Table 1. Classification of second- and third-degree AV block, and atrial fibrillation with bradyarrhythmia, based on electrocardiographic characteristics.

	AV nodal block	Infra-nodal AV block	Undetermined level of AV block
Second-degree AV block	PR increment preceding a blocked P (Wenckebach) and narrow QRS	Constant PR interval preceding blocked P	PR increment (Wenckebach) preceding a blocked P and wide QRS
2:1 AV block	Conducted impulse has long PR and narrow QRS; PR varies inversely with RP	Conducted impulse has normal PR and wide QRS; PR is constant despite varying RP	Conducted impulse has long PR and wide QRS or short PR and narrow QRS
Third-degree AV block	Escape rhythm has narrow QRS and rate ≥ 40 beats/min	Escape rhythm has wide QRS and rate < 40 beats/min	Escape rhythm has wide QRS and rate ≥ 40 beats/min
Atrial fibrillation and bradyarrhythmia	f waves with irregular narrow QRS	f waves with regular wide QRS	f waves with irregular wide QRS

AV = atrioventricular