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Mobitz Type II

What Is It, Diagnosis, Treatment, and More

Author:Anna Hernández, MD

Editors:Alyssa Haag,Ian Mannarino, MD, MBA,Kelsey LaFayette, DNP, RN

Illustrator:Abbey Richard

Copyeditor:David G. Walker


What is Mobitz type II?

Mobitz type II is a type of 2nd degree AV block, which refers to an irregular cardiac rhythm (i.e., arrhythmia) caused by a block in the electrical conduction system of the heart. 

The heart is a muscular organ composed of four chambers: two upper chambers (the right and left atria) and two lower chambers (the right and left ventricles). The chambers of the heart are wired with an electrical conduction system much like the electrical system of a house. This system consists of the sinoatrial (SA) node, the atrioventricular (AV) node, the bundle of His, the right and left bundle branches, and the Purkinje fibers. These conduction pathways are a network of highly specialized cells that generate and conduct electrical impulses, allowing the heart to contract rhythmically and pump out blood with each heartbeat. 

The heart’s electrical activity can be monitored with an electrocardiogram (ECG), which uses several electrodes (i.e., leads) placed on the patient’s limbs and chest to register the heart’s electrical activity from different angles. Electrical signals are first fired at the SA node where they then travel through the walls of the atria, triggering atrial contraction and the consequent movement of blood into the ventricles. During atrial contraction, the ECG will show a small deflection from the basal line called the P wave. Next, the electrical signal is conducted through the AV node and then to the ventricles through the bundle of His and Purkinje fibers. This triggers ventricular contraction, which pushes blood out of the heart and into circulation, resulting in a larger set of waves on the ECG that form the QRS complex. The interval between the onset of the P wave and QRS complex is known as the PR interval and reflects the typical slowed conduction through the AV node. After the QRS complex, there’s another deflection called the T wave, which corresponds to the relaxation and repolarization of the ventricles. 

An atrioventricular block (AV block), also known as a heart block, occurs when an atrial impulse gets delayed or completely blocked at some point along the electrical conduction system of the heart. There are three degrees of AV block, according to the location and severity of the nodal block. A 1st degree AV block is not technically a block but rather a delay in the conduction of atrial impulses to the ventricles, which results in a prolonged PR interval. Meanwhile, a 2nd degree AV block, which can be further divided into Mobitz I and Mobitz II, occurs when some of the atrial impulses are fully conducted to the ventricles, whereas others are blocked along the way. Blocked atrial impulses can be seen on an ECG as a P wave that is not followed by a corresponding QRS complex, resulting in a slowed heart rhythm (i.e., bradycardia) and/or missed heart beats. Finally, a 3rd degree AV block occurs when none of the atrial impulses are conducted, leaving the atria and ventricles completely disconnected from one another. On the ECG, it can be detected by a total lack of correlation between the P waves and QRS complexes.

Enlarged illustration with ECG strip showing mobitz type II.

What is the difference between Mobitz type I and Mobitz type II?

Mobitz I and Mobitz II blocks are both subtypes of a 2nd degree AV block. They can be distinguished on an ECG by the pattern in which P waves are blocked: in Mobitz I, there is a progressive prolongation of the PR interval until a P wave fails to conduct, whereas in Mobitz II, PR intervals are always the same length but are followed by a pattern of one or more non-conducted P waves. 

Mobitz I and Mobitz II also differ in the severity of the nodal block. Mobitz I is a benign rhythm that generally reflects a block at the AV node and typically results in a good prognosis. On the other hand, Mobitz II reflects a block after the AV node, either at the bundle of His or one of its branches, and often results in a poorer prognosis as it has a higher risk of progressing to a 3rd degree AV block. 

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What causes Mobitz type II?

Mobitz type II block is rarely seen in individuals without underlying structural heart disease. The most common causes include a heart attack (i.e., myocardial infarction) and disorders affecting the heart muscle walls (e.g., cardiomyopathies). Less common causes include inflammation of the heart muscle walls (i.e., myocarditis), infection of the inner layer of the heart (i.e., endocarditis), infiltrative and autoimmune disorders (e.g., amyloidosis, hemochromatosis), inflammatory conditions (e.g., rheumatic fever, Lyme disease), as well as cardiac surgical procedures (e.g., mitral valve repair). When no specific cause can be identified, the nodal block is usually attributed to progressive damage to the cardiac conduction system due to fibrosis, or scarring, of the heart tissue.

It’s important to note that most people with Mobitz type II block have a pre-existing left-bundle branch block (LBBB) or bifascicular block. In those cases, the AV block is caused by intermittent failure of the remaining fascicle (i.e., bilateral bundle-branch block). Unlike Mobitz I, which is caused by progressive fatigue of the AV nodal cells, Mobitz II is an “all or nothing” phenomenon whereby cells of the bundle of His and Purkinje system suddenly and unexpectedly fail to conduct a ventricular impulse.

What are the signs and symptoms of Mobitz type II?

People with Mobitz type II block experience symptoms related to decreased cardiac output, including fatigue, dyspnea, and chest pain, though the severity can vary between individuals. If many impulses are blocked at a time, cardiac output can be severely reduced, resulting in hypotension, bradycardia, and hemodynamic instability. The onset of hemodynamic instability may be sudden and unexpected, leaving individuals at a higher risk of syncope or sudden cardiac arrest. 

How is Mobitz type II diagnosed?

Mobitz type II block is diagnosed based on the findings on an ECG. Mobitz type II is similar to type I in that they both result in blocked atrial impulses. The key difference between them is that, in Mobitz I, there’s a progressive prolongation of the PR interval before a block occurs, whereas in Mobitz II, P waves are blocked fairly randomly, and they are not preceded by prolongation of the PR interval. Instead, the PR interval in conducted beats remains constant, and P waves appear to march through at a constant rate. In many cases, a ratio for the overall number of beats conducted to not-conducted can be seen, like 2:1 or 3:1 Mobitz II AV block.

How is Mobitz type II treated?

Treatment of Mobitz type II begins by addressing any potentially reversible causes of nodal block, including ceasing medications that can slow nodal conduction (e.g., digoxin, beta-adrenergic blockers, calcium-channel blockers, amiodarone) and addressing any electrolyte imbalances, like hyperkalemia. If the AV block is a result of another heart condition, treatment may also include addressing the specific underlying cause when possible. Ultimately, though, Mobitz type II block warrants treatment with a permanent pacemaker, which is a device that continually monitors the individual’s cardiac rhythm and, if it detects a delay, sends an electrical signal into the ventricles, causing them to contract. 

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

Mobitz type II is a type of 2nd degree atrioventricular block, which refers to a cardiac arrhythmia that reflects a conduction block at the atrioventricular (AV) node. On an ECG, Mobitz type II can be identified by the presence of intermittent, non-conducted P waves without progressive prolongation of the PR interval. Individuals with Mobitz II tend to experience more frequent and severe symptoms than those with Mobitz type I, including fatigue, dyspnea, chest pain, or syncope. Since Mobitz II has a high risk of progressing into a 3rd degree AV block, treatment typically involves the insertion of a permanent pacemaker. Complications of Mobitz type II may involve decreased cardiac output leading to syncope, symptomatic bradycardia, and sudden cardiac arrest.

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Related links

Electrical conduction in the heart
Atrioventricular block
ECG basics

Resources for research and reference

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Park, D. S. & Fishman, G. I. (2011). The Cardiac Conduction System. Circulation, 123(8): 904–915. DOI: 10.1161/circulationaha.110.942284 

Thaler, M. S. (2019). The Only EKG Book You'll Ever Need (7th ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.