What is Mobitz Type II (Second-Degree Atrioventricular Block)?

From the Guidelines

Mobitz Type II is a serious cardiac conduction abnormality that requires prompt medical attention and often necessitates permanent pacemaker implantation, regardless of symptoms, as recommended by the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1.

Key Points

• Mobitz Type II is characterized by intermittent failure of electrical impulses to reach the ventricles without progressive PR interval prolongation.
• The ECG shows normal PR intervals in conducted beats, but occasionally P waves are not followed by QRS complexes (dropped beats), creating a pattern where beats are suddenly blocked without warning.
• This arrhythmia typically occurs due to a conduction block below the AV node, usually in the His-Purkinje system, and is often associated with structural heart disease or myocardial infarction.
• Patients may experience dizziness, syncope, fatigue, or shortness of breath, though some remain asymptomatic.
• Treatment typically involves temporary transcutaneous or transvenous pacing if the patient is hemodynamically unstable, followed by permanent pacemaker placement.
• Unlike Mobitz Type I (Wenckebach), which can be benign and asymptomatic, Mobitz Type II carries a higher risk of progression to complete heart block and sudden cardiac death, making intervention necessary even in asymptomatic patients.

Recommendations

• Permanent pacing is recommended for patients with acquired second-degree Mobitz Type II atrioventricular block, high-grade atrioventricular block, or third-degree atrioventricular block not caused by reversible or physiologic causes, regardless of symptoms 1.
• In patients with a left ventricular ejection fraction between 36% to 50% and atrioventricular block, who have an indication for permanent pacing and are expected to require ventricular pacing >40% of the time, techniques that provide more physiologic ventricular activation (e.g., cardiac resynchronization therapy, His bundle pacing) are preferred to right ventricular pacing to prevent heart failure 1.

Considerations

• The presence of Mobitz Type II second-degree AV block is an indication for a permanent pacemaker, even in asymptomatic patients, due to the high risk of progression to complete heart block and sudden cardiac death 1.
• The site of the conduction block, whether it is intra- or infra-Hisian, can affect the prognosis and treatment of Mobitz Type II second-degree AV block 1.

From the Research

Definition and Characteristics of Mobitz Type II

• Mobitz type II second-degree atrioventricular block (AVB) is an electrocardiographic pattern characterized by an all-or-none conduction without visible changes in the AV conduction time or PR intervals before and after a single non-conducted P wave 2.
• An unchanged PR interval after the block is a key feature of Mobitz type II block 2.
• A 2:1 AVB cannot be classified in terms of type I or type II AVB 2.

Diagnosis and Differentiation

• The diagnosis of Mobitz type II block AVB requires a stable sinus rate, as a vagal surge can cause simultaneous sinus slowing and AV nodal block, which can resemble Mobitz type II AVB 2.
• Atypical forms of Wenckebach AVB may be misinterpreted as Mobitz type II AVB when a series of PR intervals are constant before the block 2.
• Concealed His bundle or ventricular extrasystoles may mimic both Wenckebach and/or type II AVB (pseudo-AVB) 2.
• Second-degree atrioventricular block must be distinguished from other "causes of pauses," such as nonconducted premature atrial contractions and atrial tachycardia with block 3.

Mechanism and Prognosis

• Correctly identified Mobitz type II AVB is invariably at the level of the His-Purkinje system and is an indication for a pacemaker 2.
• Mobitz type II block is more likely to progress to complete heart block and Stokes-Adams arrest, as the site of the block is almost always below the AV node 3.
• In some cases, Mobitz type II second-degree AV block with narrow QRS and junctional extrasystoles may be caused by frequent single His bundle depolarizations with multiple coupling intervals, resulting in retrograde concealed conduction and prolonged local refractoriness in the AV node 4.