What are the differences in ECG (electrocardiogram) findings between STEMI (ST-Elevation Myocardial Infarction) and NSTEMI (Non-ST-Elevation Myocardial Infarction)?

ECG Differences Between STEMI and NSTEMI

STEMI is defined by new ST-segment elevation ≥1 mm in at least 2 contiguous leads (or ≥2 mm in V2-V3 for men, ≥1.5 mm for women), while NSTEMI presents with ST-segment depression, T-wave inversion, or nonspecific changes—but never meets STEMI criteria. 1

STEMI ECG Findings

Diagnostic Criteria

• ST-segment elevation at the J point in ≥2 contiguous leads: ≥2 mm (0.2 mV) in men or ≥1.5 mm (0.15 mV) in women in leads V2-V3, and/or ≥1 mm (0.1 mV) in other contiguous chest or limb leads 1
• This pattern indicates acute total coronary occlusion with transmural myocardial ischemia requiring immediate reperfusion therapy 2

STEMI-Equivalent Patterns (Critical to Recognize)

• True posterior MI: ST depression ≥2 mm in precordial leads V1-V4 (maximal in V3-V6) with upright terminal T-waves, confirmed by ST elevation in posterior leads V7-V9 1
• Hyperacute T-wave changes: Tall, peaked T-waves that precede ST elevation 2
• Left bundle branch block with Sgarbossa criteria: New or presumed new LBBB in the appropriate clinical context 2
• Multilead ST depression with ST elevation in aVR: Suggests left main or severe three-vessel disease 1

Prognostic Significance

• ST elevation carries the highest early risk of death and indicates need for emergent reperfusion within 90 minutes (primary PCI) or 30 minutes (fibrinolysis) 2, 1

NSTEMI ECG Findings

Primary Patterns

• ST-segment depression ≥0.5 mm (0.05 mV): The hallmark finding, particularly when present in multiple leads; correlates with increased mortality risk and extent of coronary artery disease 3
• Deep T-wave inversion ≥2 mm (0.2 mV): Especially when symmetrical and deep in precordial leads, strongly suggests acute ischemia from critical LAD stenosis 3
• Nonspecific ST-T wave changes: ST deviation <0.5 mm or T-wave inversion ≤2 mm—less diagnostically helpful but may still indicate ischemia 3
• Normal ECG: Occurs in 1-6% of patients with acute MI; does not exclude ACS 3

Risk Gradient Based on ECG Abnormalities

• Highest risk: Confounding patterns (bundle-branch block, paced rhythm, LV hypertrophy) 2, 3
• High risk: ST-segment depression, with magnitude and number of leads correlating with worse outcomes 2, 3
• Moderate risk: Isolated T-wave inversion 2
• Lower risk: Normal ECG pattern (but still 1-6% have MI) 2, 3

Quantitative Prognostic Markers

• ST depression in ≥3 leads with maximal depression ≥0.2 mV increases likelihood of acute non-Q-wave MI by 3-4 times 3
• One-year incidence of death or new MI: 16.3% with ≥0.5 mm ST deviation vs 6.8% for isolated T-wave changes vs 8.2% for no ECG changes 3

Critical Distinctions and Clinical Implications

The Fundamental Difference

• STEMI: Patients with ST elevation are immediately triaged for emergent reperfusion therapy (catheterization lab activation) 2, 1
• NSTEMI: Patients with ST depression are classified as UA/NSTEMI and managed with early invasive strategy (within 12-24 hours for high-risk features) rather than emergent reperfusion 2, 3

Dynamic Nature of ECG Changes

• Transient ST changes: ST-segment changes that develop during symptoms and resolve when asymptomatic strongly suggest acute ischemia and severe underlying CAD 2, 3
• Serial ECGs: Should be obtained at 15-30 minute intervals if initial ECG is non-diagnostic but clinical suspicion remains high 3
• Continuous monitoring: Detects recurrent ischemic episodes in approximately 27% of patients within first 24-48 hours 3

Independent Prognostic Value

• ECG patterns remain independent predictors of death even after adjustment for clinical findings and cardiac biomarker measurements 2, 3
• ST depression on presenting ECG portends the highest risk of death at 6 months, despite ST elevation carrying highest early mortality risk 2

Common Pitfalls and Caveats

Diagnostic Challenges

• Up to 30% of NSTEMI patients have total coronary occlusion despite not meeting STEMI criteria—these patients have similar infarct size and mortality as STEMI patients but experience delays to angiography 4, 5, 6
• Left circumflex occlusion can present with a non-diagnostic 12-lead ECG 3
• Posterior MI may be missed if posterior leads (V7-V9) are not obtained 3, 1

Alternative Causes to Consider

• LV aneurysm, pericarditis, myocarditis, Prinzmetal's angina, early repolarization, Takotsubo cardiomyopathy, Wolff-Parkinson-White syndrome 3
• Central nervous system events and medications (tricyclic antidepressants, phenothiazines) can cause deep T-wave inversion 3
• Isolated Q waves in lead III may be normal, especially without repolarization abnormalities in other inferior leads 3

Essential Management Points

• The distinction between UA and NSTEMI is ultimately based on detection of elevated cardiac troponin (above 99th percentile), not ECG findings alone 2, 3
• Comparison with prior ECG significantly improves diagnostic accuracy 3
• Serial troponin measurements at presentation and 3-6 hours after symptom onset are essential for patients with suspected ACS but no ECG changes 3