Q. What is the difference between left and right bundle branch block?

Answered by
Dr. Muhammad Zohaib Siddiq
and medically reviewed by Dr. Divya Banu M
Published on May 20, 2019

Hello doctor,

Can you please tell me the difference between left and right bundle branch block? And why is left considered far more dangerous?

Dr. Muhammad Zohaib Siddiq

Cardiology General Practitioner


Welcome to icliniq.com.

Heart needs current to beat. Current is generated in sino-atrial node. It is like electric generator. It travels via bundle branches, which are like wires. Current goes from SA node to AV node via atrial bundles. Then from AV node to a small bundle called HIS bundle. Then goes to right ventricle via right bundle branch and to left ventricle via left bundle branch. Then it further divides down.

When there is interruption in right bundle, it is called rbbb and same for left bundle branch. Right bundle branch may not always be dangerous because right ventricle is a low pressure chamber, pumps blood to lungs which is also low pressure. Left ventricle has to pump blood to whole body, it has to force more. When lbbb occurs, left ventricle is contracted via current coming from right bundle branch and it takes some time to reach left side although only milliseconds of difference but that is significant. Thus, there is dys-synchrony between ventricle which may lead to ineffective output from left ventricle, because whole vascular circuit is interconnected and should work in rhythmic synchronized way.

Moreover, lbbb usually occurs due to heart attack. But all this is not as simple as mentioned. It is more complicated. It needs much time to make you properly understand the whole working of heart and its connections. Hope you got a little idea about lbbb and rbbb.

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