Lower-grade AV block vs High-grade AV block
How to tell Lower-grade AV block and High-grade AV block apart when a board question makes them look alike.
First-degree delays, Wenckebach lengthens then drops, Mobitz II drops without lengthening, third-degree dissociates.
Side by side
First-degree block has prolonged PR without dropped beats; Mobitz I has progressive PR prolongation before a dropped beat.
Mobitz II has dropped beats with fixed PR intervals; third-degree block has AV dissociation.
First-degree and Mobitz I often reflect slowed AV nodal conduction.
Mobitz II and complete block often reflect His-Purkinje disease with higher risk of progression.
Lower-grade block: PR over 200 ms with 1:1 conduction equals first-degree (benign); grouped beating with progressive PR lengthening before a dropped beat equals Mobitz I (Wenckebach, AV-nodal, often vagal, beta blocker, calcium channel blocker, or digoxin).
High-grade block: fixed PR intervals with sudden dropped QRS, often wide QRS, equals Mobitz II (infranodal, needs urgent pacing); completely dissociated P waves and QRS complexes at different rates equals third-degree (complete) heart block, always needs pacing. A 2:1 AV block is indeterminate without longer monitoring because PR trend is hidden.
The trap
A 2:1 block can hide PR behavior, so do not overcall Mobitz type without enough conducted beats.
Watch it explained
The repair move
For AV block, walk the four-step ladder before naming the degree and choosing therapy: (1) PR over 200 ms with 1:1 conduction equals first-degree AV block (benign, no treatment); (2) grouped beating with progressive PR lengthening before a dropped QRS equals Mobitz I (Wenckebach), usually AV-nodal, often vagal or drug-induced, rarely needs pacing; (3) fixed PR with sudden dropped QRS often with wide QRS equals Mobitz II, infranodal His-Purkinje disease, needs urgent pacing; (4) P waves and QRS complexes completely independent equals third-degree (complete) heart block, always needs pacing.
Try a free question
CoreAn ECG shows a constant PR interval greater than 200 ms, with every P wave followed by a QRS complex. Which AV block is present?
Show answer and explanation
Answer: A. First-degree AV block. First-degree AV block is delayed AV conduction with prolonged PR interval and no dropped beats.
Why the other choices are tempting
- B. Mobitz I has progressive PR prolongation before a dropped QRS.
- C. Mobitz II has dropped QRS complexes without progressive PR prolongation.
- D. Third-degree AV block has AV dissociation.
- E. Atrial fibrillation lacks organized P waves.
Flashcards
Tap a card to flip it.
What is the fastest separator for Lower-grade AV block vs High-grade AV block?
First-degree delays, Wenckebach lengthens then drops, Mobitz II drops without lengthening, third-degree dissociates.
What clue should make you choose Lower-grade AV block?
First-degree block has prolonged PR without dropped beats; Mobitz I has progressive PR prolongation before a dropped beat.
Quick answers
What is the fastest way to tell Lower-grade AV block from High-grade AV block?
First-degree delays, Wenckebach lengthens then drops, Mobitz II drops without lengthening, third-degree dissociates.
What is the key clue for Lower-grade AV block?
First-degree block has prolonged PR without dropped beats; Mobitz I has progressive PR prolongation before a dropped beat.
What is the key clue for High-grade AV block?
Mobitz II has dropped beats with fixed PR intervals; third-degree block has AV dissociation.
What is the common trap with Lower-grade AV block vs High-grade AV block?
A 2:1 block can hide PR behavior, so do not overcall Mobitz type without enough conducted beats.