The left bundle branch is actually comprised of two divisions, or "fascicles." It is technically a COMPLETE block because a true left bundle indicates disruption of the left anterior and left posterior fascicle. The left bundle branch block has a characteristic features on the 12 lead ECG. To identify a left bundle branch block, closely scrutinize lead V1.
A left bundle branch block is present if the following conditions are met:
1. The rhythm is supraventricular2. The QRS is greater than 0.12 seconds wide
3. The QRS is deflected in the opposite direction of the T wave
4. The QRS is downwardly deflected in lead V1
5. The QRS in V6 typically has an upright, and "notched" appearance.
Why so Serious?
The left bundle branch block is important to recognize because it has been traditionally associated with coronary artery disease. The latest American Heart Association / Emergency Cardiac Care guidelines place less emphasis on immediate activation of the cardiac cath lab in the setting of a left bundle branch block. However, a presumed new bundle branch block in association with concerning symptoms (chest pain, diaphoresis) is still highly suspicious for a new myocardial infarction. Patients with chest pain and a left bundle branch block of new (or unknown) onset should probably be transported to a hospital capable of percutaneous coronary intervention.
ECGs showing the LBBB
Note the following ECG features of the LBBB:
-Downwardly deflected QRS-Delayed R wave progression
-Wide QRS
-Poor or delayed R wave progression (no positive initial deflections in leads V2-V4)
-Left axis deviation
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