Chinese Medical Journal 2006;119(5):367-372
Left posterior fascicular block: a new endpoint of ablation for verapamil-sensitive idiopathic ventricular tachycardia

Correspondence to:ZHANG Shu,Center of Arrhythmia Diagnosis and Treatment, Fuwai Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100037, China (Tel: 86-10-88398943. Fax:86-10-68334688. E-mail:zsfuwai@vip.163.com)
Keywords
radiofrequency ablation; idiopathic left ventricular tachycardia; verapamil-sensitive; left posterior fascicular block
Abstract

Background Verapamil-sensitive, idiopathic left ventricular tachycardia (ILVT) with right bundle branch block configuration and left-axis deviation is known to be due to re-entry mechanism but the exact nature of reentrant circuit in ILVT is not fully elucidated. Radiofrequency (RF) ablation was applied during ventricular tachycardia (VT) and termination of the VT or abolishing the inducibility of the tachycardia was used as an endpoint for successful RF. In this study, the left posterior fascicular block in surface electrocardiogram (ECG) was used as a new endpoint of ablation to cure ILVT.
Methods  Electrophysiological studies and radiofrequency ablation were performed in 39 consecutive patients [30 men, 9 women; age ranging from 10 to 64 years, mean (29±16) years] with verapamil-sensitive ILVT and structurally normal hearts. VT could be terminated by the intravenous administration of verapamil in all patients. The target site was the midseptum of LV where the earliest Purkinje potentials were recorded during VT. RF current was applied to the target site with or without late diastolic potential (LDP) during sinus rhythm in 37 patients and during VT in 2 patients to meet the ablation endpoint: the left posterior fascicular block in the surface ECG.
Results  Thirty-seven patients with ILVT had been treated by RF ablation during sinus rhythm and two had been treated during VT. All of them met the endpoint of the left posterior fascicular block. Thirty-eight cases were symptom-free without medications during the follow-up period (range from 3 to 95 months, median 17 months). One patient developed a clinical recurrence and the left posterior fascicular block in surface ECG disappeared. The patient received another treatment. The endpoint was met and the procedure was successful.
Conclusions  The left posterior fascicular block in surface ECG used as an endpoint of RF ablation to treat ILVT is effective. It is important especially in those patients whose VT can not be induced or the inducible condition is unstable. The effective endpoint implied that the left posterior fascicle might be a critical part of the re-entrant circuit.

Verapamil-sensitive idiopathic left ventricular tachycardia with a right bundle-branch block and left-axis deviation morphology, which occurs in young adults without structural heart disease is an uncommon but well described clinical arrhythmia. This ventricular tachycardia (VT) is known to be due to re-entry mechanism and the re-entrant circuit is localized in the left ventricular septum.1-10 However, the relationship between Purkinje potential and the reentrant circuit is still controversial. Successful lesion can be introduced at the site with the earliest onset of ventricular activation during tachycardia and the best pace map concordance of the 12-lead electrocardiogram.11-13 As well, the Purkinje potential, the late or mid-diastolic electrical activity preceding Purkinje potential during VT can be used as markers for successful ablation.1-4,14-16 If the tachycardia was terminated in 10 seconds during the RF current delivery, the ablation was thought to be effective. The ablation endpoint was achieved if VT could not reveal any inducibility.11,17

During endocardial catheter mapping of this VT, in some patients with clinical typical surface ECG the VT could not be induced or the inducible condition was unstable. Here we introduce a method of RF ablation for VT. We think the left posterior fascicle is a critical part of the circuit.15 Therefore, RF catheter was guided by fluoroscopy and Purkinje potential to the target site and ablation energy was applied during sinus rhythm. When the left posterior fascicular block in body ECG is observed, we have achieved the ablation endpoint.

METHODS

Patients
The ECG recorded during VT exhibited a right bundle-branch block configuration and left-axis in all patients (Fig. 1). Intravenous verapamil (5 to 10 mg) had been found to be effective in terminating VT in all patients. Thirty-nine consecutive patients [9 women and 30 men; mean age, (29±16) years; range, 10 to 64 years] who underwent RF catheter ablation for symptomatic left idiopathic VT without any underlying heart disease were included. Two of them had sustained VT for more than 24 hours. Five of them had previously ablation attempts (range 1 to 4 times) using conventional methods and experienced recurrence. Three patients had a history of frequent episodes of palpitations with narrow complex and wide complex tachycardia in surface ECG.


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Fig. 1. Clinical surface ECG of ILVT: a right bundle-branch block, left-axis deviation morphology.

