Recognizing LBBB, RBBB, and VT on ECG
Left Bundle Branch Block (LBBB)
Complete LBBB requires QRS duration ≥120 ms with broad notched or slurred R waves in lateral leads (I, aVL, V5, V6), absent Q waves in these leads, and QS or rS pattern in V1-V2. 1
Essential Diagnostic Criteria:
- QRS duration ≥120 ms in adults 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 appear in aVL without myocardial pathology) 1
- R peak time >60 ms in leads V5 and V6 but normal in V1, V2, V3 1
- ST and T waves opposite in direction to QRS (secondary repolarization changes) 1
Key Morphologic Features:
- Lead V1/V2 must show QS or rS pattern (predominantly negative deflection with slurred S wave) 2
- Leads V5/V6 must show monophasic notched or plateau-topped R waves without preceding q waves 2
- Mid-QRS notching or slurring in at least 2 lateral leads (I, aVL, V5, V6) is highly specific 2
- QRS duration >155 ms has high specificity for true complete LBBB 2
Incomplete LBBB:
- QRS duration 110-119 ms with same morphology criteria as complete LBBB 1
- Presence of left ventricular hypertrophy pattern 1
- R peak time >60 ms in leads V4, V5, V6 1
Right Bundle Branch Block (RBBB)
Complete RBBB requires QRS duration ≥120 ms with rsr', rsR', or rSR' pattern in V1-V2, and S wave duration greater than R wave or >40 ms in leads I and V6. 1
Essential Diagnostic Criteria:
- QRS duration ≥120 ms in adults 1
- rsr', rsR', or rSR' pattern in leads V1 or V2 (classic "M-shaped" or "rabbit ears" pattern) 1
- S wave duration greater 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
Incomplete RBBB:
- QRS duration 110-119 ms with same morphology criteria as complete RBBB 1
Clinical Context:
- Bifascicular block (RBBB combined with left anterior or posterior fascicular block) or other intraventricular conduction abnormalities with QRS ≥120 ms are high-risk features requiring prompt evaluation 1
Ventricular Tachycardia (VT)
VT presents as wide QRS tachycardia (QRS >120 ms) with rate typically >100 bpm, often with AV dissociation, fusion beats, or capture beats when present. 1
High-Risk ECG Features Suggesting VT:
- Non-sustained VT (runs of wide QRS tachycardia) 1
- Wide QRS complex >120 ms with regular or irregular rhythm at rate >100 bpm 1
- AV dissociation (independent atrial and ventricular activity) when identifiable
- Fusion beats (hybrid morphology between sinus and ventricular beats) when present
- Capture beats (narrow QRS complexes during wide complex tachycardia) when present
Distinguishing VT from SVT with Aberrancy:
- RBBB-type VT with R-S ratio >1 in V1 suggests ventricular origin 3
- Activation time from QRS onset to right ventricular apex helps localize septal (shorter time: 32-71 ms) versus lateral (longer time: 71-187 ms) origin 3
- Extreme axis deviation (northwest axis) favors VT
- QRS concordance (all positive or all negative) in precordial leads favors VT
Clinical Context Requiring Urgent Evaluation:
- Syncope during exertion or supine position 1
- Palpitations at time of syncope 1
- Family history of sudden cardiac death 1
- Severe structural or coronary artery disease (heart failure, low LVEF, prior MI) 1
Common Pitfalls and Caveats
LBBB Pitfalls:
- Not all QRS >120 ms with left axis is LBBB - may represent left ventricular hypertrophy with intraventricular conduction delay 2
- Strict morphologic criteria are essential - notching/slurring in lateral leads is present in 100% of true LBBB 4
- Q waves in lateral leads exclude LBBB - their presence suggests prior infarction or other pathology 1, 4
RBBB Pitfalls:
- Incomplete RBBB is generally benign and does not require specific workup in isolation 5
- RBBB pattern with ST-elevation in V1-V3 (Brugada pattern) is high-risk and requires urgent evaluation 1
VT Recognition Pitfalls:
- Wide complex tachycardia should be assumed VT until proven otherwise in patients with structural heart disease 1
- Pre-excited QRS complex (Wolff-Parkinson-White) can mimic VT and is high-risk 1
- Inadequate sinus bradycardia <50 bpm or sinoatrial block in absence of negative chronotropic medications is high-risk 1