Incomplete Left Bundle Branch Block on ECG

What it looks like on ECG

Incomplete LBBB occupies the intermediate zone between normal intraventricular conduction and complete left bundle branch block (LBBB). It is not a strictly defined single pattern, but rather an electrocardiographic category where the key parameter is QRS duration. According to AHA/ACCF/HRS 2009 guidelines for adults, incomplete left bundle branch block is defined by a QRS duration of 110–119 ms and corresponding morphology. Certain clinical classifications cite a broader range starting from 100 ms — this discrepancy arises from different sources, and when analyzing a specific ECG, it is important to clarify which criterion is applied. With a QRS duration of 110–119 ms, features of left ventricular/left bundle activation may be present, but they do not have to be a "diminished copy" of complete LBBB — the morphology is often incomplete and variable, and the diagnosis requires evaluating the features in combination rather than relying on a single element.

In lead V1 (sometimes V2), a QS or rS pattern is most commonly observed: a small or absent initial r wave, and a deep, relatively wide S wave. The morphology in V1 is variable on its own and does not confirm the diagnosis — the decisive factor remains the combination of QRS duration with typical left ventricular morphology in leads I, aVL, V5–V6. In the left lateral precordial leads V5–V6 and limb leads I, aVL, the R wave is often broad, with a notch or slurring on the peak or descending limb — a consequence of delayed activation of the left ventricular free wall, although this feature may be weakly expressed or absent.

The Q wave in I, aVL, V5–V6 is usually absent because the interventricular septum activates from right to left, similar to complete block, though this is not an absolute mandatory finding in the incomplete form. The intrinsicoid deflection time (the interval from the onset of QRS to the peak of the R wave) in V5–V6 often exceeds 60 ms in complete LBBB — this is one of the morphological signs of complete block, but in the incomplete form, this parameter is variable and does not have to remain below this threshold, so it should not be used as a mandatory criterion for incomplete LBBB.

The ST segment and T wave may be discordant with the QRS complex (ST depression and T-wave inversion in leads with a dominant R wave); the severity of these changes is variable and not always milder than in overt block.

Real ECGs: 8 examples

ECG from the open PTB-XL dataset (Wagner P. et al., Scientific Data, 2020; PhysioNet), CC BY 4.0 license. Rendering by WhiteCoat Pro.

Mechanism

Normally, the ventricles are activated via the complex His-Purkinje fiber system, and both walls depolarize almost synchronously. Incomplete left bundle branch block is a category representing delayed or partially impaired conduction through the left bundle branch; it does not have a single mandatory mechanism. Possible mechanisms include slowed conduction through the main trunk or branches of the left bundle, block of a portion of the fibers (fascicular level) without complete interruption of conduction, and partial recruitment of the right-sided His-Purkinje system with transseptal spread of the wave to the left ventricle. In different cases, the contribution of these mechanisms varies. The net effect is a moderate prolongation of the QRS and a shift in the direction of the resultant depolarization vector, creating a complex morphology that partially resembles complete block, but without the complete loss of conduction through the bundle branch. The degree and nature of the conduction disturbance determine whether the QRS remains within the 110–119 ms range (incomplete block) or reaches 120 ms or more with full morphology (complete block).

Causes

• Isolated finding without apparent structural heart disease (more common in the elderly, but requires evaluation rather than automatic dismissal as normal)
• Early stage of degenerative fibrosis of the conduction system (Lenègre-Lev disease)
• Coronary artery disease
• Arterial hypertension with left ventricular hypertrophy
• Cardiomyopathies (dilated, hypertrophic)
• Aortic valve disease
• Myocarditis
• Dynamic or intermittent conduction block morphology in conduction system disease, including in the setting of progressive left bundle branch pathology

