachycardia documented clinically. These 17 patients were more often younger with multiple accessory pathways and with no history of orthodromic reciprocating tachycardia when compared with 25 patients in whom preexcited reciprocating tachycardia could be induced only in the laboratory. Preexcited reciprocating tachycardia was induced in the laboratory in 22 of 374 (6%) patients with single accessory pathways and in 20 of 61 (33%) of those with multiple accessory pathways. In the 20 patients with multiple accessory pathways, the spectrum of reentrant circuits included fusion over two or more accessory pathways or fusion over both an accessory pathway and the AV node. In the 22 patients with a single accessory pathway and true antidromic reciprocating tachycardia, all but two episodes were at least 4 cm from the AV node. No patient with true antidromic reciprocating tachycardia had a posterior septal accessory AV pathway. Only in patients with multiple accessory pathways was the posterior septal accessory AV pathway used as the antegrade limb.     

Permanent form of junctional reciprocating tachycardia with only even-numbered beats

An analysis of the electrocardiogram of a patient with the permanent form of junctional reciprocating tachycardia is presented. The patient demonstrated near-incessant tachycardia, with a 1:1 atrioventricular relationship and a retrograde P wave (P') occurring closer to the succeeding QRS complexes (ie, with a P'R interval that is shorter than the RP' interval). Each tachycardia episode was characterized by alternating short and long cardiac cycles due to alternation of retrograde conduction time (RP' interval), retrograde Wenckebach periodicity, and an even number of ectopic P' waves. The authors propose that there is an accessory AV connection with decremental functional properties that arborizes into two atrial branches with different conduction times. The fast branch initially exhibits a 3:2 retrograde conduction block followed by a cycle length-dependent 2:1 retrograde conduction block, thereby permitting alternate use of the slow branch, which is the weakest component of the reciprocating process.     

Radiofrequency catheter ablation of two accessory pathways with different unidirectional conduction properties

Summary Simultaneous occurrence of narrow and broad QRS complex tachycardias in patients with WPW syndrome usually indicates a macroreentry in an orthodromic atrioventricular reentry-tachycardia using the AV node as antegrade and the accessory pathway as retrograde conduction and vice versa in an antidromic circuit. We report on a 32-year-old woman with WPW syndrome presenting with both a narrow and a broad QRS complex tachycardia using two accessory pathways with different unidirectional conduction properties in combination of an exclusively antegrade conducting AV node. This case report describes conventional mapping techniques and ablation of this unusual entity of a WPW syndrome.     

Spectrum of regular tachycardias with wide QRS complexes in patients with accessory atrioventricular pathways

Reciprocating tachycardia and atrial flutter or fibrillation are the rhythm disorders most frequently documented in patients with accessory atrioventricular (A-V) pathways. Reciprocating tachycardia typically results in a regular tachycardia (140 to 250/min) with a normal QRS pattern, although on occasion bundle branch block aberration occurs. Atrial flutter or fibrillation may result in an irregular ventricular response, with the QRS configuration being normal or exhibiting bundle branch block or various degrees of ventricular preexcitation, or both. Although much less common than either reciprocating tachycardia or atrial flutter/fibrillation, regular tachycardias with a wide QRS complex suggestive of ventricular preexcitation are observed in patients with accessory pathways. Excluding functional or preexisting bundle branch block, several arrhythmias may cause these electrocardiographic findings which may mimic those of ventricular tachycardia.In the present study a variety of arrhythmias that resulted in tachycardias with a wide QRS complex were examined in 163 patients with accessory pathways who underwent clinical electrophysiologic study for evaluation of recurrent tachyarrhythmias. Twenty-six patients (15 percent) manifested a regular tachycardia with a wide QRS complex suggesting ventricular preexcitation. Atrial flutter with 1:1 anterograde conduction over an accessory pathway (15 of 26 patients, 58 percent) was the most frequent arrhythmia and was usually associated with a heart rate of 240/min or greater (12 of 15 patients). Reciprocating tachycardia with conduction in the anterograde direction over an accessory pathway (antidromic reciprocating tachycardia) occurred in 7 of 26 patients (27 percent), and resulted in a slower ventricular rate than atrial flutter (217 ± 22 versus 262 ± 42, P < 0.01). Other arrhythmias included reciprocating tachycardia with reentry utilizing a fasciculoventricular or nodoventricular connection (two patients, 8 percent), reciprocating tachycardia with reentry in the atrium or A-V node and anterograde accessory pathway conduction (one patient, 4 percent) and ventricular tachycardia (one patient, 4 percent).In this study the clinical electrophysiologic diagnostic features of several arrhythmias which cause tachycardias with a wide QRS compex suggesting ventricular preexcitation are outlined. It is apparent that definitive arrhythmia diagnosis during these tachycardias is often complex and usually requires careful study using intracardiac electrode catheter techniques.     

