Atrioventricular block is subclassified into three groups according to the degree of AV block: first degree, second degree and third degree AV block. This, in turn, relates to whether conduction is intact but slowed, whether AV conduction is partially intact or whether there is no AV conduction.
First degree is characterized by intact AV conduction but with a greater than normal delay (i.e., PR interval > 0.2 seconds).
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Second degree AV block is characterized by regular P waves with some conducting and some not conducting.
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Third degree (or complete) AV block is characterized by the complete absence of AV conduction.