What are the electrocardiographic (ECG) criteria for diagnosing an incomplete left bundle branch block (LBBB)?

ECG Criteria for Incomplete Left Bundle Branch Block

Incomplete left bundle branch block (LBBB) is diagnosed when QRS duration is between 110 and 119 ms in adults, along with left ventricular hypertrophy pattern, R peak time greater than 60 ms in leads V4-V6, and absence of q waves in leads I, V5, and V6. 1

Diagnostic Criteria for Incomplete LBBB

According to the AHA/ACCF/HRS recommendations for standardization and interpretation of the ECG, the following criteria must be met for diagnosing incomplete LBBB:

1. QRS duration:

   • 110-119 ms in adults
   • 90-100 ms in children 8-16 years of age
   • 80-90 ms in children less than 8 years of age 1

2. Presence of left ventricular hypertrophy pattern 1

3. R peak time greater than 60 ms in leads V4, V5, and V6 1

4. Absence of q waves in leads I, V5, and V6 1

Comparison with Complete LBBB

It's important to distinguish incomplete LBBB from complete LBBB, which has more stringent criteria:

Complete LBBB requires:

• QRS duration ≥120 ms in adults
• Broad notched or slurred R waves in leads I, aVL, V5, and V6
• Absent q waves in leads I, V5, and V6 (though a narrow q wave may be present in aVL without pathology)
• R peak time >60 ms in leads V5 and V6 but normal in leads V1-V3
• ST and T waves usually opposite in direction to QRS 1

Morphological Features

While not explicitly stated in the incomplete LBBB criteria, the morphological features typically include:

• QRS complexes that resemble those of complete LBBB but with shorter duration
• Broad notched or slurred R waves in lateral leads (I, aVL, V5, V6)
• Absence of normal septal q waves in leads I, V5, and V6
• Delayed intrinsicoid deflection (R peak time) in left-sided leads

Clinical Significance

Incomplete LBBB represents a partial conduction delay in the left bundle branch system. It may:

• Represent an early stage of a progressive conduction disease that could eventually develop into complete LBBB
• Be associated with underlying structural heart disease, particularly left ventricular hypertrophy
• Affect the interpretation of other ECG findings, particularly when evaluating for left ventricular hypertrophy 1

Diagnostic Challenges

Several pitfalls exist when diagnosing incomplete LBBB:

1. Differentiation from nonspecific intraventricular conduction delay:

   • Nonspecific intraventricular conduction delay is defined as QRS duration >110 ms without meeting specific morphological criteria for bundle branch blocks 1

2. Overlap with left ventricular hypertrophy:

   • The presence of LVH is part of the criteria for incomplete LBBB, making it challenging to distinguish between the two conditions 1

3. QRS duration measurement:

   • Accurate measurement of QRS duration is critical, as just a few milliseconds can change the diagnosis between incomplete and complete LBBB 1

4. Age-specific considerations:

   • Different QRS duration thresholds apply to children of different age groups 1

By applying these standardized criteria, clinicians can accurately identify incomplete LBBB on ECG and distinguish it from other conduction abnormalities.