Successful radiofrequency ablation of idiopathic left ventricular tachycardia at a site away from the tachycardia exit

OBJECTIVES: This study sought to assess the possibility of ablating verapamil-responsive idiopathic left ventricular tachycardia at a site distant from the tachycardia exit and thus to define the tachycardia circuit. BACKGROUND: The nature of the reentry circuit in idiopathic left ventricular tachycardia is unclear. If the circuit is of considerable size, then it should be possible to ablate the tachycardia at a site distant from the exit site. METHODS: Electrophysiologic studies and radiofrequency ablation were performed in 27 consecutive patients with verapamil-responsive idiopathic left ventricular tachycardia. In all 27 patients, the tachycardia exit site was defined as the site where the earliest Purkinje potential was recorded > or = 25 ms before the onset of the QRS complex during the tachycardia and where the pace map QRS complex resembled that during the tachycardia. A potential ablation site other than the exit site was then sought around the midseptum, proximal to the exit site. At such sites the tachycardia could be terminated transiently by pressure applied to the catheter tip, without induction of ventricular ectopic beats. RESULTS: The potential ablation site, other than the tachycardia exit site, was identified in seven male patients (mean [+/-SD] age 31 +/- 12 years, range 13 to 52). Application of the radiofrequency current at this site resulted in termination of the tachycardia within 1 to 5 s (mean 2.9 +/- 1.6), and successful ablation of the tachycardia was achieved in all seven patients (success rate 100%, 95% exact confidence interval 0.5898 to 1). The mean distance between the ablation site and the tachycardia exit site was 3.1 +/- 0.7 cm (range 2.0 to 4.0). A presystolic Purkinje spike was recorded 14 +/- 5 ms (range 8 to 20) before the onset of the QRS complex during the tachycardia. During the follow-up period of 24 +/- 11 months (range 12 to 39), there was no recurrence of tachycardia in these seven patients. CONCLUSIONS: Successful ablation of idiopathic left ventricular tachycardia can be achieved at sites away from the tachycardia exit site in some patients. This finding suggests that the reentry circuit is likely to be of considerable size, encompassing the middle, inferior and lower aspects of the left interventricular septum.     

Catheter ablation of ventricular tachycardia after myocardial infarction: relation of endocardial sinus rhythm late potentials to the reentry circuit

OBJECTIVES: We sought to determine whether endocardial late potentials during sinus rhythm are associated with reentry circuit sites during ventricular tachycardia (VT). BACKGROUND: During sinus rhythm, slow conduction through an old infarct region may depolarize tissue after the end of the QRS complex. Such slow conduction regions can cause reentry. METHODS: Endocardial catheter mapping and radiofrequency ablation were performed in 24 patients with VT late after myocardial infarction. We selected for analysis a total of 103 sites where the electrogram was recorded during sinus rhythm and, without moving the catheter, VT was initiated and radiofrequency current applied in an attempt to terminate VT. RESULTS: Late potentials were present at 34 sites (33%). During pace mapping, the stimulus-QRS complex was longer at late potential sites, consistent with slow conduction, than at sites without late potentials (p < 0.0001). Late potentials were present at 15 (71%) of 21 sites classified as central or proximal in the reentry circuit based on entrainment, but also occurred frequently at bystander sites (13 [33%] of 39) and were often absent at the reentry circuit exit (3 [23%] of 13). Late potentials were present at 20 (54%) of 37 sites where ablation terminated VT, compared with 14 (21%) of 66 sites where ablation did not terminate VT (p = 0.004). Ablation decreased the amplitude of the late potentials present at sites where ablation terminated VT. CONCLUSIONS: Although sites with sinus rhythm late potentials often participate in VT reentry circuits, many reentry circuit sites do not have late potentials. Late potentials can also arise from bystander regions. Late potentials may help identify abnormal regions in sinus rhythm but cannot replace mapping during induced VT to guide ablation.     

