CHAPTER 22: Atrioventricular Block
465
Often, the AV dissociation produced by third-degree AV block is “isoarrhythmic,” with
similar atrial and ventricular rates and with P waves and QRS complexes occurring al-
most simultaneously. Insight into the presence or absence of AV block can be attained
only when a P wave appears at a time sufficiently remote from a QRS complex that the
ventricular refractory period would be expected to have been completed. In Figure 22.10,
there is AV dissociation during the first three cycles, in which the independent sinus and
ventricular rhythms are similar. Then, when variation in rate caused by respiration (sinus
arrhythmia) accelerates the sinus rate but does not affect the ventricular escape focus dur-
ing the fourth cycle, atrial capture occurs. This event proves AV conduction to be possible
and eliminates complete AV block as a contributor to the AV dissociation.
Block in both the RBB and LBB (level C in Fig. 22.1), rather than block at the AV node or
in the His bundle, is usually the cause of chronic complete AV block.
7–10
Idiopathic fibrosis,
called either Lev disease or Lenègre disease, is the most common cause of chronic complete
AV block.
7,11
Acute complete AV block within the AV node results from inferior myocardial
infarction, digitalis intoxication, and rheumatic fever.
12
Acute complete AV block within
the bundle branches results from extensive septal myocardial infarction.
13,14
Complete AV
block may also be congenital, as when it results from maternal anti-Ro antibodies affecting
the AV node.
15
In the presence of chronic LBB or RBB block, the individual is at some risk of suddenly
developing complete AV block. After this occurs, the ventricles either remain inactive
(ventricular asystole; see Fig. 22.8) and the patient experiences syncope or even sudden
death, or a more distal pacing site takes over (see Fig. 22.7B) and controls the ventricles
(ventricular escape). In this event, the atria continue to beat at their own rate and the
ventricles beat at a slower rhythm. This independence (AV dissociation due to AV block)
is readily recognized in the ECG recording from the lack of relationship between the in-
frequent QRS complexes and the more frequent P waves. Each maintains its own rhythm.
I
II
V1
*
F I G U R E 2 2 . 1 0 .
Leads I, II, and V1 rhythm strips from a patient receiving digitalis therapy for
congestive heart failure.
Arrows
indicate P-wave locations (the irregularity is due to sinus arrhythmia)
and an
asterisk
indicates a QRS complex produced by atrial capture.
