The characteristic features of second degree AV block untypeable are partially intact AV conduction but no two consecutive P waves are conducted and the PR is always constant. Usually the conduction ratio is 2:1, and hence a regular rhythm, as compared with Type I and Type II second degree AV block which have grouped beating. The blocked AV conduction is either AV nodal or infranodal in origin and cannot be typed because the AV conduction is constant with a constant PR interval. The following animation illustrates what we have just described. Note in the animation the periodic complete block in the AV node.