Successful radiofrequency current catheter ablation of accessory atrioventricular pathway after tricuspid replacement in Ebstein's anomaly

A 15-year-old female with Ebstein's anomaly was referred to hospital for radiofrequency (RF) current catheter ablation of her refractory paroxysmal supraventricular tachycardia (PSVT) after tricuspid valve replacement. A surface ECG showed ventricular preexcitation of type B Wolff-Parkinson-White (WPW) syndrome. In a baseline electrophysiological study, two types of PSVT with left and right bundle branch block (LBBB and RBBB) configurations were induced. The LBBB type was antidromic and the RBBB type was orthodromic atrioventricular reciprocating tachycardia (AVRT) with a right posterolateral accessory pathway. RF current was successfully delivered at the posterolateral site above the prosthetic valve (V-delta interval = -30 msec). The patient has been free from arrhythmias during a follow-up period of 9 months. RF current ablation seems to be useful for AVRT patients with corrected Ebstein's anomaly.     

Radiofrequency catheter ablation of ventricular tachycardia in 26 patients

Electrophysiology study and radiofrequency catheter ablation (RFCA) were performed in 26 patients with refractory sustained ventricular tachycardia (VT). After induction of VT, 12-lead electrocardiogram (ECG) was recorded and QRS morphology and axis of induced VT were studied to identify the origin of VT. The precise site of VT origin were localized by pace mapping and activation mapping carefully. RF energy was delivered through a big-tip deflectable electrode catheter when the earliest site of endocardial activation and a high-frequency and low-amplitude potential of Purkinje fiber, preceding surface QRS by more than 25 ms, were identified and/or a pace map was obtained showing identical QRS complexes in at least 11 of 12 ECG leads. VTs were ablated successfully in 24 of 26 patients (success rate was 92%). For successful ablation, it is essential that the pace map QRS morphology in 12 leads should be identical with that in spontaneous or induced VT as far as possible in performing pace mapping. Pace mapping is safe, simple and has no unfavourable effect hemodynamics although it takes longer time. Activation mapping takes shorter time and has a high success rate. QRS configuration in spontaneous VT can help to localize the site of VT origin. Deliberate mapping at the site suggested to bo the origin of VT by surface ECG can shorten the duration of mapping and increase the success rate of RFCA. RFCA of VT in patients without structural heart disease is effective, safe, and has a high success rate, so it may be considered as an early therapy for these patients.     

Radiofrequency catheter ablation in monomorphic ventricular tachycardia

Twenty patients with symptomatic monomorphic ventricular tachycardia (VT) underwent radiofrequency (RF) energy catheter ablation. Four patients (20%) had underlying heart disease (1 prolapse mitral valve, 1 dilated cardiomyopathy and 2 myocarditis). Five patients (25%) had left sided VT and right sided VT in the remainder (75%). Radiofrequency catheter ablation was initially successful in all patients without major complication. Recurrence occurred in three patients (15%). In conclusion, RF ablation is an effective treatment for symptomatic monomorphic right and left sided VT especially in patients who do not want long term antiarrhythmic agents.     

Radiofrequency current caused slowing of non-reentrant idiopathic right ventricular tachycardia originating from a wide arrhythmogenic area

Radiofrequency catheter ablation was attempted in a patient with non-reentrant idiopathic right ventricular tachycardia (VT). Endocardial mapping indicated that the VT originated in the outflow tract of the right ventricle; however, an electrogram with an almost the identical activation time was recorded from an area extending to 1.0 x 2.0 cm. Each application of radiofrequency current within the area terminated VT, but a progressively slower VT with the same QRS configuration was induced until the area was covered by separate radiofrequency lesions. A progressive prolongation of VT cycle length might be related to a residual arrhythmogenic myocardium. Termination and slowing of the VT rate can be a hallmark of efficacy of each radiofrequency lesion.     

Role of Endothelin-1/Endothelin-A receptor-mediated signaling pathway in the aortic arch patterning in mice

INTRODUCTION: Several modalities of catheter ablation have been proposed to eliminate Mahaim pathway conduction. However, limited research has been reported on the electrophysiologic nature of this pathway in its entity. METHODS and RESULTS: In seven patients, electrophysiologic study was performed, and radiofrequency energy was applied to investigate the electrophysiologic clues for successful ablation. In all seven patients, the Mahaim pathway was diagnosed as a right-sided atriofascicular or atrioventricular pathway with decremental properties. In two patients, two different kinds of electrograms were recorded through the ablation catheter positioned at the Mahaim pathway location: one was suggestive of conduction over the decremental portion, demonstrating a dulled potential; and the other of nondecremental conduction, demonstrating a spiked potential. All but one of the Mahaim pathways were eliminated successfully at the atrial origin where the spiked Mahaim potential was recorded. Radiofrequency energy application was performed at the slow potential site resulting in failure to eliminate the conduction over the Mahaim pathway. Conduction block at the site between the slow and fast potential recording sites was provoked by intravenous administration of adenosine, concomitant with a decrease in the amplitude of the Mahaim potential. In one patient, the clinical arrhythmia was a sustained monomorphic ventricular tachycardia originating from the ventricular end of the Mahaim fiber. CONCLUSION: The identification of Mahaim spiked potentials may be the optimal method to permit their successful ablation. Detailed electrophysiologic assessment is indispensable for successful ablation of tachycardias associated with Mahaim fibers because tachycardias unassociated with Mahaim fibers can occur despite complete elimination of the Mahaim fiber.     

