Radiofrequency Catheter Ablation of Idiopathic Left Ventricular Tachycardia Originating in Both Left Posterior and Anterior Fascicles

A 45-year-old woman underwent radiofrequency ablation (RFA) for symptomatic idiopathic left ventricular tachycardia (ILVT). The clinical arrhythmias had two different patterns, a wide QRS tachycardia with right bundle branch block (RBBB) and left axis deviation (LAD) and another with RBBB and right axis deviation (RAD). The electrophysiology study localized the origin of tachycardias to the midinferior and superior ventricular septum, respectively. RFA terminated successfully ILVT with RBBB and LAD morphology, but another pattern could not be ablated. Noncontact mapping revealed the earliest site of activation at the superior septum. RFA at this site terminated successfully ILVT with RBBB and RAD.     

Radiofrequency catheter ablation of ventricular tachycardia originating in the left posterior and left anterior fascicles in a patient with prior myocardial infarction

A 61-year-old man with prior anteroseptal myocardial infarction (ejection fraction: 40%) presented with recurrent episodes of palpitations. Twelve-lead ECG during palpitations showed an incessant ventricular tachycardia (VT1) with right bundle branch block (RBBB) morphology and inferior axis. Electrophysiologic study revealed that the clinical VT originated from the anterolateral left ventricle. A Purkinje potential preceded onset of the QRS complex by 34 ms. Radiofrequency ablation guided by the Purkinje potential terminated the VT1. Another ventricular tachycardia (VT2) showing RBBB morphology with superior axis and originating from the posteroseptal left ventricle, was induced by programmed ventricular stimulation. A Purkinje potential preceded onset of the local ventricular potential by 120-130 ms in this VT. Radiofrequency ablation guided by the Purkinje potential terminated the VT2.     

Verapamil-Sensitive Left Anterior Fascicular Ventricular Tachycardia: Results of Radiofrequency Ablation in Six Patients

Verapamil-Sensitive Left Anterior Fascicular VT. Introduction: Verapamil-sensitive left ventricular tachycardia (VT) with a right bundle branch block (RBBB) configuration and left-axis deviation bas been demonstrated to arise from the left posterior fascicle, and can be cured by catheter ablation guided by Purkinje potentials. Verapamil-sensitive VT with an RBBB configuration and right-axis deviation is rare, and may originate in the left anterior fascicle. Methods and Results: Six patients (five men and one woman, mean age 54 ± 15 years) with a history of sustained VT with an RBBB configuration and right-axis deviation underwent electrophysiologic study and radiofrequency (RF) ablation. VT was slowed and terminated by intravenous administration of verapamil in all six patients. Left ventricular endocardial mapping during VT identified the earliest ventricular activation in the anterolateral wall of the left ventricle in all patients. RF current delivered to this site suppressed the VT in three patients (ablation at the VT exit). The fused Purkinje potential was recorded at that site, and preceded the QRS complex by 35, 30, and 20 msec, with pace mapping showing an optimal match between the paced rhythm and the clinical VT. In the remaining three patients, RF catheter ablation at the site of the earliest ventricular activation was unsuccessful. In these three patients, Purkinje potential was recorded in the diastolic phase during VT at the mid-anterior left ventricular septum. The Purkinje potential preceded the QRS during VT by 66, 56, and 63 msec, and catheter ablation at these sites was successful (ablation at the zone of slow conduction). During 19 to 46 months of follow-up (mean 32 ± 9 months), one patient in the group of ablation at the VT exit bad sustained VT with a left bundle branch block configuration and an inferior axis, and one patient in the group of ablation at the zone of slow conduction experienced typical idiopathic VT with an RBBB configuration and left-axis deviation. Conclusion: Verapamil-sensitive VT with an RBBB configuration and right-axis deviation originates close to the anterior fascicle. RF catheter ablation can be performed successfully from the VT exit site or the zone of slow conduction where the Purkinje potential was recorded in the diastolic phase.     

