A 66-year-old man with pre-existing left bundle branch block presents with 2 hours of typical ischemic chest pain. Which ECG finding best supports a diagnosis of acute STEMI in this setting?
- A New QRS widening beyond 140 ms
- B Discordant ST elevation measuring less than 25% of the depth of the preceding S wave
- C Concordant ST depression of 1 mm or more in leads V1 to V3 ✓
- D Left axis deviation
Correct answer: C. Concordant ST depression of 1 mm or more in leads V1 to V3
Explanation
In left bundle branch block the usual repolarization changes make ST-segment analysis difficult, but the Sgarbossa criteria identify infarction. Concordant ST elevation of 1 mm or more scores 5 points, concordant ST depression of 1 mm or more in V1-V3 scores 3 points, and excessively discordant ST elevation exceeding 25% of the S-wave depth scores 2 points; a score of 3 or more is diagnostic. Option D describes the expected secondary change of LBBB itself and carries no diagnostic weight.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.