Conceptual Overview

The ST segment is the isoelectric period 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), when all ventricular myocytes are depolarized and there is no net electrical movement.

The T wave immediately follows and represents ventricular repolarization - the process by which myocytes restore their resting membrane potential. Together, the ST segment and T wave form the repolarization portion of the ECG and are exquisitely sensitive to:

Ischemia and infarction: The most clinically urgent cause of ST-T changes
Electrolyte disturbances: Particularly potassium and calcium
Structural heart disease: Hypertrophy, cardiomyopathy, pericarditis
Medications and toxins: Digoxin, antiarrhythmics, tricyclic antidepressants
CNS events: Subarachnoid hemorrhage, stroke

Key concept: The ST segment should normally be isoelectric (at the same level as the TP segment baseline). Any deviation - elevation or depression - from this baseline is potentially significant. The J-point is where you measure ST deviation, typically comparing it to the TP segment or PR segment as the baseline reference.

ST Elevation

ST elevation represents a current of injury - a voltage gradient between injured and normal myocardium during the plateau phase. While STEMI is the most feared cause, there are numerous causes ranging from benign to life-threatening.

STEMI Criteria (2018 Fourth Universal Definition)

New ST elevation at the J-point in 2 contiguous leads:
Men ≥40 years: ≥1 mm in all leads except V2-V3
Men <40 years: ≥2.5 mm in V2-V3; ≥1 mm in all other leads
Women: ≥1.5 mm in V2-V3; ≥1 mm in all other leads
Reciprocal ST depression: Present in leads opposite to the territory of injury (increases specificity)

STEMI Territory Localization

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Territory	ST Elevation Leads	Reciprocal Depression	Culprit Artery
Anterior	V1-V4	II, III, aVF	LAD
Lateral	I, aVL, V5-V6	III, aVF	LCx or diagonal LAD
Inferior	II, III, aVF	I, aVL	RCA (85%) or LCx (15%)
Posterior	V7-V9 (posterior leads)	V1-V3 (ST depression = mirror)	RCA or LCx
Right ventricular	V3R-V6R (right-sided leads)	Variable	Proximal RCA

Non-Ischemic Causes of ST Elevation

Early repolarization: J-point elevation with concave-up (smiley face) ST morphology, notching at J-point; benign in most cases
Acute pericarditis: Diffuse concave-up ST elevation with PR depression; no reciprocal changes (except aVR)
Left ventricular hypertrophy: ST elevation in V1-V3 with high voltage criteria
LBBB: Appropriate discordant ST elevation in V1-V3
Brugada syndrome: Coved ST elevation >2mm in V1-V2 with RBBB pattern
Ventricular aneurysm: Persistent ST elevation with Q waves in the same territory (old MI)
Takotsubo cardiomyopathy: Anterior ST elevation mimicking LAD occlusion with apical ballooning

STEMI equivalents - don't miss these:
De Winter T waves: Upsloping ST depression with tall T waves in V1-V6 (LAD occlusion without classic ST elevation)
Wellens syndrome: Biphasic or deeply inverted T waves in V2-V3 (critical LAD stenosis)
Posterior MI: ST depression in V1-V3 = ST elevation seen from behind. Get posterior leads (V7-V9)
Isolated ST elevation in aVR: With diffuse ST depression suggests left main or proximal LAD occlusion

ST Depression

ST depression typically represents subendocardial ischemia - the innermost layer of the myocardium is most vulnerable to ischemia because it has the highest oxygen demand and poorest blood flow (farthest from epicardial vessels, compressed during systole).

