Why ST-Segment Elevation Occurs in STEMI
ST-segment elevation on the ECG occurs in STEMI because complete thrombotic occlusion of an epicardial coronary artery causes transmural myocardial ischemia, which creates electrical injury currents between the ischemic and non-ischemic myocardium that manifest as ST-segment elevation on the body surface ECG. 1, 2
Pathophysiological Mechanism
Coronary Occlusion and Transmural Ischemia
Complete thrombotic occlusion of a major epicardial coronary artery is the underlying cause in the vast majority of STEMI cases, typically resulting from physical disruption of an atherosclerotic plaque with subsequent formation of an occluding thrombus 1, 2, 3
This complete occlusion (as opposed to the subtotal occlusion seen in NSTEMI) causes severe reduction in coronary blood flow, leading to transmural myocardial ischemia—meaning the ischemia extends through the full thickness of the ventricular wall from endocardium to epicardium 1
Cellular Electrical Changes
At the cellular level, transmural ischemia causes profound alterations in the transmembrane action potential of myocardial cells, particularly shortening of the action potential duration and reduction in resting membrane potential 4
These changes in action potential characteristics create electrical heterogeneity between the ischemic (injured) myocardium and the surrounding normal myocardium 4
The ischemic cells also experience altered electrical cell-to-cell coupling, further contributing to abnormal electrical propagation 4
Surface ECG Manifestation
The electrical differences between ischemic and non-ischemic tissue generate injury currents that flow between these regions during both systole and diastole 4
These injury currents are detected on the body surface ECG as ST-segment elevation in leads overlying the ischemic territory 4
The location and extent of ST-segment elevation corresponds directly to the anatomic distribution of the transmural myocardial injury, which is determined by the territory supplied by the occluded coronary artery 1
Clinical Correlation
Distinguishing STEMI from NSTEMI
In STEMI, the completely occlusive thrombus causes transmural ischemia that produces ST-segment elevation on the ECG 1
In NSTEMI, the thrombus is subtotally occlusive, causing ischemia that is typically non-transmural (subendocardial), which manifests as ST-segment depression or T-wave changes rather than ST-segment elevation 1
This distinction is critical because most patients with ST-segment elevation ultimately develop Q-wave myocardial infarction, reflecting the transmural nature of the injury 1
Important Caveat
Approximately 30% of patients with acute total coronary occlusion may not meet standard STEMI ECG criteria, particularly with left circumflex artery occlusions, which often present without significant ST-elevation on standard 12-lead ECG 5, 6
Left circumflex occlusions present with ST-segment elevation in fewer than 50% of cases because the posterior and lateral walls may not be well-represented on standard precordial leads 6
This underscores why clinical context, serial ECGs, and liberal use of emergency angiography are essential when acute coronary occlusion is suspected despite non-diagnostic initial ECG findings 7, 5