How do you differentiate between interventricular conduction delay and bundle branch block (BBB)?

Differentiating Interventricular Conduction Delay vs Bundle Branch Block

The key distinction is QRS morphology: bundle branch blocks (BBB) have specific QRS patterns in characteristic leads, while nonspecific intraventricular conduction delay (IVCD) shows QRS duration >110 ms without meeting the morphologic criteria for right or left BBB. 1

Diagnostic Criteria

Complete Left Bundle Branch Block (LBBB)

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

Complete Right Bundle Branch Block (RBBB)

• QRS duration ≥120 ms 1
• rsr', rsR', rSR', or rarely qR pattern in leads V1 or V2 (R' or r' deflection usually wider than initial R wave) 1
• S wave of greater duration than R wave or >40 ms in leads I and V6 1
• Normal R peak time in leads V5 and V6 but >50 ms in lead V1 1

Nonspecific Intraventricular Conduction Delay (IVCD)

• QRS duration >110 ms 1
• Does NOT meet morphology criteria for RBBB or LBBB 1
• This is the critical differentiating feature—the QRS is wide but lacks the characteristic patterns described above 1

Incomplete Bundle Branch Blocks

• Same QRS morphology criteria as complete BBB 1
• QRS duration between 110-119 ms (rather than ≥120 ms) 1
• For incomplete LBBB specifically: must also have left ventricular hypertrophy pattern, R peak time >60 ms in V4-V6, and absence of Q waves in leads I, V5, V6 1

Practical Differentiation Algorithm

Step 1: Measure QRS duration

• If QRS ≤110 ms: Not BBB or IVCD (consider fascicular blocks if axis deviation present) 1
• If QRS >110 ms: Proceed to Step 2 1

Step 2: Examine QRS morphology in specific leads

• Look at V1-V2 for RBBB pattern (rsr', rsR', rSR') 1
• Look at I, aVL, V5-V6 for LBBB pattern (broad notched/slurred R waves, absent Q waves) 1
• Check R peak times: V5-V6 for LBBB (>60 ms), V1 for RBBB (>50 ms) 1

Step 3: Apply diagnostic classification

• If morphology criteria met AND QRS ≥120 ms: Complete BBB 1
• If morphology criteria met AND QRS 110-119 ms: Incomplete BBB 1
• If morphology criteria NOT met AND QRS >110 ms: Nonspecific IVCD 1

Clinical Significance of the Distinction

The differentiation matters significantly for cardiac resynchronization therapy (CRT) decisions. Patients with true LBBB and QRS ≥150 ms show substantial benefit from CRT (HR 0.58 for death/HF hospitalization), while those with non-LBBB patterns (including IVCD) show no benefit (HR 0.97) 1. This represents a highly statistically significant difference (p=0.0001) 1.

Research suggests that IVCD may represent impaired conduction in working myocardium rather than His-Purkinje system pathology, which has different implications for ventricular activation patterns and treatment response 2, 3. The presence of initial anteriorly oriented electrical forces may indicate preserved left bundle conduction with myocardial impairment rather than true bundle branch block 3.

Common Pitfalls to Avoid

• Do not diagnose BBB based solely on QRS duration—morphology criteria must be met 1
• Do not confuse left ventricular hypertrophy with LBBB—LVH diagnosis becomes unreliable in the presence of complete LBBB and should generally not be attempted 1
• Do not assume all wide QRS complexes are BBB—many represent IVCD with different clinical implications 1
• In the presence of left anterior fascicular block, standard voltage criteria for LVH become unreliable; criteria including S wave depth in left precordial leads improve accuracy 1