Page 72 - Small Animal Internal Medicine, 6th Edition

44 PART I Cardiovascular System Disorders

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A B

C

FIG 2.32
Atrioventricular (AV) conduction abnormalities. (A) First-degree AV block in an older
Cocker Spaniel (lead II, 50 mm/sec). (B) Second-degree AV block (Wenckebach) with
two ventricular escape complexes in an older West Highland White Terrier. The
differentiation between escape and premature complexes is crucial (lead II, 25 mm/sec).
(C) Complete (third-degree) heart block in a 19-year-old Lhasa Apso. There is underlying
sinus rhythm, but no P waves are conducted; a slow ventricular escape rhythm has
resulted (lead II, 25 mm/sec).

node and/or intraventricular conduction system. Three Intraventricular Conduction Disturbances
major types of AV conduction disturbance are described Abnormal (aberrant) ventricular conduction occurs in
(Fig. 2.32). First-degree (1°) AV block, the mildest AV con- association with slowed or blocked impulse transmission
duction disturbance, occurs when the atrial to ventricular within a major bundle branch or ventricular region. The
conduction time is prolonged. All impulses are conducted right bundle branch, or the left anterior or posterior fascicles
with 1° AV block, but the PR interval is longer than normal. of the left bundle branch, can be affected singly or in com-
Second-degree (2°) AV block is characterized by intermittent bination. A block in all three major branches results in 3°
AV conduction; some P waves are not followed by a QRS (complete) heart block. Electrical activation of the myocar-
complex. When many P waves are not conducted, the patient dium served by the blocked pathway occurs relatively slowly,
is described as having high-grade 2° AV block. There are two from myocyte to myocyte. Therefore the QRS complexes
subtypes of 2° AV block. Mobitz type I (Wenckebach) is appear wide and abnormal, similar to a ventricular-origin
characterized by progressive PR interval prolongation until QRS complex (Fig. 2.33). Right bundle branch block (RBBB)
conduction fails, resulting in a nonconducted P wave; it fre- is sometimes identified in otherwise normal dogs and cats,
quently is associated with high vagal tone or disorders of the although it can occur from disease or distention of the RV.
AV node itself. Mobitz type II 2° AV block is characterized Left bundle branch block (LBBB) usually is related to clini-
by uniform PR intervals preceding the blocked impulse; it is cally relevant underlying LV disease. The left anterior fas-
more likely to be associated with disease lower in the AV cicular block (LAFB) pattern is common with concentric LV
conduction system (e.g., bundle of His or major bundle hypertrophy and in cats with hypertrophic cardiomyopathy.
branches). An alternative classification of 2° AV block based
on QRS configuration has been described. Patients with type Ventricular Preexcitation
A 2° block have a normal, narrow QRS configuration; those Early activation (preexcitation) of part of the ventricular
with type B 2° block have a wide or abnormal QRS configura- myocardium can occur when there is an accessory conduc-
tion, which suggests diffuse disease lower in the ventricular tion pathway that bypasses the normal, more slowly con-
conduction system. Mobitz type I AV block usually is type ducting AV nodal pathway. Several types of preexcitation
A, whereas Mobitz type II frequently is type B. Escape com- and accessory pathways have been described. Most cause a
plexes commonly appear during long pauses in ventricular shortened PR interval. Wolff-Parkinson-White (WPW) pre-
activation. Third-degree (3°) or complete AV block means excitation also is characterized by early widening and slur-
complete failure of AV conduction; no sinus (or supraven- ring of the QRS by a so-called delta wave (Fig. 2.34). This
tricular) impulses are conducted into the ventricles. Although pattern occurs because the accessory pathway (Kent bundle)
a regular sinus rhythm or sinus arrhythmia often is evident, lies outside the AV node (is extranodal) and allows early
the P waves are completely dissociated from the QRS com- depolarization (represented by the delta wave) of part of the
plexes, which result from a (usually) regular ventricular ventricle away from where normal ventricular activation
escape rhythm. begins. Other accessory pathways may connect the atria or