STEMI Recognition and Reciprocal Changes
Key criteria, measurement technique, morphology clues, and reciprocal patterns for identifying true ST-elevation MI.
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Questions Covered in This Set
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Where is ST elevation measured?
At the J point — the junction where the QRS ends and the ST segment begins — relative to the isoelectric baseline.
What is the most reliable isoelectric baseline?
The TP segment (flat stretch from end of T wave to start of the next P wave). The PR segment is an acceptable alternative if used consistently.
Standard STEMI threshold in leads other than V2–V3
≥ 1.0 mm of new J-point elevation in two anatomically contiguous leads.
V2–V3 elevation thresholds
≥ 2.0 mm in men ≥40, ≥ 2.5 mm in men <40, ≥ 1.5 mm in women — these leads normally have some elevation.
Thresholds for posterior (V7–V9) and right-sided (V3R/V4R) leads
≥ 0.5 mm (right-sided requires ≥ 1.0 mm in men <30).
What does 'contiguous' mean?
Neighbors within a lead group, not neighbors on the printout: septal V1–V2, anterior V3–V4, lateral V5–V6 + I + aVL, inferior II, III, aVF.
Morphology suggestive of coronary occlusion
Convex upward / 'tombstone' / straightened ST segments that swallow the T wave; loss of previously present concavity.
Morphology suggestive of benign elevation
Concave upward ('smiley face') elevation with a notched J point and preserved tall T waves — early repolarization or pericarditis.
Why do reciprocal changes occur?
The injury current points toward the infarcting wall; leads facing the opposite direction (~180° away) record mirror-image ST depression.
Why are reciprocal changes so useful?
Global mimics (pericarditis, early repolarization, LVH) generally do not produce reciprocal depression, so it argues strongly for true occlusion.
