Sensitivity is low with Sgarbossa, Smith, better with Barcelona Algorithm. Don’t rely on ECG changes or troponins to activate Cath lab. Rely on prolonged ischaemic symptoms. If the story is good & reliable, don’t regret not calling the Cath lab in time.

Background

The key physiology underpinning all three systems is that LBBB produces “appropriate discordance”—ST segments normally move opposite to the QRS. The diagnostic challenge is therefore identifying when this discordance becomes “excessive” (i.e. infarction-driven injury current) rather than physiological repolarisation. The Modified Sgarbossa and Barcelona criteria both operationalise this using proportionality, which is conceptually similar to how we interpret strain patterns in LVH—contextual rather than absolute thresholds. In practice, combining ECG criteria with troponin delta and bedside echo (regional wall motion abnormality) often outperforms any scoring system alone, especially in elderly multimorbid patients where LBBB is common and baseline ECGs are abnormal.

📊 ECG Criteria for AMI in LBBB / Paced Rhythm

Feature	Sgarbossa (Original)	Modified Sgarbossa (Smith)	Barcelona Criteria
Core Concept	Absolute ST deviation rules	Proportional discordance (ST/S ratio)	Absolute + proportional + low-voltage integration
Concordant ST Elevation	≥1 mm in leads with positive QRS ✔️	Same as original ✔️	≥1 mm concordant STE ✔️
Concordant ST Depression	≥1 mm in V1–V3 ✔️	Same as original ✔️	≥1 mm concordant STD ✔️
Discordant ST Elevation	≥5 mm ❗ (fixed threshold)	≥25% of preceding S-wave depth 📏	Either: • ≥1 mm discordant STE if QRS ≤6 mm • OR ≥25% of S-wave depth
Accounts for QRS Size	❌ No	✔️ Yes (proportional)	✔️ Yes (proportional + low voltage rule)
Sensitivity	Low (~20–40%) ⚠️	Moderate (~70–80%) 👍	High (~90%) 🔥
Specificity	Very high (~90–98%) 🎯	High (~85–95%)	Slightly lower but still good
Clinical Use	Rule-in STEMI equivalent	Preferred in ED practice	Emerging / advanced use
Key Advantage	Very specific	Balances sensitivity + specificity	Best sensitivity (detects more true MI)
Main Limitation	Misses many MIs ❌	Requires calculation	More complex / less widely taught
Image

🧠 Clinical takeaway (UK practice): If any Sgarbossa-positive feature → treat as STEMI equivalent → urgent PCI. If negative but suspicion high → use Modified Sgarbossa or Barcelona + serial troponins + bedside echo. 👉 NICE/ESC emphasise clinical judgement over rigid ECG criteria in LBBB.

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Acute Coronary Syndrome: LBBB and AMI

Background

📊 ECG Criteria for AMI in LBBB / Paced Rhythm

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