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 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]. A more recent understanding of their function stresses that they primarily affect the direction of depolarization [2]:

• 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, medial, or median fascicle, is found in nearly 65 percent of people [3,4]. This fascicle runs to the interventricular septum, and can arise from the common left bundle or from the anterior, posterior or both fascicles.

Support for the trifascicular nature of the left bundle comes from the observations 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 [5,6]. In the normal heart, the three fascicles of the left bundle are simultaneously depolarized.