Cardiology · Arrhythmias
The facts most likely to be tested
Mobitz II is characterized by intermittent non-conducted P waves without progressive PR interval prolongation.
The site of the block in Mobitz II is almost always located in the His-Purkinje system below the AV node.
Mobitz II is considered a pathologic rhythm that carries a high risk of progression to complete heart block (third-degree AV block).
The QRS complex in Mobitz II is typically wide due to the underlying bundle branch block pathology.
Patients with Mobitz II are frequently symptomatic, presenting with syncope, presyncope, or dizziness due to intermittent bradycardia.
The definitive treatment for symptomatic or asymptomatic Mobitz II is the placement of a permanent pacemaker.
Atropine is generally ineffective or potentially contraindicated in Mobitz II because it may increase the atrial rate and worsen the degree of AV block.
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A 72-year-old male presents to the emergency department after a syncopal episode while walking his dog. He reports occasional lightheadedness over the past week. His physical exam reveals a regularly irregular pulse with a rate of 42 beats per minute. An ECG shows a constant PR interval preceding every conducted beat, but there are randomly dropped QRS complexes following P waves. The QRS duration is 130 ms.
What is the most appropriate next step in management?
Permanent pacemaker implantation
The ECG findings of a constant PR interval with intermittent non-conducted P waves and a wide QRS complex are diagnostic of Mobitz II, which requires a permanent pacemaker due to the high risk of progression to complete heart block.
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Etiology / Epidemiology
Caused by fibrotic disease of the conduction system or acute anterior MI. High risk of progression to complete heart block.
Clinical Manifestations
Often asymptomatic until progression; may present with syncope, Stokes-Adams attacks, or sudden cardiac death.
Diagnosis
ECG shows constant PR interval with intermittent dropped QRS complexes. Requires permanent pacemaker.
Treatment
Permanent pacemaker is the definitive treatment. Avoid AV nodal blocking agents.
Prognosis
High risk of asystole; requires urgent cardiology consultation and pacemaker implantation.
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Epidemiology & Etiology
Primarily affects older adults with underlying structural heart disease. Common causes include idiopathic fibrosis (Lenègre-Lev disease), ischemic heart disease, and acute anterior wall MI due to LAD occlusion.
Pertinent Anatomy
The block is located below the AV node, typically within the His-Purkinje system. Damage to the bundle branches explains the wide QRS often seen in these patients.
Pathophysiology
Characterized by an intermittent failure of conduction through the His-Purkinje system. Unlike Type 1, there is no PR interval prolongation prior to the dropped beat. This represents a fixed, pathological block that is inherently unstable.
Clinical Manifestations
Patients may be asymptomatic or present with syncope, lightheadedness, or Stokes-Adams attacks. Red flags include hypotension, altered mental status, or chest pain indicating hemodynamic instability.
Diagnosis
The ECG is diagnostic, demonstrating a constant PR interval followed by a non-conducted P wave. The rhythm is defined by a fixed ratio of P waves to QRS complexes (e.g., 2:1, 3:1). Electrophysiology study is rarely needed but confirms the site of block.
Treatment
Immediate management for symptomatic patients includes atropine or transcutaneous pacing. The definitive treatment is a permanent pacemaker. Avoid beta-blockers, calcium channel blockers, and digoxin as they worsen the block.
Prognosis
High risk of progression to third-degree AV block and asystole. Patients require permanent pacemaker implantation regardless of symptoms due to the high risk of sudden death.
Differential Diagnosis
Mobitz I: PR interval progressively lengthens before a dropped beat
Third-degree AV block: No relationship between P waves and QRS complexes
2:1 AV block: Cannot distinguish between Type 1 or 2 without longer rhythm strip
Sinus arrest: Absence of P waves rather than dropped QRS
Non-conducted PAC: Premature P wave interrupts the rhythm