br Discussion The mechanism of verapamil sensitive ILVT

Discussion The mechanism of verapamil-sensitive ILVT has been suggested as re-entry with an excitable gap and a slow conduction, because the VT can be induced, entrained, and terminated by programmed ventricular or atrial stimulation [1–6]. In addition, previous studies have suggested that the re-entry circuit is confined to the posterior Purkinje system and that the ventricular myocardium is involved in the circuit [5–7]. Previous studies have shown successful catheter ablation achieved at the site of the earliest recorded PP or pre-PP, particularly at the inferoposterior left ventricular septum during sustained VT [5,9], and at the site of abnormal potential in the mid-inferior septum within the posterior fascicular network during sinus rhythm [6]. In our case, catheter ablation could eliminate VT and was performed at the basal third of the mid-septum, where pre-PP and PP were recorded during tachycardia. However, the entire re-entrant circuit and slow conduction zone remain unclear, and there have been no reports describing patients with verapamil-sensitive ILVT in the presence of CLBBB. To the best of our knowledge, this is the first report of verapamil-sensitive ILVT, which was not influenced by the incidental occurrence of CLBBB.
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Introduction Patients with Brugada syndrome who have a history of ventricular fibrillation (VF) are usually implanted with implantable cardioverter defibrillators (ICDs) for secondary prevention of arrhythmia. Some patients with Brugada syndrome who do not have a history of VF are also implanted with ICDs for primary prevention of arrhythmias. In some of these cases, inappropriate shock has become a major problem in therapy. The majority of inappropriate shocks are delivered because of sinus tachycardia, atrial tachycardia, atrial flutter, or atrial fibrillation [1].
Case report A 41-year-old man with Brugada syndrome who was implanted with an ATLAS+DR V-243 (St. Jude Medical, Inc., St. Paul, MN, USA) dual-chamber ICD for secondary prevention of arrhythmia received a shock 3 years after implantation of the device. The VF zone was programmed to deliver maximum output shock therapy at a detection threshold of greater than 214 beats per min, and 12 consecutive instances of crossing the threshold would constitute 1 VF. The monitoring zone was programmed to greater than 160 beats per min with all SVT discriminators in the “ON” mode, and 12 consecutive instances of the cyclin dependent kinase rate exceeding 160 beats per min would constitute 1 SVT. The following interrogation data were normal: sensing right atrium (RA),>3.0mV; sensing right ventricle (RV),>12.0mV; RA lead impedance, 540Ω; RV lead impedance, 530Ω; high voltage (HV) lead impedance, 48Ω. The ICD shock was delivered following sinus tachycardia that continued for 3min 15s (Fig. 1). The SVT in stored electrograms (EGMs) showed a 1:1 atrioventricular (AV) conduction, and the AV interval was 180ms. In addition, the maximal onset delta measurement of sudden onset was 10ms, so elevation of the heart rate was gradual (Table 1). SVT was detected by the ICD 68 times during the sinus tachycardia episode. Fig. 2 represents the monitoring zone at the start of sinus tachycardia. EGM revealed that irregularly appearing PACs during sinus tachycardia were cumulatively counted as VFs (Fig. 3).
Discussion When an implanted ICD is set up in patients with a history of VF, only the VF zone is usually chosen. However, Annealing is also possible to set a monitoring zone. This monitoring zone is a programmed rate zone in which the device will detect ventricular tachycardia (VT) without administration of any therapeutic intervention [2]. The monitoring zone is effective for patients with asymptomatic VT or SVT. If asymptomatic VT or SVT is detected using the monitoring zone, the device can be programmed to delivery specialized therapy for these arrhythmias as necessary. In the current case, an ICD shock was delivered to the patient following sinus tachycardia that did not fulfill the 12 consecutive interval criteria for VF detection. The ICD shock was initially canceled by the SVT discriminators, but it was eventually delivered. The cause of the inappropriate ICD shock in this case was as follows: the monitoring zone was in the “ON” mode and PACs that occurred during sinus tachycardia entered the VF zone and accumulated, thereby fulfilling the programmed VF counts. This occurred because of a monitoring zone algorithm of the St. Jude Medical ICD, which adopts a binning method for arrhythmia detection. The binning method classifies the detection of VT/VF by an interval average (IA) and current interval (CI). The CI is the last interval and the IA is the average of 4 intervals, including the CI. According to Table 2, when we classified CI and IA, the count of the providing zone increased. However, one characteristic of this device is that the VF bin is counted cumulatively, even if each bin occurred separately. In this case, IAs caused by sinus tachycardia were classified as VTs. In addition, the VF count increased if a CI caused by PACs was in the VF zone. The ICD shock was delivered after 3min 15s of sinus tachycardia because the VF counter requirement was fulfilled. When the VF counter was reset, no ICD shock was delivered. Resetting of the VF counter required 5 continuous V–V intervals longer than the set monitoring zone (Fig. 4). Therefore, the IA and CI must be the intervals that occurred outside of the monitoring zone. In this case, IAs caused by sinus tachycardia were always classified in the VT bin. Therefore, it was difficult to reset the VF counter. SVT discriminators evaluated SVT or VT every 6 beats. As for the other models presented in Table 3, a similar inappropriate shock may occur. However, St. Jude Medical already modified the algorithm in more recent models (Table 4), and this type of inappropriate discharge becomes less likely in patients implanted with the recent ICD models. The modified binning method for arrhythmia detection in recent ICD models resets the VF counter when SVT discriminators evaluate the SVT.