Electrophysiological study
Written informed consent was obtained from all patients before the electrophysiological study and ablation procedure. All antiarrhythmic drugs were discontinued for >5 half-lives for each drug before the study. Two 6F quadripolar (Cordis-webster, USA) or bipolar electrode catheters were introduced to the right ventricular apex (RVA) and at the His bundle region via the femoral veins. A 6F nondeflectable decapolar catheter (Cordis-webster) with 2-5-2 mm interelectrode spacing was inserted into the coronary sinus via the right internal jugular vein. ECG leads I, II, III, aVR, aVL, aVF and V1-V6 as well as intracardiac electrograms from different sites were simultaneously displayed and recorded on a multichannel bard recorder (Bard Electrophysiology Lab System, Lowell, MA, USA) at a paper speed of 100 mm/s. The pacing stimuli were approximately twice diastolic thresholds in strength and two milliseconds in duration and were provided by a digital programmable stimulator. Complete anterograde and retrograde electrophysiological studies using incremental atrial or ventricular pacing and extrastimulus testing were performed in each patient before ablation. If the VT was not inducible, the programmed stimulation was repeated during isoproterenol infusion (1 to 4 µg/min to achieve a 20% increase in sinus rate). During VT, endocardial mapping in the LV was performed, the earliest PP and the putative VT exit site as the earliest ventricular activation site where a single fused PP was recorded before the onset of the QRS complex was determined. We chose the site where the earliest PP which represented the left posterior fascicle was recorded as the target site. In the patients whose VT could not be induced or the inducible condition was unstable, or the VT was not constant, the target site was chosen at the left ventricular midseptum where the catheter recorded a sharp PP representing the left posterior fascicle. If a narrow and complex tachycardia was found, the mechanism was studied at the same time.

Catheter ablation
A 7F steerable quadripolar electrode catheter (Cordis-webster) with a 4-mm tip and 2-mm intereletrode spacing between the distal two electrodes was used for ablation. The radiofrequency current was generated from commercially available electrosurgical unit (EP-SHUTTLE, Biosense Webster, USA) that provided a continuous, unmodulated sine wave at 500 kHz. The current was delivered between the distal electrode of the ablation catheter and a large disposable skin electrode pad that was applied to the left posterior chest under continuous digital monitoring of power strength and impedance. The ablation catheter was introduced percutaneously through the femoral artery and advanced to the left ventricular cavity. The mapping efforts were initially concentrated at the inferior apical or mid-apical septum during sinus rhythm guiding by fluoroscopy with 30 right anterior oblique and 45 left anterior oblique to record spark PP. Then, the VT was induced by atrial or ventricular programmed or burst pacing. The earliest ventricular activation site where a single fused PP was recorded before the onset of the QRS complex was determined by the ablation catheter during the VT in 26 of 39 patients at the posteroapical left ventricular septum. Then the catheter tip was carefully moved to the proximal part of the septum to map the earliest PP. This short, high-frequency potential also preceded ventricular activation during sinus rhythm, suggesting that the PP represents activation of a component of the left posterior fascicle. The PP was recorded in the area of the posterior half of the left ventricular septum, one fourth to one third of the distance from apex to base. The catheter tip located at the lowest level of the coronary sinus electrodes in the fluoroscopy image with 30 right anterior oblique and at the left edge of the vertebral column in the fluoroscopy image with 45 left anterior oblique (Fig. 2).


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Fig. 2. Fluoroscopic view of mapping and ablation catheter in LV for ablation of ILVT in right anterior oblique (RAO) 30 view (A) and left anterior oblique (LAO) 45 view (B). The large-tip electrode is positioned at the posterior left ventricular septum, one third of the distance from apex to base. The other catheters were located near the right ventricular apex (RV), His bundle region (HB), coronary sinus (CS).

The site defined in Fig. 2 was basal to the site of earliest ventricular activation. After that the VT in 24 patients was terminated by ventricular pacing. We were careful that the ablation catheter tip was not moved and/or recorded a sharp PP which represented the left posterior fascicle (Fig. 3). A test RF current, with an initial power of 20 to 25 W, was applied to the site showing a sharp PP before QRS complex with or without late diastolic potential (LDP) during the SR  for a total duration of 60 to 90 seconds. In the two patients with sustained VT which could not be terminated by ventricular or atrial pacing, RF current was delivered in VT. In the other 13 patients whose VT could not be induced or the inducible condition was unstable, we chose the midseptum by fluoroscopy described in Fig. 2 as the target site and moved the catheter subtly to record spark PP. RF current was delivered in SR. If the surface ECG showed left posterior fascicular block, we reasoned that the ablation endpoint was met. If not, the catheter was moved to the proximal site about midseptum and an area where a sharp spike in PP was recorded, this was selected as  the target site to deliver another RF current another time to meet the endpoint. The catheter tip should be located at the anatomical site which has been described in the Fig. 2 and the radiofrequency energy should be delivered at the distal site of the left posterior fascicle to avoid injuring the left bundle-branch. One to four applications of energy were needed to meet the endpoint of the left posterior fascicular block in surface ECG. When this protocol was repeated (with isoproterenol) 30 minutes after ablation, the VT could not be induced. The narrow complex tachycardia was treated by RF ablation at the same time in this study.


