Electrophysiology: Technically for left bundle branch block to be present, the QRS duration is 120 msec or greater, there is poor R wave progression in V 1 thru V4 and the T wave vector is in the opposite direction to the QRS vector (T waves are inverted in I, V5 and V6). The conduction abnormality often appears as "rabbit ears" (rsR pattern) on the left side of the chest (V4,5,6).
The meaning of incomplete left bundle branch block beyond describing an ECG pattern is unknown. For this reason the criteria for this statement are narrowly defined, and whenever a specific label such as left anterior fascicular block is available, the term incomplete left bundle branch block is suppressed.
Pathophysiology: The left bundle is a large robust structure on the left side of the septum that divides into and anterior and posterior fascicles. Conduction can be blocked or slowed by severe trauma (car accident) or by ischemia or infarction. It is often associated with left ventricular hypertrophy and/or dilation. Also, it can be due to fibrosis of the conduction system in individuals less than 40 years of age (Lenegre's disease) while in older individuals it is known as Lev's disease. An incomplete LBBB has a QRS duration of less than 120 msec with some of the LBBB characteristics and usually can be a normal finding. LAFB and LPHB both have QRS duration's of less than 120 msec with left and right axis deviation respectively. They do not necessarily progress to LBBB. LPHB is a diagnosis of exclusion, only considered in a patient without lung disease or another reason for RAD. LBBB has a weak predictive power in a young asymptomatic population (consistent with Bayes rule) but is quite ominous in an older population as a marker for an increased risk of death, stroke and CHF.
Incomplete LBBB and the hemi-blocks usually are not associated with cardiac disease. Clinical correlates including the cardiac exam direct the response to LBBB. If the patient has an enlarged heart with signs and/or symptoms of CHF then an echocardiogram is usually indicated. If systolic dysfunction is detected then an ACE inhibitor is indicated to improve survival and lessen bouts of CHF. ACE inhibitors can cause a cough, hyperkalemia and renal dysfunction requiring a switch to a long acting nitrate and hydralazine.
If a patient presents with chest pain, both the resting and exercise ECG are eliminated as diagnostic tools and enzymes or nuclear perfusion testing is required.
q When LBBB is noted in the patient presenting with prolonged anginal pain, thrombolysis is indicated particularly if there is not a quick response to NTG and there are no prior ECGs demonstrating LBBB.
View an ECG Example