What does a depressed ST segment on an electrocardiogram (ECG) indicate?

ST Segment Depression on ECG: Clinical Significance and Interpretation

ST segment depression on an electrocardiogram primarily indicates myocardial ischemia, representing electrical gradients caused by inadequate blood supply to the heart muscle. 1

Types and Criteria for ST Depression

ST segment depression is measured relative to the P-Q junction (isoelectric line) at 60-80 ms after the J point. The standard criteria for abnormal ST depression include:

• Horizontal or downsloping ST depression ≥1 mm (0.1 mV) for 80 ms after the J point 1
• In leads V2 and V3, the threshold for abnormal J-point depression is 0.05 mV for both men and women 1
• In all other leads, the threshold is 0.1 mV 1

Different patterns of ST depression carry different diagnostic significance:

• Downsloping ST depression is more specific for myocardial ischemia than horizontal or upsloping patterns 1
• Upsloping ST depression is generally considered an "equivocal" test response and less specific for ischemia 1

Clinical Significance of ST Depression

1. Myocardial Ischemia

• ST depression is the most common ECG manifestation of exercise-induced myocardial ischemia 1
• Represents subendocardial ischemia with reductions in phase 2 plateau amplitude and less negative phase 4 resting membrane potentials 1
• In acute coronary syndromes, ST depression ≥1 mm in two or more contiguous leads is highly suggestive of unstable angina 1

2. Prognostic Significance

• Patients with ST depression during acute coronary syndromes have significantly worse outcomes:
  • 8.5 times more likely to have in-hospital complications 1
  • 4.7-fold higher risk for troponin elevation in ICU patients 1
  • Each ischemic episode with ST depression predicts a 25% increase in death/MI at 5 and 30 days 1

3. Localization of Ischemia

• ST depression in leads V2-V3 may represent posterolateral wall myocardial infarction 2
• Isolated inferior ST segment depression can be an early sign of anterior wall myocardial infarction 3
• Widespread ST depression often indicates diffuse subendocardial ischemia from severe multivessel or left main coronary artery disease 4

4. Severity Assessment

• The severity of coronary artery disease correlates with:
  • The degree of ST depression
  • Time of appearance during stress testing
  • Duration of ST depression
  • Number of leads showing ST depression 1
• Lower workload and rate-pressure product at which ST depression occurs indicates worse prognosis and higher likelihood of multivessel disease 1
• Duration of ST depression in recovery phase correlates with severity of coronary artery disease 1

Other Causes of ST Depression

ST depression is not always due to ischemia and may occur with:

• Electrolyte abnormalities (particularly hypokalemia)
• Left ventricular hypertrophy
• Digoxin effect
• Secondary repolarization abnormalities in ventricular conduction disturbances
• Various cardiac and non-cardiac drugs 1

Clinical Implications for Management

• In patients with chest pain, ST depression ≥1 mm is an independent risk factor for cardiac death or MI 1
• Resolution of ST depression after intervention has prognostic value - patients with <50% resolution have worse outcomes 5
• Continuous ST-segment monitoring can identify recurrent ischemia episodes, which directly correlate with probability of adverse cardiac events 1

Diagnostic Challenges

• A completely normal ECG does not exclude acute coronary syndrome - approximately 5% of patients with normal ECGs who were discharged from emergency departments were ultimately found to have either acute MI or unstable angina 1
• Differentiating between various causes of ST depression can be challenging and requires clinical correlation 1

When evaluating ST depression, always consider the clinical context, patient demographics, and presence of other ECG abnormalities to determine the most likely diagnosis and appropriate management strategy.