Atrioventricular Blocks (AV Blocks)

Atrioventricular blocks are a set of disorders of the cardiac conduction system that cause the atrial electrical stimulus to the ventricles to be delayed or interrupted ¹.

They are caused by an alteration in the atrioventricular node or in the bundle of His, although they can also be caused by malfunctions in other heart structures (see cardiac conduction system) or metabolic alterations such as hyperkalemia.

Atrioventricular Block Classification System

There are three types of AV blocks based on their severity.

• First degree AV block
• Second degree AV block
• Second degree AV block, type I
• Second degree AV block, type II
• Third degree atrioventricular block or complete atrioventricular block

First Degree Atrioventricular Block

Related article: First degree AV block

In first degree atrioventricular block there is a delay in the stimulus as it moves through the AV node or the His-Purjinke system, delaying the appearance of the QRS complex.

First degree AV block, prolonged PR interval (0.52 s)

The distinctive alteration on the electrocardiogram is the prolonged PR interval, with a narrow QRS complex if no other alteration is present.

Furthermore, in first degree atrioventricular block there is no AV conduction interruption, so all P waves are followed by a QRS complex, in contrast to the other two types of AV blocks.

More information: First degree AV block.

Second Degree Atrioventricular Block

Related article: Second degree AV blocks

In second degree atrioventricular block a discontinued interruption of the stimulus on its way from the atria to the ventricles is observed. Non-conducted P waves (not followed by QRS complexes) can be seen on the electrocardiogram.

They are classified into second degree AV block type I and type II according to their characteristics on the electrocardiogram.

Second Degree AV Block, Type I (Wenckebach)

Progressive slowing of the atrioventricular conduction until the impulse fails to be conducted is observed during the second degree AV block, type I.

Second degree AV block, type I (Wenckebach): Progressive prolongation of the PR interval until a P wave is blocked (red)

On the EKG trace we observe:

• Progressive prolongation of the PR interval until a P wave is blocked.
• The R-R interval progressively shortens until a P wave is blocked.
• Normal QRS complex, if no other alteration is present.
• The R-R interval containing the blocked P wave is shorter than the previous two R-R intervals.

Second Degree AV Block, Type II (Mobitz II)

Second degree AV block type II is less frequent than the aforementioned and generally implies underlying cardiac pathology ³.

This type of atrioventricular block is noted for an abrupt block of the AV conduction, without previous PR interval prolongation.

On the EKG trace we observe:

• Blocked P wave with previous and subsequent PR intervals of similar duration.
• The PR interval following the blocked P wave is of similar duration to those previous.
• The R-R interval surrounding the blocked P wave is equal to the two preceding ones.

Second degree AV block, type II (Mobitz II): blocked P wave with previous PR intervals of similar duration, last beats with 2:1 conduction block

In certain cases a particular sequence might follow — a blocked P wave every three normal QRS complexes. There may be no pattern in those high grade AV blocks (advanced) where more than one consecutive P wave might appear.

This kind of AV block might unexpectedly progress to complete AV block and normally requires a permanent pacemaker.

More information: Second degree AV block

Third Degree Atrioventricular Block (Complete AV Block)

Related article: Third degree AV block

Third degree atrioventricular block — or vomplete AV block — is noted for the complete interruption of the AV conduction. No atrium-generated stimuli is conducted to the ventricle, which means that both atria and ventricles contract at their own pace.

The ventricular rate will depend on the originating point of the escape beat (AV node, bundle of His or a bundle branch). The higher the location of the AV block, the faster and and narrower the QRS complexes.

On the EKG trace we observe:

Complete AV block at 44 bpm. AV dissociation (P waves in red)

• P waves show no relation to QRS complexes — atrial rate is higher than the ventricular rate
• P waves appear near to or superimposed on the QRS complex, or superimposed on the T wave
• Morphology and frequency of the QRS complexes will depend on the originating point of the escape beat. If coming from the AV node, higher frequency and narrower QRS complexes. If coming from the bundle branches, severe bradycardia and QRS complexes similar to those in bundle branch blocks.

More information: Third degree AV block

How to Treat an AV Block

First degree and second degree atrioventricular blocks type I (Wenckebach) do not usually require treatment, although they both can progress to higher degree AV blocks.

Electronic pacemaker implantation is indicated in patients with third degree or second degree AV block type II irrespective of symptoms ⁴.

Before indicating a permanent pacemaker as treatment, other reversible causes for AV block should be discarded, such as antiarrhythmic agents or ischemic heart disease.