464
SECTION III: Abnormal Rhythms
Third-degree AV block always produces AV dissociation with its independent atrial
and ventricular rhythms; however, AV dissociation may result from other sources than
third-degree AV block. Decreased sinus automaticity (see Chapter 21), increased junctional
and ventricular automaticity (see Chapter 16), and reentrant ventricular tachycardia (see
Chapter 19) can all produce AV dissociation by creating the condition in which anterograde
impulses fail to traverse the AV node. They encounter the refractoriness that follows AV
nodal activation by retrograde impulses. Therefore, AV dissociation caused solely by im-
paired function of the AV conduction system should actually be termed “AV dissociation
due to AV block” (see Fig. 22.7A, B) and AV dissociation caused solely by an accelerated
distal pacing site should be termed “AV dissociation due to refractoriness” (Fig. 22.9A).
Both causes can coexist, producing “AV dissociation due to a combination of AV block
and refractoriness,” when there are P waves that are obviously not conducted to the ven-
tricles but the ventricular rate is slightly above the upper limit of the bradycardic range of
60 beats per minute (see Fig. 22.9B). The term “interference” is often used to describe the
condition of refractoriness that either causes or contributes to the two types of AV dissocia-
tion described here.
II
*
*
*
*
*
*
*
*
*
*
*
*
B
A
VI
F I G U R E 2 2 . 9 .
Rhythm strips from two patients receiving digitalis therapy for congestive
heart failure.
A.
Lead V1.
B.
Lead II. Note in
A
that the initial independent ventricular beat is a
“fusion beat” produced partly by conduction from the atria and partly from the ventricular pacing
site.
Arrows
indicate the varying relationships between adjacent P waves and QRS complexes, and
asterisks
indicate the regular ventricular rates. Note that the P waves are unusually small in
A
and
unusually large in
B
.
