Bundle branch blocks

Bundle branch blocks

Bundle branch blocks: Intraventricular conduction disturbances can be divided into right bundle branch block, left bundle branch block, left anterior hemiblock, left posterior hemiblock and intraventricular conduction defects which do not conform to any of these patterns, but producing widening of the QRS complex. Various anatomical studies of the infra Hisian conduction system have revealed that the branching pattern of the bundle of His could be much more complex than envisaged in this simple system of nomenclature. Moreover, the pattern or bundle branch block on the ECG could be due to a relative conduction delay between the bundles [e.g. an atrial premature complex at 330 msec may be conducted with a right bundle branch block (RBBB) pattern while another one at 300 msec may be conducted with a left bundle branch block (LBBB) pattern] or due to complete failure of conduction in a bundle. Site of block can be even in the His bundle and not in the bundle branch. In such a situation, there will be associated widened or split His potentials on intracardiac recordings. Distal His pacing can normalize the QRS in these apparent bundle branch blocks due to dissociation of bundles with the His bundle. This has in fact been demonstrated by intracardiac recordings.

Right bundle branch block

Right bundle branch block can occur due to lesions at different levels along the right bundle. Proximal RBBB is common in the spontaneous variety of RBBB and in those which occur after repair of Tetralogy of Fallot (TOF). Distal RBBB is one which occurs at the moderator band level while terminal RBBB is at distal sites like the Purkinje fibre or cardiac muscle. Direct mapping studies during TOF repair have helped in delineating various levels of block in RBBB.

Significance of Level of RBBB

Distal or terminal right bundle branch block associated with left ventricular conduction disturbances may not indicate increased risk of complete heart block (CHB) unlike the case of proximal RBBB. Terminal block in the right bundle branch can be seen in atrial septal defect (ASD). In this case, conduction is normal in the proximal right bundle while the stretching of Purkinje fibres / muscle lead to delayed activation of right ventricular outflow tract (RVOT) region. Terminal right bundle branch block may be seen in cardiomyopathies and chronic lung disease as well.

Middle or septal fascicular block

The middle or septal fascicle, more often arises from the posterior fascicle and less often from the anterior fascicle. Very rarely it can arise as an independent fascicle from the main trunk of the left bundle. Recognition of middle or septal fascicular block on ECG is difficult. It is considered in cases of intraventricular aberrant conduction and transient morphological changes of QRS, especially by comparison of ECGs before and after the proposed occurrence of middle or septal fascicular block.

Hemiblocks: diagnostic Issues

Hemiblocks of the left bundle can mimic or hide myocardial infarction, myocardial ischemia and left ventricular hypertrophy. They can also disguise or conceal a RBBB.

Left anterior hemiblock

Left anterior hemiblock (LAHB) causes only minimal QRS widening of not more than 20 ms. It shifts the QRS forces superiorly and to left and manifests as deep S waves in leads II, III, and aVF. Small Q waves are seen in I and aVL as well as small R waves in II, III and aVF. Deeper S waves may be noted in V5 and V6. Small Q waves may be seen in V2, V3 and may mimic an old anterior wall myocardial infarction.

Differential diagnosis of LAHB

The following conditions may be considered in the differential diagnosis of left anterior hemiblock: left ventricular hypertrophy (LVH), Wolff Parkinson White (WPW) syndrome, hypertrophic cardiomyopathy, inferior wall myocardial infarction, chest deformities, single ventricle and corrected transposition of great arteries (cTGA). LAHB is considered unlikely if S wave in lead II is deeper than that in lead III.

Inferior wall myocardial infarction and LAHB

Initial r in II, III and aVF is due to early activation of a portion of the myocardium by posterior fascicle. It can mask inferior myocardial infarction if this region is spared by the ischemic process. This initial r wave is lost if this region is infarcted.
LAHB can make T positive in inferior leads and hence negative T waves with LAHB is a strong indicator of inferior wall ischemia.

LAHB can obscure RBBB

LAHB can obscure RBBB and appear as what is called as precordial masquerading RBBB. This is because LAHB can cause disappearance of lateral S waves of RBBB so that RBBB becomes atypical and may appear as LBBB.

Mechanisms of LAHB

Left anterior fascicle may be injured by diseases that involve the left ventricular outflow tract (LVOT) or the anterior half of the interventricular septum or anterolateral wall of the left ventricle. LAHB can be seen in hypertensive heart disease, cardiomyopathies, aortic valve disease, degenerative conduction system diseases like Lev and Lenègre disease, following spontaneous or surgical closure of ventricular septal defects (VSD) as well as other cardiac surgical procedures. In one series, LAHB occurred in about two thirds of cases of spontaneous closure of VSD. Some authors propose that the mechanism of LAHB in those without any structural heart disease could be a previous spontaneous closure of VSD.

Left posterior hemiblock

Posterior fascicle shorter and thicker and is less often affected. It is located in left ventricular inflow which faces a less turbulent blood flow compared to the left anterior fascicle which is located in the outflow. Left posterior fascicle also has a dual blood supply from left anterior descending as well as the posterior descending arteries. Left posterior hemiblock is seldom isolated and often occurs along with RBBB. A posterior hemi block can also sometimes conceal an inferior wall myocardial infarction. Though the combination of RBBB with LPHB is more often seen in degenerative conduction system disease than in coronary artery disease, if present along with an anterior wall infarction, carries a dismal prognosis. This combination has an 80 percent risk of mortality, 45 percent risk of complete heart block and a 75 percent risk of left ventricular pump failure.

Left bundle branch block
If the left bundle branch occurs with structurally normal heart, the distal conduction system is spared. But in those with in extensive anterior wall myocardial infarction, the distal conduction system is destroyed and the endocardial conduction is muscle – muscle.

Transient bundle branch block or phasic aberrancy

Transient bundle branch block or phasic aberrancy can occur anywhere in conduction system and the site of block can shift as well. The various types are phase 3 block which is physiologic, acceleration-dependent block, phase 4 or bradycardia dependent block and aberrancy due to retrograde concealment. In acceleration dependent block, the second impulse which is aberrantly conducted is well beyond the usual refractory period.

Long-short induced RBBB

The initial site of block in long-short induced RBBB is proximal and the block shifts distally with shorter cycle lengths. This is due to the difference in shortening of refractory period or due to retrograde invasion of the right bundle. Shortening of refractory period of the right bundle is more than that of left bundle at shorter cycle lengths so that LBBB is demonstrable at short drive cycle lengths.