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Left median (middle or septal) fascicular block

INTRODUCTION

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".)

FASCICLES OF THE LEFT BUNDLE BRANCH

The classic hypothesis proposed by Rosenbaum and his coworkers was that the left bundle branch divides into two fascicles of rapidly conducting Purkinje fibers (ie, phase 0 dependent on the rapid inward sodium current) (figure 1) [1]. These fascicles primarily affect the direction of depolarization:

The left anterior fascicle crosses the left ventricular outflow tract and terminates in the Purkinje system of the anterolateral wall of the left ventricle.

The left posterior fascicle appears as an extension of the main bundle and fans out extensively posteriorly toward the papillary muscle and inferoposteriorly to the free wall of the left ventricle.

In addition, a third fascicle, called the left septal, middle, or median fascicle, is found in nearly 65 percent of people [2,3]. This fascicle runs to the interventricular septum and can arise from the common left bundle or from the anterior, posterior, or both fascicles.

      

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Literature review current through: Oct 2014. | This topic last updated: Jan 6, 2014.
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References
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  1. Rosenbaum M, Elizari MV, Lazzari JO. The Hemiblocks, Tampa Tracings, Tampa 1970.
  2. Demoulin JC, Kulbertus HE. Histopathological examination of concept of left hemiblock. Br Heart J 1972; 34:807.
  3. Uhley HN. Some controversy regarding the peripheral distribution of the conduction system. Am J Cardiol 1972; 30:919.
  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. Massing GK, James TN. Anatomical configuration of the His bundle and bundle branches in the human heart. Circulation 1976; 53:609.
  7. Gambetta M, Childers RW. Rate-dependent right precordial Q waves: "septal focal block". Am J Cardiol 1973; 32:196.
  8. 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.
  9. 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.
  10. DePadua FI, Pereirnha A, Lopes MG. Conduction defects. In: Comprehensive Electrocardiography: Theory and Practice in Health and Disease, MacFarlane P, Veitch Lawrie TD (Eds), Pergamon Press, New York 1989. p.459.
  11. Tahara Y, Mizuno H, Ono A, Ishikawa K. Evaluation of the electrocardiographic transitional zone by cardiac computed tomography. J Electrocardiol 1991; 24:239.
  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. MacAlpin RN. In search of left septal fascicular block. Am Heart J 2002; 144:948.
  14. Acunzo RS, Konopka IV, Sanchéz 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.
  15. Dabrowska B, Ruka M, Walczak E. The electrocardiographic diagnosis of left septal fascicular block. Eur J Cardiol 1978; 6:347.