CHAPTER 22: Atrioventricular Block
473
A stepwise method for determining the location of AV block is illustrated in Figure 22.18.
This algorithm does not consider the localization of AV block within the common bundle
because of the rarity of AV block in this location. (Such a location should be considered
only when a QRS complex of normal duration [step 1] is accompanied by a pattern char-
acteristic of type II block [step 4].) Note that both steps 2 and 4 may lead to situations in
which it is impossible to determine the location of a block from a particular ECG record-
ing. In this case, additional recordings should be obtained. If these are also nondiagnostic,
the patient should be managed as though the block were located in the bundle branches
because such a location has the most serious clinical consequences. This usually requires
insertion of a temporary pacemaker, which provides time for further studies to determine
the location of the AV block.
His-bundle electrograms
can be obtained via intracardiac re-
cordings. A prolonged atrial-to-His interval (from the onset of the atrial signal to the time
of the His-bundle signal), or the absence of a signal from the His bundle, indicates block in
an AV nodal location, whereas a prolonged His-to-ventricle interval (from the His-bundle
signal to the onset of the ventricular signal), or absence of a signal from the ventricles after
a His signal, indicates block in a bilateral bundle branch location (see Fig. 14.11).
Step 1
Step 2
Step 3
Step 4
QRS
duration
normal
Block in
AV node
QRS
duration
prolonged
No conducted
beats present
Conducted
beats present
Location not
indicated
Variation in
conduction time
(PR interval)
(Type I)
Block in
AV node
Constant PR intervals
only occuring with
constant RR
intervals
Location not
indicated
Block in
bundle branches
Constant PR intervals
with variable RP
intervals
(Type II)
No variation in
conduction time
(PR interval)
F I G U R E 2 2 . 1 8 .
The four-step algorithm for iden-
tifying the location of AV block from an ECG recording.
Step 1: Consider the duration of the QRS complex. Step 2:
Consider whether conducted beats are present. Step 3:
Consider whether there is variation in the conduc-
tion times. Step 4: Consider whether there are constant
PR intervals with changing RP intervals. Situations that
indicate an end point in the algorithm are indicated by
boxes
with accentuated borders
.
