Site of Accessory Pathway Block After Radiofrequency Catheter Ablation in Patients with the Wolff-Parkinson-White Syndrome

Site of Accessory Pathway Block. Introduction: Recent studies have demonstrated that the most common site of accessory pathway conduction block following the introduction of a premature atrial stimulus during atrial pacing is between the accessory pathway potential and the ventricular electrogram. consistent with block at the ventricular insertion of the accessory pathway. However, no prior study has evaluated the site of conduction block during radiofrequency catheter ablation procedures. Therefore, the objective of this study was to determine the site of conduction block after catheter ablation of accessory pathways by analyzing and comparing the local electrograms recorded before and after radiofrequency energy delivery at successful ablation sites. Methods and Results: The electrograms evaluated in this study were obtained from 85 consecutive patients who underwent successful radiofrequency catheter ablation of a manifest accessory pathway. The 50 left free-wall accessory pathways were ablated using a ventricular approach and the 35 right free-wall or posteroseptal accessory pathways were ablated using an atrial approach. The characteristics of local electrograms recorded immediately before and immediately after successful ablation of the accessory pathway were determined in each patient. The site of accessory pathway block was determined by comparing the amplitude, timing, and morphology of the local eleclrograms at successful sites of radiofrequency catheter ablation before and after delivery of radiofrequency energy. A putative accessory pathway potential was present at the successful target site in 74 of the 85 patients (87%). Conduction block occurred between the atrial electrogram and the accessory pathway potential in 66 patients (78%) and between the accessory pathway potential and the ventricular electrogram in eight patients (9%). The site of block could not be determined in 11 patients (13%) in whom an accessory pathway potential was absent. Conduction block occurred most frequently between the atrial electrogram and the accessory pathway potential regardless of accessory pathway location. No electrogram parameter or accessory pathway characteristic was predictive of the site of conduction block. Conclusion: The results of this study demonstrate that conduction block occurs most frequently between the local atrial electrogram and the accessory pathway potential during radiofrequency catheter ablation of accessory pathways. This is true regardless of whether the accessory pathway is ablated from the atrial or ventricular aspect of the mitral or tricuspid annulus.     

Catheter Ablation of the Atrial Origin of a Decrementally Conducting Atriofascicular Accessory Pathway by Radiofrequency Current

Radiofrequency Ablation of Pseudo-Mahaim Fibers. Introduction: A young woman with refractory recurrent supraventricular tachycardia was referred for catheter ablation. Methods and Results: Electrophysiologic studies revealed the mechanism of tachycardia to be atrioventricular (AV) reentry, utilizing a decrementally conducting atriofascicular accessory pathway as the anterograde limb of the circuit and the normal intraventricular conducting system as the retrograde limb. Pace mapping in the right atrium during sinus rhythm suggested an atrial origin of the accessory pathway several centimeters distant from the AV node. Multiple radiofrequency lesions at the distal insertion of the accessory pathway in or near the right bundle branch failed to abolish preexcitation. In contrast, radiofrequency current applied to the ventricular side of the anterolateral tricuspid ring, adjacent to the atrial origin of the accessory pathway, was successful in abolishing preexcitation and inducible supraventricular tachycardia without affecting AV nodal conduction. Conclusion: Radiofrequency ablation can provide curative therapy for intractable supraventricular tachycardia due to decrementally-conducting atriofascicular accessory pathways. The risk of AV block in such patients as a consequence of the procedure should be quite low.     

