Bundle Branch Blocks Flashcards
Normal Conduction Recap
SA node initiates electrical activity
Conduction across RA and LA
AV node – short delay
Bundle of His – the only A-V electrical pathway
The bundle of His divides into left and right bundle branches.
The LBB divides further into the
- left anterior superior fascicle
- Left posterior inferior fascicle
These divide further into the Purkinje fibres allowing very rapid activation of the left and right ventricles resulting in a narrow QRS complex
What are Bundle Branch Blocks
A delay in conduction in either the right or the left bundle branches Resulting in a ventricular complex that is both prolonged in duration and abnormal in morphology
Delay in conduction Can be caused by
Coronary artery disease
- A blocked coronary artery will no longer be able to supply that part of the conduction system with sufficient oxygen
Fibrosis/Calcification
- Thickening of the conduction fibres
Hypertrophy
- Increase muscle mass surrounding and encroaching upon the conduction fibres
Right Bundle Branch Block
In RBBB the right ventricle is stimulated by the impulses travelling from left ventricle by cell to cell conduction instead of through the fast conducting RBB and Purkinje fibre network
- The septum is depolarises as normal
from left to right - The left ventricle is
depolarised as normal - Finally the right ventricle is depolarised late due to slow conduction through the myocardial cells resulting in
a wide QRS complex
in an anterior direction
RBBB ECG findings
Leads oriented towards RV (V1) have RSR1 complex
- The wide distal limb of the complex (R1) is due to the stimulus spreading through the RV
- since it is late it is unopposed by LV depolarisation and is of
high magnitude.
Leads oriented towards LV (V5,V6,
aVL & lead I) have abroad slurred S wave
- due to late depolarisation of the
RV free wall away from electrode V6
RBBB ECG criteria
QRS duration exceeds 0.12 seconds
RSR1 complex in V1
Delayed slurred S wave in I, aVL, V5 & V6 (left lateral leads)
ST/T components are opposite in direction to the terminal QRS.
As depolarisation occurs abnormally, repolarisation will also be in an abnormal manner ( this is secondary to the block and does not predispose primary ST/T)
Partial or incomplete RBBB can be diagnosed when the morphology of RBBB is present but the QRS duration does not exceed 0.12s
Significance of RBBB
Occasionally seen in normal subjects
Congenital heart disease
- ASD
Coronary artery disease
- Myocardial damage
Right ventricular hypertrophy or strain
- Pulmonary embolism
- High right heart pressures
- Advanced heart failure
- Myocarditis
RBBB and MI
Any abnormal/pathological Q waves of an old MI will not be masked by the RBBB pattern.
There is no alteration of the initial part of the QRS complex resulting from Phase 1 of ventricular depolarisation (IVS) which occurs normally from left to right.
Abnormal Q wave can still be seen
Left Bundle Branch Block
In LBBB the left ventricle is activated by the impulses travelling through the RBBB and from the right ventricle by cell to cell conduction instead of through the fast conducting LBB and Purkinje fibre network
- Impulses pass abnormally to the left of the septum, therefore depolarising it from right to left.
- RV depolarisation follows, small magnitude.
- Finally the LV is depolarised late due to slow conduction through the myocardial cells
ECG Criteria for Left Bundle Branch Block
QRS duration exceeds 0.12 seconds
Wide, notched QS complexes in V1 (IVS & then LV depolarisation away from V1)
Wide, notched (M shaped) QRS in I, aVL, V5 & V6 (left lateral leads)
Significance of LBBB
Always indicative of organic heart disease.
Occurs in ischaemic heart disease.
MI
Activation criteria for further investigation
Occurs in hypertension.
LBBB and MI
MI should not be diagnosed in the presence of LBBB.
- ST elevation masked by broad bizarre QRS complex
- Require further investigations
Pathological Q waves are masked by the LBBB pattern
Partial/Incomplete LBBB is diagnosed when the pattern of LBBB is present but the duration of the QRS complex does not exceed 0.12 seconds.