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Fig. 3. Example of LDP preceding PP recorded at the middle of the left ventricular septum during SR. Tracings are ECG leads I, avF and V1, and intracardiac electrograms recorded from the CS9-10 to CS1-2, right ventricular apex (RVA), proximal and distal pairs of the ablation catheter. LDP indicates late diastolic potential. PP indicates Purkinje potential.

RESULTS 

The surface ECG of thirty-nine patients showed normal axis during SR before ablation (Fig. 4). Twenty-four patients had stable induction of clinical ILVT. Eight patients had unstable induction of ILV Tand five were not inducible. Two patients had sustained ILVT for more than 24 hours and could not be terminated by ventricular or atrial pacing. The cycle length of sustained tachycardia was (360.8±65.9) ms (range 280 to 566 ms).Duration of QRS was (125.3±6.0) ms. Twenty-three patients had no retrograde VA conduction, seven patients had constant VA conduction and four patients had intermittent VA conduction during VT. Three patients were induced with narrow and wide complex tachycardias, one of them had the concealed left-sided accessory pathway and the other two had atrioventricular nodal re-entrant tachycardia (AVNRT) at the same time. Three narrow complex tachycardias had been eliminated before treating the ILVT. One patient experienced atrial fibrillation during ILVT. The ablation energy had been applied to 37 patients during sinus rhythm and 2 during VT. All of them met the endpoint: left posterior fascicular block in surface ECG (Fig. 5). The earliest recorded PP preceded the QRS by 15 to 38 ms [mean,  (26.3±4.6) ms] during VT. During sinus rhythm, the PP preceded the QRS by 14 to 25 ms [mean,  (19.8±2.6) ms]. The number of RF applications was 1.7±0.9. The duration of QRS was (90.8±8.7) ms during SR before ablation,and it was (97.6±8.8) ms after ablation. Then we repeated the same stimulation protocol before ablation, the ILVT could not be induced.


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Fig. 4. Surface ECG during SR before ablation with normal axis and normal duration of QRS.


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Fig. 5. Surface ECG during SR after ablation with left posterior fascicular block.

The duration of follow-up ranged from 3 to 95 months (median 17 months). Thirty-eight patients were symptom-free without medications. One patient had VT recurrence after one year and the left fascicular block in surface ECG disappeared. We repeated the procedure and the endpoint was met. The left fascicular block in surface ECG of the other patients have been permanent and no complications were found.

DISCUSSION

The mechanism of verapamil-sensitive, idiopathic left ventricular tachycardia with right bundle branch block configuration and left-axis deviation has been suggested to be re-entry with an excitable gap and a slow conduction. The Purkinje fiber was a part of re-entry.5,16 Nogami et al5 speculated the slow conduction tissue is the specialized Purkinje tissue, and that has decremental properties and verapamil-sensitivity. Thakur et al18 demonstrated a false tendon is the slow conduction tissue. During sinus rhythm the activation goes from the basal septum to the apex along the left posterior fascicle. During VT, the activation goes from the basal septum to the apex along the slow conduction tissue and in the reverse direction along the left posterior fascicle. Mapping of PP, earliest activation, late or mid-diastolic potential, regional paced body surface and local activation sequence have been elucidated before. Tsuchiya et al1 reported that LDP is likely to represent the excitation at the entrance to the specialized slow conduction area with a conduction delay property associated with calcium channel- dependent conduction and it appears to be a very useful marker in guiding the successful ablation site for this VT, but RF ablation guided by LDP could result in the transient left bundle-branch block. Betts et al13 reported the site of earliest activation is likely to represent the exit from a zone of slow conduction and ablation at this site may successfully terminate the VT. Nakagawa2 and Wen et al8 reported that successful RF ablation could be guided by the earliest PP. Arya et al19 reported that compared to LDP alone, earliest PP (with or without concomitant LDP) might be superior for selection as the target site. However, ILVT can be induced and is necessary in conventional electrophysiological studies allowing the operator to decide the endpoint of ablation. We think the earliest PP represents the activation potential of the left posterior fascicle which belongs to the re-entrant circuit. If ablation results in the elimination of the conduction of the left posterior fascicle, the VT would terminate. This method can  cure the patient whose ILVT can not be induced or if the inducible condition is unstable. Ouyang et al4 reported that RF applications were delivered to the area with the earliest retro PP in three patients without inducible ILVT because the VT can not be induced before the ablation. In our study, RF current was delivered during sinus rhythm in 37 patients and the left posterior fascicular block was used as the endpoint which was clear and independent of the inducible ability of the tachycardia. In our study, since the left posterior fascicular block prohibited tachycardia, indicated that the left posterior fascicle was a critical part of the circuit. We did not prospectively study the relationship between the LDP and the earliest PP at the target and no entrainment was performed during VT.

In summary, the left posterior fascicular block can be used as the endpoint of ablation to treat verapamil-sensitive ILVT. It makes the ablation procedure simple, safe and effective. It is especially important for patients whose VT can not be induced or the inducible condition is unstable. It also implies that perhaps the left posterior fascicle is a critical part of the re-entry circuit.

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