Decreased amplitude of left ventricular posterior wall motion with notch movement to determine the left posterior septal accessory pathway in Wolff-Parkinson-White syndrome

OBJECTIVE—To determine preoperatively, by analysing asynchronous left ventricular wall motion, whether to approach through the right ventricle or the left ventricle when carrying out catheter ablation of the accessory pathway in Wolff-Parkinson-White syndrome, especially in patients with the pathway located on the septum.
METHODS—73 patients with manifest Wolff-Parkinson-White syndrome who underwent successful catheter ablation were studied. Location of accessory pathway was classified as right ventricular side: right anterior paraseptum, right anterior, right lateral, right posterior, anterior septum, midseptum, right posterior septum; left ventricular side: left posterior septum, left posterior, left lateral, left anterior. Asynchronous systolic wall motion was analysed by cross sectional echocardiography.
RESULTS—Echocardiography showed that the amplitude of left ventricular posterior systolic wall motion was reduced when the pathway was located on the left ventricular side as opposed to the right ventricular side (mean (SD), 11.1 (1.7) v 12.9 (1.1) mm, p < 0.001), especially in patients with left posterior septal accessory pathway (9.7 (0.8) mm). There were no overlapping values between the left posterior septal accessory pathway and the right ventricular side accessory pathway. Posterior wall notch motion was observed in all patients with a left posterior septal accessory pathway (9/9), but not at all in patients with pathways located on the right ventricular side of the septum. In patients with a septal accessory pathway, an ECG algorithm provided poor information (relatively low sensitivity, specificity, and predictive value) for determining whether the subsite faced either the left (left posterior septum) or the right ventricle (anterior septum, midseptum, right posterior septum).
CONCLUSIONS—Decreased amplitude of left ventricular posterior wall motion with notch movement is an important finding for accessory pathways located on the left posterior septum. These findings provided clinically useful information for determining whether to approach catheter ablation from the right or the left ventricle.


     

Variants of Preexcitation—Specialized Atriofascicular Pathways, Nodofascicular Pathways, and Fasciculoventricuiar Pathways:

Variants of Preexcitation. introduction: In the present report, the electrophysiiologic findings in patients with different types of variants of preeexcitwtion, i.e., atriofascicualr, nodofacicular, and fasciculoventricular fibers, and the results of radiofrequency catheter ablation using different target sites are described. Methods and Results: Twelve patients (mean age 36 ± 17 years) with variants of the preexcitation syndromes underwent electrophysiologic study and radiofrequency catheter ablation. The atrial origin of atriofascicular pathways remote from the normal AV node was assessed by application of late atrial extrastimuli that advanced (“reset”) the timing of the next QRS complex without anterograde penetration into the AV node. In patients with atriofacicular pathways, ablation of the accessory pathway or the retrograde fast AV node pathway was attempted. Ablation of the atriofascicular pathways was guided by a stimulus-delta wave interval mapping in the first live patients and by recording of atriofascicular pathway activation potentials in the next five patients. A nodofascicular pathway was suggested if VA dissociation occurred during tachycardia and if atrial extrastimuli failed to reset the tachycardia without anterograde penetration into the AV node. A fasciculoventricular connection was suggested if the proximal insertion of the accessory pathway was found to arise from the His bundle or bundle branches. The PR interval was expected within normal limits during sinus rhythm and the QRS complex to he slightly prolonged with a discrete slurring of the R wave, suggesting a small delta wave. Ten of the 12 patients had evidence for atriofascicular pathways and one patient each for a nodofascicular and fasciculoventricular pathway. In six patients, the atriofascicular pathways were successfully ablated, and in two patients, the retrograde fast AV node pathway. In one patient, a concealed right posteroseptal accessory AV pathway served as the retrograde limb and was successfully ablated. The nodofascicular pathway was shown to he a bystander during AV node reentrant tachycardia. After successful fast AV node pathway ablation resulting in marked PR prolongation, no preexcitation was present during sinus rhythm because of the proximal insertion of the nodofascicular pathway distal to the delay producing parts of the AV node. The proximal insertion of the fasciculoventricular pathway was suggested to arise distal to the AV node at the site of the penetrating AV bundle. The earliest ventricular activation at the His-bundle recording site indicated the ventricular insertion of this accessory connection into the ventricular summit. The fasciculoventricular connection gave rise to a fixed ventricular preexcitation and served as a bystander during orthodromic AV reentrant tachycardia incorporating a left-sided accessory AV pathway.     

