What Causes ST Elevation in STEMI?
ST-segment elevation in STEMI is caused by injury currents generated from severely ischemic or infarcted myocardium, which occur when acute coronary artery occlusion (typically from atherosclerotic plaque rupture with complete thrombotic occlusion) creates transmural myocardial ischemia that produces electrical voltage changes recorded on the body surface ECG. 1
The Electrophysiologic Mechanism
The ST elevation you see on ECG reflects injury currents emanating from the ischemic region. When coronary occlusion occurs, the affected myocardium becomes electrically abnormal, generating voltage differences between the injured tissue and normal myocardium. These currents are transmitted to the body surface where ECG leads with positive poles positioned over the ischemic region record ST-segment elevation 1.
Key Bioelectric Principles:
- All ECG leads are bipolar - they measure voltage differences between positive and negative poles
- ST elevation appears in leads whose positive electrodes face the injured myocardium
- Reciprocal ST depression simultaneously appears in leads oriented approximately 180° opposite to the injury site 1
For example, inferior wall STEMI shows ST elevation in leads II, III, and aVF (positive poles facing inferiorly) with reciprocal ST depression in lead aVL (positive pole facing superiorly and leftward) 1.
The Underlying Pathophysiology
Complete thrombotic occlusion of an epicardial coronary artery is the cause in the majority of STEMI cases 2, 3. The sequence unfolds as:
- Atherosclerotic plaque rupture
- Platelet activation and thrombus formation
- Complete coronary artery occlusion
- Transmural myocardial ischemia
- Electrical injury currents producing ST elevation
The magnitude and extent of ST elevation depend on 1:
- Size and location of the ischemic/infarcted region
- Which coronary artery is occluded and where
- Presence or absence of collateral circulation
- Distance of recording leads from the ischemic zone
- Voltage transmitted to the body surface
Critical Diagnostic Criteria
ST elevation must meet specific thresholds to diagnose STEMI 4:
- ≥0.2 mV (2 mm) in leads V2-V3 for men ≥40 years
- ≥0.25 mV (2.5 mm) in leads V2-V3 for men <40 years
- ≥0.15 mV (1.5 mm) in leads V2-V3 for women
- ≥0.1 mV (1 mm) in all other leads
- Must be present in ≥2 anatomically contiguous leads 1, 4
Important Caveats
Not all ST elevation is STEMI. Other causes include 1:
- Pericarditis
- Elevated serum potassium
- Acute myocarditis
- Early repolarization (normal variant)
- Left ventricular hypertrophy
- Bundle branch blocks
- Brugada syndrome
ST depression can indicate STEMI. Depression in leads V1-V2 often represents reciprocal changes from posterior/lateral wall STEMI - the ST elevation equivalent that would be recorded if electrodes were placed posteriorly (V8-V9 positions) 1. This is a commonly missed presentation.
For suspected circumflex occlusion with non-diagnostic initial ECG, obtain posterior leads V7-V9 where ≥0.05 mV elevation confirms posterior STEMI 4.
The changes in QRS complexes reflect alterations in electrical activation within the severely ischemic or infarcted region, with the spatial relationship between the injury site and lead orientation determining what appears on the ECG 1.