What kind of ECG (Electrocardiogram) changes can be seen in patients with Non-ST-Elevation Myocardial Infarction (NSTEMI)?

ECG Changes in Non-ST-Elevation Myocardial Infarction (NSTEMI)

In NSTEMI, the most common ECG changes include ST-segment depression, T-wave inversion, and non-specific ST-T wave changes, while a completely normal ECG does not exclude NSTEMI. The electrocardiogram provides critical diagnostic and prognostic information in patients with suspected acute coronary syndrome.

Common ECG Findings in NSTEMI

• ST-segment depression (≥0.5 mm or 0.05 mV) is a hallmark finding, particularly when present in multiple leads, and correlates with increased mortality risk and extent of coronary artery disease 1
• T-wave inversion (≥2 mm or 0.2 mV), especially when symmetrical and deep in precordial leads, strongly suggests acute ischemia, often due to critical stenosis of the left anterior descending coronary artery 1
• Nonspecific ST-segment and T-wave changes (ST-segment deviation <0.5 mm or T-wave inversion ≤2 mm) are less diagnostically helpful but still may indicate ischemia 1
• Established Q waves (≥0.04 seconds) may indicate prior MI and significant coronary artery disease 1
• Completely normal ECG occurs in 1-6% of patients with NSTEMI and does not exclude the diagnosis 1, 2

Prognostic Significance of ECG Changes

• A gradient of risk can be established based on ECG abnormalities:

  • Highest risk: Confounding ECG patterns (bundle-branch block, paced rhythm, LV hypertrophy) 1
  • Intermediate risk: ST-segment deviation (elevation or depression) 1
  • Lower risk: Isolated T-wave inversion or normal ECG patterns 1, 3

• The magnitude of ECG abnormalities provides important prognostic information:

  • Diagnosis of acute non-Q-wave MI is 3-4 times more likely with ST depression in ≥3 leads and maximal ST depression ≥0.2 mV 1
  • The sum of ST-segment depression across all leads is a powerful independent predictor of 30-day mortality 3
  • One-year incidence of death or new MI is 16.3% with ≥0.5 mm ST-segment deviation compared to 6.8% for isolated T-wave changes and 8.2% for no ECG changes 1

Special Considerations

• Posterior MI: May present with ST-segment depression in anterior precordial leads (V1-V3) and/or isolated ST-segment elevation in posterior chest leads (V7-V9) 1
• Left circumflex occlusion: Can present with a non-diagnostic 12-lead ECG 1
• Dynamic changes: Serial ECGs or continuous ST-segment monitoring can detect transient changes that may be missed on a single ECG 1
• ECG correlation with echocardiographic findings: ST-segment depression is associated with diastolic dysfunction, while T-wave inversion correlates with systolic impairment 4

Clinical Implications

• The prognostic information from ECG patterns remains an independent predictor of death even after adjustment for clinical findings and cardiac biomarker measurements 1
• Up to 25% of patients with NSTEMI and elevated cardiac biomarkers may develop Q-wave MI during hospitalization 1
• Transient ST-segment changes (≥0.05 mV) during symptoms that resolve when the patient becomes asymptomatic strongly suggest acute ischemia and severe underlying coronary artery disease 1
• ST-depression and T-wave inversion are independent predictors of new-onset heart failure within 30 days of initial presentation 5

Common Pitfalls and Caveats

• Alternative causes of ST-segment and T-wave changes must be considered, including:
  • LV aneurysm, pericarditis, myocarditis, Prinzmetal's angina, early repolarization, Takotsubo cardiomyopathy, and Wolff-Parkinson-White syndrome 1
  • Central nervous system events and medications (tricyclic antidepressants, phenothiazines) can cause deep T-wave inversion 1
• Isolated Q waves in lead III may be a normal finding, especially without repolarization abnormalities in other inferior leads 1
• Serial ECGs increase diagnostic accuracy, especially when combined with cardiac biomarker measurements 1

Remember that the distinction between unstable angina and NSTEMI is ultimately based on the detection of markers of myocardial necrosis in the blood, not solely on ECG findings 1.