Intermittent Wolff-Parkinson-White syndrome due to phase 3 and phase 4 block: disappearance of rapid ventricular response during atrial fibrillation

A 65-year-old man with Wolff-Parkinson-White syndrome type A had a rapid ventricular response over an accessory pathway during atrial fibrillation. Four months later, electrophysiological study revealed the following properties: 1) The antegrade accessory pathway conduction showed phase 3 and phase 4 block. 2) The retrograde concealed accessory pathway conduction eliminated phase 4 block in the antegrade accessory pathway conduction. 3) Premature ventricular excitation arose from the accessory pathway or from the ventricular muscle close to its distal end. 4) Atrial fibrillation during isoproterenol infusion did not show rapid ventricular response. Spontaneous impairment of accessory pathway conduction due to phase 3 and phase 4 block was suspected.     

Mapping of the Coronary Sinus and Great Cardiac Vein Using a 2-French Electrode Catheter and a Right Femoral Approach

Coronary Sinus Mapping. Introduction: Local electrogranis recorded from the coronary sinus and great cardiac vein provide important information for the diagnosis of various arrhythmias and identification of target sites for ablation of left-sided accessory pathways. One limitation of present techniques is the inability, in many cases, to probe the great cardiac vein at the anterior mitral annulus. We tested the feasibility of a new technique for catheterization of the coronary sinus and great cardiac vein by means of a small-diameter electrode catheter advanced via a right femoral approach through an angiography catheter. Methods and Results: Of 22 patients (12 men and 10 women; ages 44.5 ± 13.4 years) undergoing radiofrequency ablation of a supraventricular tachycardia, cannulation of the coronary sinus orifice using a 6-French 1L or 2L Amplatz catheter was achieved in 20 patients (91%) within 0.9 ± 0.6 minutes; after cannulation, a 2-French octapolar electrode catheter with a soft radiopaque tip and a 3-mm interelectrode distance could he advanced in all 20 patients through the guiding catheter to the great cardiac vein in the anterior region of the AV sulcus within 0.8 ± 0.7 minutes. Atrial and ventricular local potentials were recorded all along the mitral annulus during sinus rhythm, atrial and ventricular pacing, or supraventricular tachycardia. Variation of local potential amplitude never exceeded 20% of the mean and presented similar stability at all annular regions. The arrhythmogenic substrate was identified in all patients. Of 18 patients with 21 left-sided accessory pathways, an accessory pathway potential could he recorded at the ablation site by one or more adjacent epicardial electrode pairs in 10 pathways. No procedure-related complications were observed. Conclusions: The technique introduced in this study proved feasible in 91% of patients. Its main advantages are the simplicity and rapidity of coronary sinus cannulation and the ability to advance the electrode catheter to the anterior cardiac vein. In addition, closely spaced bipolar electrograms resulted in enhanced atrial, ventricular, and accessory pathway potential resolution.     

A Rare Case of Permanent Junctional Reciprocating Tachycardia Ablated on the Roof of the Left Atrium

A Rare Case of Permanent Junctional Reciprocating Tachycardia. Left‐sided anteroseptal accessory pathways that course through the aortomitral fibrous continuity are some of the rarest types of accessory pathways. At this region the atrium and ventricle are separated by their greatest distance because of the intervening aortic valve. These pathways often have a long circuitous course that may involve the root and cusps of the aortic valve. Prior reports have demonstrated the feasibility of ablating these pathways along the anteroseptal mitral annulus, the left ventricular outflow tract, or in the left or noncoronary cusps of the aortic valve. We describe a case of a concealed decremental anteroseptal accessory pathway that was ablated on the roof of the left atrium remote from the mitral or aortic valve annuli. This report indicates that when an appropriate site for ablation of a left‐sided anteroseptal accessory pathway is not found close to a valve annulus, these pathways may be amenable to ablation by targeting their atrial insertion site. (J Cardiovasc Electrophysiol, Vol. 24, pp. 464‐467, April 2013)     

Surgery for Wolff-Parkinson-White syndrome: further experience with an epicardial approach

