RT Book, Section A1 Prystowsky, Eric N. A2 Prystowsky, Eric N. A2 Klein, George J. A2 Daubert, James P. SR Print(0) ID 1176522039 T1 Bundle Branch Block T2 Cardiac Arrhythmias: Interpretation, Diagnosis, and Treatment, 2e YR 2020 FD 2020 PB McGraw Hill PP New York, NY SN 9781260118209 LK accesscardiology.mhmedical.com/content.aspx?aid=1176522039 RD 2024/03/29 AB The anatomy of the His-Purkinje system is discussed in Chapter 1. In brief, the His bundle divides into two major fascicles, the right bundle branch and left bundle branch. The left bundle branch subdivides into the left anterior division, right posterior division, and septal inputs. Bundle branch block can be permanent (fixed) or functional, that is, present only under certain conditions. Permanent bundle branch block implies that supraventricular impulses cannot conduct over that fascicle at any time to activate the ventricle. Of note, although we use the term bundle branch block, an alternative explanation in some patients may actually be very slow conduction in the apparently blocked bundle, which prevents the supraventricular impulse from activating the ventricle early enough in comparison with the other conducting fascicles. Thus, the electrocardiographic (ECG) pattern will appear as if block were present when actually very slow conduction has occurred. It is not generally possible to make this distinction, and for practical purposes the term bundle branch block will be used unless it is clearly demonstrated that a functional conduction delay is present, as discussed later in this chapter.