1. Definition and Core Concept

Second-degree atrioventricular (AV) block occurs when some atrial impulses fail to conduct to the ventricles. In Mobitz type II, one or more sinus P waves are not followed by a QRS complex, while the PR interval of conducted beats remains essentially constant before and after the blocked atrial impulse. There is no progressive PR lengthening before the dropped beat.

Mobitz II is commonly associated with disease of the His–Purkinje conduction system and may progress unpredictably to advanced or complete AV block. The term “Hay block” is an eponym used in the supplied teaching reference.

2. Electrophysiology and Mechanism

Normal atrioventricular conduction proceeds from the atria through the AV node, His bundle, bundle branches, and Purkinje network. Mobitz II is classically caused by intermittent conduction failure within the His bundle or, more often, the distal His–Purkinje system.

• Infranodal disease: Structural disease of the His–Purkinje system is the classic substrate. The block may occur below the AV node, including in the bundle branches.
• All-or-none conduction failure: A supraventricular impulse may conduct with a stable PR interval, while a subsequent impulse abruptly fails to reach the ventricles.
• QRS morphology: A broad QRS or pre-existing bundle-branch block supports more extensive distal conduction disease, but QRS width alone does not establish the site. Intra-His block can produce a narrow QRS.

Common associated causes

• Ischaemic injury, particularly septal or anterior myocardial infarction.
• Progressive conduction-system fibrosis, including Lenègre or Lev disease.
• Post-cardiac surgery or procedures involving the septal conduction system.
• Myocarditis and selected inflammatory or infectious disorders, including Lyme carditis in an appropriate epidemiological setting.
• Infiltrative disease such as cardiac sarcoidosis or amyloidosis.
• Metabolic disturbance, including hyperkalaemia.
• AV nodal or conduction-slowing drugs may contribute to bradycardia or AV block; review beta-blockers, non-dihydropyridine calcium-channel blockers, digoxin, and antiarrhythmic agents. Drug exposure should not automatically be assumed to explain a suspected infranodal Mobitz II pattern.

3. Characteristic ECG Features

How to recognize the pattern on the tracing

When the atrial cycle is regular and a single P wave is blocked, the pause between ventricular complexes may approximate twice the baseline ventricular cycle length. With two consecutive blocked P waves, it may approximate three times that cycle length. This relationship is supportive only when the underlying atrial rhythm is regular; sinus arrhythmia, ectopy, or changing conduction can invalidate the simple multiple relationship.

12-lead example supplied with the reference

4. Systematic ECG Interpretation: Step by Step

1. Check calibration and tracing quality. Confirm standard paper speed and gain when available; inspect for artifact, baseline wander, and lead misplacement.
2. Identify every P wave. Use multiple leads, especially lead II and V1 where helpful. Do not assume that a small deflection during a pause is a T wave or artifact without checking its timing and morphology.
3. Assess the atrial rhythm. Measure P–P intervals. Determine whether P waves continue regularly through the ventricular pause.
4. Map each P wave to the QRS. Mark conducted P waves and identify P waves that are not followed by a QRS. Confirm that the apparent dropped beat is not due to a premature atrial beat hidden in the preceding T wave.
5. Compare conducted PR intervals. Measure several consecutive conducted beats before and after the blocked P wave. A stable PR without progressive lengthening supports Mobitz II.
6. Measure QRS duration and morphology. Note bundle-branch block or bifascicular disease. A wide QRS increases concern for distal conduction disease but is not mandatory for Mobitz II.
7. Describe the conduction ratio. Record 3:2, 4:3, 2:1, or another pattern when discernible. Do not classify a 2:1 pattern as Mobitz II from the surface pattern alone.
8. Integrate the clinical context. Check symptoms, haemodynamic status, medication exposure, electrolytes, ischaemia, structural heart disease, and prior ECGs.

5. Diagnostic Criteria and Important Limitations

Features required for a confident surface-ECG diagnosis

• At least one non-conducted P wave that is not explained by a premature atrial impulse or artifact.
• Conducted beats show a stable PR interval before and after the blocked P wave, without the progressive PR prolongation typical of Mobitz I.
• Atrial rhythm is sufficiently stable to establish that the non-conducted P wave is not simply part of sinus slowing or a vagally mediated sequence.
• There is no evidence that the apparent block is caused by concealed ectopy or another pseudo-AV-block mechanism.

Do not overcall Mobitz II in a 2:1 pattern

With 2:1 AV conduction, every other P wave is blocked and there may be only one conducted PR interval between blocked beats. This does not provide enough consecutive conducted beats to demonstrate the defining PR behaviour of Mobitz I versus Mobitz II. Report “second-degree AV block with 2:1 conduction” and assess the likely level of block using QRS morphology, response to exercise or atropine in an appropriate monitored setting, and electrophysiology study when clinically indicated.

