HEART BLOCK

Heart Block is an interruption of impulse conduction. It may occur at the sinoatrial node, atrioventricular node, His Bundle or in its branches.

AV BLOCK

First degree AV block

Prolongation of PR interval > 0.2 second, due to delayed conduction through AV node. The delay is constant for each cardiac cycle, and each P is followed by a QRS complex.

 

Second degree AV block

• PR interval progressively lengthen with cardiac cycle, until the AV node is not penestrated ie there is no QRS complex.

• PR interval is fixed and normal, but occasionally a P wave is not followed by a QRS complex. This condition is known as Wenckebach phenomenon or Morbitz I.

• P wave is not followed by QRS complex. It is called a 2:1 AV block when every 2 P waves are followed by one QRS complex. It is called a 3:1 AV block when every 3 P waves are followed by one QRS complex. This condition is known as Morbitz II

Third degree AV block

Complete interruption of impulse conduction between atria and ventricles. As a result, both atria and ventricles are working independently. There is no relationship between P waves and QRS complexes. The QRS complexes are wide implying a ventricular origin, not from usual conduction pathways.

BUNDLE BRANCH BLOCK

A conduction block in one or both of the branches. It is commonly associated with acute myocardial infarct, these blocks also can result from certain cardiac drugs, vulvular disease, congenital anomalies, ventricular hypertrophy, myocardial disease and infectious cardiac disease.

Normally the right and left bundle branch transmit impulses to the right and left ventricles simultaneously within less than 0.1 second resulting in a narrow QRS complex. A block in either bundle branch creates a delay of depolarization of the respective ventricle.

Right Bundle Branch Block (RBBB)

In RBBB the conduction through the right bundle branch is obstructed. As a result right ventricular depolarization is delayed. In RBBB the initial left-to- right direction of depolarization is not affected. The ECG changes include:

1. QRS complex > 0.12 second.
2. Altered shape of QRS producing RSR' in the right sides leads (V1, and V2), and a reciprocal late deep S wave in the leads overlying the left ventricle (I, aVL,V5 and V6).

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Left Bundle Branch Block (RBBB)

In RBBB conduction through the left bundle branch is obstructed. As a result left ventricular depolarization is delayed. The initial right-to-left direction of depolarization is distrupted. The ECG changes include:

1. Widen QRS complex (beyond 0.12 second)
2. QRS complex in leads overlying the left ventricle (I, aVL, V5 and V6) will show a characteristic change in shape. Delayed ventricular depolarization causes prolongation in the rise of R waves and the peak of the R waves will either be broad or notch. The leads overlying the right ventricle will show broad and deep S waves.
3. Normal initial Q waves are absent in the left precordial leads.

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