We have previously reported the use of an epicardial approach for ablation of left ventricular free wall accessory atrioventricular pathways. The technique involves mobilization of the atrioventricular fat pad and exposure and cryoablation of the atrioventricular junction at the site of the accessory pathway on the normothermic beating heart. Here we describe our further experience with left ventricular free wall accessory pathways and right ventricular free wall accessory pathways. Our experience is based on 53 consecutive patients. There were 35 male and 18 female subjects, 6 to 52 (mean 41.4) years old. Forty-eight patients had a left ventricular free wall accessory pathway, and five had a right ventricular free wall accessory pathway. Two patients had an associated anterior septal accessory pathway. Five patients had associated cardiac abnormalities, including atrial septal defect, aortic insufficiency, mitral valve prolapse, Ebstein's anomaly, and cardiomyopathy. The accessory pathway was ablated in 52 patients who remain arrhythmia free without medication after a mean follow-up period of 12 months. The accessory pathway was permanently modified in one patient. There were no postoperative complications. This epicardial approach can be performed with normothermic cardiopulmonary bypass or without bypass. It does not require cross-clamping of the aorta, allowing a greater margin of safety when this is required for concomitant procedures.     

Recurrence and Late Block of Accessory Pathway Conduction Following Radiofrequency Catheter Ablation

Recurrence After RF Ablation of AP. Introduction: Many issues regarding the recurrence of accessory pathway conduction and the long-term outcome of late block of accessory pathway conduction are still unknown or controversial. Methods and Results: Data from 217 patients who underwent an initially successful radiofrequency ablation of accessory pathways and 7 patients with late block of accessory pathway conduction following an initially unsuccessful ablation were analyzed. During a mean follow-up of 19 ± 11 months, accessory pathway conduction resumed in 21 (10%) of 217 patients following an initially successful ablation and in 6 (86%) of 7 patients with late block of accessory pathway conduction (P < 0.01). After initially successful ablations, the recurrence rates of accessory pathway conduction at 1, 3, and 6 months were 5.9%, 7.4%, and 11.3%, respectively. A late electrophysiologic study at 6 months uncovered recurrence in only 1 of 124 asymptomatic patients, but failed to detect the late recurrence in 2 patients in whom the accessory pathway conduction resumed after more than 6 months. Multivariate analysis revealed that independent predictors for recurrence of accessory pathway conduction were concealed accessory pathway, presence of transient effect of radiofrequency pulse, and more than 5 pulses required for initial cure. Accessory pathway location, length of the tip electrode of the ablation catheter, and repeat radiofrequency pulses (“safety pulses”) after effective pulses did not predict resumption of accessory pathway conduction. Conclusions: After initially successful ablation, the recurrence rates of accessory pathway conduction at 1, 3, and 6 months were 5.9%, 7.4%, and 11.3%, respectively. Late electrophysiologic testing had little prognostic value in asymptomatic patients following successful ablation. Application of “safety pulses” did not prevent recurrence. Late block of accessory pathway conduction did not predict long-term efficacy.     

Current Status of Lasers for Arrhythmia Ablation

Current Status of Lasers for Arrhythmia Ablation. Wavelength specific effects and mode of laser operation allow either photocoagulation or tissue removal as a means of approaching arrhythmia ablation. Successful intraoperative ablation of ventricular tachycardias has been performed with the Nd: YAG laser (photocoagulation) and argon laser (tissue vaporization). The argon laser has been used intraoperatively for transection of accessory pathways. Experimental studies indicate a strong theoretical potential for Nd:YAG laser catheter ablation of ventricular tachycardia. Laser energy has been used experimentally to evaluate the possibility of AV junctional ablation/modification and accessory pathway ablation. Adaption of laser energy to effective catheter systems for arrhythmia ablation requires solutions to problems inherent in all catheter systems and some unique to laser energy. (J Cardiovasc Electrophysiol, Vol. 3, pp. 345–353, August 1992)     

Cardiac resynchronization therapy by ablation of right-anterolateral accessory pathway

