Clinical Overview

Right bundle branch block (RBBB) is delayed or interrupted conduction down the right bundle branch — the limb of the His-Purkinje system that carries each impulse from the bundle of His along the right side of the interventricular septum to the right ventricle. When that pathway is slowed or blocked, the left ventricle still depolarizes on schedule through the intact left bundle, and the right ventricle is activated late, by depolarization spreading cell to cell across the septum rather than travelling down its own fast pathway (StatPearls, 2023; Dodulík et al., Cardiology and Therapy, 2025).

This dataset carries three separate right-sided labels — a bare RBBB label plus separate complete (CRBBB) and incomplete (IRBBB) labels. The two graded forms are separated by QRS duration — at or above 120 ms the block is called complete; a QRS that is widened but stays below that threshold while showing the same right-precordial morphology is called incomplete (StatPearls, 2023; Merck Manual Professional Edition, 2024; Dodulík et al., 2025). Sources do not agree on exactly where the incomplete band starts: StatPearls and the 2025 narrative review put the floor at 100 ms, while the Merck Manual and the AHA/ACCF/HRS standardization statement put it above 110 ms, so a QRS between 100 and 110 ms may be read either way. [CLINICAL REVIEW NEEDED: sources disagree on the lower bound of the incomplete band (100 ms vs >110 ms); report the measured QRS duration rather than assigning a grade in that range.] In everyday clinical writing an unqualified “RBBB” usually means the complete form — the 2025 narrative review cited throughout this page contrasts incomplete RBBB against plain “RBBB” at ≥120 ms — which makes the bare label ambiguous in a way the two graded labels are not. [CLINICAL REVIEW NEEDED: the dataset does not document how it decided to file a given record under the bare RBBB label rather than under CRBBB or IRBBB, so a strip carrying this label should not be read as asserting a particular QRS duration. Measure the strip and use the complete or incomplete page for the graded criteria.]

Mechanically, the result is a QRS built in two halves: a normally timed first half, produced by septal and left ventricular depolarization, followed by a late, slurred second half as the right ventricle is reached by the detour. The degree of that delay is what separates the two grades — conduction that is slowed but not interrupted produces the milder, shorter-duration version, while conduction that is effectively interrupted produces the fully widened one (Dodulík et al., 2025). The block also does not have to be fixed. The right bundle branch has a longer refractory period than the left, so a supraventricular impulse arriving early — classically a short cycle following a long one during atrial fibrillation — can find it still refractory and conduct aberrantly, producing an RBBB-shaped complex for a single beat or a short run before normal conduction returns (StatPearls, Ashman Phenomenon, 2024). An RBBB pattern on a strip is therefore not automatically evidence of a permanent conduction defect.

Clinically, the block itself changes nothing about how the heart pumps; its significance is as a marker. Prevalence rises steadily with age, reaching roughly 11.3% by age 80 (StatPearls, 2023). The incomplete form is common — reported at 2% to 8% across general populations and 9% to 30% in athletes — and roughly 70% to 80% of incomplete cases are benign incidental findings, with the remaining 20% to 30% accompanying structural heart disease, pulmonary hypertension, or a congenital defect such as an atrial septal defect (Dodulík et al., 2025). Whether an isolated block is entirely harmless is not settled: a 2024 review reports that recent studies associate both the complete and the incomplete form with increased cardiovascular morbidity and mortality, while a standard clinical reference states that in the absence of heart disease or a more advanced conduction disturbance RBBB does not appear to indicate higher cardiovascular risk (Senftinger et al., Journal of Electrocardiology, 2024; Merck Manual Professional Edition, 2024). [CLINICAL REVIEW NEEDED: these two positions conflict; treat an isolated RBBB in a patient with cardiovascular risk factors as worth reporting rather than dismissing, and leave the risk judgement to the provider.]

Chest pain is the context that changes urgency most. Among unselected patients arriving at an emergency department with suspected acute coronary syndrome, RBBB was present in about 3%, rising to about 12% in a high-risk ST-elevation group, and it marked worse outcomes in that setting (Senftinger et al., 2024). New RBBB accompanying ischemic symptoms is associated with proximal left anterior descending artery occlusion and with larger infarcts, higher rates of cardiogenic shock, and higher mortality (Roshan, American Journal of Emergency Medicine, 2026). Unlike left bundle branch block, RBBB does not significantly interfere with the ECG criteria for prior myocardial infarction, and the diagnostic accuracy of STEMI criteria is not affected by its presence (Merck Manual Professional Edition, 2024; Senftinger et al., 2024), so the pattern is never a reason to stop looking at the ST segments — but an unremarkable ST segment does not clear the patient: RBBB patients without distinct STEMI signs on the ECG were often found to have an acute occlusion on angiography, and guidelines recommend acute invasive evaluation for high-risk RBBB regardless of ST-T deviations (Senftinger et al., 2024). A separate escalation is RBBB appearing together with a fascicular block: that bifascicular combination is associated with structural heart disease in 50% to 80% of cases, and carries a roughly 1% per year risk of progressing to complete heart block in asymptomatic patients, rising to about 17% per year in patients who have had syncope (LITFL, Bifascicular Block, 2024).

