Degrees of Atrioventricular Block

Three degrees of block are recognised and described below:

First degree AV block

Conduction is delayed within the AV node resulting in a constant prolongation of the PR interval on the ECG (which is defined as being greater than 200 milliseconds) and the QRS complex remains narrow.

This may be a sign of early fibrosis or ischaemia in the AV node but is most commonly a normal variant and is asymptomatic.  In the context of an acute coronary syndrome it requires monitoring in case of progression to other forms of heart block. It does not require treatment.

Second degree AV block

The QRS remains narrow but atrial impulses fail to conduct normally to the ventricles in one of the following ways:

Mobitz type I (Wenckebach)

The PR interval lengthens progressively after each successive P wave until a P wave is not conducted (see Figure). This is common following inferior acute myocardial infarction (AMI) when it may progress to complete heart block. It is normally asymptomatic and requires monitoring but not urgent intervention. Click the thumbnail to see a larger version.

Mobitz type I heart block (Wenckebach) is normally asymptomatic and resolves without the need for urgent intervention.

Mobitz type II

There is a constant PR interval but some P waves fail to conduct to the ventricles (see Figure).  The ratio of conducted and non-conducted beats may be fixed (e.g. 2:1 or 3:1).  This is less common than Mobitz type I, often symptomatic and of more concern. It signifies septal involvement in the setting of AMI and commonly progresses to complete heart block. Patients who have this diagnosed on pre-operative assessment are fitted with pacemakers before undergoing anaesthesia. Click the thumbnail to see a larger version.

Learning bite

Mobitz type 2 heart block commonly progresses to complete heart block which may require urgent intervention.

Third degree AV block (Complete heart block)

All P waves fail to conduct to the ventricles resulting in a broad complex ventricular escape rhythm (see Figure). A rhythm originating in the high septal region will have a rate of 40-50 beats per minute. If originating from a lower ventricular site the rate will be lower at 30-40 beats per minute. Although this may be a coincidental finding it usually presents with lethargy and syncope.  It signifies significant fibrosis or ischaemia in the AV node and requires a permanent pacemaker. Following an anterior AMI it indicates extensive damage to the septal region and indicates a worse prognosis. Click the thumbnail to see a larger version.

Learning bite

Complete heart block in the setting of acute anterior MI indicates extensive septal damage and is a poor prognostic sign.

PR interval > 200ms (five small squares).