Heart Block

Heart block or conduction block may occur at any level in the conducting system. Block in either the AV node or the His bundle results in atrioventricular (AV) block, whereas block lower in the conduction system produces bundle branch block.

Atrioventricular block

There are three forms:

First-degree AV block

This is simple prolongation of the PR interval to more than 0.22 s. Every atrial depolarization is followed by conduction to the ventricles but with delay.

Second-degree AV block

This occurs when some P waves conduct and others do not. There are several forms:

-Mobitz I block (Wenckebach block phenomenon) is progressive PR interval prolongation until a P wave fails to conduct. The PR interval before the blocked P wave is much longer than the PR interval after the blocked P wave.

-Mobitz II block occurs when a dropped QRS complex is not preceded by progressive PR interval prolongation.

- 2 : 1 or 3 : 1 (advanced) block occurs when every second or third P wave conducts to the ventricles. This form of second-degree block is neither Mobitz I nor II.

Wenckebach AV block in general is due to block in the AV node, whereas Mobitz II block signifies block at an infra -nodal level such as the His bundle. The risk of progression to complete heart block is greater and the reliability of the resultant escape rhythm is less with Mobitz II block. Therefore pacing is usually indicated in Mobitz II block, whereas patients with Wenckebach AV block are usually monitored. Acute myocardial infarction may produce second-degree heart block. In inferior myocardial infarction, close monitoring and transcutaneous temporary back-up pacing are all that is required. In anterior myocardial infarction, second -degree heart block is associated with a high risk of progression to complete heart block, and temporary pacing followed by permanent pacemaker implantation is usually indicated. 2 : 1 Heart block may either be due to block in the AV node or at an infra-nodal level. Management depends on the clinical setting in which it occurs. Complete heart block occurs when all atrial activity fails to conduct to the ventricles. In patients with complete heart block the aetiology needs to be established. In this situation life is maintained by a spontaneous escape rhythm.

Bundle branch block

The His bundle gives rise to the right and left bundle branches. The left bundle subdivides into the anterior and posterior divisions of the left bundle. Various conduction disturbances can occur.

Bundle branch conduction delay.

This produces trivial widening of the QRS complex (up to 0.11 s). It is known as incomplete bundle branch block.

Complete block of a bundle branch.

This is associated with a wider QRS complex (0.12 s or more). The shape of the QRS depends on whether the right or the left bundle is blocked. Right bundle branch block produces late activation of the right ventricle. This is seen as deep S waves in leads I and V6 and as a tall late R wave in lead V] (late activation moving towards right- and away from left- sided leads).Left bundle branch block produces the opposite – a deep S wave in lead Vj and a tall late R wave in leads I and Vfi. Because left bundle branch conduction is normally responsible for the initial ventricular activation, left bundle branch block also produces abnormal Q waves.

Hemiblock.

Delay or block in the divisions of the left bundle branch produces a swing in the direction of depolarization (electrical axis) of the heart. When the anterior division is blocked (left anterior hemiblock), the left ventricle is activated from inferior to superior. This produces a superior and leftwards movement of the axis (left axis deviation). Delay or block in the postero-inferior division swings the QRS axis inferiorly to the right (right axis deviation).

Bifascicular block.

This is a combination of a block of any two of the following: the right bundle branch, the left antero-superior division and the left postero -inferior division. Block of the remaining fascicle will result in complete AV block.

Clinical features

Bundle branch blocks are usually asymptomatic. Right bundle branch block causes wide but physiological splitting of the second heart sound. Left bundle branch block may cause reverse splitting of the second sound.Patients with intraventricular conduction disturbances may complain of syncope. This is due to intermittent complete heart block or to ventricular tachyarrhythmias.ECG monitoring and electrophysiological studies are needed to determine the cause of syncope in these patients.