The block produces no symptoms of its own. It is typically found incidentally on an ECG, and examination may reveal only a split second heart sound (StatPearls, 2023). Any chest pain, breathlessness, palpitations, or syncope a patient reports comes from the underlying condition, not from the delayed right ventricular activation.

Recognized causes and risk factors span three groups. Right-heart pressure or volume overload — pulmonary embolism, pulmonary hypertension, chronic obstructive pulmonary disease, atrial septal defect, and right ventricular hypertrophy — stretches and remodels the conduction pathway. Direct damage to the bundle comes from ischemia (particularly right ventricular infarction), myocarditis, cardiomyopathy including arrhythmogenic right ventricular cardiomyopathy, infiltrative disease such as sarcoidosis or amyloidosis, age-related fibrodegenerative change (Lenègre’s or Lev’s disease), and iatrogenic injury from right heart catheterization or transcatheter aortic valve implantation. Functional or reversible causes include hyperkalemia, rate-related aberrant conduction, and — for the incomplete form specifically — the increased vagal tone and right ventricular remodeling of endurance training (StatPearls, 2023; LITFL, 2024; Dodulík et al., 2025).

Interpretation Guide

Key Features:

• Rate: not a defining feature — RBBB is a conduction finding superimposed on whatever the underlying rate happens to be. Rate does matter in one direction: a pattern that appears only at faster rates points to rate-related aberrant conduction rather than a fixed block
• Rhythm: not a defining feature — the block describes how the impulse reaches the ventricles, not where the rhythm originates. An RBBB-shaped complex appearing on isolated beats within an irregular supraventricular rhythm is more likely aberrant conduction than a new fixed block
• P waves: within normal limits. The delay sits well below the atria, so nothing about atrial depolarization changes; a normal P wave in front of every wide complex is also what keeps a wide-complex strip in supraventricular territory rather than ventricular
• PR interval: within normal limits (0.12-0.20 s). The block is downstream of the AV node and lengthens nothing here, so a prolonged PR alongside this pattern is a second, separate conduction problem to report next to it rather than part of the grade you are assigning
• QRS complex: the defining feature and the one that assigns the grade. Look for an rSR’ or rsR’ (“M-shaped”) pattern in V1-V2 and a wide, slurred S wave in the lateral leads (I, aVL, V5-V6); the R wave peak time in V1 is prolonged (>50 ms) while it stays normal in V5-V6. Then measure the duration: at or above 120 ms the finding is the complete form, and a QRS that is widened but stays below 120 ms — with the same morphology — is the incomplete form. A normal-duration QRS with an rSR’ in V1 is not a block at all; see the look-alikes below
• ST segment: ST depression in the right precordial leads (V1-V3), discordant to the delayed terminal QRS forces — a secondary consequence of the altered depolarization sequence, not a primary ischemic finding. Because the block does not obscure the ST segments the way left bundle branch block does, concordant ST elevation or reciprocal change should still be treated as a genuine ischemic signal
• T waves: appropriately discordant T wave inversion in the right precordial leads (V1-V3), produced by the same altered depolarization sequence as the ST change above. Treat it as confirmatory rather than diagnostic here — it tells you nothing about which grade of block you are looking at, so it never substitutes for measuring the QRS
• QT interval: not measured as an independent finding on this pattern. Widening the QRS lengthens the measured QT mechanically, so a QT that looks long on a strip carrying this block may be reporting nothing beyond the QRS width you just recorded — read the two numbers together rather than reporting a QT prolongation on its own
• Other findings: check the frontal-plane axis before calling the finding isolated — a marked left or right axis shift alongside the RBBB pattern indicates an accompanying fascicular block, which is a different and higher-risk finding. Be aware of two look-alikes that share the right-precordial morphology: the crista supraventricularis pattern, a benign depolarization variant reported in 13.3% of children with high sports activity, and Brugada syndrome, which can produce an rSR’-like appearance in V1-V3 and is a genuinely dangerous overread (LITFL, 2024; Dodulík et al., 2025)

