Wagner_Marriot's Practical Electrocardiography, 12e

LOCATION OF ATRIOVENTRICULAR BLOCK As discussed in Chapter 6 and presented schematically in Figure 22.1, AV block can be located in the AV node, the common bundle, or the bundle branches. This distinction is important because both the etiology and prognosis are quite different with proximal (AV nodal) versus distal (infranodal) block. Fortunately, block within the common bundle is so rare that the clinical decision about the location of an AV block is essentially limited to the AV node versus the bundle branches. Two aspects of the electrocardiographic appearance of rhythm may help in differentiat- ing AV block at the AV node versus AV block in a bundle branch: (a) the consistency of the PR intervals of conducted impulses and (b) the width of the QRS complexes of either conducted or escape impulses. Because only the AV node has the ability to vary its con- duction time, the Purkinje cells of the common bundle and bundle branches must conduct at a particular speed or not at all. Therefore, when a varying PR interval is present, the AV block is most likely within the AV node. A QRS complex of normal duration ( 0.12 second) can occur only when the impulse producing the complex has equal access to both the RBB and LBB. Therefore, when an AV block is located at the bundle-branch level, the conducted or escape QRS complexes must be 0.12 second. The diagnosis is complicated by the possibility of either a fixed bundle-branch block accompanying AV nodal block or an aberrancy of intraventricular conduction (Chapter 20). Consequently, a QRS complex of normal duration confirms that a block has an AV nodal location, whereas a QRS complex of prolonged duration is not help- ful in locating the site of an AV block. The AV node has this ability to vary its conduction time, because its cells have uniquely prolonged periods of partial refractoriness as they return from their depolarized to their repolarized states. Therefore, in less than complete AV block, a nodal versus an infranodal (bilateral bundle) location can be determined by observing whether the PR interval is vari- able, as in the first case, or constant, as in the second. AV conduction patterns can be considered only when some conduction is present (first- or second-degree AV block). No differentiation between an AV nodal and an infranodal location is possible in complete ( third-degree ) block with wide escape QRS complexes (see Fig. 22.7B).

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CHAPTER 22: Atrioventricular Block