Lesson 16 of 21 · Heart Blocks

AV Blocks (1st–3rd Degree)

AV blocks describe how reliably atrial impulses reach the ventricles. March out the P waves with calipers (or paper), then ask how each P relates to its QRS.

011st degree

Every P conducts, PR > 0.20 s.

022nd degree, Mobitz I (Wenckebach)

Usually AV nodal. Usually benign, responds to atropine.

PR lengthens beat by beat until one P doesn't conduct; grouped beating.

032nd degree, Mobitz II

Infranodal. Can progress to complete block without warning. Needs pacing.

Constant PR, then a P suddenly fails to conduct. Often with BBB.

043rd degree (complete)

P waves and QRS complexes march independently; atrial rate > ventricular rate.

Clinical pearl. 2:1 block can't be classified as Mobitz I or II from one strip. A wide QRS suggests infranodal (treat like Mobitz II).

Key takeaways

1st: long PR
Wenckebach: longer, longer, drop
Mobitz II: constant PR, sudden drop → pace
3rd: AV dissociation → pace