Electrophysiologic characteristics and radiofrequency ablation of concealed nodofascicular and left anterograde atriofascicular pathways

INTRODUCTION: True nodoventricular or nodofascicular pathways and left-sided anterograde decremental accessory pathways (APs) are considered rare findings. METHODS AND RESULTS: Two unusual patients with paroxysmal supraventricular tachycardia were referred for radiofrequency (RF) ablation. Both patients had evidence of dual AV nodal conduction. In case 1, programmed atrial and ventricular stimulation induced regular tachycardia with a narrow QRS complex or episodes of right and left bundle branch block not altering the tachycardia cycle length and long concentric ventriculoatrial (VA) conduction. Ventricular extrastimuli elicited during His-bundle refractoriness resulted in tachycardia termination. During the tachycardia, both the ventricles and the distal right bundle were not part of the reentrant circuit. These findings were consistent with a concealed nodofascicular pathway. RF ablation in the right atrial mid-septal region with the earliest atrial activation preceded by a possible AP potential resulted in tachycardia termination and elimination of VA conduction. In case 2, antidromic reciprocating tachycardia of a right bundle branch block pattern was considered to involve an anterograde left posteroseptal atriofascicular pathway. For this pathway, decremental conduction properties as typically observed for right atriofascicular pathways could be demonstrated. During atrial stimulation and tachycardia, a discrete AP potential was recorded at the atrial and ventricular insertion sites and along the AP. Mechanical conduction block of the AP was reproducibly induced at the annular level and at the distal insertion site. Successful RF ablation was performed at the mitral annulus. CONCLUSION: This report describes two unusual cases consistent with concealed nodofascicular and left anterograde atriofascicular pathways, which were ablated successfully without impairing normal AV conduction system.     

Investigation of the preexcitation syndromes

Assessment of the localization and function of accessory atrioventricular pathways may be undertaken with noninvasive and invasive examination techniques. Noninvasive methods, however, such as electrocardiography, vectorcardiography, precordial mapping, echocardiography and scintigraphy do not enable exact delineation of the site of the accessory pathway since, in general, their use requires maximal preexcitation; moreover, they are of less value in the presence of septal bundles, multiple accessory pathways or intraventricular conduction disturbances. Accordingly, localization of accessory atrioventricular pathways is based on data obtained from intracardiac electrography such as the interval between stimulus and delta wave during atrial pacing, retrograde atrial sequence, VA conduction time at onset of right or left bundle branch block as well as responses to programmed ventricular stimulation during orthodromic reciprocating tachycardia. The most exact method for determination of the localization of the Kent bundle employs intraoperative endocardial and epicardial mapping with analysis of VA and AV conduction times during reciprocating tachycardia, ventricular stimulation and atrial pacing. The conduction properties of the accessory atrioventricular bundle may be estimated noninvasively based on the persistence of a delta wave as a function of the heart rate, the influence of antiarrhythmic agents on the anterograde conduction via the normal pathway as well as the shortest interval between two consecutive QRS complexes with delta waves during atrial fibrillation. Functional assessment is carried out invasively through recording the anterograde and retrograde refractory periods of the Kent bundle as well as observations during programmed atrial and ventricular stimulation or induced atrial fibrillation.     

Effects of intravenous sotalol in patients with atrioventricular accessory pathways

Effects of intravenous injection of 0.6 mg/kg sotalol, a beta-blocking agent with additional class III properties, were studied by means of electrophysiologic techniques in 14 patients, seven with the Wolff-Parkinson-White syndrome and seven with concealed atrioventricular (AV) accessory pathways. Sotalol brought about a significant increase in the retrograde effective refractory period of the anomalous pathway, whereas changes in the antegrade effective refractory period were more variable. In five of nine patients with electrically induced reciprocating tachycardia sotalol prevented the initiation of sustained reentry. In most cases the suppression of the circus movement was the result of the development of AV nodal block. Thus our data support the use of sotalol for the treatment of tachycardias incorporating anomalous AV conduction pathways.     

