What Does 0.9 mm ST Depression on an ECG Indicate?
0.9 mm ST depression on an ECG is clinically significant and indicates myocardial ischemia, particularly when horizontal or downsloping and present in two or more contiguous leads. This finding warrants immediate evaluation for acute coronary syndrome and carries important prognostic implications.
Clinical Significance
ST depression ≥0.5 mm (0.05 mV) in two or more contiguous leads is suggestive of non-ST elevation acute coronary syndrome (NSTE-ACS) and is linked to adverse prognosis. 1 Your finding of 0.9 mm exceeds this threshold and should be taken seriously.
The magnitude of ST depression directly correlates with risk:
- ST depression >0.1 mV (1 mm) is associated with an 11% rate of death and MI at 1 year 1, 2
- ST depression >0.2 mV (2 mm) carries approximately a six-fold increased mortality risk 1, 2
- Your 0.9 mm depression falls into the clinically significant range requiring urgent evaluation
Diagnostic Criteria
The specific characteristics that make ST depression diagnostic for ischemia include: 1
- Horizontal or downsloping ST-segment depression ≥0.5 mm at the J-point 1
- Present in 2 or more contiguous leads 1
- Measured at 80 ms after the J point during exercise testing 1
Prognostic Implications
The number of leads showing ST depression and the magnitude correlate with the extent and severity of ischemia. 1, 2 Patients with ST depression have significantly higher risk for subsequent cardiac events compared to those with isolated T-wave inversion or normal ECG. 1
ST depression combined with transient ST elevation identifies an even higher risk subgroup. 1, 2 Research demonstrates that ST depression is associated with a 100% increase in the occurrence of three-vessel or left main coronary artery disease. 3
Location-Specific Considerations
The location of ST depression provides additional prognostic information:
- ST depression in lateral leads (I, aVL, V5, V6) predicts worse outcomes with odds ratio of 3.5 for in-hospital death 4
- Lateral ST depression is associated with lower left ventricular ejection fraction and more frequent left main or three-vessel disease (60% vs 22%) 4
- ST depression in leads V1-V3 may indicate posterior myocardial infarction and warrants recording additional posterior leads (V7-V9) 5, 6
Immediate Management Required
Obtain a 12-lead ECG within 10 minutes and initiate continuous ECG monitoring immediately. 1, 6 Measure high-sensitivity cardiac troponin on admission and serially. 1, 6
Pharmacological therapy should include: 2, 6
- Aspirin 162-325 mg immediately 6
- P2Y12 inhibitor (clopidogrel or ticagrelor) 2, 6
- Anticoagulation with low molecular weight heparin or unfractionated heparin 2, 6
- Beta-blockers in absence of contraindications 2, 6
- Nitrates for ongoing chest pain 2, 6
Risk Stratification and Invasive Strategy
High-risk patients with ST depression should undergo coronary angiography within 24-48 hours. 6 High-risk features requiring immediate invasive strategy include: 5, 6
- Recurrent or ongoing chest pain
- Elevated cardiac troponin levels
- Hemodynamic instability
- Life-threatening arrhythmias
- Heart failure
Critical Pitfalls to Avoid
Do not dismiss ST depression as non-specific changes, especially when present in multiple contiguous leads. 6 However, recognize that ST depression can occur in non-ischemic conditions including: 7
- Left ventricular hypertrophy
- Left bundle branch block
- Digitalis effect
- Electrolyte abnormalities (particularly hypokalemia) 5
Never administer fibrinolytic therapy to patients with isolated ST depression, as it may increase mortality. 6 Resting ST-segment depression ≥0.5 mm precludes accurate interpretation of exercise-induced changes during stress testing. 1
Context Matters
The clinical context is essential: 1