Ventricular preexcitation caused by right-sided accessory pathways can lead to abnormal septal motion patterns and may be associated with left ventricular (LV) dysfunction and heart failure, despite the lack of a clinical arrhythmia. Hence successful ablation of the accessory pathway abolishes not only preexcitation but also ventricular dyssynchrony in these patients. We describe a case of an asymptomatic 20-year-old male presenting with ventricular Type-B preexcitation combined with LV dysfunction. The individual risk of arrhythmic events was enhanced due to competitive sport activities of the patient and a short antegrade refractory period of the accessory pathway. Hence standard radiofrequency ablation of the right anterolateral accessory pathway was performed, despite no history of tachycardia. After successful accessory pathway ablation, normalization of LV size and function was demonstrated by echocardiography with a long-term follow-up of 4 years.     

Phase 3 and phase 4 block in the accessory pathway

Pacemaker activity at a site proximal to the area of a conduction disturbance has been postulated as depressing conduction in late diastole (phase 4 block). To elucidate the correlation between the site of phase 4 depolarization and that of a conduction disturbance, we examined seven patients with intermittent Wolff-Parkinson-White syndrome who had tachycardia (phase 3) and bradycardia (phase 4) dependent on block in the accessory pathway. In each patient, antegrade conduction over the accessory pathway was absent at the sinus rate. During premature atrial stimulation a "window" of accessory pathway conduction was identified in each patient. The outer limit of the window ranged from 420 to 670 ms; the inner limit, from 330 to 620 ms. The duration of the window ranged from 20 to 160 ms. Four patients with orthodromic atrioventricular reentrant tachycardia exhibited preserved retrograde accessory pathway conduction. In one patient with unsustained orthodromic atrioventricular reentrant tachycardia, retrograde accessory pathway conduction also exhibited phase 3 and phase 4 blocks. The remaining two patients had no retrograde accessory pathway conduction. In two patients, retrograde concealed conduction in the accessory pathway induced by ventricular stimulation eliminated a phase 4 block in antegrade accessory pathway conduction. These results suggest that 1) an antegrade phase 4 block may have phase 4 depolarization in the accessory pathway and block at the ventricular insertion of the accessory pathway; 2) a retrograde phase 4 block may have a conduction disturbance at the atrial insertion of the accessory pathway; and 3) a complete retrograde block may occur at the atrial insertion of the accessory pathway.     

Atrial and accessory pathway activation direction in patients with orthodromic supraventricular tachycardia: Insights from vector mapping

Objectives. The purpose of this study was to utilize vector mapping to investigate atrial and accessory pathway activation direction during orthodromic supraventricular tachycardia.Background. Although advances have been made in the electrophysiologic evaluation and management of accessory pathways, our understanding of accessory pathway anatomy and physiology remains incomplete. Vector mapping has been validated as a method of studying local myocardial activation.Methods. In 28 patients with a left-sided or posteroseptal accessory atrioventricular (AV) pathway referred for ablation, atrial and accessory AV pathway activation direction was determined during ventricular pacing or orthodromic supraventricular tachycardia, or both, by summing three orthogonally oriented bipolar electrograms recorded from the coronary sinus to create three-dimensional vector loops. Atrial and accessory AV pathway activation direction was determined in all patients from the maximal amplitude vectors of the vector loops. Because of beat to beat variability in the directions of the vector loops, data from 8 of 28 patients could not be analyzed.Results. At 81 of 83 sites, atrial activation direction along the long axis of the coronary sinus corresponded with the direction suggested by activation time mapping. Activation direction along the anteroposterior and inferosuperior axes was variable, potentially due to variations in the level of the atrial insertion of the accessory AV pathway and in the depth or angling of pathway fibers in the AV fat pad. In eight patients, at least one recording was obtained at the site of an accessory AV pathway potential. Accessory AV pathway activation proceeded superiorly and to the right in seven of eight patients; in one patient with a posteroseptal pathway, accessory AV pathway activation proceeded superiorly and to the left.Conclusions. 1) Vector mapping is a useful technique for localizing accessory AV pathways; 2) left-sided accessory AV pathways angle from left to right as they traverse the AV groove; and 3) variable activation directions of the atrial myocardium adjacent to the coronary sinus suggest that accessory AV pathway insertion into the atrium differs from patient to patient.     

