Atrioventricular (AV) blocks slow the conduction of impulses from the atria to the ventricles. AV blocks may or may not produce escape rhythms. Wenckebach period and Mobitz block are two types of AV blocks with discernible, predictable ECG patterns. 

Blocks within or after the bundle of His, fascicular blocks, bundle branch blocks, and bilateral bifascicular blocks are possible precursors to complete AV blocks. AV blocks may occur near or at the AV node. When the block is above the bundle of His, it is termed a supra-His block. An infra-His AV block occurs somewhere within the infra-Hisian conduction system, e.g., at the bundle of His, right bundle branch, left anterior fascicular branch, or posterior fascicular branch. 

The term conduction block refers to the prolongation of impulse conduction time and does not necessarily mean there is an absolute block of the impulses. Conduction blocks can be due to a reversible medical condition.

Three Degrees of AV Blocks

1. First-degree AV block – every sinus impulse conducted to the ventricles but with a prolonged PR interval
2. Second-degree AV block – nonconducted P waves indicate an intermittent block of AV impulses, but AV conduction does occur in some beats
3. Third-degree AV block (complete AV block) – all impulses from the AV node are blocked. Impulses to the atria and ventricles come via an escape rhythm; consequently, the atria and ventricles contract independently from each other. Ventricular asystole may occur.