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  • In the present study the timing


    In the present study, the timing of the LA potentials in respect to the timing of the CSM potentials depended closely on the direction of impulse propagation across the interatrial connection at the proximal CS. Thus, a LA → CSM activation sequence was observed only when the wavefront traveled through the proximal CSM in left-to-right direction as during retrograde conduction over left-sided AP or the ongoing typical CW AFL., The CSM → LA activation sequence was observed only when it propagated in the right-to-left direction as during the ongoing CCW AFL, confirming our hypothesis. This also confirmed that the far-field LA potentials were identified on the basis of their morphologies. It was of a particular clinical interest to know whether the timing of the LA potentials on the CS electrogram recordings helps in clarifying the electrophysiological mechanism of various supraventricular tachyarrhythmias. This point has been the object of several previous reports, including our own observations. First, the activation sequence of the LA and CSM during retrograde conduction over a left-sided AP may be a clue to differentiate an AV AP from CS-ventricular accessory pathways that connect the atrium and the ventricle via CSM [7,8,15]. A LA → CSM activation sequence suggests the presence of an AV AP (Fig. 2), while, during retrograde conduction over CS-ventricular accessory pathways, the wavefront propagates via CSM before reaching the atrium, thus representing a CSM → LA activation sequence (Fig. 6) [8,16,17]. This information may be useful in choosing the best approach for ablating the left-sided AP. Second, the participation of the LA in the reentry circuit of a slow-fast AVNRT, albeit controversial, is suggested by the observation of a LA → CSM activation sequence during ongoing tachycardia [6]. Third, the activation sequence of the LA and CSM may be influenced by the direction of the reentrant wavefront during ongoing AFL. During typical CW AFL, the sequence of activation is from LA → CSM (Fig. 7), in D609 to typical CCW AFL, where the activation sequence is from CSM → LA (Fig. 3). This is because the CSM is activated by propagation of the wavefront via the superior interatrial connections, including Bachmann’s bundle and interatrial septum, reaching the LA from the RA, instead of being activated via the interatrial connection at the proximal CS, as in the case of typical CCW AFL [18]. The activation of the mid or distal CS electrograms during typical AFL is proximal-to-distal because of their RA origin, regardless of the CW or CCW reentrant direction and this sequence does not explain the mechanism of typical AFL. Thus, the timing of LA potentials may be a useful atrial indicator of the mechanism and origin of supraventricular tachyarrhythmias that cannot be mapped, particularly when non-sustained. Further studies, in larger populations are needed to verify this hypothesis.
    Conflict of interest
    Introduction Familial long QT syndrome (LQTS) is a primary arrhythmogenic disorder caused by ion channel abnormalities leading to abnormal ventricular repolarization and a prolonged QT interval on the electrocardiogram (ECG) [1]. Clinical manifestations include palpitations, syncope, or cardiac arrest due to torsade de points and ventricular fibrillation (VF) [1,2]. Familial LQTS, mostly inherited as an autosomal dominant trait, rarely presents as a recessive trait in the form of Jervell and Lange–Nielsen syndrome [3,4]. Clinical diagnosis is based on the identification of a prolonged QT interval on the ECG, presence or absence of a family history, and absence of QT-prolonging medications [2]. Advances in the understanding of molecular genetics and pathogenesis of genetic heart diseases have contributed extensively to elucidating the role of genetics in familial LQTS [5]. Compared with other genetic heart diseases, the yield of genetic testing has been highest in LQTS, and is now an integral part of clinical management of families [6]. Particularly, apart from playing a role in diagnosis, genotype also has potential therapeutic and prognostic implications, with genotype–phenotype correlations shown to explain some of the heterogeneity of the disease [6–8].