Double tachyarrhythmia: left anterior fascicular ventricular tachycardia associated with atrial flutter and fibrillation. Report of a case

A patient with episodes of palpitation in whom the electrocardiogram showed a right bundle branch (RBBB) configuration and right axis deviation underwent electrophysiologic study and radiofrequency ablation. Left ventricular endocardial mapping during ventricular tachycardia (VT) identified the earliest ventricular activation in the anterolateral wall of the left ventricle. The fused Purkinje potential was recorded at that site, and preceded the QRS complex by 47 mseg, with pace mapping showing an optimal match between the paced rhythm and the clinical VT. The stimulus to QRS time was equal to the Purkinje potential-QRS time. Several radiofrequency lesions were applied in this region, one of them resulted with termination of the tachycardia. Following delivery of this lesion the ventricular tachycardia couldn't be induced either at baseline or during isoproterenol infusion. During VT, atrial fibrillation and atrial flutter were observed, cardioversion was performed reverting to sinus rhythm.     

Clinical features of emergency electrocardiography in patients with acute myocardial infarction caused by left main trunk obstruction.

BACKGROUND: To diagnose left main trunk (LMT) infarction by 12-lead standard electrocardiogram (ECG) is an important emergency technique, but the features in LMT infarctions have not been clarified. METHODS AND RESULTS: The study enrolled 140 subjects who were divided into 4 groups according to the location of the culprit artery: 35 with LMT, 35 with left anterior descending artery (LAD), 35 with right coronary artery and 35 with left circumflex artery. Various parameters obtained from the ECGs were analyzed. Average QTc interval (0.51 +/- 0.06 s) in LMT group was markedly longer than that in the 3 other groups. Average QRS axis (-10 +/- 77 degrees) in LMT infarction showed a remarkable left deviation. ST-segment elevation in lead aVR occurred in 28 patients (80.0%) in the LMT group. The ECG features of the LMT group could be classified into 2 main groups: right bundle branch block (RBBB) with a marked left axis deviation (RBBB + LADEV type) and ST-segment elevation in leads V2-5, I and aVL without abnormal axis deviation (LAD type). CONCLUSION: Either ST-segment elevation in lead aVR and marked prolongation of both the QRS width and QTc interval with a prominent abnormal axis deviation or ST-segment elevation in the broad anterior precordial lead with a normal QRS axis strongly suggests LMT infarction.     

Narrow QRS Tachycardia with Ventriculoatrial Dissociation Mediated by a Left Fasciculoventricular Fiber

A 30-year-old man presented with narrow QRS tachycardia. The intracardiac electrocardiogram showed an atrial-HIS (AH) interval of 75 msec and a HIS-ventricular (HV) interval of 44 msec during baseline. Atrial incremental pacing revealed HV shortening, with apparent incomplete right bundle branch block (RBBB) morphology without QRS complex axis deviation. The induced tachycardia exhibited several QRS morphologies: a narrow QRS, complete RBBB and complete left bundle branch block (LBBB) morphology. Spontaneous conversion of the QRS pattern from wide to narrow was observed. The cycle length of the tachycardia was significantly shortened (from 316 to 272 ms) from LBBB morphology to narrow QRS complex. The atrial activation was dissociated from the ventricular activation during all tachycardias. Each QRS complex during tachycardia was preceded by a HIS deflection and HV interval was 35 ms, which was shorter than that of sinus rhythm. HIS deflection was earlier than right bundle potential during all kinds of tachycardia. This tachycardia is most likely mediated by a left fasciculoventricular fiber which connects the HIS bundle below the atrioventricular node to the myocardial tissue of the left ventricle. The HIS-Purkinje system is used as an antegrade conduction limb and the fasciculoventricular fiber as a retrograde limb in the tachycardia circuit.     

Radiofrequency catheter ablation of coexistent atrioventricular reciprocating tachycardia and left ventricular tachycardia originating in the left anterior fascicle

Coexistence of supraventricular tachycardia and ventricular tachycardia is rare. A patient with no structural heart disease and wide QRS complex tachycardia with a right bundle block configuration and right-axis deviation underwent electrophysiological examination. A concealed left atrioventricular pathway (AP) was found, and atrioventricular reciprocating tachycardia (AVRT) and left ventricular tachycardia (VT) originating in or close to the anterior fascicle of the left ventricle were both induced. Radiofrequency (RF) catheter ablation of the concealed left AP was successfully performed. Ten months later, VT recurred and was successfully ablated using a local Purkinje potential as a guide. Coexistent AVRT and idiopathic VT originating from within or near the left anterior fascicle were successfully ablated.     

