See quick 12 lead ECG interpretation
Characteristics of complete AV block
In third degree AV block – also called complete AV block – the conduction from the atria is completely blocked and the atria and ventricles contract independently. On the ECG there is no relationship between the p waves and the QRS complexes (AV dissociation). The ventricles are instead activated from a more distal focus, either from the AV node or the ventricles depending on the location of the lesion causing the block.
The rhythms originating are called escape rhythms. AV nodal (junctional) escape rhythms are characterized by narrow QRS complexes (<0.12 sec) and a rate from 40 to 60 beats/min. Ventricular escape rhythms are characterized by wide QRS complexes (>0.12 sec) and a rate from 20 to 40 beats/min. If the escape rhythm fails ventricular standstill or torsades de pointes can be seen.
Here is a detail from the 12 lead EKG:
The atria and the ventricles work independently. The ventricular rhythm is regular at a rate of approximately 37 beats/min with QRS complexes more than 0.12 sec indicating a ventricular escape focus.
Ventriculophasic sinus arrhythmia
Note that the atrial rhythm in the above strip – somehow unexpected – is irregular. It is (of course) possible that the p waves in the strip are incorrectly identified. However, there is a phenomenon called ventriculophasic sinus arrhythmia, which is characterized by shorter PP intervals of the p waves surrounding the QRS complexes compared to PP intervals not containing a QRS complex. This phenomenon probably occurs as the sinus node is influenced by changes in blood supply or due to mechanical influence of the sinus node by ventricular contraction. Ventriculophasic sinus arrhythmia is not uncommon in patients with third degree AV block so do not necessarily expect a regular atrial rhythm.
Third degree AV block or Wenkebach block?
Third degree AV block can be confused with Wenkebach block as the PR interval in both blocks vary. When first looking at the 12 lead ECG above it actually seems that the PR interval progressively lengthens. The key is to check if the ventricular rhythm is regular. See a discussion of this in the article second degree AV block Mobitz type I (Wenkebach).
Finally, if you are having trouble analyzing what seems to be blocks of the conduction system a longer rhythm strip than the ones above may be appropriate.
– Mathias
Third degree AV blogging
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