Retrograde Purkinje potential activation during sinus rhythm following catheter ablation of idiopathic left ventricular tachycardia

We describe two patients with idiopathic left ventricular tachycardia that were cured by radiofrequency catheter ablation. Tachycardia was inducible by ventricular stimulation and was verapamil sensitive. Two distinct presystolic potentials (P1 and P2) were recorded during tachycardia in the mid-septal or inferoapical area, but only one potential (P2) was recorded during sinus rhythm. After catheter ablation at this site, the P1 potential was noted after the QRS complex during sinus rhythm, while the P2 was still observed before the QRS complex. The P1 potential showed a decremental property during atrial or ventricular pacing. These data suggest that Purkinje tissue with decremental properties was responsible for the tachycardia mechanism, and that the reentry circuit involving this tissue is likely to be of considerable size.     

Ablation of manifest left free wall accessory pathways with polarity reversal mapping: ventricular approach

Polarity reversal mapping for localization of the left free wall accessory pathway (AP) at the atrial insertion site has been shown to be effective for successful ablation, but this technique requires atrial septal puncture. We evaluated the safety, efficacy, and reproducibility of two dimensional polarity reversal mapping at the ventricular insertion site of the accessory pathway without atrial septal puncture in symptomatic patients with manifested left free wall AP. Polarity reversal mapping under the mitral annulus by transaortic approach was performed in 10 consecutive patients with conventional ablation catheter (6 French, 4 mm tip, 2 mm interelectrode distance), during sinus rhythm or atrial pacing. A low set high, bandpass filter (0.005-400Hz) was used. Radiofrequency (RF) ablation was performed at the site of ventricular electrocardiogram polarity reversal during sinus rhythm. Polarity reversal was identified in all patients at the ventricular side of the mitral annulus. Ablation was successful in all patients without complications. The procedure time was 86.0 +/- 21.1 min, the fluoroscopic exposure time was 16 +/- 12 min, the number of RF applications was 8 +/- 6, the power level 21 +/- 7 watts, and the time to initial AP block was 3.0 +/- 0.9 sec. Polarity reversal mapping is a safe and efficient technique at the ventricular insertion site. This technique might be complementary to the currently-utilized activation mapping technique.     

Repetitive monomorphic tachycardia from the left ventricular outflow tract: electrocardiographic patterns consistent with a left ventricular site of origin

OBJECTIVES: This study sought to characterize the electrocardiographic patterns predictive of left ventricular sites of origin of repetitive monomorphic ventricular tachycardia (RMVT). BACKGROUND: RMVT typically arises from the right ventricular outflow tract (RVOT) in patients without structural heart disease. The incidence of left ventricular sites of origin in this syndrome is unknown. METHODS: Detailed endocardial mapping of the RVOT was performed in 33 consecutive patients with RMVT during attempted radiofrequency ablation. Left ventricular mapping was also performed if pace maps obtained from the RVOT did not reproduce the configuration of the induced tachycardia. RESULTS: Pace maps identical in configuration to the induced tachycardia were obtained from the RVOT in 29 of 33 patients. Application of radiofrequency energy at sites guided by pace mapping resulted in elimination of RMVT in 24 (83%) of 29 patients. In four patients (12%), pace maps obtained from the RVOT did not match the induced tachycardia. All four patients had a QRS configuration during RMVT with precordial R wave transitions at or before lead V2. In two patients, RMVT was mapped to the mediosuperior aspect of the mitral valve annulus, near the left fibrous trigone; catheter ablation at that site was successful in both. In two patients, RMVT was mapped to the basal aspect of the superior left ventricular septum. Catheter ablation was not attempted because His bundle deflections were recorded from this site during sinus rhythm. CONCLUSIONS: RMVT can arise from the outflow tract of both the right and left ventricles. RMVTs with a precordial R wave transition at or before lead V2 are consistent with a left ventricular origin.     

