Third degree AV block, also called complete AV block or AV dissociation occurs when the atria become totally separated electronically from the ventricles. Both end up depolarizing at their own intrinsic rate. The atria are usually initiated by the SA node while the ventricles are typically run by the junction since that has the fastest intrinsic rate out of all the sub-AV-nodal structures. Intrinsic depolarization rates are typically:
50-60 beats/min: AV node
30-40 beats/min: Conduction pathway
20-30 beats/min: Ventricular myocardium
On EKG P waves and QRS waves are found but are not associated. There is a fixed P-P distance as well as a fixed R-R interval but the P waves and the QRS complexes bear no relation to each other.
See examples of AV dissociation