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Arrhythmias · Atrioventricular conduction disease

Second-degree AV block

Lesson 2 of 214 <2% of the exam 5 min 5 sources

What the exam asks

Diagnosis on the ECG and where the block is, because that decides the pacemaker. Expect a strip to classify (Mobitz I, Mobitz II, 2:1, high-grade), a vagal or premature-beat mimic, and a choice between watching and pacing.

The one-minute version

  • Second-degree AV block means some P waves are conducted and some are not; the pattern names the type [1].
  • Mobitz I (Wenckebach): the PR lengthens until a P wave is dropped; it is usually in the AV node and often benign [1][2].
  • Mobitz II: the PR is the same before and after a single dropped P at a steady sinus rate; it is always in the His-Purkinje system and needs a pacemaker, symptoms or not [3].
  • 2:1 block cannot be called type I or type II from that strip alone; localise it with the QRS width and other recordings [1][3].
Evidence (6)
  • "Second-degree or incomplete AV block occurs when there is intermittent AV conduction." [1] PMID 29083636
  • "Wenckebach is a phenomenon commonly associated with the AV nodes" [1] PMID 29083636
  • "When every other P wave is blocked (not conducted), it is impossible to differentiate between Wenckebach and Mobitz type II AV block" [1] PMID 29083636
  • "Mobitz type I (Wenckebach) is often a normal variant and seen in individuals with a high vagal tone without evidence of structural heart disease." [2] PMID 29493981
  • "Correctly identified Mobitz type II AVB is invariably at the level of the His-Purkinje system and is an indication for a pacemaker." [3] PMID 39267806
  • "A 2:1 AVB cannot be classified in terms of type I or type II AVB." [3] PMID 39267806

The four patterns

Pattern What the ECG shows Usual level
Mobitz I PR lengthens beat by beat, then one P is not conducted; the beats come in groups [2] AV node [1]
Mobitz II Constant PR before and after one non-conducted P, constant sinus rate, at least two conducted P waves in a row [3] His-Purkinje [3]
2:1 Every other P wave not conducted [1] Either [3]
High-grade (advanced) Two or more consecutive P waves not conducted, with some conduction left [1] Either [3]

The PR after the dropped beat is the key: in Mobitz II it is unchanged, which is the defining feature; in Wenckebach it shortens [3].

Evidence (7)
  • "This phenomenon leads to a grouped beating." [2] PMID 29493981
  • "Wenckebach is a phenomenon commonly associated with the AV nodes" [1] PMID 29083636
  • "Advanced (high-grade) AV block is defined as the presence of ≥2 consecutive, non-conducted P waves occurring at a constant physiologic rate" [1] PMID 29083636
  • "When every other P wave is blocked (not conducted), it is impossible to differentiate between Wenckebach and Mobitz type II AV block" [1] PMID 29083636
  • "there are at least two consecutive conducted P waves (i.e., 3:2 AV block) to reveal the behavior of the PR interval" [3] PMID 39267806
  • "An unchanged PR interval after the block is a sine qua non of Mobitz type II block." [3] PMID 39267806
  • "which can occur in either the AV node or the His-Purkinje system" [3] PMID 39267806

Mimics of Mobitz II

A vagal surge slows the sinus rate and the AV node together; if the P-P interval lengthens around the blocked beat, it is not Mobitz II, even when the PR intervals look constant [3].

A non-conducted premature atrial beat looks like a dropped P wave; check that the blocked P comes on time [3].

Wenckebach with tiny PR increments can look like Mobitz II; seeing obvious Wenckebach elsewhere on the same recording almost rules Mobitz II out [3].

Evidence (3)
  • "Mobitz type II AVB is not present if the P-P interval lengthens before or after the blocked beat (as in vagally induced block), even if the PR intervals remain constant before and after the block." [3] PMID 39267806
  • "Non-conducted atrial premature beats masquerading as AVB." [3] PMID 39267806
  • "when faced with the combination of Wenckebach AVB and Mobitz II AVB-like recordings, one can be almost certain that such a tracing is showing only Wenckebach AVB" [3] PMID 39267806

Finding the level

A wide QRS points below the node: Mobitz II comes with a bundle branch block in about 70 to 80% of cases [3].

Exercise helps with 2:1 block: an exercise ECG can show where the block is [1]. A nodal block improves as sympathetic tone rises, while His-Purkinje block can worsen as the sinus rate rises; atropine, by speeding the sinus node more than the AV node, can increase a His-Purkinje block [3].

When the level is still unclear in a symptomatic patient, an electrophysiology study measures it directly [2].

