What magnitude of ST‑segment elevation measured in small 1‑mm boxes on a standard 12‑lead ECG is considered significant?

ST-Segment Elevation Thresholds for STEMI Diagnosis

For STEMI diagnosis, ST-elevation must measure ≥2.0 mm (2 small boxes) in leads V2-V3 for men ≥40 years, ≥2.5 mm (2.5 boxes) for men <40 years, and ≥1.5 mm (1.5 boxes) for women in these same leads; all other leads require ≥1.0 mm (1 box) elevation in ≥2 anatomically contiguous leads. 1, 2

Standard Measurement Criteria by Patient Demographics

The thresholds vary by age, sex, and lead location because normal J-point elevation differs across these groups:

Leads V2-V3 (Right Precordial)

• Men ≥40 years: ≥2.0 mm (0.2 mV) elevation 1, 2
• Men <40 years: ≥2.5 mm (0.25 mV) elevation 1, 2
• Women (all ages): ≥1.5 mm (0.15 mV) elevation 1, 2

The higher threshold in younger men accounts for benign early repolarization patterns that are physiologically normal in this population. 1

All Other Standard Leads (I, II, III, aVF, aVL, V4-V6)

• All patients: ≥1.0 mm (0.1 mV) elevation 1, 2

Special Posterior and Right Ventricular Leads

• Leads V7-V9 (posterior): ≥0.5 mm (0.05 mV) elevation 1, 3
• Leads V3R-V4R (right ventricular): ≥1.0 mm elevation 3, 2

Critical Technical Requirements

Contiguous Lead Requirement

ST-elevation must appear in ≥2 anatomically contiguous leads to qualify as STEMI—this requirement reduces false-positives from measurement artifact or normal variants. 1, 2 The contiguous lead groups are:

• Anterior: V1-V6 3
• Inferior: II, III, aVF 3
• Lateral: I, aVL, V5-V6 3
• Septal: V1-V2 3

Measurement Technique

Measure ST-elevation at the J-point (junction of QRS complex and ST segment) relative to the PR segment baseline. 1, 2 Standard ECG calibration is 10 mm/mV, so 1 mm = 0.1 mV = 1 small box. 2

Important caveat: Measurement location matters—ST-elevation measured 60 milliseconds after the J-point yields significantly higher values than J-point measurement (mean difference 5.2 mm in anterior MI), potentially affecting eligibility for reperfusion. 4 Guidelines specify J-point measurement. 1

Non-Diagnostic ST-Elevation (<1 mm)

ST-elevation <1.0 mm in non-precordial leads or below the sex/age-specific thresholds in V2-V3 is considered non-specific and unreliable for STEMI diagnosis. 1, 2 However:

• Do not exclude acute coronary syndrome—obtain serial ECGs at 5-10 minute intervals to detect evolving changes 3
• Consider continuous ST-segment monitoring 3
• Hyperacute T-waves may precede overt ST-elevation in the earliest phase of occlusion 3, 2

STEMI Equivalents Requiring Immediate Reperfusion

These patterns mandate urgent catheterization even without classic ST-elevation:

Posterior MI Pattern

Isolated ST-depression ≥0.5 mm in V1-V3 with upright terminal T-waves represents posterior wall STEMI; confirm with posterior leads V7-V9 showing ≥0.5 mm elevation. 1, 3, 2 This pattern reflects circumflex occlusion and is frequently missed on standard 12-lead ECG. 3

New or Presumed New LBBB

New left bundle branch block with ischemic symptoms is a STEMI equivalent requiring immediate reperfusion. 1, 3 Emergency angiography should not be delayed even if specific ECG criteria are not met. 3

Reciprocal Changes Improve Specificity

When ST-elevation is accompanied by reciprocal ST-depression in opposite leads, the positive predictive value for true STEMI increases from 49% to 93-95%. 5 The criterion of ≥1 mm ST-elevation with reciprocal changes captured 86% of MI patients who received thrombolysis within 5 hours. 5

Common Pitfalls to Avoid

Left Ventricular Hypertrophy and LBBB

These conditions produce baseline ST-elevation that mimics STEMI—33% of false-positive prehospital STEMI activations were due to LVH, and 21% to LBBB. 5 In LBBB, use the modified Sgarbossa criterion: ST/S-wave ratio ≤-0.25 (sensitivity 91%, specificity 90% for coronary occlusion). 6

Circumflex Occlusions Are Frequently Missed

Standard leads I-V6 show diagnostic ST-elevation in only 46-61% of circumflex occlusions (versus 85-96% for LAD occlusions). 7 Recording posterior leads V7-V9 increases sensitivity by 6-14% in circumflex territory infarctions. 7

Inferior MI Requires Right-Sided Leads

In all inferior STEMIs (II, III, aVF elevation), record right-sided leads V3R-V4R to detect right ventricular involvement, which mandates specific hemodynamic management (avoid nitrates/diuretics, maintain preload). 3, 2

Prognostic Implications

The magnitude of ST-elevation correlates with infarct size and mortality risk. 2 Patients with ST-elevation carry higher mortality than those with ST-depression or isolated T-wave changes. 1 Never administer fibrinolytics for isolated ST-depression (increases mortality) unless posterior MI is confirmed. 2

ST-depression ≥1 mm in ≥2 contiguous leads indicates high-risk NSTEMI/unstable angina requiring urgent catheterization but not fibrinolysis. 1, 2