Bundle Branch Blocks

Bundle branch blocks (BBBs) are significant findings on an electrocardiogram (ECG) that are characterized by a wide QRS complex. These blocks indicate that the electrical conduction through the bundle branches of the heart is impaired. The underlying mechanisms for the ECG patterns of Right Bundle Branch Block (RBBB) and Left Bundle Branch Block (LBBB) can be explained through principles of cardiac depolarization. Understanding how the septum is depolarized from left to right is foundational, as the left ventricle (LV) has a greater mass, which influences the resulting ECG pattern. When excitation occurs towards a lead, it produces an upward deflection on the ECG.

Left Bundle Branch Block (LBBB)

LBBB occurs when the normal direction of septal depolarization is reversed, shifting from left to right to a right to left orientation. This reversal causes a small Q wave to appear in lead V1, and a prominent R wave to be observed in lead V6. The electrical impulse first spreads to the right ventricle (RV) through the intact right bundle branch before activating the LV via the septum. This alteration in the sequence of activation extends the QRS duration to greater than 120 ms, leading to the absence of the normal septal Q waves in the lateral leads.

ECG Features of LBBB

The overall electrical depolarization direction (from right to left) results in:

Tall R waves in the lateral leads (I, aVL, V5-V6)
Deep S waves in the right precordial leads (V1-V3)
Usually a left axis deviation

As the ventricles are activated sequentially (right first, then left), this causes a broad or notched ('M'-shaped) R wave in the lateral leads.

Diagnostic Criteria for LBBB

The specific diagnostic criteria for identifying LBBB on an ECG include:

QRS duration of >120 ms
Dominant S wave present in V1
Broad monophasic R wave in lateral leads (I, aVL, V5-V6)
Absence of Q waves in the lateral leads (while small Q waves may still be present in aVL)

Associated Features

Additional features often seen with LBBB involve:

T wave inversion in the lateral leads (I, aVL, V5, V6)
Poor R wave progression in chest leads
Left axis deviation

QRS Morphology in LBBB

In LBBB, the morphology of the QRS complex presents variations:

R wave in the lateral leads may appear as:

‘M’-shaped
Notched
Monophasic
RS complex

In lead V1, the QRS complex morphology may be:

rS complex (small R wave, deep S wave)
QS complex (deep Q/S wave with no preceding R wave)

Right Bundle Branch Block (RBBB)

RBBB occurs when the activation of the right ventricle is delayed since depolarization must spread from the left ventricle across the septum. In this pattern, the early portion of the QRS complex remains unchanged due to normal activation of the left ventricle. However, the delayed activation of the right ventricle generates a secondary R wave (R’) seen in the right precordial leads (V1-V3) and produces a wide, slurred S wave in the lateral leads.

Dynamic Changes and Features in RBBB

The delayed activation of the right ventricle also leads to secondary changes in repolarization, noticeable as ST segment depression and T wave inversion in the right precordial leads. In isolated instances of RBBB, the cardiac axis remains unaffected because the left ventricle activates normally through the left bundle branch.

Diagnostic Criteria for RBBB

To diagnose RBBB, certain ECG criteria must be met:

Broad QRS duration >120 ms
RSR’ pattern noted in V1-V3 (often described as ‘M-shaped’ QRS complex)
Wide, slurred S wave in lateral leads (I, aVL, V5-V6)

Associated Features of RBBB

There are common features associated with RBBB, including:

ST depression and T wave inversion in the right precordial leads (V1-V3)

Variations can occur, such as instead of the RSR’ pattern in V1, one may observe a broad monophasic R wave or a qR complex.

Examples

Examples of both LBBB and RBBB can be analyzed through specific case studies or patient ECGs, allowing for practical understanding and recognition of these conditions on an ECG.

Understanding Bundle Branch Blocks

Understanding Bundle Branch Blocks