Percentage of NSTEMI Patients with Normal ECG
Approximately 1-6% of patients with NSTEMI present with a completely normal ECG, while a significant proportion (up to 22.8% in some studies) may present with unremarkable or non-diagnostic ECG findings. 1, 2
ECG Findings in NSTEMI
- ST-segment depression (≥0.5 mm or 0.05 mV) is a hallmark finding in many NSTEMI cases, particularly when present in multiple leads, and correlates with increased mortality risk 3
- T-wave inversion (≥2 mm or 0.2 mV), especially when symmetrical and deep in precordial leads, strongly suggests acute ischemia 3
- Nonspecific ST-segment and T-wave changes (ST-segment deviation <0.5 mm or T-wave inversion ≤2 mm) are less diagnostically helpful but may still indicate ischemia 1, 3
- A completely normal ECG does not exclude NSTEMI, as 1-6% of such patients will eventually be proven to have had an MI 1, 3
- A recent study from Pakistan found that 22.8% of patients with confirmed NSTEMI had normal ECG findings within 24 hours of symptom onset 2
Risk Stratification Based on ECG Findings
A gradient of risk can be established based on ECG abnormalities 1:
- Highest risk: Patients with confounding ECG patterns (bundle-branch block, paced rhythm, LV hypertrophy)
- Intermediate risk: Patients with ST-segment deviation
- Lowest risk: Patients with isolated T-wave inversion or normal ECG patterns
The magnitude of ECG abnormalities provides important prognostic information 1:
- One-year incidence of death or new MI is 16.3% with ≥0.5 mm ST-segment deviation
- 6.8% for isolated T-wave changes
- 8.2% for no ECG changes
Special Considerations
- Posterior MI may present with ST-segment depression in anterior precordial leads (V1-V3) without classic ST elevation 1, 3
- Left circumflex coronary artery occlusion can present with a non-diagnostic 12-lead ECG in approximately 4% of acute MI patients 1
- Recent research suggests that a significant proportion of NSTEMIs (10-25%) show angiographic evidence of complete occlusion at rates comparable to STEMIs 4
- One-third of NSTEMI patients have an acutely occluded culprit coronary artery (occlusion myocardial infarction), which can lead to poor outcomes due to delayed identification 5
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
- Serial ECGs increase diagnostic accuracy, especially when combined with cardiac biomarker measurements 1
- 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
- High-sensitivity troponin measurements are essential for patients with suspected ACS but no ECG changes 3, 6
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, 3
- Central nervous system events and medications (tricyclic antidepressants, phenothiazines) can cause deep T-wave inversion 1, 3
- Isolated Q waves in lead III may be a normal finding, especially without repolarization abnormalities in other inferior leads 1, 3
- Certain patient populations, such as the elderly, diabetics, and women, are more likely to present with atypical symptoms and non-diagnostic ECGs 3