Wagner_Marriot's Practical Electrocardiography, 12e

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.

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SECTION III: Abnormal Rhythms