Unifascicular block is a conduction block that affects one of the three major infranodal conduction pathways: right bundle branch (RBB), left anterior superior fascicle (LASF), or left posterior inferior fascicle (LPIF). A wide variety of disease processes can produce conduction block in the fascicles: ischemia, cardiomyopathies, valvular (especially aortic), myocarditis, cardiac surgery, congenital conditions, and degenerative processes affecting the conduction tissue (Lenegre or Lev diseases). The RBB and the LASF are relatively small and easily affected parts of the conduction system. The LPIF is very broad and is caused by disease affecting a large area of myocardium.
In LASF block, left ventricular activation is by way of the LPIF and proceeds in an inferior-to-superior and right-to-left direction. The ECG characteristics of LASF block are (1) normal QRS duration, (2) frontal plane mean QRS axis of less than -45°, (3) R wave in lead I greater than the R waves in leads II or III, (4) a qR complex in lead AVL, and (5) deep S wave in leads II, III, and AVF (Fig 2.4.-31). The LASF is small and easily affected by focal lesions. Other causes of left axis deviation should be excluded—inferior MI, hyperkalemia, preexcitation syndromes, or body habitus. Left ventricular hypertrophy can cause left axis deviation as seen with LASF block; however, the axis is infrequently less than -30.
In LPIF block, left ventricular activation is by way of the LASF and proceeds in a superior-to-inferior and left-to-right direction. The ECG characteristics of LPIF block are (1) normal QRS duration, (2) frontal plane mean QRS axis greater than 110°, (3) small r and deep S waves in lead I, (4) an R wave in lead III larger than the R
wave in lead II, and (5) a qR complex in lead III (Fig 24-32). The LPIF is broad and not affected by focal lesions; its presence indicates widespread organic heart disease. Other causes of right axis deviation are chronic cor pulmonale, right ventricular hypertrophy, and lateral MI.
In RBB block, ventricular activation is by way of the left bundle branch, proceeding from the left to the right ventricle. The ECG characteristics of RBB block are (1) prolonged QRS duration (greater than 0.12 s); (2) triphasic QRS complexes (RSR') in lead V I; (3) wide S waves in the lateral leads I, V5, and V6; and (4) normal onset of ventricular activation in lead V6 (Fig 2,4:33). The frontal plane mean QRS axis is usually not deviated to the right unless there is associated right ventricular hypertrophy or LPIF block.
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