In a case similar to ours
In a case similar to ours, presented by Seres et al.  in which LVNC was associated with polymorphic VT, the value of QT interval was also difficult to ascertain because of the advanced LBBB. The onset of the polymorphic VT was similar to ours, but neither pause-dependence nor post-PVB lengthening of QT was described. In our case, the initiating mechanism for polymorphic VT was a ‘long-short’ sequence generated by a PVB and its compensatory pause. A normally conducted QRS complex followed the PVB and its “post pause U wave” potentially created the arrhythmic substrate for VT. As described by Jackman et al. , this could be the initiation sequence for a torsade de pointes tachyarrhythmia, but in our case the QT interval was normal and the presence of LBBB made difficult to ascertain the post pause TU waveform amplitude and morphology pre- as well as post-VT.
Since LVNC appears to be a genetically heterogeneous disease with multiple gene mutations reported, an altered ion channel activity or an enhanced intercellular communication could be the basis for this arrhythmia mechanism. Shan et al. , screened 62 patients with LVNC for variants in SCN5A, a well know gene involved in the pathophysiology of multiple cardiac arrhythmias like Brugada syndrome, long QT type III and Lev–Lenegre syndrome. In his study, the frequency of SCN5A variants was significantly higher in the patients with arrhythmias than those without (50% vs 7%), supporting the hypothesis that gene encoding ion thiamine hcl are involved in LVNC pathophysiology. The most frequent arrhythmias were VT and PVBs. The LVNC patients with heart failure also had high occurrence of SCN5A variants (53%), suggesting the presence of SCN5A variants and/or arrhythmias increase the severity of LVNC. Conceivably, an SCN5A variant in our patient could underline her susceptibility to ventricular arrhythmias and/or her conduction system disease.
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Case presentation A 77-year-old man with a history of healed inferior myocardial infarction underwent radiofrequency catheter ablation for drug-refractory ventricular tachycardia (VT) with right bundle branch block and superior axis. VT was reproducibly induced by programmed ventricular stimulation. Pacing during VT at the basal edge of the low voltage zone, where a low-amplitude diastolic potential (DP) was recorded, showed concealed entrainment, in which the pacing stimulus-QRS was nearly equal to the DP-QRS interval, and the post-pacing interval (PPI) of the DP was equal to the tachycardia cycle length (TCL), consistent with pacing on the essential pathway of the reentrant circuit (Fig. 1). The delivery of radiofrequency energy at that site terminated and eliminated the induction of VT. Before ablation, overdrive pacing with identical output and a cycle length slightly shorter than that of the VT near the successful ablation site caused alternans of the DP to DP intervals (Fig. 2). What is the mechanism of this DP alternans?
Commentary Understanding the electrophysiological phenomenon illustrated in Fig. 2 requires the accurate identification of the myocardial tissue captured by pacing in the reentry circuit. First, the morphology, amplitude, and timing of the local ventricular electrograms and DP relative to the QRS complex recorded from the ablation catheter during VT in Fig. 2 are slightly, though distinctly, different from those shown in Fig. 1. This suggests that the location of the tip of the ablation catheter, while near, was not strictly the same during both episodes of overdrive pacing. Second, the very short intervals between the DP and the following pacing stimuli suggest that, unlike in Fig. 1, pacing could not capture the essential pathway because it was refractory. Third, in contrast to Fig. 1, every even-numbered paced cycle in Fig. 2 shows (a) no latency of the pacing stimulus-QRS complex, (b) a slight change in the morphology of the QRS, and (c) a shortening of the pacing stimulus-ventricular electrogram, suggesting that the even-numbered cycles represent direct capture of the ventricular myocardium outside the zone of slow conduction. The wavefront of the 2:1 captures propagated antidromically to the ventricle and orthodromically advanced the DP and subsequent ventricular electrogram, while the interval between the DP preceding and that following the non-captured stimuli was similar to the TCL, causing 2:1 alternans of the DP cycles and ventricular electrograms. Furthermore, an oscillation in the intervals between the DPs preceding and those following captured stimuli was observed, whereas the interval between the DP preceding and that following non-captured stimuli was relatively constant. These phenomena were observed even during overdrive pacing at a shorter cycle length, and probably represented decremental conduction properties between captured ventricular myocardium and the recording site of the DP.