Clinical and electrophysiologic observations in patients with concealed accessory atrioventricular bypass tracts

In 12 of 46 consecutive patients with paroxysmal supraventricular tachycardia or atrial flutter-fibrillation, without electrocardiographic evidence of ventricular preexcitation, electrophysiologic studies suggested the presence of accessory atrioventricular (A-V) pathways capable only of retrograde conduction (concealed Wolff-Parkinson-White syndrome). The ages of these patients ranged from 29 days to 71 years (mean 39.2 years). Most patients were clinically symptomatic with palpitations, dizziness, weakness or congestive heart failure. One patient had “cardiac dysrhythmia” described by an obstetrician during intrauterine life. Eleven patients manifested A-V reciprocating tachycardia involving the normal pathway for anterograde conduction and the accessory pathway for retrograde conduction. The remaining patient manifested recurrent paroxysms of atrial flutter-fibrillation as a result of rapid ventriculoatrial activation through the accessory pathway during the atrial vulnerable phase.

The electrophysiologic observations were analyzed with regard to clinical and electrocardiographic characteristics in these patients. The presence of concealed accessory pathways should be suspected in patients presenting with (1) an “incessant” form of tachycardia, (2) spontaneous onset of A-V reciprocal rhythms or reciprocating tachycardias after acceleration of the sinus rate without antecedent atrial extrasystoles or P-R interval prolongation, (3) slowing of the tachycardia rate consequent to the development of functional bundle branch block, (4) retrograde P waves (negative in leads II, III and aVF) discernible after the QRS complexes, with the R-P interval being shorter than the P-R interval during both A-V reciprocal rhythm and reciprocating tachycardia, and (5) oc-currence of atrial flutter-fibrillation in association with A-V reciprocal rhythms.

It is suggested that medical treatment in patients having concealed accessory pathways should be aimed at increasing the refractoriness of either the A-V node or the accessory pathway for reciprocating tachycardia, while increasing the refractoriness of the atrium and the accessory pathway in cases with atrial flutter-fibrillation. Pacemaker therapy and surgical intervention may be indicated in selected patients refractory to antiarrhythmic agents.     

Multiple reentry tachycardia in patients with ventricular preexcitation: report of three cases

The electrophysiologic studies of three patients with accessory pathways and multiple reentrant circuits are reported. The first patient had two atrioventricular accessory pathways: a left posterior capable of bidirectional conduction and a right paraseptal with retrograde conduction only. Four atrioventricular reentry circuits were documented: left and right orthodromic circuits and a left antidromic circuit with retrograde conduction over the right paraseptal accessory pathway. The second patient had a left lateral atrioventricular accessory pathway with type A preexcitation. Two reentrant tachycardias were noted: an atrial tachycardia where the accessory pathway remained concealed and an orthodromic atrioventricular tachycardia. The third patient had dual atrioventricular nodal pathways and a right nodofascicular accessory pathway. The accessory pathway became manifest only when a critical atrioventricular delay was reached, indicating its association with the slow atrioventricular nodal pathway. Wide QRS tachycardia with left bundle branch block contour was documented, by means of the slow atrioventricular nodal pathway and nodofascicular fiber antegradely, and the proximal right bundle branch, the His bundle, and the fast atrioventricular nodal pathway retrogradely.     

Mode of initiation of reciprocating tachycardia during programmed ventricular stimulation in the Wolff-Parkinson-White syndrome: With reference to various patterns of ventriculoatrial conduction

