What are the criteria to diagnose ST-Elevation Myocardial Infarction (STEMI) in patients with Left Bundle Branch Block (LBBB)?

Criteria to Diagnose STEMI in LBBB

The Sgarbossa criteria remain the most validated tool for diagnosing STEMI in patients with LBBB, with concordant ST-segment elevation ≥1 mm having the highest specificity (92%) for acute myocardial infarction. 1

Sgarbossa Criteria for STEMI in LBBB

The following ECG findings are independently predictive of acute myocardial infarction in patients with LBBB:

1. ST-segment elevation ≥1 mm concordant with the QRS complex (sensitivity 73%, specificity 92%) 1
2. ST-segment depression ≥1 mm in leads V1-V3 (sensitivity 25%, specificity 96%) 1
3. ST-segment elevation ≥5 mm discordant with the QRS complex (sensitivity 19%, specificity 82%) 1

Clinical Application and Interpretation

When evaluating a patient with chest pain and LBBB:

• Normal ECG findings in LBBB include discordant ST-segment and T-wave configurations (opposite direction from the major terminal portion of the QRS complex) 1
• Loss of this normal QRS complex-T-wave axes discordance may indicate myocardial injury or ischemia 1
• A Sgarbossa score ≥3 has excellent specificity (98%) and positive predictive value for acute myocardial infarction and angiographically confirmed coronary occlusion 2

Important Considerations

• The 2025 ACC/AHA/ACEP/NAEMSP/SCAI guidelines note that "new or presumably new LBBB at presentation occurs infrequently and should not be considered diagnostic of AMI in isolation; clinical correlation is required" 1
• A new LBBB in an asymptomatic patient does not constitute a STEMI equivalent 1
• Patients with new LBBB and suspected ischemia are 5 times more likely to have MI than patients with LBBB of chronic or unknown duration 3

Limitations and Pitfalls

• The Sgarbossa criteria have high specificity but limited sensitivity, meaning they cannot be used to exclude MI in patients with LBBB 3
• Only a minority of patients with LBBB are ultimately diagnosed with acute myocardial infarction, regardless of LBBB chronicity 4
• False positive diagnoses can lead to unnecessary cardiac catheterization laboratory activation and inappropriate fibrinolytic therapy 2

Alternative Approaches

Newer algorithms have been proposed to improve sensitivity:

• Smith criteria: ST elevation ≥25% of the S-wave amplitude (sensitivity 67%, specificity 90%) 5
• QRS area-based criteria: ST elevation ≥100 μV + 1050 μV/Ash * QRS area (sensitivity 23.8%, specificity 95.8%) 6

However, these newer approaches still have suboptimal sensitivity and require further validation before widespread implementation 5.

Decision Algorithm

1. Apply Sgarbossa criteria to ECG in patients with chest pain and LBBB
2. If any criteria are positive (especially concordant ST elevation), consider immediate reperfusion therapy
3. If criteria are negative but clinical suspicion remains high, obtain serial ECGs and consider emergency coronary angiography 7
4. Consider additional leads (posterior V7-V9) to identify potential posterior MI that may be missed on standard 12-lead ECG 1

Remember that the diagnosis of STEMI in LBBB remains challenging, and clinical judgment should complement ECG findings when making reperfusion decisions.