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SECTION II: Abnormal Wave Morphology
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.
A
B
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
).