The mode of initiation of reciprocating tachycardia in relation to various patterns of ventriculoatrial (V-A) conduction induced by programmed right ventricular stimulation was systematically analyzed in 29 patients with the Wolff-Parkinson-White (WPW) syndrome, type A. His bundle activity and atrial electrograms near the atrial ends of the normal and accessory pathways were simultaneously recorded. V-A conduction time was plotted as a function of premature ventricular coupling intervals. Four groups of patients were observed. In Group I, four patients with absence of V-A conduction, reciprocating tachycardia could not be elicited. In Group II, three patients with persistent retrograde atrial fusion from the normal and accessory pathways, reciprocating tachycardia could not be elicited because of a constant impulse collision at the atrial level. In Group III, seven patients with predominant or exclusive V-A conduction over the normal pathway, concealed retrograde penetration of the accessory pathway might have prevented the initiation of reciprocating tachycardia in all patients. In Group IV, 15 patients with predominant or exclusive V-A conduction over the accessory pathway, reciprocating tachycardia could be elicited only in 7, and its initiation appeared to depend upon the depth of retrograde penetration of the normal pathway; the less the depth of such penetration, the easier it was to elicit a reciprocating tachycardia. Furthermore, with the production of progressively less deep retrograde penetration of the normal pathway, development of repetitive ventricular responses at short ventricular coupling intervals and shortening of the ventricular driving cycle length favored the induction of reciprocating tachycardia in patients in this group. The study demonstrates that the initiation of reciprocating tachycardia during programmed ventricular stimulation in patients with the WPW syndrome is related to various patterns of V-A conduction.     

His bundle block 2:1 during paroxysmal supraventricular tachycardia involving an occult accessory atrio-His pathway

A high degree AV block during paroxysmal supraventricular tachycardia is an uncommon and short-lasting finding in patients with intranodal re-entry circuits. The AV block is the result of temporary inadequacy of the infranodal conduction system--which is not part of the circuit--to cope with the sudden increase in heart rate in terms of refractory periods and conduction velocity. In a patient with paroxysmal reciprocating tachycardia at a very high rate (230-250 bpm), 2:1 intra-hisian AV block was constantly observed after arrhythmia initiation by atrial extrastimuli. This persisted for 1-4 minutes and then progressively subsided passing through a period of 3:2 intra- infra-hisian AV block until it reached 1:1 conduction with transient left bundle branch block. The latter occasionally disappeared as result of retrograde activation of the area of functional anterograde block and of subsequent noncompensatory pause, following a premature ventricular depolarization. Tachycardia-dependent AV block was abolished by verapamil and flecainide, because of lengthening of the tachycardia cycle length. Functional and electropharmacological features of retrograde conduction were consistent with an extranodal concealed atrio-hisian accessory pathway acting as the retrograde limb of the re-entry circuit. Moreover, His bundle electrogram was prolonged and polyphasic even in sinus rhythm. Therefore, tachycardia-dependent advanced AV block can occur not only in truly intranodal re-entry but also in very fast atrio-hisian re-entry tachycardias with evidence of transient impairment of intra-hisian conduction, distal to the insertion of the accessory pathway. Localized morphofunctional pathological changes in the His bundle are probably involved in the mechanism of this uncommon pattern.     

"Slowing" right bundle-branch block in reciprocating tachycardia from a latent right lateral Kent bundle

The authors report the case of a 56 year old man with paroxysmal reciprocating tachycardia. The participation of a right lateral Kent bundle, latent in sinus rhythm and with retrograde atrioventricular conduction during tachycardia was proved by : 1) the slowing of the tachycardia rhythm and lengthening of the ventriculo-atrial conduction time by 50 ms during right bundle branch block ; 2) atrial mapping during tachycardia showing right lateral atrial pre excitation ; 3) the spontaneous termination of some attacks after a blocked Hisian depolarisation. Analysis of the mechanisms of spontaneous termination of tachycardia showed a block in the accessory pathway in 80% of cases, leading to the successful use of Amiodarone. The particular electrophysiological mechanism of functional bundle branch block makes it the most reliable positive diagnostic criterion in reciprocating tachycardia. A review of previously reported series shows participation of right lateral and septal accessory pathways to be uncommon during reciprocating tachycardia. Functional bundle branch block does not necessarily lengthen the ventriculo-atrial interval with septal accessory pathways. Left lateral Kent bundles are much more common. These points are analysed together with the mechanism of functional bundle branch block in the discussion.     

Electrocardiogram during tachycardia in patients with anterograde conduction over a Mahaim fiber: old criteria revisited.

