Clinical Overview

Incomplete left bundle branch block (ILBBB) is a partial delay in conduction through the left bundle branch of the His-Purkinje system — the pathway that normally carries the depolarizing impulse from the bundle of His down the left side of the interventricular septum to activate the left ventricle (Patra, Zhang, Brady, StatPearls, “Physiology, Bundle of His,” 2023). In ILBBB the left bundle is partially, not fully, impaired: the initial depolarization of the left ventricle is delayed, but the impulse still passes through the bundle itself rather than being forced to reach the left ventricle indirectly, cell-to-cell, across the septum from the right bundle branch the way a complete block requires. That difference in degree is what separates ILBBB from complete left bundle branch block: both share the same general morphology, but ILBBB’s QRS stays narrower, and its lateral-lead R waves and any V1/V2 S waves are less pronounced than the deep, fully developed pattern of the complete form (StatPearls, “Left Bundle Branch Block,” 2024). Sources converge on a QRS duration of roughly 110-120 ms for the incomplete grade, distinct from the ≥120 ms threshold for complete block: StatPearls (2024) states the incomplete band as 110-120 ms, and the Merck Manual Professional Edition (2024) states it as greater than 0.11 s but less than 0.12 s — the same band expressed the same way. LITFL’s ECG Library (2024) confirms the sub-120 ms upper bound but does not independently state a lower bound.

This dataset carries ILBBB as its own label, distinct from the complete-grade left-sided entry (labeled “LFBBB” in this dataset despite that label’s literal, fascicular-sounding name) and from the two true left-sided fascicular blocks (left anterior and left posterior fascicular block), which are narrower, non-ILBBB findings on their own pages. That three-way split on the left mirrors this dataset’s separate incomplete- and complete-grade labels on the right side (IRBBB and CRBBB).

Mechanistically, ILBBB reverses the normal septal activation sequence only partially: the left bundle still conducts, so left ventricular activation is delayed rather than rerouted, and the resulting broad or notched R wave in the lateral leads and, when present, S wave in V1-V2 are both milder than in complete block (StatPearls, 2024). Clinical sources describe left bundle branch block in general — complete and incomplete together — as less often an isolated, benign finding than right bundle branch block, and more often accompanied by identifiable structural or electrical heart disease (Merck Manual Professional Edition, 2024; WebMD, “Bundle Branch Block,” 2025). [CLINICAL REVIEW NEEDED: none of the sources reviewed for this page report prevalence, mortality, or heart-failure-association figures specific to the incomplete grade in isolation from complete LBBB; the general LBBB figures cited in this dataset’s Left Front Bundle Branch Block (complete LBBB) page should not be assumed to apply unchanged to ILBBB, and the isolated incomplete finding is far less studied in the literature than isolated incomplete right bundle branch block.] Because ILBBB is a milder degree of the same left-sided conduction delay, any right precordial or lateral-lead repolarization change it produces should likewise be read as milder than, and discordant in the same direction as, the corresponding change in complete LBBB — not as a separate ischemic signal on its own.

ILBBB produces no symptoms of its own; it is a conduction finding, not a rhythm disturbance, and most people with an isolated bundle branch block have no idea it is present until it is found incidentally on a routine ECG (Cleveland Clinic, “Left Bundle Branch Block,” 2022; MedicineNet, “How Serious Is Left Bundle Branch Block?,” 2026). Any symptoms a patient reports — fainting, presyncope, dizziness, or breathlessness — come from whatever underlying condition is causing or accompanying the block, not from the mildly delayed left ventricular activation itself (Cleveland Clinic, 2022; MedicineNet, 2026).

Causes and risk factors for left-sided bundle branch block, at either grade, include coronary artery disease (including prior myocardial infarction), hypertensive heart disease, valvular disease (notably aortic stenosis), cardiomyopathy, myocarditis, and age-related fibrodegenerative change in the conduction system (Merck Manual Professional Edition, 2024; Cleveland Clinic, 2022; WebMD, 2025). A block can also occur with no identifiable structural cause, though this is less common on the left side than on the right (Cleveland Clinic, 2022).