Use of biplane transesophageal echocardiographic guide in radiofrequency catheter ablation of Wolff-Parkinson-White syndrome with left side Kent bundle

We report 2 patients with Wolff-parkinson-White Syndrome, who underwent radiofrequency (RF) catheter ablation under observation by biplane transesophageal echocardiography. One of the patients had Kent bundle in the lateral wall of the left ventricle, and the tip of the catheter could be easily confirmed with transverse views. Since perforation was observed where the valve was attached to the posterior leaflet of the mitral valve during the 39th administration of RF energy, ablation was discontinued. The other patient had Kent bundle in the posterior septum of the left ventricle. The tip of the catheter could be easily confirmed with sagittal views. Ablation was successful after the 4th administration of RF energy, but particulate contrast echoes appeared from the site of the tip of the electrode which had been used for ablation during the 4th RF delivery. When the delivery of RF energy was discontinued, the contrast echo disappeared, and a thrombus was found at the tip of the removed catheter. TEE in RF catheter ablation was helpful for confirming the site of the tip of the catheter and for early detection of complications.     

Effects on cardiac performance of atrioventricular node catheter ablation using radiofrequency current for drug-refractory atrial arrhythmias

Patients with atrial fibrillation or atrial flutter (AF) are candidates for radiofrequency (RF) catheter ablation of the atrioventricular (AV) node with the aim being to control heart rate. As patients with AF can have markedly impaired ventricular function, information concerning the hemodynamic effects of AV node ablation using RF current would be valuable. Fourteen consecutive patients (mean age 65 +/- 3 years) with drug-resistant AF underwent AV node catheter ablation with RF current and had permanent pacemaker implantation. The mean left ventricular ejection fraction (EF) by two-dimensional echocardiography immediately before ablation was 42 +/- 3% (range 14%-54%) and their mean exercise time was 4.4 +/- 0.4 minutes. Complete AV block was achieved in all 14 patients with 6 +/- 2 RF applications (range 1-18). There was no evidence of any acute cardiodepressant effect associated with delivery of RF current, and EF 3 days after ablation was 44 +/- 4%. By 6 weeks after ablation, the left ventricular EF was significantly improved compared to baseline (47 +/- 4% postablation vs 42 +/- 3% preablation; P < 0.05), and this modest increase in EF was accompanied by an improvement in exercise time (5.4 +/- 0.4 min). In conclusion, delivery of RF current for AV node catheter ablation in patients with AF and reduced ventricular function is not associated with any acute cardiodepressant effect. On the contrary, improved control of rapid heart rate following successful AV node ablation is associated with a modest and progressive improvement in cardiac performance.     

Analysis of the immunoglobulin heavy-chain gene rearrangement providing molecular evidence of second lymphoma in a patient in apparent relapse after autotransplantation

Thirteen consecutive patients with idiopathic ventricular tachycardia underwent radiofrequency catheter ablation. This group included 9 idopathic left ventricular tachycardia (ILVT) and 4 idiopathic right ventricular tachycardia (IRVT). Five ILVT patients with left axis deviation and one with right axis deviation were ablated successfully. By pace mapping, two IRVT patients with ventricular tachycardia originating from right ventricular out-flow tract were ablated. No complications occured. By means of follow-up of 3-22 months one case showed recurrence with successful reablation. It indicates that radiofrequency catheter ablation therapy is an effective and safe procedure in patients with idiopathic ventricular tachycardia.     

Decreasing parasympathetic tone activity and proarrhythmic effect after radiofrequency catheter ablation--differences in ablation site

Holter ECG was used to evaluate changes in heart rate variability (HRV), indicators of the autonomic nervous system, and arrhythmia before and after radiofrequency (RF) catheter ablation in patients with symptomatic supraventricular tachycardia. Ablation targets in 43 patients included the atrioventricular (AV) nodal pathway (AVNRT, n = 17), a right free wall accessory pathway (n = 10), a septal accessory pathway (n = 6), and a left free wall accessory pathway (n = 10). The High frequency component (0.15 - 0.40 Hz) or pNN50 of HRV analysis, indicating parasympathetic activity, significantly decreased immediately after RF ablation in the AVNRT and septal accessory pathway groups, but not in the right or left wall groups. In contrast, in all four groups, ventricular premature contractions (VPCs) significantly increased in most of the patients, and ventricular tachycardia occurred in a few of the patients immediately after RF ablation. There was no serious arrhythmia. These alterations in HRV analysis and arrhythmia returned to the control level after 1 week or more. VPCs after RF ablation did not consistently increase as a result of the reduced parasympathetic tone activity, but at the lesion near the conduction system, the increase in VPCs was inhibited by higher parasympathetic tone activity, because the parasympathetic nerve fibers and receptors were distributed in these lesions.     