Evidence (5)
  • "Mobitz type II AVB is associated with bundle branch block in approximately 70-80% of cases." [3] PMID 39267806
  • "If the effect is greater on the sinus rate than the AV node, then atropine will increase the degree of block in the His-Purkinje system" [3] PMID 39267806
  • "Mobitz type II AVB, like any block in the His-Purkinje system, can be tachycardia-dependent" [3] PMID 39267806
  • "An exercise ECG may be considered in selective individuals to assess the exercise-induced AV block and localize the site of the block in patients with 2:1 AV block." [1] PMID 29083636
  • "Electrophysiologic testing is sometimes done to determine the level of the block and the need for a pacemaker." [2] PMID 29493981

Management

First stop what can be reversed: AV nodal blocking drugs, ischaemia, hyperkalaemia, Lyme disease [1][2].

Asymptomatic Mobitz I, or 2:1 block at the node, needs no pacemaker; pace if symptoms come from the block [1][4].

Mobitz II and high-grade block without a reversible cause get a permanent pacemaker, with or without symptoms [3][4].

In an unstable patient, atropine helps a nodal (type I) block but often fails in Mobitz II; pace if it fails [2].

Evidence (6)
  • "this rhythm can result from inferior myocardial ischemia, medication toxicity (AV nodal blocking agents), hyperkalemia, cardiomyopathy (Lyme disease), or following cardiac surgery" [2] PMID 29493981
  • "Unlike Mobitz type I (Wenckebach), patients that are bradycardic and hypotensive with a Mobitz type II rhythm often do not respond to atropine." [2] PMID 29493981
  • "If unresponsive to atropine, pacing (transcutaneous or transvenous) should be initiated for stabilization." [2] PMID 29493981
  • "it is crucial to discontinue any medications triggering the AV block and address any reversible factors contributing to its occurrence" [1] PMID 29083636
  • "Asymptomatic patients with first-degree AV block, second-degree type I AV block, or 2:1 AV block at the AV node level usually do not require a pacemaker." [1] PMID 29083636
  • "is universally accepted as being due to His-Purkinje disease and is an indication for a pacemaker, regardless of symptoms" [3] PMID 39267806

Board traps

  • 2:1 block is not Mobitz II: calling it so is a common error, and the answer depends on the level [3].
  • Sinus slowing with a dropped beat at night or during vomiting is vagal block, not Mobitz II, and needs no pacemaker [3].
  • Atropine for a wide-QRS second-degree block can make it worse, because the faster sinus rate meets a His-Purkinje block [3].
  • Mobitz II is not a symptom-driven decision: the pacemaker is indicated without symptoms [3][4].
Evidence (4)
  • "Unfortunately, a 2:1 AV block remains widely and erroneously called Mobitz type II AVB in the literature." [3] PMID 39267806
  • "A vagal surge (generally a benign condition) causes simultaneous sinus bradycardia and depression of AV nodal conduction." [3] PMID 39267806
  • "If the effect is greater on the sinus rate than the AV node, then atropine will increase the degree of block in the His-Purkinje system" [3] PMID 39267806
  • "is an indication for a pacemaker, regardless of symptoms" [3] PMID 39267806

Europe differs

They mostly agree. The ESC pacing guideline paces type II, infranodal 2:1 and high-grade block whatever the symptoms, and considers pacing for type I block that causes symptoms or is found below the node at an electrophysiology study [5].

Not yet checked against the source's own words: the guideline's full text could not be fetched by our build. The citation is right; the wording check is pending.

Think it through

Case. A 72-year-old man had near-syncope while gardening. ECG: sinus rhythm 78/min, right bundle branch block with left anterior fascicular block, and one non-conducted P wave; the PR intervals before and after it are both 180 ms and the P-P interval does not change. Potassium and drugs are normal.

Question. What is the rhythm and what follows?

Show the answer

Answer. Mobitz II: constant PR before and after a single blocked P at a steady sinus rate, with a bundle branch block, so the block is in the His-Purkinje system and a permanent pacemaker is indicated [3].

  • "Single non-conducted P wave associated with constancy of all the PR intervals before and that of the conducted after the blocked impulse." [3] PMID 39267806
  • "Mobitz type II AVB is associated with bundle branch block in approximately 70-80% of cases." [3] PMID 39267806
  • "Correctly identified Mobitz type II AVB is invariably at the level of the His-Purkinje system and is an indication for a pacemaker." [3] PMID 39267806

References

  1. 1. Ahmed I, Goyal A, Chhabra L. Atrioventricular Block. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2024. StatPearls PMID 29083636
  2. 2. Mangi MA, Jones WM, Mansour MK, et al. Second-Degree Atrioventricular Block. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. StatPearls PMID 29493981
  3. 3. Barold SS, Herweg B. Mobitz type II second-degree atrioventricular block: a commonly overdiagnosed and misinterpreted arrhythmia. Front Cardiovasc Med. 2024;11:1450705. Review PMID 39267806 doi:10.3389/fcvm.2024.1450705
  4. 4. Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Circulation. 2019;140(8):e382-e482. Guideline PMID 30586772 doi:10.1161/CIR.0000000000000628
  5. 5. Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2021;42(35):3427-3520. Guideline PMID 34455430 doi:10.1093/eurheartj/ehab364

After the lesson

Flashcards

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