Interpretation Guide

Key Features:

• Rate: not a defining feature — ILBBB is a conduction finding superimposed on whatever the underlying rate happens to be
• Rhythm: not a defining feature — ILBBB describes a conduction delay, not the rhythm origin or regularity
• P waves: within normal limits; unaffected by the block itself
• PR interval: within normal limits (0.12-0.20 s) unless a separate, coexisting AV conduction disturbance is present
• QRS complex: mildly widened, roughly 110-119 ms per StatPearls and the Merck Manual (both state a sub-120 ms threshold; LITFL confirms the upper bound without independently stating a lower one); morphology shows a broad or notched R wave in leads I, aVL, V5, and V6, less marked than the fully developed monophasic R wave of complete LBBB, with an R wave peak time greater than 60 ms in the left-sided leads; the deep, well-formed S wave (QS or rS) seen in V1-V2 in complete LBBB may be present but is typically less pronounced (StatPearls, 2024)
• ST segment: any ST change is appropriately discordant to the delayed terminal forces, in the same direction complete LBBB produces but milder, given the smaller conduction delay
• T waves: any T wave change is similarly discordant and subtle; confirmatory at most, never the basis for grading complete versus incomplete
• QT interval: not independently diagnostic; the measured QT lengthens only slightly, in proportion to the modest QRS widening, so QT prolongation should not be read as a separate finding on this pattern
• Other findings: confirm the QRS duration to distinguish ILBBB (110-119 ms) from complete LBBB (≥120 ms, this dataset’s “LFBBB” label); left ventricular hypertrophy can produce a similarly widened QRS with delayed intrinsicoid deflection in the lateral leads that resembles this pattern — Sokolow-Lyon voltage criteria and LVH-typical ST-T changes, rather than the LBBB-typical discordant pattern above, help distinguish the two (StatPearls, 2024; LITFL, “Left Ventricular Hypertrophy,” 2026)

Key Leads

• Leads I, aVL, V5-V6 – Primary leads for diagnosis; show the broad or notched R wave with an R wave peak time over 60 ms that identifies delayed left ventricular activation, though the notching is typically more subtle than in complete LBBB
• Lead V1 – Secondary view; may show a small or shallow S wave, in contrast to the deep, well-formed QS or rS complex that marks complete LBBB in this lead
• Whichever lead shows the widest QRS complex – Used to confirm the measured duration actually falls in the 110-119 ms incomplete band rather than at or above the 120 ms complete-block threshold

Differential Diagnosis

• Left Front Bundle Branch Block (this dataset’s label for complete left bundle branch block) — the fully developed grade of the same conduction delay, with the same general lateral-lead and V1 morphology but a QRS duration at or above 120 ms and a deeper, more fully formed S wave in V1-V2; measuring the QRS is what separates the two
• Incomplete Right Bundle Branch Block (IRBBB) — the mirror-image incomplete block on the opposite side, producing an rsR’/rSR’ pattern in V1-V2 and a terminal S wave in the lateral leads rather than ILBBB’s monophasic lateral R wave and small or absent V1 S wave
• Left Ventricle Hypertrophy (LVH) — can produce a similarly widened QRS with delayed intrinsicoid deflection in the lateral leads, but LVH’s own voltage criteria (e.g., Sokolow-Lyon) and its characteristic ST-T pattern, rather than ILBBB’s discordant repolarization pattern, distinguish the two; the two can coexist, so QRS morphology and voltage should both be checked
• Bundle Branch Block (BBB) — this dataset’s bare, ungraded label spanning either side and either grade; a strip carrying only this label should not be assumed to meet the incomplete-left-sided criteria without measuring the QRS and checking the lateral-lead and V1 morphology directly
• Intraventricular Block (IVB) — this dataset’s label for a non-specific, widened QRS that does not meet either bundle branch block family’s defined morphology; it is a diagnosis of exclusion, so a strip meeting ILBBB’s own lateral-lead and V1 criteria belongs on this page instead

Treatment Brief

ILBBB found as an isolated finding in an asymptomatic patient generally needs no direct treatment — the conduction delay itself is not treated; any underlying cause is.

• Confirm lead placement and repeat the strip if the pattern is new or unexpected.
• Measure the QRS duration and check the lateral-lead and V1 morphology together; that pairing is what assigns the strip to ILBBB rather than complete LBBB or LVH, and reporting both is more useful than reporting a label alone.
• Compare against a prior ECG when available — a longstanding, unchanged pattern in an asymptomatic patient is reassuring, and a new one is not.
• Note whether the finding is isolated or accompanied by symptoms, a family history of cardiac disease, or other ECG abnormalities; left-sided conduction delay more often accompanies identifiable structural or electrical heart disease than an isolated right-sided finding does, so any of these raises the likelihood of an underlying cause and supports referral for further evaluation (e.g., echocardiography).
• Consider reversible causes rather than accepting the pattern at face value; correcting an underlying metabolic or drug-related cause changes the ECG appearance along with the underlying problem.
• If a strip that was previously ILBBB now measures at or above 120 ms with the fully developed lateral and V1 morphology, note the progression to complete block rather than re-applying the old label.