Retrograde coronary venous infusion of ethanol for ablation of canine ventricular myocardium

INTRODUCTION: Permanent cure of reentrant ventricular tachycardia (VT) associated with coronary artery disease is difficult to achieve. Retrograde coronary venous infusion of ethanol for ablation of ventricular myocardium associated with reentrant tachyarrhythmias has several potential advantages, including use of physiologic mapping techniques and production of deeper, wider necrotic zones. METHODS AND RESULTS: Nine anesthetized dogs had baseline hemodynamic measurement, left ventriculography, coronary arteriography, occlusive coronary venography, and programmed electrical stimulation of the right ventricular apex and outflow tract. A balloon-tipped infusion catheter was advanced into a distal coronary venous branch, the balloon slowly inflated, and pure ethanol infused at volumes of 1.5, 3, or 5 cc. Hemodynamic measurements, angiography, ventriculography, and programmed electrical stimulation were repeated immediately and 1 week following ablation. Formalin-perfused hearts were serially sectioned and lesion volumes determined. Histologic examination of ablation beds then was performed. No significant difference was found in any hemodynamic measurement before or after ablation. Coronary arteriograms and left ventriculograms were unchanged after ablation. Nonsustained VT occurred in eight dogs during ethanol infusion; however, VT was not inducible in any dog before or after ablation. Infusion volumes of 3 cc or more were required to produce transmural lesions. CONCLUSION: Retrograde coronary venous infusions of ethanol using a balloon-tipped infusion catheter were effective in ablating ventricular myocardium. Retrograde chemical ablation did not itself result in inducible VT or adversely affect hemodynamic measurements or coronary arteries. Transmural myocardial necrosis, necessary in the ablation of VT associated with coronary artery disease, can be produced by higher infusion volumes.     

Radiofrequency catheter ablation of sustained monomorphic ventricular tachycardia in hypertrophic cardiomyopathy

INTRODUCTION: Incessant monomorphic ventricular tachycardia (VT) with a right bundle branch block morphology and a northwest axis is a rare arrhythmic complication in a patient with hypertrophic cardiomyopathy and apical left ventricular aneurysm. METHODS AND RESULTS: The origin of this VT was localized using the following criteria: the presence of entrainment without fusion, equal intervals from the stimulus to the beginning of the QRS complex and from the electrogram to the QRS complex during VT, and the first postpacing interval identical to the tachycardia cycle length. Radiofrequency energy applied to the septoapical part of the apical left ventricular aneurysm terminated the tachycardia within 2 seconds. CONCLUSION: Using criteria to guide radiofrequency (RF) ablation of VT in patients with coronary artery disease, an incessant monomorphic VT in a patient with hypertrophic cardiomyopathy was successfully ablated.     

Catheter ablation for the common type of atrioventricular nodal reentrant tachycardia

Radiofrequency (RF) catheter ablation of the slow AV nodal pathway was attempted in 34 patients with common type of AV nodal reentrant tachycardia (AVNRT). Radiofrequency energy of 18-32 watts was applied for 30-60 seconds at sites exhibiting atrial-slow pathway potentials or slow potentials. These potentials were recorded at the mid or posterior septum, anterior to the coronary sinus ostium. A mean of two radiofrequency applications successfully eliminated AVNRT in all patients. The incidence of junctional ectopy was significantly higher during 34 effective applications of radiofrequency energy than during 36 ineffective applications (100% versus 17%). Thus, the recording of atrial-slow pathway potentials or slow potentials, and the development of junctional ectopy can be used as a marker for successful ablation. Slow AV nodal conduction was eliminated in 22 patients and persisted without inducible AVNRT in 12. None of the patients had recurrences of AVNRT over a mean follow-up interval of 12 months, and all had preserved AV conduction. Long-term follow-up studies with an electrophysiological method confirmed that the ablation was effective. Transient AV block was observed in only 1 patient, and no major complications were noted. Thus, radiofrequency catheter ablation of the slow AV nodal pathway is highly effective and safe, with a low rate of complication, for the treatment of common type of AVNRT.     

