Wenckebach vs Mobitz II
How to tell Wenckebach and Mobitz II apart when a board question makes them look alike.
Wenckebach (Mobitz I): progressive PR lengthening before a dropped beat, usually benign; Mobitz II: fixed PR with a sudden drop, needs pacing.
Side by side
Mobitz I second-degree block with progressive PR interval prolongation before a dropped QRS.
Second-degree block with constant PR intervals before sudden dropped QRS complexes and higher progression risk.
Wenckebach usually reflects AV nodal fatigue and often has a more benign course.
Mobitz II often reflects infranodal His-Purkinje disease and can progress to complete heart block.
Grouped beating and lengthening PR intervals point Wenckebach.
Fixed PR intervals, wide QRS, syncope, or post-MI conduction disease point Mobitz II.
The trap
Dropped QRS complexes alone do not equal Mobitz II. Lay a ruler on three PR intervals leading into the drop. Any progressive lengthening, even by 20 to 40 ms, is Wenckebach and usually does not need a pacemaker. The block location is the actual driver: AV-nodal block (Wenckebach) is often vagal or drug-induced (beta blockers, calcium channel blockers, digoxin) and reversible; infranodal block (Mobitz II) is structural His-Purkinje disease and needs urgent pacing.
Watch it explained
The repair move
Before deciding on a pacemaker, do three things in order: compare three consecutive PR intervals before the dropped QRS, check QRS width, and look for reversible causes. Lengthening PR with narrow QRS is Wenckebach: treat the cause, often no pacing. Fixed PR with wide QRS is Mobitz II: transcutaneous pacing immediately, transvenous bridge, and permanent pacemaker urgently.
Try a free question
CoreA rhythm strip shows progressive PR interval prolongation before a dropped QRS complex. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Wenckebach. Wenckebach is Mobitz I second-degree AV block: progressively lengthening PR intervals before a dropped beat.
Why the other choices are tempting
- B. Mobitz II has dropped beats with constant PR intervals.
- C. Third-degree block has AV dissociation.
- D. Atrial fibrillation lacks organized P waves.
- E. WPW has short PR and delta wave.
Flashcards
Tap a card to flip it.
What PR trend separates Wenckebach from Mobitz II?
Wenckebach gradually lengthens before the dropped beat; Mobitz II drops beats after fixed PR intervals.
What anatomy makes Mobitz II higher risk?
It usually reflects disease below the AV node in the His-Purkinje system.
Quick answers
What is the fastest way to tell Wenckebach from Mobitz II?
Wenckebach (Mobitz I): progressive PR lengthening before a dropped beat, usually benign; Mobitz II: fixed PR with a sudden drop, needs pacing.
What is the key clue for Wenckebach?
Mobitz I second-degree block with progressive PR interval prolongation before a dropped QRS.
What is the key clue for Mobitz II?
Second-degree block with constant PR intervals before sudden dropped QRS complexes and higher progression risk.
What is the common trap with Wenckebach vs Mobitz II?
Dropped QRS complexes alone do not equal Mobitz II. Lay a ruler on three PR intervals leading into the drop. Any progressive lengthening, even by 20 to 40 ms, is Wenckebach and usually does not need a pacemaker. The block location is the actual driver: AV-nodal block (Wenckebach) is often vagal or drug-induced (beta blockers, calcium channel blockers, digoxin) and reversible; infranodal block (Mobitz II) is structural His-Purkinje disease and needs urgent pacing.