Cardiac Conduction System Disease

Bundle branch block

The heart is a muscular pump that has to be electrically activated by a specialized wiring system to permit it to beat regularly and to ensure that all parts of the hearts beat virtually synchronously. The electrical activation is achieved by a specialized conduction system. After it exits from the upper part of the heart, the electrical impulse originating from the superior “pacemaker” of the heart engages a thin bundle (bundle of His) which is the beginning of the ventricular specialized conduction system. This bundle then divides into two branches: The right bundle branch and the left bundle branch. They travel on the inside of the left ventricle which is the main pumping chamber of the heart. The left bundle branch then divides into 2 branches: left anterior and left posterior divisions. Conduction blocks of the divisions of the left bundle branch are known as hemiblocks. All the specialized branches or divisions display marked ramification to encompass the muscle areas to be activated. In effect the specialized conduction system of the heart consists of a three-pronged or trifascicular system in the left ventricle, one on the right and two on the left.

 

Hemiblocks

Isolated left anterior hemiblock or left posterior hemiblock produce minor changes in cardiac activation. Damage to one bundle branch is easily recognizable in an electrocardiogram. Activation of the heart then proceeds via the other bundle branch so that there is a slight delay of activation on the side of bundle branch block. Activation moves from the normally activated side to the abnormal side via slow conduction utilizing normal myocardium and not via fast-conducting specialized conduction tissue thereby accounting for the activation delay. It is possible for damage to involve two of the three prongs or fascicles (bifascicular block). Thus right bundle branch block may be associated with block of the left anterior division of the left bundle (left anterior hemiblock). Or right bundle branch block may be associated with block of the left anterior division of the left bundle (left posterior hemiblock). Left bundle branch block is considered a form of bifascicular block. All bifascicular blocks are easily identifiable on an ECG. All types of bifascicular block allow near-normal activation of the heart with only a slight delay as long as the third fascicle is intact. This three-pronged conduction system therefore provides safety.

 

Trifascicular block and complete heart block

Trifascicular block occurs when all three fascicles are non-functional. It may rarely be manifested as intermittent bilateral bundle branch block. If trifascicular is permanent, the ventricles are no longer controlled by the trifascicular conduction system and this leads to complete heart block where the ventricles create a very slow unstable rhythm (20 - 40/min) and beat independently of the rate generated by the natural pacemaker high in the right atrium. If there is bifascicular block and intermittent failure of the third fascicle, symptoms will also be intermittent.

 

Cause of conduction system disease

There are many causes of fascicular blocks but most occur with age with no specific etiology. They are associated with fibrotic degeneration of the specialized conduction system.

 

Test for documentation

In suspected cases, Holter recordings and an implanted loop recorder may be useful. An invasive electrophysiologic study can define a very prolonged ventricular conduction time an important indication for a permanent pacemaker.

 

Symptoms
In most people, bundle branch block does not cause any symptoms. Sometimes, people with the condition do not even know they have a bundle branch block. For those people who do have signs and symptoms, they may include: Fainting (syncope) or feeling as if you are going to faint (presyncope). These symptoms suggest the presence of intermittent trifascicular block. It is important to do an echocardiogram to evaluate left ventricular function. If left ventricular function is poor, the symptoms may be due to ventricular tachycardia (a rapid rhythm from the ventricles). The most likely diagnosis is intermittent trifascicular block if left ventricular function is normal.

 

Management
The asymptomatic patient with bifascicular block requires no treatment. The implantation of a permanent pacemaker is required in symptomatic patients provided there is strong evidence of intermittent trifascicular block. However, ventricular tachycardia must be ruled out in patients with poor left ventricular function.


S. Serge Barold MD, Tampa, US 


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