Second degree AV block

Refers to a disease of the electrical conduction system of the heart.

It is a conduction block between the atria and ventricles.

The presence of second-degree AV block is diagnosed when one or more of the atrial impulses fail to conduct to the ventricles due to impaired conduction.

Most people with Wenckebach (Type I Mobitz) do not show symptoms, but those that do usually display light-headedness, dizziness or syncope.

There are two non-distinct types of second-degree AV block, called Type 1 and Type 2.

In Type 1, there are increasing delays in each cycle before the omission, whereas, in Type 2, there is no such pattern.

Type 1 second-degree heart block is considered a more benign entity than type 2 second-degree heart block.

Type 1 (Mobitz I/Wenckebach 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 (ECG) on consecutive beats followed by a blocked P wave.

After the dropped QRS complex, the PR interval resets and the cycle repeats.

When determining if an individual has Mobitz I heart block the atrial rhythm has to be regular.

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

In symptomatic cases, intravenous atropine or isoproterenol may transiently improve conduction.

Type 2 Second-degree AV block, also known as “Mobitz II,” is almost always a disease of the distal conduction system of His-Purkinje System.

Mobitz II heart block is characterized on lECG by intermittently nonconducted P waves not preceded by PR prolongation and not followed by PR shortening.

There is usually a fixed number of non-conducted P waves for every successfully conducted QRS complex, and this ratio is often specified in describing Mobitz II blocks.

In a Mobitz II block in which there are two P waves for every one QRS complex, it may be referred to as “2:1 Mobitz II block”.

The medical significance of this Mobitz II type of AV block is that it may progress rapidly to complete heart block, in which no escape rhythm may emerge.

II the cases of complete heart block a patient may experience a Stokes-Adams attack, cardiac arrest, or sudden cardiac death.

The treatment for complete AV Block is an implanted pacemaker.

Type I Mobitz block occurs in regular cycles, there is always a fixed ratio between the number of P waves and the number of QRS complexes per cycle.

This ratio is often specified when describing the block.

Type II Mobitz block also usually occurs with a fixed P:QRS ratio.

Type II Mobitz block has a set number of P waves for every successfully elicited QRS.

Type II Mobitz block is frequently specified in referring to “3:1”, “4:1”, “5:1”, or higher Mobitz type II block.

Higher numbers of P waves for every QRS indicate more severe block.

Because type II Mobitz block is unstable, it is common for the P:QRS ratio in Mobitz type II block to change over time.

The P:QRS ratio is always of the form “X:(X-1)” in type I Mobitz block and of the form “X:1” in type 2 Mobitz block.

In the case of 2:1 block, 2 P waves for every QRS complex,it is not possible to differentiate type I from type II Mobitz block based solely on the P:QRS ratio or on a pattern of lengthening PR intervals.

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