Chapter 3: Bundle Branch Block

INTRODUCTION

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.

PERMANENT BUNDLE BRANCH BLOCK

Right Bundle Branch Block

Under normal circumstances, initial activation of the interventricular septum starts on the left side and proceeds rightward.¹ Activation of the right ventricular endocardial surface occurs at least 5 to 10 ms after activation of the left side of the septum.¹ Thus, in a patient with right bundle branch block, initial activation occurs normally and proceeds from left to right across the interventricular septum; then activation of the left ventricle occurs and activation of the right ventricle is last (Figure 1-4, Chapter 1). The ECG representation of right bundle branch block is shown in Figure 3-1. The duration of the QRS complex is approximately 130 to 140 ms, and initial left-to-right septal activation results in a small r wave in lead V₁, followed by an S wave reflecting left ventricular activation, and finally by a predominant R´ wave representing activation of the right ventricle. ECG leads I, V₅, and V₆ typically demonstrate a wide S wave, reflecting terminal activation away from the left ventricle (Table 3-1).²