Normally, there is a 1:1 relationship between the P waves and the QRS complexes. The PR interval, which includes inter and intra-atrial, AV nodal and His-Purkinje conduction, has a normal duration of 0.12 to 2.0 seconds. Conduction slowing within the AV node and His-Purkinje system, which together comprise the AV junction, is manifest by prolongation of the PR interval beyond 0.20 seconds and/or the failure of some or all of the atrial impulses to conduct to the ventricles. Prolongation of the PR interval beyond 0.20 seconds is referred to as 1st degree AV block, although some readers refer to it merely as “prolonged PR interval.” Failure of some, but not all of the atrial impulses to propagate to the ventricles is referred to as 2nd degree AV block, and failure of most or all of the impulses to propagate to the ventricles is variously referred to as high grade, 3rd degree, or complete AV block.