A bifascicular block on ECG is defined by the combination of a right bundle branch block and either a left anterior fascicular block or left posterior fascicular block. When these occur in combination, significant conduction disease is usually present, and there is a risk for higher degrees of atrioventricular block in the future causing symptomatic bradycardia and requiring pacemaker implantation.
Note: A bifascicular block is related to a trifascicular block, which also includes a first-degree AV block. Even though technically incorrect, the AV node in this situation is considered the third fascicle.
A bifascicular block can occur as a part of the ischemic heart disease or as a part of the normal degeneration of the conduction system (Lev's disease). Although the 2009 American College of Cardiology/American Heart Association scientific statement on ECG interpretation does not recommend the use of the terms “bifascicular” or “trifascicular,” they are quite commonly used.
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