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Second Degree Atrioventricular Block

Unlike first-degree atrioventricular (AV) block in which the atrial impulse is delayed in reaching the ventricles, some atrial impulses in second-degree AV block fail to reach the ventricles. There are three types of second degree atrioventricular block: Mobitz type I, Mobitz type II, and advanced. In Mobitz types I and II, two or more consecutive P waves are conducted to the ventricle, and only one P wave is blocked. In advanced second-degree AV block, two or more consecutive P waves are blocked. On the ECG, Mobitz type I, also called Wenckebach block, is characterized by progressive PR prolongation that culminates in a nonconducted P wave. In Mobitz type II, the PR interval that precedes…

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 Woldemar Mobitz and His 1924 Classification of Second-Degree Atrioventricular Block

Woldemar Mobitz, an early 20th century German internist, analyzed arrhythmias by graphing the relationship of changing atrial rates and premature beats to AV conduction. Through an astute mathematical approach, he was able to classify second-degree atrioventricular block into 2 types, subsequently referred to as Mobitz type I (Wenckebach) and Mobitz type II (Hay).

Articles of Interest

AV Block: 2nd degree, Mobitz I (Wenckebach Phenomenon)

Mobitz I is usually a benign rhythm, causing minimal haemodynamic disturbance and with low risk of progression to third degree heart block. Asymptomatic patients do not require treatment. Symptomatic patients usually respond to atropine. Permanent pacing is rarely required.

AV block: 2nd degree, “high-grade” AV block

Unlike 3rd degree heart block there is still some relationship between the P waves and the QRS complexes. High-grade AV block may result from either Mobitz I or Mobitz II AV block.

Do Not Confuse Mobitz Type I and Mobitz Type II Atrioventricular (AV) Block

Mobitz type II AV Block is always abnormal, usually associated with significant heart disease, and has a high rate of progression to complete heart block (CHB). Mobitz type II AVB should never be considered a benign ECG finding. In contrast to Mobitz type I AVB where the conduction block is at the level of the AV node, the conduction delay of Mobitz type II is usually below the AV node.

Rhythms Review: Is this Second Degree AV Block 2:1 Mobitz I or Mobitz II?

Second-degree AV block is rarely symptomatic and by itself is not dangerous. The question is whether second-degree block indicates increased risk for complete AV block and if so whether asystole is likely to occur.

Second Degree Atrioventricular Block – Mobitz II

Mobitz II occurs when the infranodal conduction system intermittently fails, resulting in intermittently “dropped” P waves but consistent PR intervals when conducted. Mobitz II usually occurs with preexisting conduction disease (e.g., combination of bundle branch and fascicular blocks), is always pathologic, and more likely to progress to complete heart block.

Second Degree AV Block: Mobitz Type I or Wenckebach Block

Second degree AV .block is due to an increasing delay in AV conduction with each successive complex until ultimately failure of conduction occurs. The site of the block is usually relatively high in the junctional tissues, near or within the AV node, although the precise site of the block cannot be determined from the surface ECG. This type of AV block is a relatively common finding after inferior infarction when ischaemia affects the conducting tissue. Wenckebach block following inferior infarction does not usually require treatment and progression to more advanced degrees of AV block is unusual.