What Does a Right Bundle Branch Block Look Like on ECG?

Electrical signals traveling through the right-sided wiring of the heart can become delayed, producing a widened QRS complex and a characteristic rSR’ shape in the right chest leads. The diagnostic signature sits in lead V1, where a small initial r, a deep S wave, and a tall second R’ form the “rabbit-ear” complex, paired with a wide, slurred terminal S wave in lead V6 and a QRS duration of 120 ms or more.

This page walks you through the conduction physiology behind that tracing, the waveform features in each lead, the complete-versus-incomplete cutoff, the repolarization changes that travel with it, and how to keep RBBB from being mistaken for something more dangerous.

The Conduction System Behind the Tracing

Electrical activation of your ventricles normally splits into two rapid highways after the signal leaves the bundle of His. The left bundle branch fans out across the muscular wall almost immediately, while the right bundle branch tracks along the right side before breaking into a thinner network that reaches the right ventricular myocardium.

When the right bundle branch fails to conduct, the right side of your heart loses its fast pathway. The signal still reaches the right ventricle, but it must spread slowly from one muscle cell to the next instead of zipping through the His-Purkinje system. That detour adds roughly 40 to 60 ms to ventricular activation compared with the normal route.

The surface ECG picks up that delay because right ventricular activation now happens after left ventricular activation. Earlier left-sided forces produce the first portion of the QRS complex, and late right-sided forces add an extra bump at the end. Once that mechanism is clear, every diagnostic feature below makes immediate sense for your interpretation.

Why the timing shift shows up on the tracing

ECG electrodes sense moving wave fronts that travel toward (positive deflection) or away from (negative deflection) the recording electrode. A delay on the right pushes the rightward forces to the end of ventricular activation, where they no longer blend with leftward forces but stand alone as a separate terminal deflection you can pick out by eye.

Anatomy of the Classic RBBB Waveform

The hallmark of RBBB lives in the right precordial leads, particularly V1. Look there first because V1 sits over the right ventricle and records its activation from a closer vantage than any other standard lead.

The rSR’ pattern in V1

Sometimes called the “M-shaped” complex or the rabbit-ears sign, an rSR’ pattern serves as the signature of RBBB in lead V1. The lowercase r is a small initial upward deflection from normal septal and left ventricular depolarization. The deep S wave that follows reflects the bulk of left ventricular activation moving away from V1. The R’, the second tall upward deflection, represents the late right ventricular forces arriving after left-sided activation has completed.

The slurred S wave in V5 and V6

Lead V6 looks at the heart from the left lateral side, so right ventricular activation there is moving away from the electrode. RBBB produces a wide, slurred terminal S wave in V5 and V6 because the right-sided signal is finishing late and dragging the final portion of the complex downward. The width and slurring of that terminal portion, not its depth, is what identifies RBBB in those leads.

Anatomy at a glance

• ▸V1 morphology: rSR’ (“M shape”) with a wide R’ as the dominant terminal feature.
• ▸V6 morphology: qRS pattern with the S wave wider than 40 ms and slurred on its downstroke.
• ▸QRS width: 120 ms or more in complete RBBB, 110 to 120 ms in incomplete RBBB.
• ▸Lead I: A wide, slurred S wave commonly mirrors what happens in the lateral chest leads.
• ▸Axis: Usually unchanged from your baseline, one feature separating RBBB from right ventricular hypertrophy.

QRS Duration and the Complete Versus Incomplete Cutoff

Measuring QRS width correctly is the difference between calling a pattern complete RBBB, incomplete RBBB, or no block at all. Lead selection and artifact handling both matter for your final read.

Measuring QRS width correctly

Use the lead showing the widest complex, typically V1, V2, or V6, because that lead captures the late rightward forces without cancellation. Start the calipers at the very first deflection away from the isoelectric baseline, even if the deflection is tiny, and end them where the complex returns to baseline. Do not include the T wave, which can blend with the S wave in tachycardic tracings.

The 120 ms threshold

A QRS duration of 120 ms or more is required for a tracing to be classified as complete RBBB. Anything between 110 and 120 ms in the setting of an rSR’ pattern in V1 falls into the incomplete RBBB category, sometimes called partial RBBB, and can be a normal variant in young people, athletes, or thin-chested individuals.

The 110 to 120 ms borderline zone

Borderline cases happen because of filter settings, paper speed calibration, or atrial enlargement stretching the QRS onset. Guidance from the AHA/ACC/HRS emphasizes morphology over duration at the border: an rSR’ in V1 with a terminal slurred S in V6 still qualifies as RBBB morphology even when the measured width sits right at the cutoff. Repeat the ECG at standard paper speed (25 mm/s) and gain (10 mm/mV) before finalizing the call.

ST and T-Wave Changes That Travel With RBBB

ST-segment and T-wave changes almost always appear in the same leads where the QRS abnormality lives whenever RBBB is present. These are expected, secondary repolarization changes that reflect the abnormal activation sequence rather than ischemia.

Expected discordant T-wave inversions

Leads V1 through V3 typically show ST depression and T-wave inversion when RBBB is present. Discordant is the rule: in leads where the QRS points up (V1, V2), the T wave points down. In leads where the QRS points down (V5, V6), the T wave points up. This is a normal consequence of the abnormal depolarization sequence.