6. Differential Diagnosis

7. Clinical Presentation and Significance

Presentation ranges from an incidental ECG finding to presyncope, syncope, fatigue, dyspnoea, exercise intolerance, hypotension, or acute haemodynamic compromise. The ventricular response may become critically slow if conduction worsens or complete AV block develops.

• Potential consequences: severe bradycardia, reduced cardiac output, syncope, progression to complete AV block, asystole, and cardiac arrest.
• Clinical context: evaluate for acute myocardial infarction, structural or infiltrative heart disease, recent cardiac surgery, infection/inflammation, medication effects, and electrolyte abnormalities.
• QRS context: associated bundle-branch or bifascicular block increases concern for extensive conduction-system disease.

The supplied LITFL document describes Mobitz II as more likely than Mobitz I to cause haemodynamic compromise and progression to complete heart block. It also quotes an annual asystole risk of approximately 35%; this numeric estimate is reproduced as a claim from that educational source and should not be treated as a contemporary individualized risk prediction.

8. Clinical Management Principles

Management depends on haemodynamic status, the presence of reversible or physiological causes, and confirmation that the rhythm is truly Mobitz II. The following is an educational overview, not a patient-specific protocol.

Immediate assessment

• Assess airway, breathing, circulation, perfusion, blood pressure, mental status, symptoms, and evidence of shock or ischaemia.
• Continuous cardiac monitoring; obtain IV access and a 12-lead ECG. Apply external pacing/defibrillation pads when clinically appropriate.
• Escalate promptly to cardiology/electrophysiology and a setting capable of urgent pacing, particularly with syncope, hypotension, ischaemia, heart failure, very slow ventricular response, or wide-QRS conduction disease.
• Investigate potentially reversible contributors, guided by presentation: medication review, potassium and other electrolytes, renal function, ischaemia evaluation, and targeted testing for inflammatory, infectious, or infiltrative disease.

Pacing

For acquired Mobitz II AV block that is not attributable to a reversible or physiological cause, the 2018 ACC/AHA/HRS guideline recommends permanent pacing regardless of symptoms (Class I recommendation). The 2021 ESC pacing guideline likewise recommends pacing for second-degree type II AV block in sinus rhythm irrespective of symptoms, when not due to a reversible cause.

In an unstable patient, urgent temporary pacing may be required as a bridge while reversible causes are addressed and definitive management is arranged. Atropine may be ineffective in infranodal disease; do not allow a trial of medication to delay pacing or specialist escalation when clinically indicated. Choice of temporary and permanent pacing strategy must be individualized by the treating team.

9. Common Pitfalls

• Calling any dropped QRS Mobitz II without identifying the preceding P wave and measuring PR intervals.
• Classifying 2:1 AV block as Mobitz II despite insufficient conducted beats to determine the subtype.
• Assuming a wide QRS is mandatory. Narrow QRS does not exclude intra-His Mobitz II.
• Assuming a wide QRS proves infranodal block; it is supportive, not definitive.
• Missing a premature P wave hidden in the T wave, which may produce blocked atrial ectopy rather than AV block.
• Ignoring sinus slowing or changing P–P intervals, which can suggest vagally mediated AV nodal block rather than intrinsic Mobitz II.
• Relying on automated ECG interpretation without manual confirmation of P waves, PR intervals, and conduction sequence.
• Using an old or single ECG without comparing prior tracings or correlating with the clinical state.

10. ECG Interpretation Practice

Illustrative question

A rhythm strip shows regular P waves. Most P waves are followed by QRS complexes with a stable PR interval. One P wave is not followed by a QRS; the PR intervals of conducted beats before and after the pause are unchanged. What is the most appropriate interpretation?

Explanation: A non-conducted P wave with stable PR intervals in conducted beats and no progressive PR lengthening is the characteristic Mobitz II pattern. In real ECG interpretation, also exclude blocked premature atrial beats, concealed ectopy, and vagally mediated slowing. This is a constructed teaching scenario, not a real patient case.

11. Clinical Pearls and Quick Reference

• Find and count P waves before interpreting the ventricular rhythm.
• Mobitz II is defined by intermittent non-conduction without progressive PR prolongation in conducted beats.
• QRS width is supportive for localization, not a defining criterion.
• A 2:1 pattern cannot be confidently subclassified as Mobitz I or II from the ratio alone.
• Exclude blocked atrial ectopy and vagally mediated AV block before diagnosing intrinsic Mobitz II.
• Confirmed acquired Mobitz II without a reversible cause warrants permanent pacing even without symptoms.

12. References

Image licensing and attribution

The supplied LITFL PDF states that the material is licensed under Creative Commons Attribution–NonCommercial–ShareAlike 4.0 International (CC BY-NC-SA 4.0). The three ECG figures above are attributed to that source and have been cropped only to remove surrounding page whitespace; the tracing itself has not been redrawn or altered. This licence restricts commercial use and requires ShareAlike compliance. Confirm that the intended Appcardio deployment and distribution comply with the licence or obtain permission from the rights holder before publishing these images on a commercial/ad-supported service.