OBJECTIVES: The aim of this study was to prospectively evaluate the sensitivity, specificity, and positive and negative predictive values of previously described ECG criteria to identify preexcited tachycardia due to decrementally conducting accessory pathways (QRS axis between 0 and -75 degrees , QRS width < or = 0.15 seconds, an R wave in lead I, an rS pattern in lead V(1), RS > 1 QRS transition > V(4), and cycle length between 220 and 450 ms). BACKGROUND: Preexcited tachycardia associated with decrementally conducting right-sided accessory pathways usually shows a rather "narrow" QRS complex and can be difficult to differentiate from supraventricular tachycardia (SVT) with left bundle branch block (LBBB) aberrant conduction. METHODS: We analyzed three groups of patients: 32 patients with an atriofascicular pathway (group I); 8 patients with long (n = 3) or short (n = 5) decrementally conducting right-sided AV pathway (group II); and a control group that consisted of 35 patients with SVT and LBBB (group III). RESULTS: Presence of all six criteria had 87.5% sensitivity in group I and a 0% sensitivity in group II. There were four false negatives in group I. The negative predictive value was 82.5%, with six false positives in group III (five patients with an aberrant LBBB-shaped tachycardia with ventriculoatrial conduction over an accessory AV pathway). The criterion cycle length was not helpful. CONCLUSIONS: Criteria for identifying a tachycardia with anterograde conduction over a Mahaim fiber are helpful only in atriofascicular pathways, with a sensitivity of 87.5% and a negative predictive value of 82.5%. The major cause of false positives was a tachycardia with aberrant LBBB conduction and ventriculoatrial conduction over an accessory AV pathway.     

Functional bundle branch block and orthodromic reciprocating tachycardia cycle length: do not bet on accessory pathway location.

AIMS: To show 2 examples in which the analysis of the effect of bundle branch block on orthodromic reciprocating tachycardia cycle length was misleading. METHODS AND RESULTS: We performed an electrophysiological study in two patients with orthodromic reciprocating tachycardia showing a transition from wide to narrow QRS during tachycardia. Our two cases of left bundle branch block during reciprocating tachycardia using infero-septal pathways show that ventricular to atrial conduction time prolongation may be larger than 30 ms and may be concealed by a shortening of atrial to ventricular conduction time. In the 2 cases, the atrial insertion of the accessory pathway could be successfully ablated from the right atria at the ostium of the coronary sinus. CONCLUSIONS: The observation of the association between left bundle branch block and cycle length prolongation during reciprocating tachycardia may be associated with a successful ablation at the ostium of the coronary sinus.     

Differential effects of adenosine on antegrade fast pathway,antegrade slow pathway,and retrograde fast pathway in atrioventricular nodal reentry

Adenosine has a potent negative dromotropic effect. However, comparative effects of adenosine on the three pathways of atrioventricular (AV) nodal reentry remain unclear. In this study, we sought to determine the effects of adenosine on the antegrade fast, antegrade slow, and retrograde fast pathway conduction in patients with AV nodal reentrant tachycardia (AVNRT). Twenty patients with common slow-fast AVNRT (mean cycle length 360 +/- 49 ms) were studied. The effects of adenosine on the antegrade slow pathway and on the retrograde fast pathway conduction were determined during sustained AVNRT and constant right ventricular pacing at identical cycle lengths (mean 360 +/- 49 ms), respectively. Incremental doses of adenosine were rapidly administered: initial dose of 0.5 mg, followed by stepwise increases of 0.5 or 1.0 mg given at 5-min intervals until termination of AVNRT or second-degree ventriculoatrial block occurred. After the antegrade slow pathway conduction was selectively and completely ablated by radiofrequency catheter ablation, the effect of adenosine on the antegrade fast pathway conduction was evaluated. The dose-response curve of adenosine and the dose of adenosine required to produce AV or ventriculoatrial block among the representative three conduction pathways were compared. The dose-response curve for the effect of adenosine on the antegrade fast pathway lies to the left and upward to that of the effect of adenosine on the antegrade slow pathway which in turn lies to the left and upward to that of the retrograde fast pathway. The mean dose of adenosine required to produce conduction block at antegrade fast, antegrade slow, and retrograde fast pathways were 1.4 +/- 0.5, 4.2 +/- 1.6, and 8.5 +/- 2.6 mg, respectively (p < 0.01). Adenosine has a differential potency to depress antegrade fast, antegrade slow, and retrograde fast pathway conduction in patients with AVNRT. The depressant effect of adenosine on the antegrade fast pathway is more potent than that on the antegrade slow pathway which in turn is more potent than that on the retrograde fast pathway conduction.