ECG Differentiators: Complete vs. Incomplete RBBB—The Key Difference and What It Means Clinically

Understanding the distinction between Complete and Incomplete Right Bundle Branch Block (RBBB) is essential for accurate ECG analysis. While they share a common “look,” the difference lies entirely in the speed of conduction through the right ventricle.

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1. The Key Diagnostic Differentiator: QRS Duration

The primary way to distinguish between the two is the measurement of the QRS duration. Both patterns involve a delay in right ventricular depolarization, but the extent of that delay defines the category.

Feature	Incomplete RBBB	Complete RBBB
QRS Duration	Between 100 ms and 120 ms	≥ 120 ms (0.12s)
V1 Morphology	rsr’, rSR’, or rsR’	rsr’, rSR’, or rsR’ (M-shape)
Lead I / V6	Shallow, slurred S wave	Wide, deep, slurred S wave

Morphological Similarities

In both cases, you will see the classic “Rabbit Ears” in the right precordial leads (V1-V2). This happens because the left ventricle depolarizes first (the ‘r’), followed by a delayed signal traveling to the right ventricle (the ‘R”).

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2. Electrophysiology: What is Happening?

The Right Bundle Branch (RBB) is a long, thin, and fragile part of the conduction system.

• Complete RBBB: The electrical impulse is entirely blocked in the RBB. The right ventricle must wait for the impulse to travel through the left bundle and then “seep” through the myocardium (muscle-to-muscle conduction). This is a slow process, hence the QRS width ≥ 120 ms.
• Incomplete RBBB: The RBB is not completely blocked but conducts more slowly than usual, or there is mild right ventricular hypertrophy (RVH) causing a slight delay. The signal eventually gets through the conduction system, but it takes just a bit longer than the standard 100 ms.

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3. Clinical Significance

The implications of these findings vary significantly based on the patient’s presentation.

Incomplete RBBB

• Normal Variant: Often found in healthy, asymptomatic individuals (especially athletes) with no underlying heart disease.
• Right Ventricular Strain: Can be a sign of early or mild RVH, often seen in conditions like Atrial Septal Defect (ASD).
• Progression: It rarely progresses to a complete heart block on its own.

Complete RBBB

• Pathological Associations: More frequently associated with structural heart disease, such as coronary artery disease, hypertensive heart disease, or pulmonary embolism (acute onset).
• Diagnostic Interference: Unlike Left Bundle Branch Block (LBBB), a Complete RBBB does not usually hide an acute myocardial infarction (STEMI) on an ECG, as the initial phase of the QRS (septal depolarization) remains intact.
• Surgical Context: New-onset RBBB is a common occurrence following certain cardiac surgeries.

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Summary Checklist

When you see an rSR’ pattern in V1:

1. Check the width: Is it <120 ms? (Incomplete) or ≥120 ms? (Complete).
2. Check the S-wave: Is there a slurred S-wave in Lead I or V6? (Confirms RBBB).
3. Check the Axis: Is there a concomitant Axis Deviation? (Consider Bifascicular Block).