3* AV Block

3* AV block forms complete AV dissociation. This means the QRS complexes are conducted completely independent of the P waves. This produces a regular P-P interval and regular R-R interval. The QRS complexes are formed from either a junctional escape or a ventricular escape. The ventricular escape produces wide (>120ms) and slow (20-40bpm) QRS complexes. The junctional escape produces slightly wide (100-120ms) and somewhat slow (40-60bpm) QRS complexes. 

These escape rhythms are what produce perfusion as they cause the ventricles to contract; therefore, patients with slower escape rhythms are more likely to be symptomatic or have more severe symptoms. The PR interval is often completely variable unless the escape rate is a fraction of the atrial activity (ie the P waves occur around 80bpm and the QRS around 40bpm). 

This can be differentiated from 1* AV block as a 1* block produces synchrony (P for every QRS and vice versa). However, it can be quite difficult, sometimes, if a 3* AV block produces ventricular and atrial rates that are similar. However, this would be quite rare. If one cannot differentiate on an EKG, reviewing the monitor over a longer period of time will usually help differentiate these. 2* AV blocks will have variable R-R intervals (Mobitz I's R-R interval variability is usually more noticeable).  

Sometimes, patients have intermittent 3* AV block. This tends to occur more with ischemia (ie inferior OMI progressing from 2* to 3* AV block).  It is always important to closely monitor patients with a high-degree block for progression into a 3* block as those with higher degree blocks are more likely to progress to 3* AV block. 

Examples:

Further Reading:

https://litfl.com/av-block-3rd-degree-complete-heart-block/

http://hqmeded-ecg.blogspot.com/search/label/Third%20%283rd%29%20degree%20AV%20block

https://www.sciencedirect.com/science/article/abs/pii/S0733862705000763?via%3Dihub