[[Image:AV_block_sequence.png|thumb|Sequence of AV block from incomplete to complete]]
Conduction disturbances can occur at the level of the sinoatrial (SA) node, the atrioventricular (AV) node and the bundle branch system.
In atrioventricular block the conduction between atria and ventricles is disturbed leading to an
incread [[Conduction#PQ interval|PQ interval]] or to drop out of QRS complexes: atrial activity that is not followed by ventricular activity. Three degrees of block can be distinguished.
[[Image:PR_interval_buildup.svg|300px|thumb|right|The PR duration depends on the conduction velocity in the atria, AV node, His bundle, bundle branches and Purkinje fibers.]]