RT Book, Section A1 Zimmerman, Franklin H. SR Print(0) ID 1200100134 T1 Bundle Branch Block and Related Conduction Abnormalities T2 ECG Core Curriculum YR 2023 FD 2023 PB McGraw Hill Education PP New York, NY SN 9780071785211 LK accesscardiology.mhmedical.com/content.aspx?aid=1200100134 RD 2024/03/29 AB Let’s again review how the normal sinus impulse reaches the ventricles. The SA node depolarization first conducts through the atria, the AV node, and the bundle of His. Next, the stimulus proceeds through the right bundle branch and the left bundle branch and its divisions. The left common bundle branch divides into two major fascicles called the left anterior-superior fascicle and left posterior-inferior fascicle, as well as a smaller mid-septal fascicle (Figure 8-1). Remember that this is a simplified description of the left bundle branch because the two major divisions and one minor branch are arranged more like a fan-shaped array rather than distinct, isolated fibers. The right and left bundle branches arborize widely, dividing into an extensive network of Purkinje fibers that interconnects with the ventricular myocytes. Transmission of the sinus impulse through the atria and the remainder of the specialized conduction system (AV node and His-Purkinje system) is recorded on the ECG as the PR interval. Electrical transmission through the His-Purkinje system is extremely rapid, with the majority of the duration of the PR interval representing conduction through the atria and AV node (Figure 8-2). The QRS complex records depolarization of the ventricular myocardium, a process that is normally completed within 0.10 seconds. Recall from Chapter 6 that we can simplify ventricular depolarization into three stages, each represented by a vector (Figure 8-3).