Atrioventricular Block

This section will provide an overview of atrioventricular blocks, in which problems either at or below the level of the AV node prevent signals from travelling from the atria to the ventricles, or vice-versa.

To be covered:

• The difference between intranodal and infranodal blocks, including their respective clinical characteristics and associations.

• We will cover the various manifestations of anterograde AV block, such as:

  • First degree AV block, which is typically a mild form of intranodal disease, but can sometimes represent significant infranodal disease in the context of balanced bundle branch delay. We will cover some uncommon mimickers, such as concealed conduction or dual AV nodal physiology.

  • Second degree Mobitz I AV block, also known as AV Wenckebach, a form of incomplete AV block characterized by progressive fatiguing of AV nodal conduction and PR prolongation. 

    • We will go over typical Wenckebach periodicity and RP-PR reciprocity. We will also cover several variants of atypical Wenckebach periodicity, such as that with sinus arrhythmia, having higher grade blocks due to concealed conduction, obliteration of the normal periodicity due to reciprocating beats in dual AV nodal physiology, and so on.

    • We will highlight atrial flutter with variable block as sometimes being due to Wenckebach block overlying an underlying flutter rhythm.

    • While the condition is normally intranodal, we will also cover uncommon examples of AV Wenckebach associated with (infranodal) bilateral bundle branch disease.

  • Second degree Mobitz II AV block, an incomplete AV block usually localizing below the AV junction, leading to sporadically dropped beats without evidence of PR prolongation.

  • 2:1 AV block, which can be difficult to subclassify as intranodal or infranodal in origin.

  • High grade AV block, in which several consecutive atrial beats are blocked from conducting into the ventricles. 

  • Third degree AV block, which is a complete disconnect between the atria and ventricles, with a block that can either be intranodal or infranodal. We will go over specific manifestations of complete heart block in cases of underlying atrial flutter or fibrillation.

  • All the sections will involve a discussion on relevant differential diagnoses, such as concealed conduction, dual AV nodal physiology, physiologic block of atrial arrhythmias, and so on.

• We will also cover a few unique manifestations of atrioventricular block, such as:

  • The presence of anterograde AV block with preserved retrograde conduction.

  • The presence of multi-level blocks, wherein a signal incurs multiple conduction delays on its path from the atria to the ventricles, causing a layered and complex rhythm.

• Similarly to atrioventricular blocks, we will cover ventriculoatrial blocks. These are conditions where retrograde conduction across the AV node is affected, usually manifesting as changing RP intervals or  missing retrograde p waves.