Catheter ablation of a concealed right anteroseptal AV
Catheter ablation of a concealed right anteroseptal AV bypass tract was successfully performed in our patient. Catheter ablation of the anteroseptal accessory pathway did not affect normal AV conduction. The effective refractory period of the fast AV node was 380ms at a pacing buy N6022 length of 700ms, and there was no AH jump after catheter ablation of the bypass tract. The patient developed a 2:1 RBBB when the atrium was paced at a cycle length of 450ms, which was the same length as the tachycardia cycle.
Discussion Because the ventricle is part of the AV reentrant circuit, development of a bundle branch block ipsilateral to the free wall bypass prolongs the tachycardia cycle secondary to prolongation of the VA conduction time if AV conduction time is unchanged. However, development of a bundle branch block in AV reentrant tachycardia by using a septal bypass tract does not increase the VA interval by less than 25ms . As observed in our case, reciprocal shortening of the AV interval can lead to no change or decrease of the tachycardia cycle length after bundle branch block development ipsilateral to the bypass tract. A 2:1 bundle branch block is reported in patients with AV nodal reentrant tachycardia, sinus tachycardia, atrial flutter, atrial fibrillation, and pulmonary embolism [1–5]. This phenomenon can be attributed to multiple mechanisms including first- and second-degree bundle branch blocks (prolonged refractory period of bundle branch block), aberrancy, or supernormal conduction [7–10]. A 2:1 bundle branch block can be also attributed to phase-3 (or tachycardia-dependent) antegrade block associated with supernormal conduction in the affected bundle branch. If the prolonged refractory period of an impaired bundle branch includes an early short phase of supernormal conduction (improved responsiveness), at a defined rate, the supraventricular impulses fall alternately within this period (producing normal QRS complexes) and outside it (resulting in a bundle branch block). This phenomenon only occurs if the antegradely blocked bundle branch is penetrated in a retrograde direction by the impulse coming from the contralateral bundle branch [8,9]. However, supernormal conduction should be considered only after failing to explain the phenomenon by a known physiologic mechanism . Considering that the fisrt-mentioned physiologic mechanism can explain the 2:1 RBBB, supernormal conduction would be less likely. Phase-4 (or bradycardia-dependent) antegrade block of bundle branch can be excluded by the existence of bundle branch block only during tachycardia .
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Introduction We report an episode of device-related arrhythmia observed with a dual-chamber implantable cardioverter defibrillator (ICD). A repetitive nonreentrant ventriculoatrial synchrony (RNRVAS) [1,2] occurred in a patient with poor left ventricular function due to a dilated cardiomyopathy and caused the worsening of heart failure. This pacemaker-mediated arrhythmia can be induced in patients with retrograde conduction, high basic heart rate, and long postventricular atrial refractory period (PVARP), and an algorithm called noncompetitive atrial pacing (NCAP) is effective to prevent pacemaker-mediated tachycardia.
Discussion Various types of pacing system malfunctions can occur with dual-chamber pacemakers. The typical form of pacemaker-mediated tachycardia (PMT) is endless loop tachycardia, which is characterized as a repetitive reentrant VA synchrony, repeated when the pacemaker senses retrograde atrial activation and triggers another ventricular stimulus. The other uncommon form of PMT is RNRVAS, which is a repetitive sequence of ventricular pacing, undersensed retrograde P wave due to PVARP, and uncaptured atrial pacing due to effective refractory period after atrial escape interval (AEI; Fig. 3A). For each type of PMT, intact VA conduction is essential. A sufficiently long PVARP to prevent endless loop tachycardia is another factor to predispose patients to RNRVAS. A long AV interval and a relatively rapid base rate are also required to shorten the AEI, which allows atrial noncaptured pacing within the absolute atrial myocardial refractory period .