Second degree AV block classification
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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. The distinction between Mobitz I and II can be made only when the ratio of atrial to ventricular conduction is not 2:1, because in 2:1 conduction every other beat is conducted to the ventricle and there is no opportunity to observe the PR prolongation that defines type I and II second degree AV 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.
- Mobitz II heart block is characterized on a surface ECG by intermittently non-conducted 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.
Differentiating Mobitz I from Mobitz II in the Presence of a 2:1 Conduction
Likely EKG findings that help differentiate Mobitz type I from type II in the presence of a 2:1 conduction ratio include:
- Very long PR interval (> 300 msec) or narrow QRS complex - indicates the block is at the level of AV node
- Administration of atropine enhances AV nodal conduction resulting in less frequent nonconducted beats - this confirms type I Mobitz
- Mobitz I is worsened by carotid sinus massage which slows AV nodal conduction, unlike Mobitz II. Carotid sinus massage paradoxically eliminates infranodal block by slowing the sinus rate.
Another type of classification used to classify second degree AV block is 2:1 block and high grade block (not third degree AV block). In 2:1 block every other atrial impulse is conducted down the ventricle. Higher grade blocks (eg., 3:1) unlike third degree AV block conduct few beats down the ventricle.
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