Hemiblocks
Block can occur in either the anterior or posterior fascicle of the Left Bundle Branch
The Anterior and Posterior Fascicles of the LBB conduct to the anterosuperior and posteroinferior regions of the Left Ventricle
**Affects the direction not the duration of the QRS complex **as the conduction disturbance primarily involves the early phases of activation
The diagnosis of hemiblock depends on the hexaxial reference system
Mean frontal QRS axis basics
Normal mean frontal axis lies between Lead aVL (-30 degrees) and aVF (+90 degrees)
If the axis is to the left of aVL <-30 degrees = abnormal Left Axis Deviation Lead I +ve and Lead II and III are -ve
If the axis is to the right of aVF >+90 degrees = abnormal Right Axis Deviation Lead I –ve and Lead II and III are +ve
Left Anterior Hemiblock
Leads to a delay in activation of the anteriosuperior portion of the left ventricle
Initial LV activation will be via the posterior fascicle to the posteroinferior region and will be directed inferiorly and to the right
QRS duration less than 0.12s
LAD
rS complex in II,III,aVF
Left anterior hemiblock on ECG
On the ECG this results in an initial small positive deflection or r wave in the inferior leads II, III and aVF
The anterosuperior region is activated by conduction from the posteroinferior region, the resultant depolarisation will be superiorly directed
There will be an R wave in I and aVL and an S wave in II, III and aVF, conduction will be slightly slower through the myocardium
Activation of the anterosuperior region is delayed and unopposed by the activity from the rest of the ventricles
The superiorly directed wave will be larger than the initial inferiorly directed wave
The mean frontal QRS axis will be superiorly directed resulting in Left Axis Deviation
What criterias must be satisfied to diagnose left anterior hemiblock?
To diagnose Left Anterior Fascicular Block two criteria must be satisfied
1. The initial direction of the ventricular activation must be inferior and to the right therefore there must be initial r waves in leads II, III and aVF
2. Left Axis Deviation
Lead I must be predominantly +ve and both leads II and III must be predominantly -ve
Left Posterior Hemiblock
Causes a delay in activation of the posteroinferior portion of the left ventricle
Initial septal depolarisation occurs abnormally from right to left as the left bundle now conducts less quickly due to the LPFB.
- Initial +ve r wave in leads I and aVL and
- Initial –ve q waves in leads II, III and aVF
QRS duration less than 0.12s
Late activation of the posterioinferior portion of the left ventricle will be via the anterior fascicle and will be directed inferiorly and to the right
There will be right axis deviation
How to diagnose left posterior fascicular block
A diagnosis of left posterior fascicular block can only be made in the absence of other causes of right axis deviation
- right ventricular hypertrophy or strain,
- lateral MI,
- tricyclic overdose
- acute pulmonary embolus.
Bifascicular Block
Most common is right bundle branch block and left anterior fascicular block
RBBB pattern in V1 and V6 and a LAFB pattern in lead II, III and aVF
QRS duration ≥ 0.12seconds Axis is -45 to -120 degrees, LAD
Trifascicular Block
1st degree atrio-ventricular block – PR interval >0.2secs
Right bundle branch block
Left anterior fascicular block
RBBB pattern in V1 and V6 and a LAFB pattern in lead II, III and aVF
QRS duration ≥ 0.12seconds Axis is -45 to -120 degrees, LAD
RBBB
- QRS > 0.12s, RSR1 V1, slurred S V6
LBBB
- QRS > 0.12s, Deep S V1, notched M shape V6
Incomplete RBBB
- Normal QRS <0.12s
- RSR1 V1, slurred S V6
Incomplete LBBB
- Normal QRS <0.12s
- Deep S V1, notched M shape RsR1 V6
Left Anterior Fascicular Block
- Normal QRS duration, Initial r waves in II, III and aVF initial q waves in I and aVL with LAD
Left Posterior Fascicular Block
- Normal QRS duration, Initial r waves in I and aVL initial q waves in II, III and aVL with RAD
Bifascicular Block
- RBBB + LAFB or LPFB
Trifascicular Block
- 1st degree AV block, RBBB + LAFB or LPFB
