What is the pathophysiology of second-degree atrioventricular (AV) block?

Updated: Jul 05, 2018
  • Author: Akanksha Agrawal, MBBS; Chief Editor: Jeffrey N Rottman, MD  more...
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Second-degree AV block exists when more P waves than QRS complexes are seen on the ECG, but a relationship between the P waves and QRS complexes still exists. In other words, not all P waves are followed by QRS complexes (conducted). Traditionally, this type of AV block is divided into two main subcategories, Mobitz type I (Wenckebach) and Mobitz type II.

In the Mobitz I second-degree AV block, the PR interval is prolonged until the P wave is not followed by a QRS complex. In a typical Mobitz I block, the PR interval prolongation from beat to beat is greatest in the first interval and progressively less with subsequent intervals. This is reflected in shortening of the R-R interval and an increase in the overall PR interval. Also, the R-R interval enveloping the pause is less than twice the duration of the first R-R interval following the pause.

On the ECG tracing, Mobitz I second-degree AV block results in the characteristic appearance of grouping beats; conversely, the presence of grouped beats should prompt a careful evaluation for Wenckebach conduction (although it should be noted that not all such conduction is pathologic).

In Mobitz II second-degree AV block, the PR interval is constant, but occasional P waves are not followed by the QRS complexes (nonconducted). Occasionally, the first PR interval following nonconducted P waves may be shorter by as much as 20 msec.

To differentiate between Mobitz I block and Mobitz II block, at least three consecutive P waves must be present in the tracing. If only every other P wave is conducted (2:1), a second-degree block cannot be classified into either of these categories and thus is best described as a 2:1 AV block, unless the mechanism can be inferred from surrounding patterns of atrial-to-ventricular conduction.

An AV block resembling second-degree AV block has been reported with sudden surges of vagal tone associated with cough, hiccups, swallowing, carbonated beverages, pain, micturition, or airway manipulation in otherwise healthy subjects. The distinguishing feature is simultaneous slowing of the sinus rate. This condition is paroxysmal and benign, but it must be carefully differentiated from a true second-degree AV block because the prognosis is very different.

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