THE CONTEXT
Damage to ventricular conducting branches alters the route and timing of ventricular depolarisation. Complete bundle branch block and fascicular block produce different electrocardiographic changes.
Causes and conduction mechanism
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Bundle branch damage may result from ischaemic heart disease, hypertensive heart disease or cardiomyopathy.
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Right bundle branch block (RBBB)* can also be a normal variant in healthy individuals.
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In RBBB and left bundle branch block (LBBB), depolarisation in the affected ventricle proceeds through slow myocardium instead of rapidly conducting Purkinje tissue.
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Delayed ventricular conduction broadens the QRS complex to at least 0.12 seconds and produces characteristic changes in QRS morphology.
Hemiblock and bifascicular block
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Damage after the left bundle divides into its anterior and posterior fascicles is called hemiblock.
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In hemiblock, the QRS remains narrow, but the direction of ventricular depolarisation changes.
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Left anterior hemiblock causes left axis deviation; left posterior hemiblock causes right axis deviation.
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RBBB combined with either left anterior or left posterior hemiblock is called bifascicular block.
LBBB findings and clinical implications
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LBBB often signifies underlying heart disease and causes ventricular incoordination that may aggravate heart failure symptoms.
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Selected patients can be treated with cardiac resynchronisation therapy pacemakers.
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The described LBBB electrocardiogram has wide QRS complexes, loss of the Q wave or septal vector in lead I, and M-shaped QRS complexes in V5 and V6.
Atrial ectopic beat appearance
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The accompanying atrial ectopic beat description identifies abnormal, sometimes barely visible, P waves with QRS complexes identical to those of the normal sinus beats.