ECG Level 3 Tutorial: Bundle Branch Blocks
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Tutorial: Bundle Branch Blocks
Understand why a right bundle branch block and left bundle branch block pattern are created. Learn also the diagnostic criteria.
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Tutorial: Bundle Branch Blocks General Concepts
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
General Concepts
There are a few key concepts that you need to remember and understand. They include:
  • if one bundle branch is blocked, the the opposite ventricle will be depolarized first. For example:
    • in left bundle branch block (LBBB), LV depolarization is delayed and the RV is depolarized first
    • in right bundle branch block (RBBB), RV depolarization is delayed and the LV is depolarized first
  • the ventricle that has the blocked bundle branch is innervated by slow conduction between contracting myocytes. The electrical signal must travel from the ventricle that is depolarized first over to the ventricle with the blocked bundle branch, but when this happens, the fast conducting cells are not utilized. Therefore, the conduction occurs slowly. Slow conduction takes a long time and therefore the QRS will be wide. Remember that width on the ECG represents time.
  • the LV and the RV are not mirror opposites of each other. They are different sizes, different shapes, and are not oriented in a simple "left and right" position in the thorax. Therefore, the ECG patterns of RBBB and LBBB are not mirror images of each other as you might initially think.

Please note that we are focusing on abnormalities of depolarization. It might not shock you to also learn that when an abnormality of depolarization exists, abnormalities of repolarization will also probably exist. Remember that repolarization is represented by the T wave on the ECG, but sometimes, repolarization starts a little early and can cause mischief with the ST segment as well. Changes in the ST segment are extremely important because some important diagnoses (like myocardial ischemia and infarction) focus on the ST segment, so diagnostic confusion can occur if ST changes are also caused by other factors.

It is critical that you look for all diagnoses that interfere with the ST segment before making a diagnosis of myocardial ischemia or infarction. Therefore, your approach of 12 lead ECG interpretation should be carried out in a specific order to avoid confusion. Bundle branch blocks should be searched for early in your analysis.

In the next 2 lessons, we will go into some details that are specific to LBBB and RBBB and will give you some opportunities to practice your bundle branch diagnostic skills.