When is Left Bundle Branch Block (LBBB) on an electrocardiogram (ECG) considered a likely or possible myocardial infarction (heart attack) and what ECG changes suggest it?

LBBB and Myocardial Infarction on ECG

When LBBB Suggests Acute MI

LBBB should be considered indicative of acute myocardial infarction when concordant ST-segment elevation ≥1 mm is present in leads with positive QRS deflections (Sgarbossa criteria), which has >90% specificity for ongoing MI with an occluded infarct artery. 1, 2

Specific ECG Changes Indicating MI in LBBB

High-Specificity Findings (Sgarbossa Criteria)

The following ECG changes are highly specific (>90%) for acute MI in the presence of LBBB 2, 3:

• Concordant ST-segment elevation ≥1 mm in leads with positive QRS deflections (sensitivity 73%, specificity 92%) 3
• ST-segment depression ≥1 mm in leads V1-V3 (sensitivity 25%, specificity 96%) 3
• Excessive discordant ST-segment elevation ≥5 mm opposite to the QRS direction (sensitivity 19%, specificity 82%) 3

Additional Supportive Findings

• Marked ST abnormalities beyond what is expected from LBBB alone suggest acute ischemia 1
• Dynamic ST-segment changes on serial ECGs (obtained at 15-30 minute intervals) strongly support acute MI 1, 4
• New LBBB (confirmed by comparison with prior ECG) in the setting of chest pain raises high suspicion for acute MI 1

Clinical Context Requirements

LBBB alone is insufficient for MI diagnosis and requires clinical correlation 2:

• Chest pain lasting >20 minutes not responding to nitroglycerin 1
• Elevated cardiac biomarkers (troponin positive 1-2 hours after symptom onset) confirm the diagnosis 1, 5
• Atypical presentations including nausea, shortness of breath, fatigue, or syncope (seen in up to 30% of STEMI patients) 1

Critical Management Pitfalls

Common Errors to Avoid

• Do not wait for "perfect" ECG criteria - the Sgarbossa criteria have low sensitivity (10% in some studies) and should not be used to rule out MI 6, 7
• Do not assume all LBBB patients have MI - most LBBB patients in the emergency department do not have acute coronary occlusion 1
• Do not rely solely on discordant ST elevation - this finding is neither sensitive nor specific for MI 7

Recommended Diagnostic Approach

1. Obtain 12-lead ECG within 10 minutes of first medical contact 1, 2
2. Compare with prior ECG if available to determine if LBBB is new 1
3. Apply Sgarbossa criteria - if positive, treat as STEMI equivalent 2, 3
4. Perform serial ECGs at 15-30 minute intervals if initial ECG is non-diagnostic but clinical suspicion remains high 1, 8
5. Obtain point-of-care troponin 1-2 hours after symptom onset to aid decision-making 1
6. Consider emergency coronary angiography for patients with clinical suspicion of ongoing ischemia, new or presumed new LBBB, and positive biomarkers 1

Reperfusion Therapy Decision

In patients with clinical suspicion of ongoing MI and new or presumed new LBBB, reperfusion therapy should be initiated promptly, preferably with primary PCI (target door-to-device time ≤90 minutes) or fibrinolytic therapy if PCI unavailable, especially within 3 hours of symptom onset. 1, 2, 3

This aggressive approach is justified because 1, 5:

• LBBB patients have higher baseline mortality than those without bundle branch block
• These patients receive the greatest survival benefit from reperfusion therapy
• LBBB patients are historically undertreated despite their high-risk status