QRS Axis and the Benefit of Cardiac Resynchronization Therapy in Patients with Mildly Symptomatic Heart Failure Enrolled in MADIT‐CRT

Cardiac Resynchronization Therapy and QRS Axis . Background: Mildly symptomatic heart failure (HF) patients derive substantial clinical benefit from cardiac resynchronization therapy with defibrillator (CRT‐D) as shown in MADIT‐CRT. The presence of QRS axis deviation may influence response to CRT‐D. The objective of this study was to determine whether QRS axis deviation will be associated with differential benefit from CRT‐D. Methods : Baseline electrocardiograms of 1,820 patients from MADIT‐CRT were evaluated for left axis deviation (LAD: quantitative QRS axis ‐30 to ‐90) or right axis deviation (RAD: QRS axis 90–180) in left bundle branch block (LBBB), right bundle branch block (RBBB), and nonspecific interventricular conduction delay QRS morphologies. The primary endpoints were the first occurrence of a HF event or death and the separate occurrence of all‐cause mortality as in MADIT‐CRT. Results: Among LBBB patients, those with LAD had a higher risk of primary events at 2 years than non‐LAD patients (20% vs 16%; P = 0.024). The same was observed among RBBB patients (20% vs 10%; P = 0.05) but not in IVCD patients (22% vs 23%; P = NS). RAD did not convey any increased risk of the primary combined endpoint in any QRS morphology subgroup. When analyzing the benefit of CRT‐D in the non‐LBBB subgroups, there was no significant difference in hazard ratios for CRT‐D versus ICD for either LAD or RAD. However, LBBB patients without LAD showed a trend toward greater benefit from CRT therapy than LBBB patients with LAD (HR for no LAD: 0.37, 95% CI: 0.26–0.53 and with LAD: 0.54, 95% CI: 0.36–0.79; P value for interaction = 0.18). Conclusions: LAD in non‐LBBB patients (RBBB or IVCD) is not associated with an increased benefit from CRT. In LBBB patients, those without LAD seem to benefit more from CRT‐D than those with LAD. (J Cardiovasc Electrophysiol, Vol. 24, pp. 442‐448, April 2013)     

Verapamil-sensitive left posterior fascicular ventricular tachycardia after myocardial infarction

Verapamil-sensitive fascicular ventricular tachycardia (VT) of right bundle branch block (RBBB) and superior axis pattern is typically seen in young patients with structurally normal hearts and considered “idiopathic”. Recently, involvement of the Purkinje system in post-infarction monomorphic VT that mimics such idiopathic fascicular VT has been described. In this report we describe a case of a patient who following myocardial infarction developed left posterior fascicular Purkinje reentrant VT that was sensitive to verapamil. The VT was successfully treated by radiofrequency ablation guided by three dimensional electroanatomical CARTO™ mapping. Our case highlights that involvement of Purkinje fibers should be considered in post infarction patients with VT of narrow QRS duration, RBBB morphology and superior axis. Recognition of such VT is clinically important, as this arrhythmia is amenable to curative catheter ablation.     

Prolongation of QRS duration and axis deviation in the right bundle branch block are not markers for left ventricular systolic dysfunction

BACKGROUND:

Right bundle branch block (RBBB) is not commonly associated with structural heart disease and left ventricular (LV) systolic dysfunction. The purpose of the present study was to determine whether the QRS duration and degree of right axis deviation (RAD) or left axis deviation (LAD) in patients with RBBB predicted a subset of patients with significant LV systolic dysfunction.

METHODS:

In the present prospective study, 75 of 200 consecutive patients with RBBB had their ejection fraction (EF) evaluated by echocardiography. The relationship among QRS duration, axis and EF was derived.

RESULTS:

There were no significant differences in sex and EF among the patients with a normal axis, RAD or LAD. The EFs of patients with a normal axis (n=27), RAD (n=15) and LAD (n=33) were 52±15%, 49±14% and 46±17%, respectively (P=0.35). The mean EF (46±16%) of patients with a QRS duration of 150 ms or greater (n=53) was not significantly different from the mean EF (49±18%) of patients with a QRS duration of less than 150 ms (n=22) (P=0.54). For patients with a QRS of 120 ms or greater and less than 150 ms (n=22), QRS of 150 ms or greater and 180 ms or less (n=48), and QRS of greater than 180 ms (n=5), the mean EFs were 49±18%, 47±16% and 44±7%, respectively (P=0.78). There was no significant correlation between QRS duration and EF in all patients (r=0.03, P=0.83), EF and RAD (r=0.38, P=0.16) or EF and LAD (r=0.26, P=0.14).

