Wagner_Marriot's Practical Electrocardiography, 12e

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F I G U R E 7 . 2 .

Two types of aberrant conduction. A. Late ventricular activation ( dashed line ).

B. Early ventricular activation ( dashed line ).

Figure 7.2 illustrates the two types of altered or “aberrant” conduction from the atria (PR interval) to the ventricles (QRS interval) that results from bundle-branch block (BBB) and ventricular preexcitation. Right or left BBB does not alter the PR interval, but prolongs the QRS complex by delaying activation of one of the ventricles (see Fig. 7.2A). Ventricular preexcitation, due to a connection of the ventricle to the atria via an accessory muscle bundle, shortens the PR interval and produces a “delta wave” in the initial part of the QRS complex (see Fig. 7.2B). The total time from the beginning of the P wave to the end of the QRS complex remains the same as in the normal condition, because conduction via the abnormal pathway does not interfere with conduction via the normal AV conduction system. Therefore, before the entire ventricular myocardium can be activated by progres- sion of the preexcitation wavefront, electrical impulses from the normal conduction system arrive to activate the remainder of the ventricular myocardium.

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SECTION II: Abnormal Wave Morphology