What is LBBB?

What is LBBB?

LBBB stands for left bundle branch block. It is an electrical block in the pathway taking electrical signals from the upper chamber of the heart to the left lower chamber. Left and right bundle branches are the branches of the bundle of His, coming down from the AV node in the right upper chamber of the heart. AV node in turn receives signals from the SA node situated in the upper part of right upper chamber. SA node is the natural pacemaker of the heart which controls the heart rate.

In the normal sequence of electrical activation of the lower chambers, left bundle branch is activated first. Hence a block in in the left bundle branch can affect the sequence of activation recorded in the ECG, the recording of the electrical activity of the heart. This leads to errors in interpretation of ECG changes of a heart attack.

Unlike the block in right bundle branch which can occur in individuals without heart disease, LBBB is usually associated with other heart diseases. LBBB has great significance in the setting of heart failure. LBBB causes delay in the electrical activation of the left side of the left lower chamber. This causes lack of synchrony between the contractions of the inner and outer portions of the left ventricle, the lower left chamber. Lack of synchrony causes one part to bulge out when the other part contracts. This reduces the effective pumping function of the heart and can cause heart failure.

Lack of synchrony of different parts of left ventricle can be treated by giving electrical signals with a special type of artificial pacemaker implanted under the skin. The electrical leads from this device is connected to the inner and outer parts of the left ventricle through the blood vessels. This procedure is known as cardiac resynchronization therapy (CRT). CRT improves the synchrony of contraction of different parts of the left ventricle and hence the pumping function. CRT is useful in persons with LBBB and heart failure not responding to medical treatment.

Recently, instead of pacing the two sides of the left ventricle, pacing the left bundle branch directly has also come into vogue. This is also called LBBB correction as the pacing is done below the block so that LBBB pattern in ECG is corrected. LBBB correction may soon be the optimal treatment of heart failure in those with LBBB, ahead of CRT. Potential advantage is that only a single site of pacing is needed compared to CRT in which two sides of the left ventricle has to be paces using two leads.