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8. Intraventricular Conduction Abnormality and Bundle Branch Blocks
Current normally flows from sinoatrial node to ventricular myocardium through atrioventricular node (AV node), common bundle of His, bundle branches (left and right), fascicles, and the Purkinje fibers at a certain speed. Any delay or block in the conduction pathways below the bundle of His leads to an intraventricular conduction abnormality popularly known as bundle branch blocks (BBB). Intraventricular conduction system consists of the right bundle branch, left bundle branch, and anterior and posterior fascicle of the left bundle branch. Any of the pathways may be affected. A block or delay may occur in any one, two, or three pathways at a time leading to delayed depolarization causing changes in “QRS” configuration. Well-accepted electrocardiographic features of bundle branch blocks are as follows.
8.1 Left Bundle Branch Block (LBBB)
Mean electrical axis (MEA) on frontal plane may remain within normal limits without any significant change.
“QRS” complex in different leads (lead I, II, III, aVF, CV6LL, and CV6LU) is broad and positive (Fig. 8.1).
Sometimes a small “Q” wave is seen in different leads (lead II, III, and aVF). It is suggestive of incomplete LBBB (Fig. 8.1).
Because of slow conduction due to block, QRS complex may become broad and sloppy (Fig. 8.2).
Left bundle branch block is also suggested by the presence of a small “Q” wave in lead I, CV6LL, and CV6LU.
Inverted “QRS” in lead aVR, aVL, and CV5RL is another indication of left bundle branch block.
Left bundle branch block is indicated when above electrocardiographic features are present in the electrocardiogram with the absence of left ventricular enlargement in radiographs (lateral, ventrodorsal, and dorsoventral views).
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