What does ST depression on an EKG indicate and how is it managed, especially in patients with risk factors for coronary artery disease?

Understanding ST Depression on EKG

I cannot provide an actual image, but I can describe what ST depression looks like on an EKG and explain its clinical significance and management.

What ST Depression Looks Like on EKG

ST depression appears as a downward displacement of the ST segment below the baseline (isoelectric line) on the EKG tracing. 1

Visual Characteristics:

• Horizontal ST depression: The ST segment is flat and parallel to the baseline, displaced downward by ≥0.5 mm (0.05 mV) 2
• Downsloping ST depression: The ST segment slopes downward from the J point, which is more specific for ischemia 1, 2
• Upsloping ST depression: The ST segment slopes upward but remains below baseline—this is generally considered "equivocal" and less specific for ischemia 1

Measurement Technique:

• Measure ST depression at 60-80 milliseconds after the J point (the junction where the QRS complex ends and ST segment begins) 1
• Significant ischemic ST depression is defined as ≥0.5 mm (0.05 mV) horizontal or downsloping depression in two or more contiguous leads 2, 3

Clinical Significance

ST depression indicates subendocardial ischemia and represents a high-risk marker for adverse cardiac events. 4

Risk Stratification by Magnitude:

• ≥0.5 mm depression: Indicates myocardial ischemia and warrants evaluation for acute coronary syndrome 2, 3
• ≥2 mm depression: Indicates more extensive coronary artery disease, worse prognosis, and higher likelihood of multivessel disease 2, 5
• Greater extent of ST depression (sum across all leads): Correlates with increased 30-day mortality, three-vessel disease, and left main coronary disease 5

Special Pattern - Posterior MI:

• ST depression in leads V1-V4 with upright T waves may represent posterior myocardial infarction rather than simple ischemia 4
• Always obtain posterior leads (V7-V9) when this pattern is present 4

Diagnostic Approach

Immediate Actions:

1. Obtain 12-lead EKG within 10 minutes of presentation with chest pain or anginal equivalents 3
2. Perform serial EKGs at 30-60 minute intervals if initial EKG is non-diagnostic and symptoms persist 1
3. Measure troponin immediately, with repeat at 1-2 hours if initial value is non-diagnostic 4

Exercise Testing Context:

• During exercise stress testing, horizontal or downsloping ST depression ≥1 mm at 60-80 ms after the J point defines a positive test 1
• ST depression appearing at lower workloads indicates more severe coronary disease 1, 2
• Marked ST depression (≥2 mm) is a relative indication to terminate exercise testing, while ≥4 mm is an absolute indication 1

Important Pitfalls to Avoid:

• Never rely on a single normal or non-diagnostic EKG to rule out acute coronary syndrome—this is a common and dangerous mistake 4
• Do not administer fibrinolytic therapy to patients with ST depression (except suspected posterior MI), as mortality may increase 4
• Consider non-ischemic causes: left ventricular hypertrophy, left bundle branch block, digitalis use, electrolyte abnormalities, and ventricular paced rhythm all cause false-positive ST depression 1, 6

Management Algorithm

For Suspected Acute Coronary Syndrome with ST Depression:

Step 1: Immediate Medical Therapy 4

• Aspirin 162-325 mg immediately
• P2Y12 inhibitor (ticagrelor or prasugrel preferred over clopidogrel)
• Anticoagulation (fondaparinux, enoxaparin, or unfractionated heparin)
• Beta-blocker (unless contraindicated)
• Nitrates for ongoing chest pain

Step 2: Risk Stratification 1, 4

High-risk features requiring urgent angiography (within 2-24 hours):

• Elevated troponin with ST depression
• Recurrent ischemia despite medical therapy
• Hemodynamic instability
• Life-threatening arrhythmias

Step 3: Invasive Strategy 4

• Urgent coronary angiography within 2-24 hours for high-risk patients with confirmed NSTE-ACS
• Immediate angiography (<2 hours) for hemodynamic instability, ongoing ischemia, or life-threatening arrhythmias 4

For Stable Angina with Exercise-Induced ST Depression:

Patients with intermediate-to-high pre-test probability of coronary disease should undergo exercise ECG testing unless unable to exercise or have uninterpretable baseline EKG changes 1

• Positive test (≥1 mm horizontal/downsloping ST depression): Proceed to coronary angiography or advanced imaging based on clinical context 1
• Exercise ECG has 68% sensitivity and 77% specificity for detecting significant coronary disease 1

Continuous Monitoring:

• 15-30% of patients with unstable coronary artery disease have transient ST segment changes on continuous monitoring, predominantly ST depression 1
• Continuous ST monitoring provides additional prognostic information and identifies patients at increased risk 4

Key Clinical Correlations

ST depression severity correlates directly with:

• Extent of coronary artery disease (three-vessel or left main disease prevalence increases from 22% without ST depression to 45% with ST depression) 7
• 30-day mortality risk (continuous increase with greater extent of ST depression) 5
• Peak creatine kinase levels during acute coronary syndrome 5

In patients with prior myocardial infarction, resting ST depression indicates 2.5 times higher prevalence of severe coronary disease and larger left ventricular volumes 8