The significance of electrocardiography in locating original sites of ventricular tachycardia

The sites of origin of ventricular tachycardia (VT) in 12 patients were located by ECG during the episode and further confirmed by catheter mapping. The results showed that there were 14 sites of origin of VT in the 12 patients from ECG in which 1 site was incompletely mapped by catheter and 12 of the other 13 original sites were confirmed by the catheter endocardial or epicardial mapping. Of the 12 original sites of VT, the locating of 11 ones were completely consistent with those from ECG, which was 84.6% of the 13 original sites. Moreover, 8 of the 12 patients had been successfully treated by catheter direct or radiofrequency current ablation and 1 of the 12 by successful surgical operation. Thus, the original sites of VT located by ECG was reliable and could shorten the time of catheter mapping during non-pharmacological therapy of VT.     

Radiofrequency catheter ablation of septal accessory pathways within the triangle of Koch: importance of energy titration testing other than the local electrogram characteristics for identifying the successful target site

Radiofrequency (RF) catheter ablation of accessory atrioventricular (AV) connections in the proximity of His bundle or AV node is at high risk of developing complete heart block. A safe and effective protocol has not been well established. Nineteen consecutive patients with 19 septal accessory pathways within the triangle of Koch underwent a protocol with power-titrated RF energy testing to identify the target site for successful catheter ablation. At every potential target site preselected by local electrogram characteristics, RF energy was started at 5 W for 10 seconds, with an increment of 5 W (duration remained at 10 s) until maximally 30 W or the observation of transient interruption of accessory pathway conduction. By this stepwise RF energy testing, we successfully localized and ablated 18 (94.7%) of the 19 septal accessory pathways, 10 close to His bundle (zone I) and 8 away from it (zone II). The test-effective RF power was 20 W or less in 9 of all 11 septal accessory pathways in zone I, and 5 of the 8 in zone II (P = 0.68). Meanwhile, the final RF power for successful ablation was 30 W or less in 9 of the 10 zone I and 6 of the 8 zone II septal accessory pathways (P = 0.83). One patient with an accessory pathway in zone I was complicated with complete AV block after final ablation at 30 W. None of the local electrogram characteristics except continuous electrical activity during retrograde mapping was helpful in the prediction of ablation outcome. Careful RF energy titration testing could effectively help identify the target site for successful RF catheter ablation of septal accessory pathways within the triangle of Koch. The dependence on local electrogram manifestations could be frustrated by a low probability of success.     

Posterior fast atrioventricular node pathways: implications for radiofrequency catheter ablation of atrioventricular node reentrant tachycardia

OBJECTIVES: This study sought to present evidence that fast atrioventricular (AV) node pathways with posterior exit sites may participate in typical AV node reentry. BACKGROUND: Catheter ablation of the slow AV node pathway in the posteroseptal right atrium is the preferred therapeutic approach in patients with AV node reentrant tachycardia. Despite the success achieved with this approach, electrophysiologic changes consistent with fast pathway ablation are occasionally observed. One potential explanation is the presence of an aberrant posterior fast pathway. METHODS: The location of fast and slow AV node pathways was determined by atrial activation mapping along the tricuspid valve annulus during tachycardia and was further confirmed by the effect of radiofrequency catheter ablation. RESULTS: Seven patients with AV node reentrant tachycardia had evidence of a posterior fast pathway near the coronary sinus os. Abolition of anterograde and retrograde fast pathway conduction followed radiofrequency ablation in the posteroseptal region in six patients. Consistent with fast pathway ablation, the AH interval increased from 70 +/- 24 to 195 +/- 35 ms (mean +/- SD), and tachycardia was no longer inducible. Selective slow pathway ablation was performed in one other patient with a posterior fast pathway. CONCLUSIONS: Functionally fast AV node pathways may be located in the posteroseptal right atrium, where slow pathway modification is performed. These data delineate the limitation of an anatomically guided slow pathway ablative approach and emphasize the importance of detailed mapping and localization of the retrograde fast pathway exit site before ablation. Failure to recognize the presence of posterior fast AV node pathways may account for sporadic examples of AV block, complicating posteroseptal ablation in patients with AV node reentry.