ECG Differentiation: Stable vs Unstable Angina
The key ECG difference is that unstable angina typically shows ST-segment depression ≥1 mm in two or more contiguous leads and/or T-wave inversions >1 mm in leads with predominant R waves, while stable angina characteristically has a normal ECG between episodes or only non-specific changes <1 mm. 1
Critical ECG Findings in Unstable Angina
ST-segment depression ≥1 mm (0.1 mV) in two or more contiguous leads is highly suggestive of unstable angina and represents the most reliable electrocardiographic indicator of unstable coronary disease 1, 2. This finding, when present in the appropriate clinical context, strongly differentiates unstable from stable angina.
Additional ECG features that indicate unstable angina include:
- T-wave inversions >1 mm in leads with predominant R waves (though less specific than ST-depression) 1
- Deep symmetrical T-wave inversions in anterior chest leads, often indicating significant proximal left anterior descending artery stenosis 1, 2
- Transient ST-segment elevation, particularly in variant (Prinzmetal's) angina 1
- New or dynamic horizontal or down-sloping ST-segment depression ≥0.5 mm 3
- Transient bundle branch block during ischemic episodes 1, 2
ECG Findings in Stable Angina
Stable angina patients typically present with:
- Normal ECG between symptomatic episodes 4
- Non-specific ST-segment and T-wave changes <1 mm, which lack diagnostic specificity 1
- Old Q waves from previous MI (indicates coronary disease but not current instability) 1, 2
Critical Clinical Caveat
A completely normal ECG does NOT exclude unstable angina or acute coronary syndrome—approximately 5% of patients with normal ECGs discharged from emergency departments were ultimately found to have either acute MI or unstable angina 1, 2. However, a completely normal ECG recorded during an episode of significant chest pain should direct attention to other possible causes 1.
Dynamic Nature and Serial ECG Monitoring
The ECG represents only a "snapshot" of a dynamic process—ST-segment changes are frequently unstable in early stages of acute coronary syndromes 5. Therefore:
- Perform repeat ECG at 30-60 minute intervals in patients with initial non-diagnostic ECG who have ongoing or recurrent symptoms 5
- Obtain ECG during symptomatic episodes and compare with tracings when symptoms resolve 1, 2
- Compare with previous ECGs when available, especially in patients with left ventricular hypertrophy or prior MI 1, 2
Risk Stratification Based on ECG
An abnormal ECG is the single most predictive clinical indicator of complex coronary lesion morphology (odds ratio 4.2-4.9) and predicts triple vessel disease and cardiac events 6, 7. Multivariate analysis demonstrates that ECG abnormalities combined with recent onset angina best identify high-risk unstable lesions 7.
The presence of ST-segment depression on ECG identifies high-risk patients requiring aggressive management, while those with normal ECG and normal cardiac troponins represent a low-risk group 8.