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LBBB
LBBB is best seen in leads I and/or V6, where there is M pattern. LBBB always means heart
disease and prevents further interpretation of the ECG.
Left bundle branch block produces a deep S wave in lead V1 and a tall late R wave in leads I
and/or V6.
Left bundle branch block is almost always caused by disease of the left ventricle and causes
include:
* Coronary artery disease: acute MI, severe two- to three-vessel disease
* Cardiomyopathy
* Hypertension
* Aortic stenosis
* Conduction system fibrosis.
LBBB always indicates significant cardiac disease. It is seen most commonly in ischaemic
heart disease, hypertension, aortic stenosis and fibrous degeneration of the conducting tissue.
It may also occur in congestive and hypertrophic cardiomyopathy, myocarditis, acute
rheumatic fever, syphilis, cardiac tumours, post-cardiac surgery and in congenital heart
disease.
Indicates total failure of conduction in the left bundle branch system.
Complete reversal of the direction of depolarisation of the interventricular septum.
Delay in the initiation and velocity of depolarisation of the free wall of the left ventricle
Diagnostic criteria for LBBB:
1) Total QRS duration >0.12 s.
2) No secondary R wave in V1 to indicate RBBB.
3) No septal q wave in V5, V6 or in leads further to the left (lead I and aVL in horizontal
hearts).
Additional features frequently seen in LBBB include:
1) The QRS complexes in some leads may be notched (leads I, aVL, V5 and V6).
2) The QRS complexes in V5, V6, I and aVL tend to have rsR’, "M" pattern or broad
monophasic R waves.
3) Secondary ST depression and possibly T wave inversion may be seen in the left precordial
leads (and in leads I and aVL).
4) ST segment elevation and abnormally tall T waves may be seen in the right precordial
leads.
5) Abnormally deep S waves in the right precordial leads.
6) Initial r waves in the right precordial leads may be very small or absent.
7) In the precordial leads, the dominant direction of the ST segments and the T waves tends to
be opposite to the dominant direction of the QRS complexes in any given lead.
The mean frontal plane QRS axis is usually within the normal range in uncomplicated
complete LBBB. The axis may move 15-30° towards the left when LBBB develops but
abnormal left axis deviation is not a routine feature of LBBB. The axis may sometimes be
indeterminate. When LBBB is combined with abnormal left axis deviation, extensive disease
of the left ventricular conducting system is likely to be present.