Morphology Patterns

Horizontal ST depression: Most specific for ischemia. Flat ST segment ≥1 mm below baseline
Downsloping ST depression: Also highly suggestive of ischemia. ST segment slopes downward from J-point
Upsloping ST depression: Least specific for ischemia. ST segment slopes upward from the J-point. Common with tachycardia and may be physiologic, but if ≥1 mm at 80 ms after J-point, it is still significant

Causes of ST Depression

Subendocardial ischemia / NSTEMI: The most clinically important cause
Reciprocal changes: ST depression opposite a STEMI territory (mirror image)
Posterior MI: ST depression in V1-V3 is ST elevation seen from behind - obtain posterior leads
Digoxin effect: "Salvador Dali" sagging ST depression, reverse tick morphology
LVH strain pattern: Asymmetric ST depression with T wave inversion in lateral leads
Hypokalemia: ST depression with T wave flattening and prominent U waves
Rate-related: Tachycardia-induced demand ischemia or physiologic J-point depression

"Depression in V1-V3 = think posterior": Isolated ST depression in V1-V3 without ST elevation elsewhere is a pattern that screams posterior STEMI. The anterior leads are looking at the back of the heart through the chest wall - what appears as depression is actually elevation on the posterior surface. Always obtain right-sided (V4R) and posterior (V7-V9) leads in inferior STEMI or isolated anterior depression.

Acute Pericarditis vs. STEMI

Differentiating pericarditis from STEMI is a critical skill. Both produce ST elevation, but the patterns are dramatically different.

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Feature	Acute Pericarditis	STEMI
ST elevation distribution	Diffuse (most leads)	Regional (vascular territory)
ST morphology	Concave up (smiley face)	Convex up or straight (tombstone)
Reciprocal depression	Absent (except aVR and sometimes V1)	Present in opposite territory
PR segment	PR depression (highly specific)	Normal or PR elevation in aVR
Q waves	Absent	May develop (pathologic Q waves)
Evolution	4 stages over days-weeks	Rapid evolution (hours): STE → Q waves → T inversion
Spodick sign	TP segment downsloping	Absent

Pericarditis ECG Stages

Stage 1 (acute): Diffuse ST elevation + PR depression (except aVR: ST depression, PR elevation)
Stage 2: ST segments normalize; T wave flattening
Stage 3: Diffuse T wave inversions (after ST normalization)
Stage 4: ECG returns to normal (or T wave inversions may persist)

The Spodick sign: A downsloping TP segment (the baseline between the T wave and the next P wave slopes downward) is a subtle but highly useful sign of acute pericarditis. It's best seen in lead II and the lateral precordial leads. When present, it strongly favors pericarditis over STEMI.

STEMI Equivalents & Mimics

Several ECG patterns represent acute coronary occlusion without meeting classic STEMI criteria. Missing these can delay life-saving intervention.

De Winter T Waves

Pattern: 1-3 mm upsloping ST depression at the J-point with tall, symmetric, peaked T waves in precordial leads
Significance: Represents 2% of acute LAD occlusions; true STEMI equivalent
Action: Immediate cath lab activation - do NOT wait for ST elevation to develop
Key distinguishing feature: The upsloping ST depression differentiates this from simple hyperkalemia (which has peaked T waves but different ST morphology)

Wellens Syndrome

Type A (25%): Biphasic T waves (initial positive, terminal negative) in V2-V3
Type B (75%): Deeply inverted symmetric T waves in V2-V3 (may extend to V4-V6)
Context: Patient with recent chest pain that has now resolved; minimal troponin elevation
Significance: Critical proximal LAD stenosis; 75% will develop extensive anterior MI within weeks without intervention
Contraindication: Do NOT stress test these patients - this can precipitate MI

Posterior STEMI

Standard ECG: ST depression in V1-V3 (mirror image of posterior ST elevation)
Tall, upright R waves in V1-V2: Mirror image of posterior Q waves
Confirm with posterior leads: V7-V9 showing ≥0.5 mm ST elevation
Often accompanies inferior STEMI: Always check posterior leads with inferior MI

Left Main / Proximal LAD Occlusion

ST elevation in aVR ≥1 mm with diffuse ST depression in multiple leads
Significance: Suggests severe proximal disease (left main, proximal LAD, or three-vessel disease)
Prognosis: Very high mortality without emergent revascularization

Don't be fooled by "non-diagnostic" ECGs: Up to 30% of acute coronary occlusions do NOT present with classic ST elevation. Serial ECGs (every 15-30 minutes), posterior leads, and right-sided leads can unmask STEMI equivalents. When clinical suspicion is high and the ECG is inconclusive, repeat it. The first ECG is often the worst ECG.

