There are three conducting pathways from the AV node to the tissue of the ventricles: The right bundle branch, the left anterior fascicle, and the left posterior fascicle. The combined left anterior and posterior fascicles are referred to as the left bundle.
Bundle branch block occurs when electricity is not conducted down the one or more of the specialized conducting paths in the ventricles. This may occur due to degeneration with age, or may be due to specific pathology such as myocardial infarction. Bundle branch block widens the QRS, because electricity passes slower through muscle than through the special conducting pathways. The form or shape of the QRS will change. Partial bundle block or hemiblock may be diagnosed by a change in QRS axis or change in QRS shape, as the QRS is not widened in either partial block or hemiblock.
Right bundle branch block:
When the right bundle branch is blocked, activation of the right ventricle begins when electrical activity spills over from the left ventricle. Depolarization of the right ventricle is delayed.
The QRS is prolonged (over 0.1 sec) in right bundle branch block (RBBB). This extra length of the QRS is caused by late activation of the right ventricle, which is then seen after the left ventricle activity. Normally, right ventricle activity is not seen, as it is overshadowed by the larger left ventricle.
In RBBB, a typical RsR wave occurs in lead V1. Also, a wide S wave is seen in leads I, V5, and V6, along with a broad R in lead R. When RBBB occurs in a patient with old or new septal infarction, the initial septal R wave may not be seen in lead V1. Instead, a wide QR complex is seen.
When the typical RsR wave is seen in V1 without widening of the QRS complex, this is called right ventricular conduction defect rather than RBBB. [Chapter Menu]
Left bundle branch block:
LBBB usually indicates widespread cardiac disease. When the left bundle is blocked, activation of the left ventricle proceeds through the muscle tissue, resulting in a wide (.12 msec) QRS complex.
In left bundle branch blockage (LBBB), the QRS usually has the same general shape as the normal QRS, but is much wider and may be notched or deformed. Voltage (height of the QRS complex) may be higher.
In LBBB, look for wide (possibly notched) R waves in I, L, or V5-V6, or deep broad S waves in V1-V3. There is left axis deviation. Septal Q waves sometimes seen in I, L, and V5-V6 disappear in LBBB.
T waves in LBBB are usually oriented opposite the largest QRS deflection. That is, where large R waves are seen, T waves will be inverted. ST segment depression may occur. [Chapter Menu]
The two major branches of the left bundle may be blocked individually. When only one branch is blocked, this is called hemiblock either anterior hemiblock or posterior hemiblock depending on whether the anterior or posterior fascicles are involved.
Left anterior fascicular block (anterior hemiblock, LAH) is diagnosed by NORMAL QRS duration with marked left axis deviation, usually minus 60 degrees. S waves are larger than Rs in II, III, and F.
Left posterior fascicular block (posterior hemiblock, LPH) may be difficult to diagnose without prior ECGs. The QRS axis shifts substantially rightward. An axis of over +120, with no evidence of RVH or anterior infarction, is presumed LPH.
Fascicular block can occur with RBBB. Most commonly, LAH may be accompanied by RBBB. On ECG, signs of RBBB are accompanied by a leftward axis of -60 degrees.
When the QRS is prolonged without features of eather RBBB or LBBB, this is called nonspecific intraventricular conduction block. [Chapter Menu]
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