Bundle branch blocks

Question 1

Right bundle branch block (RBBB)

Answer 1

Features according to the manual:

• > Supraventricular impulse.
• > Wide QRS (>120ms)
• > M complex or notched R in the V1 (R-S-Rprime complex)
• > Wide S in lead 1 and V
• > Prolonged intrisicoid deflection time in V1 (determined by chest leads only)
• > Secondary repolarization abnormalities

Lifeinthefastlane:

In RBBB, activation of the right ventricle is delayed as depolarisation has to spread across the septum from the left ventricle.
The left ventricle is activated normally, meaning that the early part of the QRS complex is unchanged.
The delayed right ventricular activation produces a secondary R wave (R’) in the right precordial leads (V1-3) and a wide, slurred S wave in the lateral leads.
Delayed activation of the right ventricle also gives rise to secondary repolarization abnormalities, with ST depression and T wave inversion in the right precordial leads.
In isolated RBBB the cardiac axis is unchanged, as left ventricular activation proceeds normally via the left bundle branch.

Diagnostic Criteria
Broad QRS > 120 ms
RSR’ pattern in V1-3 (‘M-shaped’ QRS complex)
Wide, slurred S wave in the lateral leads (I, aVL, V5-6)
Associated Features

ST depression and T wave inversion in the right precordial leads (V1-3)
Variations

Sometimes rather than an RSR’ pattern in V1, there may be a broad monophasic R wave or a qR complex.

Question 2

Left bundle branch block

Answer 2

According to manual:

• > Supraventricular impulse.
• > Wide QRS (> 120ms)
• > Slurred or notched R in V5, V6, Lead 1 and aVL.
• > Lack of Q in V5 and V6
• > rS or QS in v1 till v4.
• > Prolonged ID time in V6
• > Secondary repolarization abnormalities.

Frontal axis: usually left deviation, but this is not obligatory.

Lifeinthe..

Normally the septum is activated from left to right, producing small Q waves in the lateral leads.
In LBBB, the normal direction of septal depolarisation is reversed (becomes right to left), as the impulse spreads first to the RV via the right bundle branch and then to the LV via the septum.
This sequence of activation extends the QRS duration to > 120 ms and eliminates the normal septal Q waves in the lateral leads.
The overall direction of depolarisation (from right to left) produces tall R waves in the lateral leads (I, V5-6) and deep S waves in the right precordial leads (V1-3), and usually leads to left axis deviation.
As the ventricles are activated sequentially (right, then left) rather than simultaneously, this produces a broad or notched (‘M’-shaped) R wave in the lateral leads.

Question 3

Hemiblocks/ Fascicular blocks

Left anterior hemiblock-

Answer 3

Left axis deviation (usually between -45 and -90 degrees)
Small Q waves with tall R waves (= ‘qR complexes’) in leads I and aVL
Small R waves with deep S waves (= ‘rS complexes’) in leads II, III, aVF
QRS duration normal or slightly prolonged (80-110 ms)
Prolonged R wave peak time in aVL > 45 ms
Increased QRS voltage in the limb leads

**https://www.youtube.com/watch?v=nKohbPclvbk

In left anterior fascicular block (aka left anterior hemiblock), impulses are conducted to the left ventricle via the left posterior fascicle, which inserts into the infero-septal wall of the left ventricle along its endocardial surface.
On reaching the left ventricle, the initial electrical vector is therefore directed downwards and rightwards (as excitation spreads outwards from endocardium to epicardium), producing small R waves in the inferior leads (II, III, aVF) and small Q waves in the left-sided leads (I, aVL).
The major wave of depolarisation then spreads in an upwards and leftwards direction, producing large positive voltages (tall R waves) in the left-sided leads and large negative voltages (deep S waves) in the inferior leads.
This process takes about 20 milliseconds longer than simultaneous conduction via both fascicles, resulting in a slight widening of the QRS.
The impulse reaches the left-sided leads later than normal, resulting in a increased R wave peak time (the time from onset of the QRS to the peak of the R wave) in aVL.

Question 4

Left posterior hemiblock

Answer 4

Right axis deviation (> +90 degrees)
Small R waves with deep S waves (= ‘rS complexes’) in leads I and aVL
Small Q waves with tall R waves (= ‘qR complexes’) in leads II, III and aVF
QRS duration normal or slightly prolonged (80-110ms)
Prolonged R wave peak time in aVF
Increased QRS voltage in the limb leads
No evidence of right ventricular hypertrophy
No evidence of any other cause for right axis deviation

In left posterior fascicular block (aka left posterior hemiblock), impulses are conducted to the left ventricle via the left anterior fascicle, which inserts into the upper, lateral wall of the left ventricle along its endocardial surface.
On reaching the ventricle, the initial electrical vector is therefore directed upwards and leftwards (as excitation spreads outwards from endocardium to epicardium), causing small R waves in the lateral leads (I and aVL) and small Q waves in the inferior leads (II, III and aVF).
The major wave of depolarisation then spreads along the free LV wall in a downward and rightward direction, producing large positive voltages (tall R waves) in the inferior leads and large negative voltages (deep S waves) in the lateral leads.
This process takes up to 20 milliseconds longer than simultaneous conduction via both fascicles, resulting in a slight widening of the QRS.
The impulse reaches the inferior leads later than normal, resulting in a increased R wave peak time (= the time from onset of the QRS to the peak of the R wave) in aVF.

Question 5

bifascicular blocks…

Answer 5

right bunddle branch block: Wide QRS.
RSR’ in V1 and V2..

With either:
A) Left anterior hemiblock- Left axis deviation between -30–90degrees.
B) Left poster hemiblock- Right axis deviation.