CONCLUSIONS:

In patients with RBBB, the QRS duration and axis do not have a significant relationship with EF. Furthermore, prolongation of the QRS duration (150 ms or greater) in the presence of RBBB is not a marker of significant LV systolic dysfunction.     

Idiopathic left ventricular tachycardia: assessment and treatment

Idiopathic left ventricular tachycardia (VT) has been classified into three subgroups according to mechanism: verapamil-sensitive, adenosine-sensitive, and propranolol-sensitive types. VT can be categorized also into left fascicular VT and left outflow tract VT. Although the mechanism of fascicular VT is verapamil-sensitive reentry, the mechanism of left outflow tract VT is not homogeneous. Fascicular VT can be classified into three subtypes: (1) left posterior fascicular VT with a right bundle branch block (RBBB) and superior axis configuration (common form); (2) left anterior fascicular VT with RBBB and right-axis deviation configuration (uncommon form); and (3) upper septal fascicular VT with a narrow QRS and normal axis configuration (rare form). Posterior and anterior fascicular VT can be successfully ablated at the mid-septum guided by a diastolic Purkinje potential or at the VT exit site guided by a fused presystolic Purkinjepotential. Upper septal fascicular VT also can be ablated at the site indicated by a diastolic Purkinje potential. The mechanism of left ventricular outflow tract VT is most likely adenosine-sensitive triggered activity. This VT can be classified into three subtypes according to the location where catheter ablation is successful, i.e., (1) endocardial origin; (2) coronary cusp origin; and (3) epicardial origin. The R-wave duration and R/S-wave amplitude in V1/V2 can be used to differentiate coronary cusp VT from other types of outflow tract VT. Recognition of the characteristics of the various forms of this group of arrhythmias should facilitate appropriate diagnosis and therapy.     

Phase image characterization of ventricular contraction in left anterior hemiblock

We investigated whether or not left anterior hemiblock is present in patients with left axis deviation using first-harmonic Fourier analysis of gated blood-pool images. Gated blood-pool images were taken in 50 patients without contraction abnormality. They included 14 normal subjects, 8 patients with right bundle branch block (RBBB), 20 with left axis deviation (LAD) and 8 with both RBBB and LAD (RBBB + LAD). ECG gated blood-pool scans were acquired in the anterior and "best septal" left anterior oblique projections. First, the phase images were displayed cinematically as a continuous-loop movie. Next, for quantitative analysis of the phase image, the whole left ventricular and left ventricular high lateral regions of interest were drawn. The "regional phase shift" (RPS) was then defined as (RPS = A-a) where "A" is the mean value of the whole left ventricular phase angles and "a" is that of phase angles in the high lateral region. The left ventricular phase changes and the RPSs in the RBBB and LAD groups were similar to those in the normal group. In the RBBB + LAD group, the latest phase changes occurred in the high anterolateral region. The RPSs of this group were significantly lower than those in the other 3 groups (p less than 0.01). These data suggest that left anterior hemiblock might coexist with RBBB in patients with RBBB + LAD, whereas left anterior hemiblock might not exist in the majority of patients with LAD alone.     

Non-contact mapping and linear ablation of the left posterior fascicle during sinus rhythm in the treatment of idiopathic left ventricular tachycardia.

AIM: The reentry circuit of idiopathic left ventricular tachycardia (ILVT) has been demonstrated to be confined to the left posterior Purkinje network. We hypothesized that mapping and linear ablation of the left posterior fascicle (LPF) during sinus rhythm guided by non-contact mapping can effectively modify the arrhythmogenic substrate in patients with ILVT and abolish the tachycardia. METHODS: Six patients with ILVT, consisting of one case in which conventional mapping failed three times, one recurrent case, one non-inducible case and three common cases, were included in the study. After a three-dimensional endocardial geometry of the left ventricle (LV) was created, the conduction system in the LV was mapped during sinus rhythm using a filter setting of 8 Hz. The His bundle area, left bundle branch, fascicles and sinus breakout point (SBO) were mapped in detail and tagged as special landmarks in the geometry. A linear lesion was placed perpendicular to the wave front propagation direction of the LPF, 1cm above the SBO. There was a small Purkinje potential preceding the ventricular activation at its starting and ending point. RESULTS: The mean tachycardia cycle length of ILVT in this study was 340.3+/-51.4ms. After a mean of 5.5+/-1.6 radiofrequency deliveries, the clinical tachycardias could not be induced and the 12-lead surface ECG showed right QRS axis deviation (mean 39.7+/-26.0 degrees) in all patients. The total procedure time was 160.0+/-32.2 min with fluoroscopic time of 26.0+/-6.8 min. No ILVT was inducible during control stimulation, and none recurred during a mean follow-up of 13.0+/-4.8 months. CONCLUSION: Mapping and linear ablation of the Purkinje network in LPF area guided by non-contact mapping is an effective and safe treatment of ILVT with radiofrequency energy, especially for those ILVTs which were unsuccessfully treated by conventional means or were non-inducible or non-sustained during the procedure.     

