7/5/23, 10:36 AM Left septal fascicular block - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Left septal fascicular block : William H Sauer, MD : Ary L Goldberger, MD : Susan B Yeon, MD, JD, FACC All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jun 2023. This topic last updated: Jun 08, 2022. INTRODUCTION Left septal fascicular block (LSFB; also called left middle or left median fascicular block) results when one of the earliest phases of normal electrical activity in the His-Purkinje system is delayed or interrupted ( figure 1). Since the normal sequence of activation is altered in LSFB, it may be associated with alteration in the electrocardiogram (ECG). This form of block is one of the causes of a "counterclockwise rotation" pattern (early R wave transition) in the precordial leads and is quite variable, sometimes associated with a loss, not a gain, of anterior forces ( waveform 1). This variability has contributed to some of the controversies and unresolved issues regarding LSFB. (See 'Electrocardiographic findings' below.) The anatomy, clinical manifestations, differential diagnosis, prognostic implications, and treatment of LSFB will be reviewed here. In the discussion that follows, it is assumed that the reader understands the general concepts of cardiac vectors, asynchronous activation of the ventricles (delayed as in fascicular or bundle branch block or early as in preexcitation), and the effects that asynchrony has on the duration, morphology, and amplitude of the QRS complex. (See "ECG tutorial: Physiology of the conduction system" and "General principles of asynchronous activation and preexcitation".) ANATOMY AND ELECTROPHYSIOLOGY Anatomy The bundle of His divides at the juncture of the fibrous and muscular boundaries of the interventricular septum into the right and left bundle branches. The main left bundle branch https://www.uptodate.com/contents/left-septal-fascicular-block/print 1/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate penetrates the membranous portion of the interventricular septum under the aortic ring and then divides into several fairly discrete branches ( figure 1) [1]. There is a large amount of individual variability in the size and distribution of the left fascicles [2]. However, in most patients, there are two main fascicles: The left anterior fascicle that crosses the left ventricular outflow tract and terminates in the Purkinje system of the anterolateral wall of the left ventricle. The left posterior fascicle that fans out extensively inferiorly and posteriorly into Purkinje fibers. In up to 65 percent of hearts, a left septal fascicle (also called left middle or left median fascicle) to the interventricular septum. This has been reported in nearly 65 percent of people and can arise from the common left bundle or from the anterior, posterior, or both fascicles. The anatomy of the septal fascicle, when found, is more variable than the other fascicles, with a large number of interconnections [2,3]. Support for the trifascicular nature of the left bundle comes from the observation in animals and humans that depolarization of the left ventricle begins in three areas corresponding to the terminal portions of the anterior, posterior, and septal fascicles [4,5]. In the normal heart, the three fascicles of the left bundle are simultaneously depolarized. Further evidence of simultaneous activation of the fascicles can be found with routine electroanatomic mapping of a structurally normal heart. (See "Invasive diagnostic cardiac electrophysiology studies", section on 'Mapping and ablation'.) Blood supply The left anterior and septal fascicles are supplied either by septal branches of the left anterior descending (LAD) artery or by the atrioventricular (AV) nodal artery ( figure 2). The proximal part of the left posterior fascicle is supplied by the artery to the AV node and, at times, by septal branches of the LAD artery. The distal portion has a dual blood supply from both anterior and posterior septal perforating arteries. EPIDEMIOLOGY ECG findings consistent with isolated LSFB are an extremely rare finding, particularly among otherwise healthy persons. There are only a few reported estimates of isolated LSFB. In a review of some 26,000 ECGs, criteria for LSFB were found in approximately 0.5 percent, a prevalence similar to left posterior fascicular block (between 0.1 and 0.6 percent) but less than left anterior fascicular block (between 1 and 2.5 percent) [6]. https://www.uptodate.com/contents/left-septal-fascicular-block/print 2/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate ETIOLOGY A variety of underlying disorders may be responsible for LSFB, including ischemic or hypertensive heart disease, hypertrophic cardiomyopathy, and diabetes mellitus. LSFB can be induced by ischemia, fibrosis, or sclerodegenerative changes, which are generally associated with alterations in the other fascicles [2,7-10]. In addition, patients with no known structural heart disease can have transient LSFB with premature atrial ectopy [11]. Patients with prior catheter ablation of fascicular arrhythmias may also have LSFB as a consequence of the ablative treatment. ELECTROCARDIOGRAPHIC FINDINGS Similar but even more subtly than left anterior and posterior fascicular blocks, LSFB mainly affects the direction, but not the duration, of the QRS complex because the conduction disturbance primarily involves the early phases of activation ( waveform 1). The major clinical implication of LSFB is that the ECG mimics the changes induced by a septal or posterior myocardial infarction. (See "Left anterior fascicular block" and "Left posterior fascicular block".) Definition