ECG Appearance of V1 in Left Bundle Branch Block (LBBB)
In Left Bundle Branch Block (LBBB), lead V1 typically shows a predominantly negative QRS complex with an rS or QS pattern, characterized by a small initial r wave (if present) followed by a deep, wide S wave.
Characteristic Features of V1 in LBBB
The appearance of lead V1 in LBBB has several distinctive features:
QRS Morphology
- QS pattern: Many patients with LBBB show a QS pattern in V1 (complete absence of initial r wave) 1
- rS pattern: When an r wave is present, it is typically small (≤20 ms in duration) 2
- When an rS pattern is present, the ratio between the descending and ascending S wave components can predict whether the LBBB is likely to be correctable with His bundle pacing 1
Other V1 Characteristics
- Slurred or notched downstroke of the S wave
- QRS duration ≥120 ms in adults (part of LBBB criteria) 3
- ST segment and T wave typically opposite in direction to the QRS complex (discordant) 3
Diagnostic Criteria Involving V1
According to AHA/ACCF/HRS recommendations, complete LBBB includes the following criteria that involve lead V1 3:
- QRS duration ≥120 ms in adults
- Slurred predominant S wave in the right precordial leads (including V1)
- Normal R peak time in V1 but prolonged (>60 ms) in V5 and V6
- Absent q waves in leads I, V5, and V6 (though a narrow q wave may be present in aVL)
Clinical Significance of V1 Appearance
The specific morphology in V1 has important clinical implications:
- A QS pattern in V1 has been shown to predict correction of LBBB with His bundle pacing with a positive predictive value of 94.1% 1
- An initial r wave ≥1 mm in V1 suggests intact left-to-right ventricular septal activation and may indicate incomplete LBBB 4
- The presence of an r wave ≥1 mm in V1 in patients with otherwise typical LBBB may identify patients at lower risk of complete heart block during right heart catheterization 4
Distinguishing LBBB from Other Conditions
The appearance of V1 helps differentiate LBBB from:
- Right Bundle Branch Block (RBBB): RBBB shows an rSR' pattern in V1 (rather than rS or QS) 3
- Left Ventricular Hypertrophy (LVH): LVH with intraventricular conduction delay can mimic LBBB but may have different V1 morphology 3
- Bilateral Bundle Branch Block: Shows features of both RBBB (terminal R wave in V1) and LBBB (absence of S wave in leads I and aVL) 5
Strict vs. Conventional LBBB Criteria
Recent research suggests that strict LBBB criteria (including specific V1 morphology requirements) improve specificity in diagnosing true LBBB, which is important for selecting patients who would benefit from cardiac resynchronization therapy 6.
The strict criteria include:
- LV conduction delay
- QRSd ≥140 ms in men or ≥130 ms in women
- Mid-QRS notching in at least two of leads I, aVL, V1, V2, V5, and/or V6 6
Myocardial Infarction Detection in LBBB
In patients with LBBB, the Sgarbossa criteria use ST-segment changes in V1-V3 to help identify acute myocardial infarction:
- ST-segment depression ≥1 mm in leads V1-V3 is one criterion that suggests myocardial infarction in the presence of LBBB 3
Understanding the normal appearance of V1 in LBBB is crucial for correctly identifying abnormal ST-segment changes that may indicate acute coronary syndromes.