Reciprocating tachycardia and anterograde conduction by a nodoventricular pathway

Two patients were investigated for paroxysmal regular tachycardia with left bundle branch block centrifugation. A right-sided nodo-ventricular accessory pathway was demonstrated in both cases at electrophysiological investigation with His bundle recording and atrial and ventricular programd pacing techniques. However, the function of this pathway was different in the two cases. In the first case, there were no signs of an accessory pathway on the surface ECG in sinus rhythm but it could be unmasked by simple right atrial pacing at the same rhythm (widening of the QRS and shortening of HV from 40 to 25 ms). The tachycardias could be only initiated by ventricular extrastimulus. They showed major pre-excitation with left sided delay and a 1/1 atrio-ventricular response. There was no His potential before the ventriculogramme which retained the same configuration throughout the attack. The investigations also suggested the presence of a dual nodal pathway with the accessory pathway connected to the slow pathway. In the second case, the presence of an accessory pathway could be suspected from the appearance of the QRS complex in sinus rhythms. Tachycardia was initiated by an atrial extrastimulus with initially a first complex showing slightly more marked pre-excitation and a distinct His potential before the QRS but with a shorter HV interval than in sinus rhythm. Then the reciprocating tachycardia had appearances of major pre-excitation, left-sided delay and a 1/1 atrio-ventricular response. However, in contrast to the first case, all ventricular complexes were preceded by a His potential and the degree of pre-excitation was variable with a HV interval ranging from 0 to 15 ms. These two cases merit attention because of: --their points in common: nodal duality and an accessory pathway which was not atrio-ventricular (decremental conduction) but nodo-right ventricular, conducting well in the anterograde direction but more or less masked in sinus rhythm; the presence of the accessory pathway was clearly visible during reciprocating tachycardia; --the differences: in the first case the nodo-ventricular pathway formed part of the circuit of the reciprocating tachycardia which was antidromic: descending limb, the slow nodal pathway and then the accessory nodo-ventricular pathway; ascending limb, the His bundle and then the rapid nodal pathway. In the second case, the reciprocating tachycardia was entirely intranodal, the accessory pathway not being involved in the circuit but connected to it in parallel with the normal Hisian pathway.     

心室起搏拖带对房室结折返性和间隔旁路参与的房室折返性心动过速的鉴别作用

目的　研究心动过速时心室起搏拖带对房室结折返性心动过速 (AVNRT )和间隔旁路参与的顺向型房室折返性心动过速 (间隔旁路ORT)的鉴别意义。方法　 30例AVNRT和 2 5例间隔旁路ORT病人在心动过速发生后 ,采用较心动过速的周长 (TCL)短 10～ 4 0ms的周长行右心室起搏拖带心动过速。测量右心室起搏之前的心室 心房 (VA)间期和TCL。停止起搏后 ,测量最后一次刺激信号至最后起搏拖带的心房激动 (SA)间期 ,以及起搏后间期 (PPI)。结果　所有 30例AVNRT病人的SA -VA间期 >85ms、PPI-TCL >115ms,而 2 5例ORT病人的SA -VA间期 <85ms、PPI-TCL <115ms。结论　PPI TCL和SA VA间期是鉴别AVNRT和间隔旁路ORT的非常可靠的指标 ,具有较高的特异性。     

Entrainment of circus movement tachycardia utilizing an accessory pathway with long retrograde conduction times during ventricular and atrial stimulation

