## Why Concordant ST elevation ≥1 mm in any lead with a positive QRS complex is right The Sgarbossa criteria (1996) were specifically designed to identify acute MI in the presence of LBBB, which otherwise masks or mimics ischemic ST/T changes. Concordant ST elevation ≥1 mm (ST elevation in the same direction as the major QRS deflection) in any lead with a positive QRS is the MOST SPECIFIC Sgarbossa criterion, scoring 5 points and achieving >90% specificity for acute MI. This finding is highly reliable because it violates the expected "appropriate discordance" pattern of LBBB, where ST segments normally point opposite to the QRS. The wide notched R-waves in V5/V6 (marked **A**) are the hallmark of LBBB morphology; when concordant ST elevation appears in these lateral leads or any other lead with positive QRS, it strongly indicates acute transmural ischemia rather than the baseline LBBB pattern (Harrison 21e Ch 235; Braunwald 12e Ch 27). ## Why each distractor is wrong - **Discordant ST elevation ≥5 mm in any lead with a negative QRS complex**: This is the third and LEAST SPECIFIC Sgarbossa criterion (2 points only), with low specificity because discordant ST elevation can occur as part of the normal LBBB pattern itself. The modified Sgarbossa (Smith-modified) criteria replaced this with an ST/S ratio to improve sensitivity, but it remains much less specific than concordant ST elevation. - **ST depression ≥2 mm in leads V1–V3 with upright T-waves**: While ST depression in V1–V3 is part of the normal LBBB pattern (appropriate discordance), this finding alone is NOT a Sgarbossa criterion. The second Sgarbossa criterion specifically requires CONCORDANT ST DEPRESSION ≥1 mm in V1–V3 (not depression with upright T-waves), and it scores only 3 points—less specific than concordant ST elevation. - **Progressive deepening of the S-wave in V1 with T-wave inversion in lateral leads**: This represents the expected baseline LBBB morphology (appropriate discordance) and is NOT a Sgarbossa criterion. Progressive S-wave deepening and T-wave inversion in lateral leads are part of the normal LBBB pattern and do not indicate acute MI. **High-Yield:** Concordant ST elevation ≥1 mm in LBBB = Sgarbossa criterion 1 (5 points, >90% specificity) — this is the most reliable sign of acute STEMI in LBBB and should trigger immediate cath lab activation. [cite: Harrison 21e Ch 235; Braunwald 12e Ch 27; Sgarbossa et al. 1996; 2013 ACC/AHA STEMI Guidelines; 2018 ACC/AHA Chest Pain Guidelines]
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