ST Segment
The ST segment is the flat, isoelectric portion of the ECG between the end of the QRS complex (J point) and the beginning of the T wave. It represents the plateau phase of the ventricular action potential (phase 2), during which the myocardium is uniformly depolarized and there is minimal net current flow. Deviations from the isoelectric baseline are among the most clinically important findings in electrocardiography.
Also known as: ST Interval, ST Level
Measurement
| Normal Range | The ST segment is normally isoelectric (at the baseline), with no more than 1 mm (0.1 mV) of elevation or depression in limb leads and no more than 1–2 mm in precordial leads (with early repolarization variants allowed up to 2 mm in some guidelines). The J point may be slightly elevated in early repolarization, particularly in young males. |
| Measurement | Measure ST deviation at the J point (the junction between the end of the QRS and the beginning of the ST segment) and at 60–80 ms after the J point. The isoelectric baseline is the PR segment (the flat portion between the end of the P wave and the start of the QRS). Measure in all 12 leads and note the distribution pattern of any elevation or depression, as the pattern identifies the clinical syndrome. |
Clinical Significance
ST segment analysis is the cornerstone of acute coronary syndrome evaluation. ST elevation in a regional distribution indicates transmural ischemia and is the ECG criterion for STEMI (ST-elevation myocardial infarction), requiring emergent reperfusion therapy. Diffuse ST elevation with PR depression suggests acute pericarditis. ST depression indicates subendocardial ischemia, digoxin effect, or reciprocal changes. The morphology, distribution, and evolution of ST changes over time are critical diagnostic clues.
Abnormalities
STEMI (ST-Elevation Myocardial Infarction)
Regional ST elevation in two or more contiguous leads meeting voltage criteria (≥1 mm in limb leads, ≥2 mm in V1-V3 in males or ≥1.5 mm in females) indicating acute transmural myocardial ischemia. The distribution of elevation identifies the culprit artery: inferior leads (II, III, aVF) for the RCA, lateral leads (I, aVL, V5-V6) for the circumflex, and anterior leads (V1-V4) for the LAD. Reciprocal ST depression in anatomically opposite leads strongly supports the diagnosis.
Acute Pericarditis
Diffuse, saddle-shaped ST elevation in most leads (except aVR and V1, which show ST depression) with concurrent PR segment depression. Unlike STEMI, the ST elevation is widespread and does not correspond to a single coronary territory. PR depression is highly specific for pericarditis and reflects atrial inflammation. Spodick's sign (downsloping TP segment) may also be present.
Subendocardial Ischemia / NSTEMI
Horizontal or downsloping ST depression of 0.5 mm or more in two or more contiguous leads indicating subendocardial ischemia. Subendocardial ischemia does not localize to a specific territory on ECG; widespread ST depression with ST elevation in aVR may indicate left main or proximal LAD stenosis causing global subendocardial hypoperfusion.
Benign Early Repolarization
J-point elevation (often with ST elevation up to 2 mm) in precordial leads, especially V2-V4, with an upwardly concave ST morphology and prominent notching or slurring at the J point. Common in young, athletic males. Generally benign, though some patterns (particularly inferior or lateral J-point elevation with horizontal or descending ST morphology) have been associated with idiopathic ventricular fibrillation.
Digitalis Effect (Dig Effect)
Characteristic downsloping ST depression with a scooped or sagging morphology, best seen in leads with tall R waves (V5-V6, I, II). This is a pharmacodynamic effect of digitalis on repolarization and does not necessarily indicate toxicity or ischemia. The pattern is sometimes described as a 'reverse tick' or 'Salvador Dali mustache.'
Frequently Asked Questions
How do I distinguish STEMI from pericarditis on ECG?
Several features help differentiate these two causes of ST elevation. In STEMI, ST elevation is regional (confined to a coronary territory), the morphology is often convex or tombstone-shaped, reciprocal ST depression in opposite leads is typically present, and evolution occurs over hours with development of Q waves. In acute pericarditis, ST elevation is diffuse and saddle-shaped, PR segment depression is often present, reciprocal changes are absent (except in aVR and V1 which show ST depression and PR elevation), and the pattern does not evolve into Q waves.
What does horizontal ST depression mean compared to downsloping?
The morphology of ST depression provides diagnostic information. Horizontal ST depression (the segment runs flat and parallel to the baseline) is highly suggestive of ischemia and is the most specific morphology for subendocardial ischemia during stress testing. Downsloping ST depression (the segment descends after the J point) is even more worrisome and carries a worse prognosis during exercise testing. Upsloping ST depression (the segment ascends from a depressed J point) is less specific and may be a normal finding at high heart rates, requiring at least 1.5–2 mm at 80 ms post-J point to be considered abnormal.
Why is ST elevation seen in right-sided leads during right ventricular MI?
Isolated right ventricular myocardial infarction occurs when the proximal right coronary artery (RCA) is occluded before the branch supplying the RV. Since standard precordial leads V1-V6 are positioned on the left side of the chest, right ventricular ischemia may be missed or show only subtle changes. Right-sided chest leads (V3R, V4R) are positioned over the right ventricle and are required to demonstrate the ST elevation of RV MI. V4R is the most sensitive single lead. The clinical significance is that nitroglycerin and diuretics must be used cautiously in RV MI because the right ventricle is preload-dependent.
What is the Wellens' syndrome and why is it a critical ECG finding?
Wellens' syndrome describes a specific T-wave and ST pattern in V2-V3 that indicates critical stenosis of the proximal left anterior descending artery (LAD) in a pain-free patient. Type A (less common) shows biphasic T waves, while Type B (more common) shows deeply symmetrically inverted T waves. These changes represent reperfusion after transient LAD occlusion. The danger is that these patients appear stable but are at extremely high risk for massive anterior STEMI if the LAD re-occludes. They should not undergo stress testing and require urgent cardiology evaluation and likely angiography.
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