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 Penaloza-Tranchesi vector). 10 Next, the stimulus is directed to the mid-septal or left paraseptal region, blocked by numerous Purkinje passage areas, thus shifting the forces to the front and the left, resulting in prominent anterior forces (PAF). Figure 4 shows two cases where the trifascicular anatomy of the left His system is evident. Ironically, both cases come from the school of electrocardiography that coined the bifascicular concept of the left His system. 11 Left fascicles blood supply LAF: the blood supply to the LAF of the LBB originated in 50 % of the cases not only from the anterior septal branch of the LAD, but also from the atrioventricular (AV) nodal artery, a branch of the right coronary artery (RCA) in 90 % of the cases and of the LCX in 10%. 12 Thus, anatomic data support the observation that occlusion of the proximal segment of the LAD is not a prerequisite for the occurrence of LAFB. The appearance of LAFB during acute myocardial infarction is not a sign of a coexistent significant stenosis of the LAD or of more severe or extensive coronary artery disease. In these patients, other mechanisms such as the degree of the coronary collateral circulation may play a role in the occurrence of this conduction disturbance and supports the experimental and clinical reports that LAFB may be due to lesions involving the His bundle by means of a longitudinal dissociation of this structure. 13 LPF: the broad nature of the LPF, its protected location in the left ventricular inflow tract, as well as its dual blood supply 14 makes isolated LPFB very rare. 15 The PMPM where LPF ends is supplied by arterial branches that terminate on the diaphragmatic surface of the LV, and most commonly by a junction of terminal branches of the LCX and of the RCA. When the LCX supplies nearly all the diaphragmatic surface of the LV (10 % of human hearts), its branches provide the entire blood supply for the PMPM. The LPF is irrigated in 10% of cases by LAD only, in 40% of cases by LAD and RCA and in 50% of cases by RCA only. Figure 1 – ECG at admission (A) and the injury vector in the frontal and horizontal planes (B). A) Widespread ST-segment depression in I, II, III (II>III) and VF and from V2 to V6 waves. Diffuse ST-segment depression in the inferolateral leads (≥7 leads with ST-depression) and reciprocal ST-segment elevation in the aVR lead. In addition, atypical left anterior fascicular block (LAFB), QRS axis -40°, SIII>SII, and absence of initial q wave in I and aVL by absence of the first left middle septal vector (in typical LAFB the first 10-20 ms vectors are directed to +120°). 4 B) Frontal plane (FP): The ST injury vector (arrow) is directed upward and rightward pointing towards the aVR lead (-150°). When this vector is located between -90° and ±180° in the FP, it is indicative of LMCA obstruction in up to 100% of cases; 5 ST-segment depression in the inferior leads with STII>STIII; Horizontal plane: the ST injury vector is directed to the right and leftward (arrow), perpendicular to V1. ST segment depression from V2 to V6. 2