Significance of a Slightly Depressed ST Segment on ECG
Horizontal or downsloping ST-segment depression ≥0.5 mm at the J-point in 2 or more contiguous leads is suggestive of myocardial ischemia and requires further evaluation. 1
Definition and Clinical Significance
- ST-segment depression is defined as horizontal or downsloping depression ≥0.5 mm at the J-point in 2 or more contiguous leads according to the American College of Cardiology guidelines 1
- ST depression may be caused by various physiological, pathological, and pharmacological interventions that affect the plateau phase of the ventricular action potential 1
- Primary causes include myocardial ischemia, hypokalemia, and effects of various cardiac and non-cardiac drugs 1
- ST-segment depression can also occur concurrently with T-wave changes, such as in ventricular hypertrophy or as secondary repolarization abnormalities in ventricular conduction disturbances 1
Ischemic vs. Non-Ischemic Causes
- ST-segment depression during acute ischemia is due to the flow of current across the boundary between ischemic and non-ischemic zones (injury current) 1
- The pattern of ST depression can help differentiate ischemic from non-ischemic causes:
Prognostic Significance
- In patients with unstable coronary artery disease, ST-segment depression is associated with a 100% increase in the occurrence of three-vessel/left main disease and an increased risk of subsequent cardiac events 2
- The anatomic and functional severity of coronary artery disease can be related to the time of appearance of ischemic ST-segment depression - the lower the workload at which it occurs, the worse the prognosis 1
- Among patients with acute chest pain in the emergency department, those with ST-segment depression were 8.5 times more likely to have in-hospital complications 1
Diagnostic Approach
- When evaluating ST depression, consider:
- Degree of depression (≥0.5 mm is significant) 1
- Pattern (horizontal or downsloping is more specific for ischemia) 1
- Number of leads affected (more leads suggest more extensive ischemia) 1
- Time of appearance during stress testing (earlier appearance suggests more severe disease) 1
- Duration of ST depression in recovery phase (longer duration correlates with severity of coronary artery disease) 1
Special Considerations
- ST depression may represent reciprocal changes when ST elevation is present in other leads 3
- In patients with inferior Q wave myocardial infarction, stress-induced ST depression in high lateral leads should be recognized as a reciprocal change for ST elevation in the inferior leads 3
- Widespread ST-segment depression is commonly attributed to diffuse subendocardial ischemia precipitated by severe multivessel or left main coronary artery disease 4
- ST depression can also occur in non-cardiac conditions such as central nervous system events 5
Management Implications
- Patients with ST-segment depression in the setting of acute chest pain should be considered for early invasive strategy with coronary angiography/revascularization 2
- Continuous ST-segment monitoring can be valuable for detecting silent ischemia and predicting outcomes 1
- Serial ECGs over very short intervals are useful to assess for progression of ischemic changes 1
Common Pitfalls to Avoid
- Misinterpreting upsloping ST depression as definitive evidence of ischemia (it's considered an equivocal finding) 1
- Failing to recognize reciprocal ST depression when ST elevation is present in other leads 3
- Overlooking non-cardiac causes of ST depression such as electrolyte abnormalities, medications, or neurological events 1, 5
- Ignoring the pattern, magnitude, and distribution of ST depression when determining clinical significance 1