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Third degree AV block


See brief interpretation of 12 lead EKG above

In complete heart block (or third degree atrioventricular, AV, block) the conduction from the atria is completely blocked and the atria and ventricles contract independently. On the EKG 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. Junctional escape rhythms are characterized by narrow QRS complexes (<0.12 sec, unless bundle branch block is present) 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:

detail coplete heart block Third degree AV block

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.

Note that the atrial rhythm in the above strip – somehow unexpected – is irregular. This can be explained by 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.

Ventriculophasic sinus arrhytmia 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. The phenomenon is not uncommon in patients with heart blocks.

Complete heart block can be confused with Wenckebach heart block as the PR interval in both blocks vary. When first looking at the 12 lead EKG above it actually seems that the PR interval progressively lengthens. The key is to check if the ventricular rhythm is regular. In complete heart block the ventricular rhythm is regular while Wenckebach block shows group beating.

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

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