CHAPTER 22: Atrioventricular Block
469
When second-degree AV block appears during an acute inferior myocardial infarction,
the elevation of the ST segment in the ECG may obscure many of the P waves, as seen
in Figure 22.13. The visible P waves with prolonged PR intervals during the pauses allow
diagnosis of first-degree block, but only the typical RR-interval pattern allows a diagnosis
of second-degree AV nodal block.
The features described are typical of a classic Wenckebach period, but AV nodal block
rarely fits this pattern, because both the sinus rate and the AV conduction are under the
constant influence of the autonomic nervous system.
17,18
Among common variations from
the classic pattern are (a) the first incremental increase in PR interval may not be the
greatest, (b) the PR intervals may not lengthen progressively, (c) the last PR increment may
be the longest of all, and (d) a nonconducted atrial beat may not occur.
17
The only criterion
needed to identify the form of AV block that typically occurs in the AV node is a variation
in the PR intervals. The term
Mobitz type I
or simply
type I
AV block is used when variation
of the PR intervals is virtually diagnostic of block in the AV node.
II
F I G U R E 2 2 . 1 3 .
A continuous recording of lead II from a patient with acute inferior myocardial
infarction.
Arrows
indicate both the obvious and the assumed locations of sinus-originated P waves.
The earlier an impulse arrives during the prolonged partial refractory period of the
AV node, the longer the time required for conduction of the impulse through to the ventri-
cles. Therefore, when the AV node remains in its refractory period, the shorter the interval
between a conducted QRS complex and the next conducted P wave (
RP interval
), the lon-
ger is the following conduction time (PR interval). This inverse or reciprocal relationship
between RP and PR intervals is illustrated schematically in Figure 22.14.
F I G U R E 2 2 . 1 4 .
A ladder diagram illustrating the effect of progressively earlier entry of atrial
impulses
1
to
5
into the AV node (
AV
)
.
The
light stippled area
indicates the AV node’s relative refrac-
tory period, during which impulses
2
,
3
, and
4
encounter progressively slower conduction. The
dark
stippled area
indicates the node’s absolute refractory period, during which impulse
5
cannot be con-
ducted to the ventricles.
