Left bundle branch block, LBBB
See brief interpretation of 12 lead EKG above
– broad, notched R waves and absent Q waves in V5 and V6 (and aVL)
– QRS ≥ 0.12 sec
– also: repolarization changes (T wave inversion) in aVL and V6 (not absolute diagnostic criteria for LBBB)
As outlined in the article about the electrical conduction system of the heart the left bundle branch originates from the bundle of His and split into an anterior and a posterior fascicle. The concept of left bundle branch block (LBBB) can be illustrated using the same diagram:
In an EKG of the normal heart a Q wave is seen in the left chest leads as the depolarization of the ventricular septum proceeds from left to right (the depolarization wave is pointing away from the left chest leads). In left bundle branch block the vector points from right to left as the right bundle branch is unaffected. This produces broad and notched R waves in lead V5, V6 and aVL and the Q waves in lead V5 and V6 are absent. Like with right bundle branch block (RBBB) the duration of the QRS complex ≥ 0.12 seconds and ST segment and repolarization changes can be seen.
A detail from the 12 lead EKG above:
While suspicion of RBBB is primarily raised looking at lead V1, left bundle branch block is seen in lead V5 and V6. This makes sense given where the electrodes are placed. Left bundle branch block is almost always associated with heart disease.
Please refer to the 12 lead EKG above and the brief interpretation below it to summarize the diagnostic criteria for LBBB.