When ST-T behavior should raise suspicion

Concordant ST elevation, meaning the T wave points the same direction as the dominant QRS deflection, in the setting of RBBB is abnormal and raises concern for acute ischemia.

The Sgarbossa criteria, originally developed for LBBB, also flag suspicious patterns in RBBB: concordant ST elevation of 1 mm or more, concordant ST depression of 1 mm or more in V2 or V3, and excessively discordant ST elevation of 5 mm or more above baseline all warrant urgent evaluation for acute MI.

“Heads up: T-wave inversions in V1 to V3 with RBBB are expected. Concordant ST deviation in those same leads is not.

Patterns that suggest pulmonary embolism or right heart strain

When RBBB appears with sinus tachycardia, S1Q3T3 morphology, and right precordial T-wave inversions, the picture shifts toward pulmonary embolism. Right heart strain from PE can produce a new RBBB on a tracing because sudden pressure overload disrupts conduction in the right bundle, a pattern the AHA/ACC/HRS guidelines flag as clinically meaningful.

RBBB Against Its Closest Mimics

RBBB on ECG has a small set of look-alikes that trip up even experienced eyes. A side-by-side view keeps each trap visible so you can sort them quickly.

RBBB versus ventricular tachycardia

The rSR’ in V1 superficially resembles the monomorphic R wave seen in some VT rhythms, but VT usually shows a much wider complex (>140 ms), extreme axis deviation, AV dissociation, and a fusion or capture beat. If your patient is stable and the QRS sits at 120 to 140 ms with a clear rSR’, RBBB is far more probable than VT.

RBBB versus Brugada pattern

ST elevation in V1 to V2 with a coved or saddleback appearance defines Brugada syndrome, and an accompanying rSR’ pattern can closely mimic RBBB. Distinguish the two by looking for a true terminal R’ characteristic of RBBB versus a high takeoff followed by downsloping ST elevation characteristic of Brugada.

RBBB versus right ventricular hypertrophy

Tall R waves in V1 paired with deep S waves in V5 and V6 point toward right ventricular hypertrophy, yet the QRS usually stays narrower than in RBBB and the axis tends to shift rightward beyond +90°. RBBB without RVH keeps the axis in its normal range.

Because the axis is a key separator, recognizing when it stays normal versus shifts right becomes the deciding factor.

Clinical Significance and When RBBB Demands Attention

RBBB on ECG ranges from a benign incidental finding in a young, healthy person to a marker of acute right ventricular strain or progressive conduction disease. Reading the clinical context makes the difference between reassurance and urgent workup.

Incidental RBBB in healthy hearts

A long-standing RBBB in an asymptomatic person with no structural heart disease usually carries no clinical significance. The conduction system has simply developed a delay on the right side, and the heart compensates through muscle-to-muscle spread. Follow-up is generally unnecessary unless new symptoms appear.

Underlying causes that warrant workup

• ▸Pulmonary embolism: Acute pressure overload can interrupt right bundle conduction and produce a new RBBB.
• ▸Right heart strain: Pulmonary hypertension, severe COPD, or sleep apnea can stretch the right bundle.
• ▸Ischemic disease: A new RBBB in the setting of anterior MI signals a large infarct involving the septal perforators.
• ▸Congenital heart disease: Atrial septal defect, Ebstein anomaly, and repaired tetralogy of Fallot commonly produce RBBB.
• ▸Degenerative conduction disease: Progressive fibrosis of the conduction system, sometimes progressing to bifascicular block.

Red flags that turn a tracing into a clinical priority

Syncope, chest pain, dyspnea, or hemodynamic instability paired with a new RBBB all push the tracing toward urgent evaluation. A new RBBB in the setting of acute chest pain is a high-risk finding because the same artery, the proximal left anterior descending, supplies the right bundle’s septal perforators and much of the anterior wall.

The 30-second bedside recognition workflow

1. 1Step 1: Look at V1. Spot the rSR’ pattern with a dominant terminal R’ deflection.
2. 2Step 2: Look at V6. Confirm a wide, slurred terminal S wave longer than 40 ms.
3. 3Step 3: Measure QRS. Use the widest lead; complete RBBB requires ≥120 ms.
4. 4Step 4: Check ST-T. Expect discordant changes in V1 to V3; flag concordant deviation.
5. 5Step 5: Read the clinical context. Stable, asymptomatic, chronic pattern calls for reassurance. New, symptomatic, or paired with PE/MI features calls for urgent workup.

You’ll encounter the ICD-10 code I45.1 in medical records for RBBB not elsewhere classified, useful when matching the diagnosis to documentation requirements. The Minnesota Code, used in epidemiology research, classifies RBBB under code 7.2 with a QRS threshold of 120 ms.

That coding threshold of 120 ms doubles as a clinical benchmark, and it is worth distilling into a take-home summary.

The Bottom Line

A recognizable tracing, not an illness itself, is what RBBB on ECG actually represents. The rSR’ in V1, the slurred terminal S in V6, and a QRS of 120 ms or more together identify it. ST-T changes in the right precordial leads are expected and secondary, while concordant changes point toward ischemia. Match the tracing to the clinical story and the pattern falls into place.

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