Incomplete vs. Complete Bundle Branch Blocks: Understanding the Degrees of Conduction Delay

Bundle branch blocks (BBBs) occur when there's a delay or interruption in the electrical conduction through either the right or left bundle branch of the His-Purkinje system. These conduction abnormalities affect the timing and sequence of ventricular depolarization, leading to characteristic changes on the ECG.

But not all BBBs are created equal. They exist along a spectrum—with incomplete and complete forms—depending on how much the conduction is slowed or disrupted. This post breaks down the difference between incomplete and complete bundle branch blocks, how to recognize each on the ECG, and what they imply clinically.

Understanding the Bundle Branch System

After the electrical impulse exits the AV node, it travels through the bundle of His, which bifurcates into:

• The right bundle branch (RBB), leading to the right ventricle

• The left bundle branch (LBB), which further divides into:

  • The left anterior fascicle

  • The left posterior fascicle

Each branch is responsible for the rapid, synchronous activation of its respective ventricle.

When conduction through a bundle is delayed or interrupted, the ventricles depolarize asynchronously, and the ECG reflects this disturbance.

Complete Bundle Branch Block

Definition:
A complete BBB occurs when conduction through a bundle branch is entirely blocked, and one ventricle is activated late by impulse spread from the opposite side.

ECG Criteria:

• QRS duration ≥ 120 ms (≥ 3 small boxes)

• Characteristic morphology depending on which bundle is affected:

  • Right Bundle Branch Block (RBBB):

    • rsR' in V1

    • Wide, slurred S wave in leads I and V6

  • Left Bundle Branch Block (LBBB):

    • Broad, notched or slurred R wave in leads I, V5, and V6

    • Deep, broad S wave in V1

    • Absence of septal Q waves in left-sided leads

Mechanism:

• One ventricle depolarizes normally (via the intact bundle)

• The other ventricle is depolarized cell-to-cell, which is slower and less coordinated, leading to widened QRS and altered morphology

Incomplete Bundle Branch Block

Definition:
An incomplete BBB occurs when the affected bundle is partially impaired, allowing for delayed but not fully blockedconduction.

ECG Criteria:

• QRS duration between 100–119 ms

• Morphology resembling complete BBB, but without full prolongation of QRS

  • Incomplete RBBB:

    • rsR' in V1

    • QRS duration 100–119 ms

  • Incomplete LBBB:

    • LBBB-like morphology with QRS < 120 ms

Mechanism:

• Conduction through the bundle is slowed, but not interrupted entirely

• Ventricular depolarization is less delayed, resulting in mild QRS widening and intermediate morphology

Key Differences: Incomplete vs. Complete BBB

Clinical Implications

• Incomplete BBB is often seen in healthy individuals, particularly young adults or athletes (in the case of incomplete RBBB). It may also be a precursor to a complete block or appear in right heart strain (e.g., pulmonary embolism).

• Complete BBB may indicate underlying structural heart disease, especially LBBB, which is frequently associated with:

  • Coronary artery disease

  • Cardiomyopathy

  • Hypertension-related remodeling

• Recognition is essential because LBBB can obscure ischemic changes on ECG and may alter management, particularly in acute chest pain scenarios.

Important Considerations

• Rate dependence: Incomplete or complete BBBs can be rate-dependent, appearing only at certain heart rates.

• Fixed vs. intermittent: BBBs may be persistent or transient, depending on the underlying cause.

• Progression: Incomplete BBB may progress to complete block over time, particularly in degenerative conduction disease.

Summary Table

Clinical Takeaway

Bundle branch blocks are not simply "on or off"—they exist on a spectrum of conduction delay. Incomplete BBB represents partial impairment, while complete BBB indicates total disruption of fast conduction through one bundle. Recognizing these patterns on the ECG and understanding their anatomical and physiological underpinnings helps clinicians differentiate benign findings from more ominous ones.