CHAPTER 22: Atrioventricular Block
457
SEVERITY OF ATRIOVENTRICULAR BLOCK
First-Degree Atrioventricular Block
The “normal” PR interval has a duration of 0.12 to 0.20 second.
First-degree
AV
block
is
generally defined as a prolongation of AV conduction time (PR interval) to 0.20 second.
In analyses of records from normal young persons, the incidence of first-degree block by
this definition ranged from 0.5%
1
to 2%.
2
In healthy middle-aged men, a prolonged PR in-
terval in the presence of a normal QRS complex was found not to affect prognosis and to
be unrelated to ischemic heart disease.
3
Figure 22.2 illustrates two examples of first-degree
AV block. The first of these (see Fig. 22.2A) is minor, with a PR interval of 0.24 second, and
the second (see Fig. 22.2B) shows extreme PR lengthening. Note that in Figure 22.2B, the
P wave is superimposed on the T wave of the preceding cycle.
Rhythm V1
Rhythm II
Rhythm V5
Rhythm V1
Rhythm II
Rhythm V5
A
B
F I G U R E 2 2 . 2 .
Simultaneous three-lead (V1, II, and V5) rhythm strips showing examples of
first-degree AV block from healthy patient
(A)
and a
(B)
woman receiving no medications.
Arrows
indicate PR intervals of 0.25
(A)
and 0.35 second
(B)
.
