Mobitz Type 1 vs Mobitz Type 2: Second-Degree AV Block on ECG
In second-degree AV block some P waves reach the ventricles and some don't. In Mobitz I (Wenckebach) the PR interval lengthens beat by beat until a P wave fails to conduct; in Mobitz II the PR stays fixed and a P wave fails without warning. Mobitz I usually sits in the AV node and is often benign; Mobitz II sits below it and usually needs a pacemaker.
At a glance
- Mobitz I: the PR lengthens, then a P wave is not conducted, and the PR after the pause is the shortest. Usually in the AV node and often benign.
- Mobitz II: the PR is identical before and after the non-conducted P wave. Usually below the AV node, often with a wide QRS, and it can progress to complete heart block.
- 2:1 block can't be called Mobitz I or II from the strip alone. A long PR and narrow QRS point to the AV node; a normal PR and wide QRS point below it.
- Two or more non-conducted P waves in a row is high-grade (advanced) AV block: manage it like complete heart block.
- Most patients with Mobitz II or high-grade block need a permanent pacemaker once reversible causes are excluded.
Free calculator
AV block helper
March out the P waves on a long rhythm strip first, then answer each question. The answer names the block and where it most likely sits.
Worked out in your browser. Nothing you enter is sent or saved.
Criteria
Mobitz I (Wenckebach)
The PR lengthens over successive beats until a single P wave is not conducted. The PR after the pause is shorter than the one before it. The RR intervals often shorten as the PR lengthens, which groups the beats.
Mobitz II
A constant PR, normal or long, in the beats before and after a single non-conducted P wave, at a steady sinus rate. The pause equals two PP intervals.
2:1 AV block
Every second P wave is not conducted. With only one conducted beat in a row, there is no way to see whether the PR lengthens.
High-grade (advanced) AV block
Two or more consecutive P waves are not conducted at a regular sinus rate, while other P waves still conduct.
What it looks like
Mobitz I (Wenckebach)
Mobitz II
Mobitz I (Wenckebach)
Mobitz II
How to measure it
- Get a long rhythm strip in lead II or V1, where P waves are easiest to see.
- March out the P waves with calipers. They should come at a regular rate; look for P waves hidden in the T waves.
- Measure the PR of every conducted beat, above all the beats just before and just after each non-conducted P wave.
- PR lengthens and then resets after the pause: Mobitz I. PR identical before and after the pause: Mobitz II.
- Check the QRS width. A Mobitz pattern with a wide QRS makes block below the AV node more likely.
- Rule out mimics: a blocked premature atrial complex comes early and looks different, and a sinus pause has no P wave at all.
Causes
At the AV node, usually Mobitz I
- High vagal tone: sleep, trained athletes, vomiting
- AV nodal blocking drugs: beta-blockers, verapamil, diltiazem, digoxin
- inferior myocardial infarction
- myocarditis and Lyme carditis
- hyperkalemia
Below the AV node, usually Mobitz II
- Degenerative fibrosis of the conduction system
- anterior myocardial infarction
- after cardiac surgery or TAVR
- infiltrative disease such as sarcoidosis and amyloidosis
- class I antiarrhythmic drugs
What to do
- Mobitz I without symptoms: no treatment. Stop or reduce AV nodal blocking drugs and look for reversible causes.
- Symptomatic bradycardia: give atropine (1 mg IV, repeated every 3 to 5 minutes to 3 mg). It often fails in block below the AV node; then use transcutaneous pacing or a dopamine or epinephrine infusion while arranging transvenous pacing.
- Mobitz II, high-grade block, or 2:1 block with a wide QRS: continuous monitoring with pacing pads on, same-day cardiology review, and a permanent pacemaker in most patients once reversible causes are excluded.
- 2:1 block of uncertain level: exercise or atropine can help, because conduction improves in nodal block and often worsens below the node. An electrophysiology study settles it.
Pitfalls and mimics
- Calling every 2:1 block Mobitz II: it can be either.
- Missing P waves buried in T waves and calling the rhythm sinus bradycardia.
- Mistaking blocked premature atrial complexes for AV block: the non-conducted P wave comes early.
- Comparing PR intervals far from the pause in a long Wenckebach cycle, where the PR barely changes: compare the PR just before and just after the dropped beat.
- Vomiting, suctioning or pain can cause vagal AV block; sinus slowing at the same moment points to vagal tone rather than conduction disease.
Key studies
Kusumoto 2019, 2018 ACC/AHA/HRS bradycardia guideline
Recommends permanent pacing for Mobitz II, high-grade and third-degree AV block that isn't caused by a reversible or physiologic problem, whether or not there are symptoms. Mobitz I needs pacing mainly when it causes symptoms.
Barold 2001, Mayo Clin Proc
A review that defines Mobitz I and II by the PR around a single non-conducted P wave and explains how 2:1 block and long Wenckebach cycles get misclassified.
Panchal 2020, AHA adult life support guidelines
Atropine is the first drug for symptomatic bradycardia; transcutaneous pacing or a dopamine or epinephrine infusion follows if it fails.
EvidencePulse™by 2 Minute Medicine®
Go deeper into the evidence.
Ask how a finding applies to your patient, or what the studies behind a criterion found. No ads, no sponsored answers.
2MM+New ECG research, summarized for clinicians
Read our physician-written summaries of new ECG and arrhythmia studies and of the landmark trials behind these criteria. 2MM+ includes every article, EvidencePulse and HypeCheck.
In our archive
Get new ECG studies by email
The free 2MM Daily Brief: new research in cardiology and every other specialty, summarized by physicians.
Opens Mailchimp to confirm. Unsubscribe anytime.
More ECG pages
Questions clinicians ask
What is the difference between Mobitz type 1 and type 2?
In type 1 (Wenckebach) the PR lengthens beat by beat before a P wave fails to conduct, and the block is usually in the AV node. In type 2 the PR is fixed before and after the non-conducted P wave, and the block is usually below the AV node, where it can progress to complete heart block.
Is Mobitz type 2 an emergency?
It needs prompt attention because it can turn into complete heart block without warning. Monitor the patient with pacing pads available and involve cardiology the same day; most patients need a permanent pacemaker.
Can Mobitz type 1 be normal?
Yes. Wenckebach is common during sleep, in trained athletes and with high vagal tone. It needs treatment only when it causes symptoms, or when a wide QRS suggests disease below the AV node.
What is 2:1 heart block?
Every second P wave fails to conduct. Because there are never two conducted beats in a row, you can't see whether the PR lengthens, so it can be Mobitz I or Mobitz II. A longer strip, the QRS width and the PR of the conducted beats help place it.
Does atropine work in Mobitz II?
Often not. Atropine speeds the sinus node and AV node, which helps nodal block, but below the AV node the faster atrial rate can block even more beats. Pacing is the dependable treatment.
Sources
- 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. Circulation. 2019;140(8):e382-e482. doi.org/10.1161/CIR.0000000000000628
- Barold SS, Hayes DL. Second-degree atrioventricular block: a reappraisal. Mayo Clin Proc. 2001;76(1):44-57. doi.org/10.4065/76.1.44
- Panchal AR, Bartos JA, Cabanas JG, et al. Part 3: Adult basic and advanced life support: 2020 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation. 2020;142(16 suppl 2):S366-S468. doi.org/10.1161/CIR.0000000000000916
Written in our own words by 2 Minute Medicine's physician editors from the sources listed here. The diagrams are schematic drawings made for 2MM, not patient tracings. This page supports, and does not replace, a full ECG reading and clinical judgment.
For health care professionals. Values are for adults unless a page says otherwise.
Get the app