458
SECTION III: Abnormal Rhythms
Second-Degree Atrioventricular Block
By definition,
second-degree AV
block
is present when one or more, but not all, atrial im-
pulses fail to reach the ventricles. Examples of atrial premature beats that are not conducted
because they occur early were presented in Chapter 15 (see Figs. 15.7 and 15.8). This situ-
ation is not considered AV block because it is normal. Figure 22.3 presents an example of
both first- and second-degree AV block in which on-time P waves either have delayed AV
conduction (first and second cycles of the series) or no AV conduction (third cycle).
I
II
*
*
*
III
F I G U R E 2 2 . 3 .
Leads I, II, and III rhythm strips from an elderly patient receiving digitalis ther-
apy for chronic heart failure.
Arrows
indicate first-degree AV block and
asterisks
indicate second-
degree AV block.