An unusual case is presented in which a circus movement tachycardia incorporating an accessory pathway with long retrograde conduction time was transiently entrained. Overdrive high right atrial stimulation produced entrainment without atrial fusion since collision of anterograde and retrograde impulses took place within the accessory pathway. Tachycardia termination occurred when, at a faster pacing rate, an atrial impulse that collided in the accessory pathway was blocked at the atrioventricular (AV) node. In contrast, the entrainment seen during right ventricular apical stimulation was characterized by the occurrence of both fusion and collision within the ventricles. The tachycardia was terminated when a pure paced impulse that collided in the normal pathway was blocked in a retrograde direction in the accessory pathway. These data indicate that: 1) transient entrainment of this arrhythmia (circus movement tachycardia) can be identified by the classical criteria used to diagnose it, provided that fusion and collision occur within the ventricles; and 2) the accessory pathway is the weak link for tachycardia termination only during ventricular pacing since the AV node is the weak link during atrial stimulation.     

Patterns of atrial activation during right ventricular pacing in patients with concealed left-sided Kent pathways.

A 'concealed' accessory pathway was suspected in 12 patients because of eccentric left atrial activation during tachycardia. Retrograde conduction during ventricular pacing may occur over the atrioventricular node, the accessory pathway, or both. There were 4 patterns of ventriculoatrial conduction in response to ventricular extrastimuli (V2) at various coupling intervals: (1) exclusive accessory pathway conduction throughout the cardiac cycle in 2 patients; (2) exclusive accessory pathway conduction at long coupling intervals and exclusive atrioventricular node conduction at short coupling intervals in 2 patients; (3) variably fused accessory pathway/atrioventricular node conduction at long coupling intervals but exclusive accessory pathway conduction at short coupling intervals in 4 patients; (4) fused accessory pathway/atrioventricular node conduction at long coupling intervals but exclusive atrioventricular node conduction at short coupling intervals in 4 patients. With increased prematurity of V2 the ventricle to right atrial interval prolonged conspicuously in 11 of 12 patients whereas the ventricle to left atrial interval remained constant until the refractory period of the accessory pathway in all but 2 instances where intraventricular delay occurred. This study emphasises the importance of left atrial recordings in these patients.     

Demonstration of phase-3 and phase-4 retrograde block in a second concealed accessory pathway after an initial successful radiofrequency ablation of a 'normal' concealed accessory pathway

We report a patient with concealed Wolff-Parkinson-White syndrome who, following catheter ablation, demonstrated phase-3 and phase-4 retrograde block in a concealed accessory pathway. After an initial 'apparently successful' ablation, retrograde conduction was through the atrioventricular node during constant ventricular pacing. Ventricular extrastimulus testing was performed at a basic drive cycle length of 600 ms. Unexpectedly, ventricular extrastimuli at coupling intervals of 440-380 ms were conducted retrogradely over an accessory pathway, consistent with a phase-3 and phase-4 retrograde block in the accessory pathway. Residual accessory pathway conduction was eliminated in a single ablation session.     

Rate-dependent accessory pathway conduction due to phase 3 and phase 4 block. Antegrade and retrograde conduction properties.

Six patients who had antegrade phase 3 and phase 4 block in the accessory pathway were examined. In each patient, antegrade conduction over the accessory pathway was absent both at the sinus rate and at slower heart rates. During premature atrial stimulation a "window" of accessory pathway conduction was identified in all patients. The outer limits of the window ranged from 480 ms to 670 ms. The inner limits ranged from 410 ms to 620 ms. The durations of the window ranged from 20 ms to 160 ms. Three patients with orthodromic atrioventricular reentrant tachycardia showed preserved retrograde accessory pathway conduction. The remaining three patients had impaired retrograde accessory pathway conduction. One of the patients showed retrograde phase 4 block in the accessory pathway. In two patients, retrograde concealed conduction in the accessory pathway induced by ventricular stimulation prolonged the outer limit of the window in the antegrade accessory pathway conduction. These findings suggest that the mechanism of antegrade phase 3 and phase 4 block in the accessory pathway may be spontaneous diastolic depolarization in the accessory pathway and conduction disturbance at the ventricular and/or atrial insertion of the accessory pathway.