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SECTION III: Abnormal Rhythms
Third-Degree Atrioventricular Block
I
II
III
*
*
*
*
*
A
I
II
III
*
*
*
*
B
F I G U R E 2 2 . 7 .
Leads I, II, and III rhythm strips from two patients presenting with complaints
of dyspnea on exertion.
Arrows
indicate the varying PR-interval relationships and
asterisks
indicate
the regular junctional
(A)
and ventricular
(B)
escape rates.
When no atrial impulses are conducted to the ventricles, the cardiac rhythm is termed
“third-degree AV block,” and the clinical condition is determined by the escape capability of
the more distal Purkinje cells. The junctional or ventricular escape rhythm in the presence
of third-degree AV block is almost always precisely regular because these sites are not as
influenced by the sympathetic/parasympathetic balance as is the sinus node. Figure 22.7A
shows junctional escape in a case of third-degree AV block, but Figure 22.7B shows ven-
tricular escape, which occurs at a slower rate.
