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Second degree AV block is a disease of the electrical conduction system of the heart. It refers to a conduction block between the atria and ventricles.

The presence of second degree AV block is diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction.

Types

There are two distinct types of second degree AV block, called Type 1 and Type 2. The distinction is made between them because type 1 second degree heart block is considered a more benign entity than type 2 second degree heart block.

Type 1 (Mobitz I/Wenckebach)

Type 1 Second degree AV block, also known as Mobitz I or Wenckebach periodicity, is almost always a disease of the AV node.

Mobitz I heart block is characterized by progressive prolongation of the PR interval on the electrocardiogram (EKG) on consecutive beats followed by a blocked P wave (i.e. a 'dropped' QRS complex). After the dropped QRS complex, the PR interval resets and the cycle repeats.

One of the baseline assumptions when determining if an individual has Mobitz I heart block is that the atrial rhythm has to be regular. If the atrial rhythm is not regular, there could be alternative explanations as to why certain P waves do not conduct to the ventricles.

This is almost always a benign condition for which no specific treatment is needed.

Type 2 (Mobitz II)

Type 2 Second degree AV block, also known as Mobitz II is almost always a disease of the distal conduction system (His-Purkinje System). Although the terms infranodal block or infrahisian block are often applied to this disorder, they are not synonymous with it. Infranodal block and infrahisian block are terms which refer to the anatomic location of the block, whereas Mobitz II refers to an electrocardiographic pattern associated with block at these levels.

Mobitz II heart block is characterized on a surface [ECG] by intermittently nonconducted P waves not preceded by PR prolongation and not followed by PR shortening. The medical significance of this type of AV block is that it may progress rapidly to complete heart block, in which no escape rhythm may emerge. In this case, the person may experience a Stokes-Adams attack, cardiac arrest, or Sudden Cardiac Death. The definitive treatment for this form of AV Block is an implanted pacemaker.

In horses, a Mobitz type II heart block is considered a normal variant finding, frequently encountered in relaxed, athletically fit animals.

Symptoms

Most people with Wenckebach (Type I Mobitz) do not show symptoms. However, those that do usually display one or more of the following:

• Light-headedness
• Dizziness
• Syncope (Fainting)

See also

• Electrical conduction system of the heart
• Electrocardiogram (ECG or EKG)
• SA node
• AV node
• First degree AV block
• Third degree AV block
• Karel Frederik Wenckebach
• de:Woldemar Mobitz

External links

• synd/2824 at Who Named It
• http://www.youtube.com/watch?v=GVxJJ2DBPiQ - AV Block Parody Video by the University of Alberta
• Emergency Cardiovascular Medicine - eMedicine article on 2nd Degree Heart Block

Wikipedia content modification information:

• This page was last modified on 3 August 2008, at 03:50.

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