Radiofrequency Catheter Ablation of Idiopathic Ventricular Tachycardia Originating in the Anterior Fascicle of the Left Bundle Branch

Ablation of an Anterior Fascicular Idiopathic VT. Introduction : Idiopathic ventricular tachycardia (VT) originating in or close to the anterior fascicle of the left bundle is rare. A patient with no structural heart disease and VT with a right bundle branch block configuration and right-axis deviation underwent an electrophysiologic examination.
Methods and Results : Both endocardial activation mapping during VT and pacemapping were performed via a transseptal approach to localize the site of origin of the VT. Endocardial recordings of the His bundle and the posterior and anterior fascicles of the left bundle branch revealed an origin of the VT in or close to the anterior fascicle. The Purkinje potential at that site preceded the QRS complex by 20 msec, with pacemapping showing an optimal match between the paced rhythm and the clinical VT. RF energy delivered at this site terminated the VT. A left anterior nemiblock appeared after RF ablation. Ten months later, the patient is free from recurrences of VT.
Conclusions : Idiopathic VT originating in or close to the anterior fascicle was cured by RF ablation. A Purkinje potential preceding the QRS during tachycardia and an optimal pacemap were used to guide RF ablation.     

Idiopathic Left Ventricular Tachycardia with Left and Right Bundle Branch Block Configurations

Fascicular Tachycardia. Introduction : Idiopathic left ventricular tachycardia typically has a right bundle branch block configuration. The purpose of this case report is to demonstrate that idiopathic ventricular tachycardia arising in or near the left posterior fascicle also may have a left bundle branch block configuration.
Methods and Results : A 27-year-old woman underwent an electrophysiologic procedure because of recurrent, verapamil-responsive, wide QRS complex tachycardia. Two types of ventricular tachycardia (cycle lengths 330 to 340 msec) were reproducibly inducible, one with a right bundle branch block configuration and left-axis deviation that had been documented clinically, and the other with a left bundle branch block configuration and axis of zero. A Purkinje potential recorded at the junction of the left ventricular mid-septum and inferior wall preceded the ventricular complex by 40 msec in both tachycardias. A single application of ra-diofrequency energy at this site successfully ablated both ventricular tachycardias.
Conclusion : The findings of this case report demonstrate that idiopathic ventricular tachycardia arising in or near the left posterior fascicle may have a left bundle branch block configuration     

Left posterior fascicular tachycardia: a diagnostic and therapeutic challenge

A wide QRS complex tachycardia with right bundle-branch block morphology and left axis deviation observed in a young patient without structural heart disease may pose a diagnostic and therapeutic challenge. The surface ECG may provide several diagnostic clues to make a correct diagnosis of left posterior fascicular tachycardia and may help to differentiate it from both a supraventricular tachycardia with aberrant conduction and a typical ventricular tachycardia related to coronary artery disease. Although this tachycardia is sensitive to verapamil, this medication may probably cause transient infertility in males. The presence of a Purkinje potential preceding the QRS complex during tachycardia and optimal pace mapping may guide radio-frequency ablation resulting in a definite cure.     

Pilsicainide-induced Brugada-type ECG and ventricular arrhythmias originating from the left posterior fascicle in a case with Brugada syndrome associated with idiopathic left ventricular tachycardia.

The patient was a 50-year-old male in 2002, who was first suspected of having a Brugada-type electrocardiogram (ECG). A drug challenge test using pilsicainide was performed and unmasked a typical coved type ST elevation followed by ventricular arrhythmias (VAs) manifesting a QRS pattern with a right bundle branch block and left axis deviation. Three years later, he was transferred to the emergency room due to a wide QRS tachycardia with the same QRS morphology as the VA that previously occurred in the drug challenge test. An ECG just after the recorded termination of the tachycardia exhibited a typical Brugada-type ECG. In an electrophysiological study, ventricular fibrillation could be easily induced with reproducibility. Since the clinical tachycardia could not be sustained by an isoproterenol infusion, mapping and catheter ablation targeting the pilsicainide-induced VAs was performed. The successful ablation site was the left mid-lower septal wall where a Purkinje potential was recorded and a false tendon was attached just to it.     