A task force from the American Heart Association, the American College of Cardiology, and the Heart Rhythm Society has defined the ECG features of left anterior and posterior fascicular blocks. This document acknowledges the lack of universally accepted criteria for LSFB, and therefore this condition was not electrocardiographically defined by this group [12]. In a separate consensus report, LSFB was specifically addressed with recognition of a lack of true universal ECG criteria [13]. There are several proposed ECG criteria including the loss of a septal q wave with an R pattern in leads V5 and V6 and the presence of an RS pattern in V1 to V2 indicating a gain in prominent anterior forces. However, because there are other conditions that can result in these ECG patterns, the consensus document is in agreement with the aforementioned task force conclusions. ECG activation patterns Myocardial activation may be affected in two ways by a conduction disturbance in the left septal fascicle: apparent loss or gain of anterior forces. In fact, the demonstration of these changes was one of the early observations suggesting the existence of a left septal fascicle. The ECG pattern seen with LSFB is probably determined by the differences in the sites of insertion of the septal, anterior, and posterior fascicles: Apparent gain of anterior forces with early precordial transition/counterclockwise rotation (CCWR) In marked contrast, prominent R waves are seen in the right precordial leads when LSFB leads to a gain of anterior forces [8,14]. These changes are similar to those that https://www.uptodate.com/contents/left-septal-fascicular-block/print 3/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate occur in true posterior (dorsal) myocardial infarction. The prominence of the R waves may be increased when LSFB occurs in association with right bundle branch block. The relationship between shift of the transitional zone on the standard 12-lead ECG and anatomic rotation of the heart in one plane was studied by cardiac computed tomography (CT) [15]. The left-sided angle between the interventricular septum and horizontal axis of the body (the septal angle) was determined on the cardiac CT. The mechanism of CCWR could be attributed to the septal angle in approximately two-thirds of the cases, but the remainder was thought to be due to LSFB. In one description of transient LSFB occurring during proximal occlusion of the left anterior descending artery, an acute gain of anterior forces was observed along with preservation of septal q waves [16]. Apparent loss of anterior forces Functional or hyperkalemia-induced dysfunction in the left septal fascicle can lead to the loss of anterior forces, resulting in the transient development of q waves in leads V1 and V2, which normally have a positive initial deflection due to septal depolarization [17,18]. Similar changes occur with permanent LSFB and are indistinguishable from septal fibrosis or infarction. EVALUATION, TREATMENT, AND FOLLOW-UP Patients with an ECG raising consideration of isolated LSFB are generally asymptomatic and do not require further diagnostic evaluation or placement of a pacemaker or any other specific therapy. Therapy should be considered only in patients with persistent bifascicular or trifascicular block or in certain neuromuscular disorders. (See "Chronic bifascicular blocks".) Any symptoms consistent with the development of cardiac disease (eg, coronary heart disease, heart failure, atrial fibrillation, etc) should immediately be evaluated. PROGNOSIS Similar to isolated left posterior fascicular block, isolated LSFB is very rare. Thus, unlike left anterior fascicular block, left bundle branch block, and right bundle branch block, there are no studies evaluating this ECG pattern with subsequent atrial fibrillation or other cardiovascular morbidity risk. SUMMARY AND RECOMMENDATIONS https://www.uptodate.com/contents/left-septal-fascicular-block/print 4/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate Left septal fascicular block (LSFB; also called left middle or left median fascicular block), a pattern seen on the surface electrocardiogram (ECG), results when normal electrical activity in the His-Purkinje system is delayed or interrupted ( figure 1). The normal sequence of activation is altered in LSFB, with a frequent counterclockwise rotation and a resultant appearance on the ECG that is variable, sometimes manifesting with a gain of anterior forces, but at other times with an apparent loss of anterior forces. (See 'Introduction' above.) Among experts and professional societies, there is an acknowledgment of the lack of universally accepted criteria for LSFB. Proposed ECG criteria include the loss of a septal q wave with an R pattern in lead V5 and V6 and presence of an RS pattern in V1 to V2 indicating a gain in anterior forces. However, there are other conditions that can result in these nonspecific ECG patterns. (See 'Electrocardiographic findings' above.) Isolated LSFB is an extremely rare finding, particularly among otherwise healthy persons. Criteria for LSFB have been reported only in approximately 0.5 percent of the general population. (See 'Epidemiology' above.) Patients with an ECG raising consideration of isolated LSFB are generally asymptomatic and do not require further diagnostic evaluation or placement of a pacemaker or other specific therapy. Any symptoms consistent with the development of cardiac disease (eg, coronary heart disease, heart failure, atrial fibrillation, etc) should immediately be evaluated. (See 'Evaluation, treatment, and follow-up' above.) Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Elizari MV, Acunzo RS, Ferreiro M. Hemiblocks revisited. Circulation 2007; 115:1154. 2. Demoulin JC, Kulbertus HE. Histopathological examination of concept of left hemiblock. Br Heart J 1972; 34:807. 3. Massing GK, James TN. Anatomical configuration of the His bundle and bundle branches in the human heart. Circulation 1976; 53:609. 4. Myerburg RJ, Nilsson K, Gelband H. Physiology of canine intraventricular conduction and endocardial excitation. Circ Res 1972; 30:217. 5. Durrer D, van Dam RT, Freud GE, et al. Total excitation of the isolated human heart. Circulation 1970; 41:899. 6. MacAlpin RN. In search of left septal fascicular block. Am Heart J 2002; 144:948. https://www.uptodate.com/contents/left-septal-fascicular-block/print 5/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate 7. Uhley HN. Some controversy regarding the peripheral distribution of the conduction system. Am J Cardiol 1972; 30:919. 8. Nakaya Y, Hiasa Y, Murayama Y, et al. Prominent anterior QRS force as a manifestation of left septal fascicular block. J Electrocardiol 1978; 11:39. 9. P rez-Riera AR, Barbosa-Barros R, Daminello-Raimundo R, et al. Transient left septal fascicular block and left anterior fascicular block as a consequence of proximal subocclusion of the left anterior descending coronary artery. Ann Noninvasive Electrocardiol 2019; 24:e12546. 10. Ibarrola M, Chiale PA, P rez-Riera AR, Baranchuk A. Phase 4 left septal fascicular block. Heart Rhythm 2014; 11:1655. 11. Acunzo RS, Konopka IV, Sanch z RA, et al. Right bundle branch block and middle septal fiber block with or without left anterior fascicular block manifested as aberrant conduction in apparent healthy individuals: Electro-vectorcardiographic characterization. J Electrocardiol 2013; 46:167. 12. Surawicz B, Childers R, Deal BJ, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances: a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society. Endorsed by the International Society for Computerized Electrocardiology. J Am Coll Cardiol 2009; 53:976. 13. Bay s de Luna A, Riera AP, Baranchuk A, et al. Electrocardiographic manifestation of the middle fibers/septal fascicle block: a consensus report. J Electrocardiol 2012; 45:454. 14. DePadua FI, Pereirnha A, Lopes MG. Conduction defects. In: Comprehensive Electrocardiogr aphy: Theory and Practice in Health and Disease, MacFarlane P, Veitch Lawrie TD (Eds), Perg amon Press, New York 1989. p.459. 15. Tahara Y, Mizuno H, Ono A, Ishikawa K. Evaluation of the electrocardiographic transitional zone by cardiac computed tomography. J Electrocardiol 1991; 24:239. 16. Andreou AY. Transitory R wave growth in the midst of ST-segment elevation myocardial infarction: A case of left septal fascicular block with atypical electrocardiographic presentation. J Electrocardiol 2022; 72:39. 17. Gambetta M, Childers RW. Rate-dependent right precordial Q waves: "septal focal block". Am J Cardiol 1973; 32:196. 18. Arnsdorf MF. Electrocardiogram in Hyperkalemia: electrocardiographic pattern of anteroseptal myocardial infarction mimicked by hyperkalemia-induced disturbance of impulse conduction. Arch Intern Med 1976; 136:1161. https://www.uptodate.com/contents/left-septal-fascicular-block/print 6/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate Topic 2111 Version 26.0 https://www.uptodate.com/contents/left-septal-fascicular-block/print 7/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate GRAPHICS Normal conduction system Schematic representation of the normal intraventricular conduction system (His- Purkinje system). The Bundle of His divides into the left bundle branch and right bundle branch. The left bundle branch divides into anterior, posterior, and, in some cases, median fascicles. AV: atrioventricular; RA: right atrium; LA: left atrium; RV: right ventricle; LV: left ventricle. Graphic 63340 Version 6.0 https://www.uptodate.com/contents/left-septal-fascicular-block/print 8/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate Surface 12-lead ECG showing left septal fascicular block (LSFB) Surface 12-lead ECG depicting second-degree atrioventricular block and phase 4 or bradycardia-dependent L ECG: electrocardiogram; LSFB: left septal fascicular block. Original gure modi ed for this publication. From: Ibarrola M, Chiale PM, P rez-Riera AR, Baranchuk A. Phase 4 left septal fascicular b the permission of Elsevier Inc. All rights reserved. Graphic 118370 Version 2.0 https://www.uptodate.com/contents/left-septal-fascicular-block/print 9/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate Coronary artery anatomy Graphic 70956 Version 2.0 https://www.uptodate.com/contents/left-septal-fascicular-block/print 10/11 7/5/23, 10:36 AM Left septal fascicular block - UpToDate Contributor Disclosures William H Sauer, MD Grant/Research/Clinical Trial Support: Biosense-Webster [Catheter ablation]; Boston Scientific [Catheter ablation]. All of the relevant financial relationships listed have been mitigated. Ary L Goldberger, MD Other Financial Interest: Elsevier book royalties [Clinical electrocardiography]. All of the relevant financial relationships listed have been mitigated. Susan B Yeon, MD, JD, FACC No relevant financial relationship(s) with ineligible companies to disclose. Contributor disclosures are reviewed for conflicts of interest by the editorial group. When found, these are addressed by vetting through a multi-level review process, and through requirements for references to be provided to support the content. Appropriately referenced content is required of all authors and must conform to UpToDate standards of evidence. Conflict of interest policy https://www.uptodate.com/contents/left-septal-fascicular-block/print 11/11