What degree of ST deviation on an electrocardiogram (ECG) is concerning?

ST Deviation Thresholds on ECG

ST-segment elevation ≥1 mm (0.1 mV) in two or more contiguous limb leads, or ≥2 mm (0.2 mV) in two or more contiguous precordial leads is concerning and warrants immediate assessment for acute myocardial infarction and reperfusion therapy. 1

ST Elevation Criteria

Standard Thresholds for STEMI

• Limb leads: ST elevation ≥1 mm (0.1 mV) in at least 2 contiguous leads indicates acute MI requiring reperfusion therapy 1
• Precordial leads: ST elevation ≥2 mm (0.2 mV) in at least 2 contiguous precordial leads is the threshold for fibrinolytic therapy 1
• These criteria apply when presenting within 12 hours of symptom onset and lacking features of non-infarction causes (early repolarization, pericarditis, left ventricular hypertrophy, incomplete bundle branch block) 1

Age and Gender-Specific Normal Variants

• Young white men (<40 years): J-point elevation up to 0.3 mV in V2 can be normal 1
• White men ≥40 years: Upper normal limit is approximately 0.25 mV in V2 1
• White women: Upper normal limit remains approximately 0.15 mV in V2 across age groups 1
• Black men: Upper normal limit is approximately 0.20 mV in V2 1
• Black women: Upper normal limit is approximately 0.15 mV in V2 1

ST Depression Criteria

Concerning ST Depression

• ≥0.5 mm (0.05 mV) ST depression in 2 or more contiguous leads indicates significantly increased risk, with 1-year mortality/MI rate of 16.3% versus 8.2% in patients without ECG changes 1
• ≥2 mm (0.2 mV) ST depression in anterior precordial leads (V1-V4) with upright T-waves suggests posterior wall MI and warrants assessment for reperfusion therapy 1
• ≥2 mm (0.2 mV) ST depression with ≥3 leads involved increases likelihood of acute non-Q-wave MI by 3-4 fold 1

Horizontal or Downsloping Pattern

• Horizontal or downsloping ST depression ≥0.5 mm at the J-point in 2 or more contiguous leads is diagnostic for myocardial ischemia and classified as UA/NSTEMI when symptomatic 2

Bundle Branch Block Considerations

High-Risk Patterns

• Any bundle branch block (right, left, atypical—new or old) obscuring ST analysis in patients with strongly suggestive clinical presentation warrants reperfusion assessment 1
• LBBB with concordant ST deviation ≥1 mm (0.1 mV) toward the major QRS deflection in ≥2 contiguous leads 1
• LBBB with discordant ST deviation ≥5 mm (0.5 mV) away from the major QRS deflection in ≥2 contiguous leads 1

T-Wave Changes

Concerning T-Wave Inversions

• ≥2 mm (0.2 mV) symmetrical T-wave inversion in precordial leads strongly suggests acute ischemia, particularly critical LAD stenosis 1
• These patients often exhibit anterior wall hypokinesis and are at high risk with medical treatment alone 1
• Dynamic T-wave changes that develop during symptoms and resolve when asymptomatic strongly suggest acute ischemia and very high likelihood of severe CAD 1

Non-Specific Changes

• ST deviation <0.5 mm or T-wave inversion <2 mm are considered non-specific and less diagnostically helpful 1, 3
• However, these should not be dismissed as benign without clinical correlation, as they may represent early or resolving ischemia 3

Risk Stratification Hierarchy

Highest Risk (Immediate Action Required)

• Patients with confounding ECG patterns (bundle branch block, paced rhythm, LV hypertrophy) are at highest risk for death 1
• ST-segment deviation (elevation or depression meeting above thresholds) represents second-highest risk 1

Intermediate Risk

• Isolated T-wave inversion ≥2 mm 1
• Non-specific ST-T changes with symptom correlation 3

Lower Risk

• Isolated T-wave changes <2 mm 1
• Normal ECG patterns (though 5% of acute coronary syndromes present with normal initial tracings) 3

Critical Pitfalls to Avoid

• Never rely on a single normal ECG to exclude ACS, as the ECG provides only a snapshot of a dynamic process 3
• Always compare with prior ECGs when available, as patients with unchanged ECGs have reduced MI risk and very low risk of life-threatening complications 1
• Obtain serial ECGs to detect evolving changes, as this increases diagnostic ability though cardiac biomarkers have higher yield 1
• Consider posterior MI: approximately 4% of acute MI patients show ST elevation isolated to posterior leads, presenting as non-diagnostic on standard 12-lead ECG 4
• Recognize that ECG prognostic information remains an independent predictor of death even after adjustment for clinical findings and cardiac biomarker measurements 1