Diagnosis and Ablation of Multiform Fascicular Tachycardia

Ablation Multiform Fascicular Tachycardia . Introduction: Fascicular tachycardia (FT) is an uncommon cause of monomorphic sustained ventricular tachycardia (VT). We describe 6 cases of FT with multiform QRS morphologies. Methods and Results : Six of 823 consecutive VT cases were retrospectively analyzed and found attributable to FT with multiform QRS patterns, with 3 cases exhibiting narrow QRS VT as well. All underwent electrophysiology study including fascicular potential mapping, entrainment pacing, and electroanatomic mapping. The first 3 cases describe similar multiform VT patterns with successful ablation in the upper mid septum. Initially, a right bundle branch block (RBBB) VT with superior axis was induced. Radiofrequency catheter ablation (RFCA) targeting the left posterior fascicle (LPF) resulted in a second VT with RBBB inferior axis. RFCA in the upper septum just apical to the LBB potential abolished VT in all cases. Cases 4 and 5 showed RBBB VT with alternating fascicular block compatible with upper septal dependent VT, resulting in bundle branch reentrant VT (BBRT) after ablation of LPF and left anterior fascicle (LAF). Finally, Cases 5 and 6 demonstrated spontaneous shift in QRS morphology during VT, implicating participation of a third fascicle. In Case 6, successful ablation was achieved over the proximal LAF, likely representing insertion of the auxiliary fascicle near the proximal LAF. Conclusions : Multiform FTs show a reentrant mechanism using multiple fascicular branches. We hypothesize that retrograde conduction over the septal fascicle produces alternate fascicular patterns as well as narrow VT forms. Ablation of the respective fascicle was successful in abolishing FT but does not preclude development of BBRT unless septal fascicle is targeted and ablated. (J Cardiovasc Electrophysiol, Vol. 24, pp. 297‐304, March 2013)     

Idiopathic verapamil-sensitive sustained left ventricular tachycardia

Idiopathic verapamil-sensitive left ventricular tachycardia (VT) has characteristic QRS configurations during VT: right bundle-branch block with either left axis or right axis (less common) deviation. QRS duration is relatively narrow (0.13-0.16s) and frequently endocardial activation prior to QRS is recorded during VT, which is the basis of its being called fascicular tachycardia. The mechanism is probably reentry, but the nature of the slow conduction necessary for the occurrence of reentry is quite different from that of other sustained monomorphic VT associated with structural heart disease. Chronic oral verapamil therapy is the drug of choice for alleviation of symptoms. Long-term prognosis is good.     

Effects of right bundle branch block on the antidromic circus movement tachycardia in patients with presumed atriofascicular pathways

BACKGROUND: The typical and most common tachycardia in patients with atriofascicular pathways is a macro reentrant tachycardia, with anterograde conduction over the decrementally conducting bypass tract and retrograde conduction over the right bundle branch-His-AV node axis resulting in a short V-right bundle branch and short V-H interval. OBJECTIVES: To report on changes in rate and QRS configuration when right bundle branch block (RBBB) develops spontaneously during antidromic tachycardia using an atriofascicular fiber. METHODS: Three of 25 patients with an antidromic circus movement tachycardia using a right-sided atriofascicular pathway showed episodes of right bundle branch block (RBBB) during ventriculo-atrial conduction. Effect of retrograde RBBB on tachycardia rate and QRS configuration was studied using intracardiac and extracardiac recordings. RESULTS: All 3 patients showed prolongation of their V-A interval when retrograde RBBB occurred during tachycardia, resulting in a longer tachycardia cycle length. The VA time increase ranged from 85 to 100 msec, with a mean 346 +/- 5 msec. Two of the 3 patients also showed a change in QRS configuration due to a more leftward shift of the frontal plane QRS axis. CONCLUSION: Rate changes in antidromic tachycardia in patients with atriofascicular fibers can be based on a shift in VA conduction from one bundle branch to the other. This may be accompanied by changes in the frontal plane QRS axis because of a change in ventricular activation sequence.