ABC | Volume 115, Nº1, Suplement, July 2020

Case Report Pérez-Riera et al. Transient left anterior fascicular block Arq Bras Cardiol 2020; 115(1Suppl.1):1-5 Left septal fascicle (LSF): it is irrigated exclusively by the septal perforating artery from the LAD, which supplies the upper 2 / 3 portion of the interventricular septum (IVS) at this site. Most of the blood supply to the IVS is provided by the LAD. Branches into the septum from the posterior descending artery rarely penetrate more than 10 mm from the epicardium (slightly more than the normal thickness of the LV free wall), so that for practical purposes one may consider the entire blood supply of the IVS to be derived from four to six nearly equal size septal perforating branches of the LAD (Table 1). In the recent Brazilian consensus paper, the following criteria for LSFB were established. They are as follows, with modifications and clarifying comments by our group: Presence of prominent anterior forces (PAFs) of the QRS, being transient in sequential tracings. The transitory nature of PAF and the leads involved in it indicate a high likelihood of critical proximal obstruction of the left anterior descending coronary artery (LAD). When this pattern is observed in the scenario of acute coronary syndrome or during a stress test, urgent coronary angiography should be considered; Normal QRS duration or discrete increase (up to 110 ms) when not associated to other blocks; Unaltered frontal plane leads; R-wave-peak time in V1 and V2 ≥40 ms. 16 (Note: the term intrinsicoid deflection is not recommended, 17 R-wave voltage in V1 ≥5 mm; R/S ratio in V1 and V2 >2; S-wave depth in V2 <5 mm; Possible heart-rate dependent, embryonic and/or transient q wave 18 in V2 or V1 and V2; R-wave voltage in V2 >15 mm; RS or Rs patterns in V2 and V3 (frequently, rS in V1) with R wave “in crescendo” from V1 through V3 and decreasing from V5 to V6; Absence of q wave in V5, V6 and I (by absence of 1 AM septal vector) 18 ; confirmed experimentally in explanted human hearts by Durrer et al. 19 Conclusion To our knowledge, this is the first case in the literature describing ECG features compatible with LSFB associated with LMCA subocclusion. This evolution should alert clinicians about the possibility of severe coronary artery disease in patients with an ECG pattern of LSFB associated with wide- spread ST-segment depression both in patients with stable angina and those with acute coronary syndrome. Coronary angiography without delay should be considered. Author contributions Conception and design of the research and Analysis and interpretation of the data: Pérez-Riera AR, Barbosa-Barros R; Writing of the manuscript: Pérez-Riera AR; Critical revision of the manuscript for intellectual content: Pérez- Riera AR, Barbosa-Barros R, Raimundo RD, Abreu LC, Almeida MC, Nikus K. Potential Conflict of Interest No potential conflict of interest relevant to this article was reported. Figure 4 – Endocardial lateral view of the IVS in the human heart. 11 In this example the LSF originates from the main LBB. Additionally, the LAF conducts to the ALPM of the mitral valve and the LPF straight to the PMPM of the mitral valve (A). Figure extracted from the original book by Rosenbaum, 11 the LSF originates from the LPF. Rosenbaum considered these as “false sinews or tendons” originating from the LPF (B). LBB: left bundle branch; RBB: right bundle branch; LAF: left anterior fascicle; LPF: left posterior fascicle; LSF: left septal fascicle; LSFB: left septal fascicular block. Table 1 – Artery responsible for the irrigation of the three fascicles of the LBB Responsible system LAF LPF LSF LAD only 40% 10% 100% LAD & RCA 50% 40% 0% RCA only 10% 50% 0% 4