Differential diagnosis

• Complete LBBB — QRS duration ≥ 120 ms combined with typical morphology: broad/notched R in I, aVL, V5–V6, absence of normal q waves in I, V5–V6, predominantly negative QRS in V1–V3; a QRS duration of ≥ 120 ms alone is insufficient for the diagnosis of complete block, as such a complex may correspond to a nonspecific intraventricular delay, hyperkalemia, ventricular rhythm, and other conditions — the specific combined morphology is required
• Left ventricular hypertrophy — QRS duration alone does not reliably differentiate LVH from incomplete LBBB (in LVH, the QRS can reach 110–119 ms and be associated with intraventricular delay); the diagnosis of LVH relies on a combination of voltage criteria (Sokolow-Lyon index, Cornell criteria) and repolarization abnormalities rather than the width of the complex
• Nonspecific intraventricular conduction delay — QRS duration is increased, but the morphology does not match either the left or right bundle branch pattern
• Left anterior fascicular block — changes primarily affect the electrical axis (marked leftward deviation) and QRS morphology in the limb leads, while the precordial leads appear different
• Normal QRS variant in young individuals — with a QRS width of about 100 ms and without typical morphology in V1 and V5–V6, this represents a borderline normal variant rather than a block

Clinical significance and management

Isolated incomplete LBBB in an individual without signs of organic heart disease may be a benign finding, but it should not be routinely labeled as a normal variant, especially in the elderly: such morphology can reflect structural heart disease, ischemia, cardiomyopathy, or conduction system disease. The role of the functional diagnostics physician is not to make a diagnosis based on a single tracing, but to evaluate the finding in context: patient age, history of coronary artery disease, arterial hypertension, heart failure, and previous ECGs.

In newly detected incomplete LBBB, it is advisable to compare it with archival ECGs (if available). For newly detected or unclear incomplete LBBB, echocardiography should reasonably be considered, especially in the presence of symptoms, risk factors, physical examination abnormalities, or concomitant ECG changes; for a long-standing, stable, isolated finding in an asymptomatic patient, the necessity of the test is determined by the clinical context rather than performed automatically.

Persistent progression of the QRS duration to 120 ms or more on repeat ECGs may indicate progressive conduction system disease, but reversible and rate-dependent causes of QRS widening must first be excluded—such as rate-related aberration, drug effects, electrolyte disturbances, and measurement variability. QRS widening alone does not prove the progression of structural heart disease. The urgency of response is determined by symptoms, hemodynamics, and suspicion of acute coronary syndrome, rather than merely crossing the 120 ms QRS threshold; a new bundle branch block morphology in the setting of ischemic symptoms warrants urgent evaluation regardless of whether the QRS has reached 120 ms.

If acute coronary syndrome is suspected, prompt clinical evaluation, serial ECGs, and high-sensitivity troponin are required. Sgarbossa criteria and their modified version (Smith) were historically developed and validated for assessing ischemia in complete LBBB and paced rhythms; their validated thresholds primarily apply to wide QRS complexes, and applying them to incomplete block requires caution. Block morphology can obscure ST-T interpretation, so diagnosing ischemia in the setting of incomplete LBBB should not rely on a mechanical application of general ST criteria—decisions must be based on clinical presentation, ECG dynamics, troponin, and imaging when necessary.

Common interpretation errors

• Diagnosing complete left bundle branch block based solely on a QRS duration ≥ 120 ms without checking for typical morphology in V1–V3 and I, aVL, V5–V6
• Ignoring the finding as "insignificant" without assessing the clinical context, age, symptoms, and changes compared to prior ECGs
• Confusing it with left ventricular hypertrophy without checking voltage and repolarization criteria
• Mechanically applying Sgarbossa or Smith criteria to incomplete block when assessing ischemia without considering clinical presentation and serial changes
• Treating any QRS widening up to 120 ms on a repeat ECG as definitive progression of conduction system disease without ruling out rate-dependent aberration, medications, and electrolyte disturbances
• Failing to account for the fact that QRS morphology in V1–V2 during incomplete block may be uninformative if chest electrodes are misplaced

Frequently asked questions

Related terms

More in the section "Bundle Branch and Fascicular Blocks"

Entire section

Sources

• AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
• Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
• Orlov V.N. Guide to Electrocardiography

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.