What magnitude of ST‑segment depression in millimeters is considered significant?

ST-Segment Depression Significance Threshold

Horizontal or downsloping ST-segment depression ≥0.5 mm (0.05 mV) at the J-point in 2 or more contiguous leads is considered significant for myocardial ischemia. 1

Context-Specific Thresholds

The significance of ST-segment depression varies by clinical context:

Acute Coronary Syndrome (Emergency Setting)

• ≥0.5 mm (0.05 mV) horizontal or downsloping ST depression at the J-point in 2 or more contiguous leads indicates myocardial ischemia and classifies the patient as UA/NSTEMI 1
• This threshold applies to leads V2-V3 with J-point depression ≥0.05 mV and ≥0.1 mV (1 mm) in all other leads for both men and women 1
• The morphology matters critically: horizontal and downsloping patterns are pathologic, while upsloping depression requires different interpretation 1

Exercise Stress Testing

• ≥1.0 mm (0.1 mV) horizontal or downsloping ST depression measured at 60-80 ms after the J-point constitutes a positive test for ischemia 1
• Upsloping ST depression ≥1.0 mm is considered equivocal (not diagnostically useful for separating normal from abnormal) 1
• Markedly depressed upsloping ST depression (≥2.0 mm at 80 ms after J-point) may identify underlying CAD in highly symptomatic patients with angina, but this is not generalizable 1

Special Circumstances: Left Bundle Branch Block

• ≥1 mm concordant ST-segment depression in leads V1-V3 scores 3 points in the modified Sgarbossa criteria and is highly specific for acute MI 1

Critical Measurement Details

Proper measurement technique is essential 1:

• Measure ST level relative to the end of the PR segment (P-Q junction), not the T-P segment
• Assess 3 or more consecutive beats in the same lead with stable baseline
• Measure at 60-80 ms after the J-point for exercise testing 1
• Measure at the J-point for acute ischemia evaluation 1
• Visually verify all automated computer measurements 1

Prognostic Significance

The extent of ST depression correlates with outcomes 2:

• The sum of ST-segment depression across all leads is a powerful independent predictor of 30-day mortality in NSTE-ACS (P<0.0001) 2
• Greater ST depression correlates with three-vessel disease (P<0.0001), left main disease (P<0.0001), and higher peak creatine kinase levels (P<0.0001) 2
• However, quantitative assessment beyond presence/absence may not provide incremental value when comprehensive risk stratification (like GRACE score) is already performed 3

Common Pitfalls to Avoid

Do not confuse ST depression patterns 1:

• ST depression maximal in V1-V4 (versus V5-V6) has 97% specificity for occlusion MI and likely represents posterior wall OMI requiring emergent intervention 4
• ST depression during supraventricular tachycardia is common (85% of patients) but has only 51% specificity and 6% predictive value for significant CAD 5, 6

Recognize that additional ST depression matters when baseline abnormalities exist 1:

• When modest resting ST depression is present, only additional ST depression during exercise should be measured for analysis 1
• In the presence of resting ST elevation from early repolarization, only ST changes below the P-Q baseline should be analyzed 1