Key Leads

• Lead V1 – Primary lead for the finding; carries the rSR’ (“M-shaped” or “rabbit ears”) pattern and the prolonged R wave peak time that identify delayed right ventricular activation. Read the relative heights of the two deflections as well as the notch: the R’ is often taller than the initial r, and is usually the wider of the two deflections (Dodulík et al., 2025; Surawicz et al., AHA/ACCF/HRS standardization statement, Circulation, 2009). Electrode position matters here more than anywhere else on the chest — V1 and V2 placed above the fourth intercostal space can manufacture a right-precordial pattern that is not there, most often an incomplete-RBBB appearance (LITFL, Misplacement of V1 and V2, 2024; Abobaker and Rana, Annals of Noninvasive Electrocardiology, 2021)
• Lead V2 – The second of the two leads the morphology criterion is actually written against: published criteria write the rsr’/rsR’/rSR’ morphology as present in V1 or V2, not V1 alone, so an equivocal V1 does not by itself settle whether the pattern is present (Surawicz et al., AHA/ACCF/HRS standardization statement, Circulation, 2009; Dodulík et al., 2025)
• Leads I, aVL, V5-V6 – Show the reciprocal finding, a wide and slurred terminal S wave, confirming the delayed forces are directed toward the right ventricle rather than reflecting a left-sided process. The lateral S wave is a required criterion rather than a bonus — published criteria list a prolonged terminal S in I, V5, and V6 alongside the right-precordial morphology — so a strip showing the rSR’ with normal lateral S waves deserves a harder look at the look-alikes before you grade it. These are also the leads where R wave peak time stays normal while V1’s is prolonged (Surawicz et al., AHA/ACCF/HRS standardization statement, Circulation, 2009)
• Leads II, III, aVF read against I and aVL – Not needed to recognize the block, but these are the leads that reveal the frontal-plane axis shift of an accompanying fascicular block, which changes the finding from an isolated RBBB into a bifascicular one
• Whichever lead shows the widest QRS complex – Used to measure the duration that assigns the complete or incomplete grade. Any one lead can under-read the QRS, because the earliest onset and the latest offset of the complex are not both visible in the same lead; the standard is a global measurement from the earliest onset to the latest offset across all leads, which is by definition longer than a single-lead reading (Surawicz et al., AHA/ACCF/HRS standardization statement, Circulation, 2009)

Differential Diagnosis

• Complete Right Bundle Branch Block (CRBBB) — not a different finding but the fully developed grade of this one. Distinguishing clue: measure the QRS. A duration at or above 120 ms with the rSR’ pattern in V1-V2 and a wide slurred S wave in the lateral leads is the complete form, and the CRBBB page carries its criteria and its specific risk picture.
• Incomplete Right Bundle Branch Block (IRBBB) — the milder grade of the same block. Distinguishing clue: the same right-precordial morphology with a QRS that is widened but stays below the complete-block threshold. Sources disagree on the floor of that band (100 ms per StatPearls, above 110 ms per the Merck Manual and the AHA/ACCF/HRS 2009 standardization statement), so a duration in the low 100s should be reported as measured rather than forced into one grade.
• Right Ventricle Hypertrophy (RVH) — also produces a tall R wave in V1 from a right-sided cause. Distinguishing clue: RVH gives a single dominant R wave rather than the notched rSR’, and the QRS stays under 120 ms unless a bundle branch block coexists; the two can be present together, so V1 morphology alone does not exclude either.
• Intraventricular Block (IVB) — this dataset’s label for a non-specific intraventricular conduction delay. Distinguishing clue: that finding is defined by exclusion — a widened QRS whose morphology does not reproduce the rSR’-in-V1 pattern of a right bundle branch block or the criteria for a left one. If the classic right-precordial morphology is present, the finding belongs on this page rather than there.
• Ventricular Premature Beat (VPB) — a wide, early complex that can be mistaken for an aberrantly conducted supraventricular beat, and vice versa. Distinguishing clue: an aberrant beat typically follows a long-then-short cycle sequence, takes a right bundle branch morphology, and lacks the fixed coupling interval and fully compensatory pause that characterize a true ventricular ectopic beat (StatPearls, Ashman Phenomenon, 2024).

Treatment Brief

The block itself is not treated. What a monitoring technician or nursing student owns here is measuring it correctly, deciding whether it is old or new, and recognizing the two or three contexts where it stops being an incidental finding.

• Measure and record the QRS duration. That single measurement is what assigns the strip to the complete or incomplete grade, and reporting the number is more useful than reporting a label the dataset may have applied loosely.
• Confirm lead placement — particularly the height of V1 and V2 — and repeat the tracing if the pattern is new or unexpected, since misplaced right precordial electrodes can manufacture the appearance of a block (LITFL, Misplacement of V1 and V2, 2024; Abobaker and Rana, 2021).
• Compare against a prior ECG whenever one exists. A longstanding, unchanged pattern in an asymptomatic patient is reassuring; a new one is not.
• Notify the provider promptly if the pattern is new and the patient has chest pain or other symptoms suggestive of acute coronary syndrome. This combination is handled as a high-risk presentation rather than as an incidental conduction finding (Senftinger et al., 2024; Roshan, 2026).
• Check the frontal-plane axis before calling the finding isolated. RBBB with a fascicular block is a bifascicular pattern with a real risk of progressing to complete heart block, and that risk rises sharply if the patient has had syncope or presyncope — flag it rather than treating it as routine (LITFL, Bifascicular Block, 2024).
• Consider reversible and time-critical causes rather than accepting the pattern at face value: hyperkalemia, acute pulmonary embolism, and acute right ventricular ischemia can all produce or unmask it, and each changes management immediately.
• Watch whether the pattern is constant or intermittent. A wide complex that appears only on early beats or only at faster rates is aberrant conduction, not a newly acquired fixed block, and should be described that way.