[[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 increased [[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.]]
==First degree AV block==
[[Image:Rhythm_1stAVblock.png|thumb| 1st degree AV block. Although the PQ interval is prolonged all
p-waves are followed by QRS complexes: there is no dropout of beats]]In first degree AV block there is a prolongation of PQ duration (PQ time > 0.20 sec). Still every p-wave is being followed by a QRS complex.
First degree AV block is present in 16% of >90-year olds <cite>kelley</cite> and is mostly caused by a degeneration of the conduction system. First degree AV block is relatively harmless.