Atrioventricular Blocks (AV blocks) - ECG Diagnosis Summary

First Degree AV Block

 • The P wave is normal in morphology.

 • PR interval greater than 0.20 sec that remains constant.

 • The QRS is normal in duration or wide if there is an existing bundle branch block.

Second Degree AV Block Type I aka Mobitz I / Wenckebach 

 • Normal P waves.

 • The PRI progressively lengthens until a P wave is not followed by a QRS.

 • As the PRI lengthens, there is shortening of the RR interval.

 • The RR interval containing the dropped P wave is less than 2x of the shortest RR interval.

 • The PRI (may be normal or prolonged) of the first conducted P wave is shorter than the last conducted PRI.

 • The largest increment in the PRI is usually on the second conducted P wave.

 • There is "group-beating" on the ECG.

Second degree AV Block type II (aka Mobitz II)

 • There is constant PR interval (normal or prolonged) before a P wave is dropped.

 • The QRS is usually widened because the location of the block is often infranodal.

 • The QRS complex maybe narrow indicating a more proximal location of the block (AV node).

High-grade AV Block aka Advanced AV Block

 • During sinus rhythm, when 2 or more P waves are not conducted the term given is advanced or high-grade AV block.

 • The QRS may be wide or narrow.

 • This is a clinically concerning variant of Mobitz II and often implies advanced conduction disease and may progress to complete heart block.

Third Degree AV Block aka Complete Heart Block

 • In sinus rhythm with complete AV block, the PP and RR intervals are regular but the P wave has no relationship with the R wave.

 • The PR interval varies because there is really no P and QRS relationship.

 • The ventricular rate is usually 40-60 bpm and narrow when it is driven by a junctional pacemaker (AV node).

 • The QRS is wide and less the 40 bpm when an infra-Hisian pacemaker takes over.

Paroxysmal AV Block aka Ventricular Standstill

 • Abrupt and persistent AV block ( multiple P waves with no QRS) in the presence of otherwise normal AV conduction.

 • May be initiated by a conducted or blocked PAC or PVC, acceleration or slowing of sinus rhythm.

 • Once the block is initiated, the block will persist until terminated by an escape, usually ventricular, with a predictable relationship of the escape to the following P wave.



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