Overview
The left and right bundle branches emerge from the bundle of His in the heart. They transmit impulses from the bundle of His to the Purkinje fibres. The bundle branches are found along the interventricular septum and each bundle branch depolarises their respective ventricles. The interventricular septum itself is depolarised by the left bundle branch and is depolarised from left to right. The ventricles contract simultaneously.
Left Bundle Branch Block
Overview
Left bundle branch block (LBBB) describes slowed or absent conduction through the left bundle branch. This leads to delayed depolarisation of the left ventricle. This results in the left ventricle being depolarised via the right bundle branch, whose impulses travel through the right ventricle, and then to the left ventricle via the septum.
This leads to:
- Prolonged, positive R waves in the left ventricular leads (I, V5-6)
- These are usually negative, but since depolarisation is happening in the opposite direction,
- Secondary R waves in the left ventricular leads (I, V5-6) giving “M-shaped” R waves
- This is due to delayed activation between the right ventricle and left ventricle
- QRS prolongation
- Due to delayed conduction as the left ventricle now needs to be depolarised via the right ventricle through a slower and less efficient pathway
A quick and easy way to remember this is WiLLiaM:
- There is a ‘W’ in V1 and an ‘M’ in V6
Figure 5: left bundle branch block
Causes
A new LBBB is always pathological. Causes include:
- Myocardial infarction (MI)
- This can make it difficult to diagnose an MI in a patient with a pre-existing LBBB.
- The Sgarbossa criteria can help with this
- Aortic stenosis
- Hypertension
- Cardiomyopathy
Right Bundle Branch Block
Overview
Right bundle branch block (LBBB) describes slowed or absent conduction through the right bundle branch. This leads to delayed depolarisation of the right ventricle. This results in the right ventricle being depolarised via the left bundle branch, whose impulses travel through the left ventricle first, then to the right ventricle via a slower and less efficient pathway.
This leads to:
- Secondary R waves in the right ventricular leads (V1 and V2) giving “M-shaped” R waves
- This is due to delayed activation between the left ventricle and right ventricle
- QRS prolongation
- Due to delayed conduction as the right ventricle now needs to be depolarised via the left ventricle through a slower and less efficient pathway
- Wide, slurred S waves in leads I and V6
A quick and easy way to remember this is MaRROW:
- There is an ‘M’ in V1 and a ‘W’ in V6
Figure 6: right bundle branch block
Causes
- RBBB can be a normal variant
- Ischaemic heart disease
- Pulmonary disorders (e.g. COPD)
- Right ventricular hypertrophy
- Pulmonary embolism
Other Blocks
Overview
Other types of block include:
- Bifascicular block:
- RBBB and blockage of one of the fascicles of the left bundle branch
- Trifascicular block:
- Bifascicular block and third-degree heart block
