Mobitz Type II Block

What is a Mobitz type II block?

Mobitz type II, also called Mobitz type 2, is a form of second-degree atrioventricular block (AV block) in which some atrial impulses fail to conduct to the ventricles. On ECG, the conducted beats have a constant PR interval, followed by a sudden nonconducted P wave and dropped QRS complex.

The Mobitz classification separates second-degree AV blocks into type I and type II. This distinction matters because Mobitz type II usually reflects disease below the atrioventricular node, within the His-Purkinje system. Compared with Mobitz type I, Mobitz type II is more unstable and has a higher risk of progressing to complete heart block.

In clinical settings, this rhythm may also be called Mobitz type 2 or Mobitz 2. The key feature is that the PR interval stays constant before a suddenly dropped QRS complex. The contrast between Mobitz type I and II helps clinicians recognize which rhythm is more likely to require urgent treatment.

This conduction defect is more common in older adults and in people with structural heart disease. Mobitz type II is less common than Mobitz type I but is more clinically dangerous because it can progress suddenly to complete heart block.

What causes Mobitz type II blocks?

Mobitz type II usually occurs when the His-Purkinje conduction system is damaged. Common causes include age-related fibrosis or sclerosis of the conduction system, ischemia, anterior myocardial infarction, cardiomyopathy, and recent cardiac procedures involving the septum or valves.

Other possible causes include medication effects, Lyme carditis, myocarditis, infiltrative diseases such as sarcoidosis or amyloidosis, electrolyte abnormalities, and certain neuromuscular or congenital conditions. Because some causes are reversible, clinicians look for medication toxicity, acute ischemia, infection, and metabolic abnormalities before deciding on long-term treatment.

What are the signs and symptoms of Mobitz type II blocks?

People with Mobitz type II may have lightheadedness, dizziness, fatigue, exercise intolerance, shortness of breath, palpitations, presyncope, or syncope. These symptoms occur when intermittent dropped beats reduce cardiac output and cerebral perfusion.

Some people are initially asymptomatic, especially if the block is intermittent and the ventricular rate remains adequate. However, Mobitz type II is concerning because it can progress unpredictably to complete heart block, severe bradycardia, hemodynamic collapse, or asystole.

How is a Mobitz type II block diagnosed?

A 12-lead ECG or continuous cardiac monitoring shows the rhythm by demonstrating constant PR intervals in conducted beats, followed by intermittent nonconducted P waves and dropped QRS complexes. The QRS complex is often wide, which suggests disease in the His-Purkinje system.

A 2-to-1 AV block means every other atrial impulse is not conducted to the ventricles. This pattern can be concerning, but it cannot always be classified as Mobitz type I or Mobitz type II from a surface ECG alone because there are no consecutive PR intervals to compare. A wide QRS complex or electrophysiology testing may suggest infranodal disease.

Continuous telemetry, Holter monitoring, or an event monitor can help capture intermittent episodes when symptoms do not occur during a standard ECG. If the rhythm is difficult to classify, especially in 2:1 AV block, an electrophysiology study can help identify whether the block is located in the AV node, His bundle, or below the His bundle.

The workup also looks for underlying or reversible causes. This may include medication review, electrolyte testing, thyroid testing, Lyme testing when appropriate, cardiac biomarkers if acute coronary syndrome is suspected, and echocardiography to evaluate structural heart disease.

How is a Mobitz type II block treated?

Acute management of Mobitz type II focuses on cardiac monitoring and pacing when the patient is symptomatic, unstable, or at high risk of progression. Temporary transcutaneous or transvenous pacing may be needed while the cause is evaluated or while permanent treatment is arranged.

Atropine is often ineffective because Mobitz type II usually occurs below the AV node. Catecholamines such as epinephrine or isoproterenol may be used as temporary measures in selected cases while pacing is arranged.

Clinicians review medications and other reversible causes, such as acute ischemia, Lyme carditis, and electrolyte abnormalities. If the block is not due to a reversible or physiologic cause, permanent pacemaker implantation is recommended regardless of symptoms. Permanent pacing is the definitive treatment for acquired type II Mobitz heart block.

What are the most important facts to know about a Mobitz type II block?

• Mobitz type II is a dangerous form of second-degree AV block characterized by constant PR intervals with sudden nonconducted P waves.
• His-Purkinje system disease, often from fibrosis or ischemia, makes this rhythm prone to sudden progression to complete heart block.
• Symptoms may include dizziness, fatigue, shortness of breath, presyncope, or syncope, but some people are initially asymptomatic.
• A 2:1 AV block cannot always be classified as Mobitz type I or Mobitz type II from a surface ECG alone.
• ECG monitoring remains the diagnostic cornerstone, and the evaluation should also look for reversible causes.
• Treatment centers on monitoring, temporary pacing when needed, correction of reversible causes, and permanent pacemaker implantation when the block is not reversible.