Early Repolarization

Early repolarization (ER) is characterized by J-point elevation with ST elevation and is common in young, healthy individuals. While historically considered benign, certain patterns have been linked to increased risk of ventricular fibrillation.

Classic (Benign) Pattern

J-point elevation: ≥1 mm in lateral leads (V4-V6) or inferior leads
J-point morphology: Notching or slurring of terminal QRS
ST elevation: Concave upward (smiley face morphology)
Tall, concordant T waves: Prominent and upright
Demographics: Young males, athletes, Black individuals

Higher-Risk Features

Inferior leads (II, III, aVF): ER in inferior leads carries greater risk than lateral leads
Horizontal or downsloping ST segment: After J-point elevation (vs. upward concavity)
J-point elevation >2 mm: Greater magnitude associated with increased risk
Notching (vs. slurring): Notched J-point may carry slightly higher risk

ER vs. STEMI differentiation: Early repolarization has concave-up ST segments, no reciprocal depression, no evolutionary changes on serial ECGs, and a normal ST/T amplitude ratio. If the ST elevation to T wave amplitude ratio exceeds 0.25 in any lead, STEMI becomes more likely. Clinical context is paramount - chest pain + risk factors + ST elevation = assume ischemia until proven otherwise.

Quick Reference

ST elevation + reciprocal depression: STEMI until proven otherwise - activate cath lab
Diffuse ST elevation + PR depression: Acute pericarditis
ST depression V1-V3: Think posterior MI - obtain V7-V9
ST elevation aVR + diffuse depression: Left main or severe three-vessel disease
Concave-up ST elevation, young patient: Consider early repolarization (but rule out STEMI first)
Upsloping ST depression + tall T waves: De Winter pattern - STEMI equivalent
Deep T wave inversion V2-V3 post chest pain: Wellens syndrome - critical LAD, no stress test
Sagging ST depression: Digoxin effect (Salvador Dali mustache)
ST elevation with LBBB: Apply modified Sgarbossa criteria for concordance

Clinical Pearls

"Morphology matters more than magnitude": A concave-up (smiley face) ST elevation is more likely benign (early repolarization, pericarditis). A convex-up (frowning) or "tombstone" ST elevation is highly concerning for STEMI. The shape of the ST segment often tells you more than the millimeters of elevation.

"Reciprocal changes are your friend": The presence of reciprocal ST depression dramatically increases the specificity for STEMI. Early repolarization and pericarditis do NOT produce reciprocal changes. When you see ST elevation, immediately check the opposite leads.

"Serial ECGs save lives": The initial ECG is non-diagnostic in up to 30% of acute MIs. Repeat ECGs every 15-30 minutes in patients with ongoing chest pain. Subtle changes that evolve over serial tracings can unmask STEMI before classic criteria are met.

"Check aVR": Lead aVR is the most neglected lead. ST elevation in aVR with diffuse ST depression suggests left main or proximal LAD occlusion - one of the highest-mortality patterns. Similarly, PR elevation in aVR with diffuse PR depression is pathognomonic for pericarditis.

The posterior MI trap: Up to 15-20% of acute MIs involve the posterior wall. With standard 12-lead ECG alone, posterior STEMI presents as ST DEPRESSION in V1-V3, which gets labeled "NSTEMI" and may not receive emergent cath. Always obtain posterior leads (V7-V9) when you see isolated anterior ST depression, especially alongside inferior STEMI.

Right ventricular infarction matters: In inferior STEMI, always obtain right-sided leads (V4R). ST elevation ≥1 mm in V4R diagnoses RV infarction. These patients are preload-dependent - nitroglycerin and diuretics can cause profound hypotension. Treatment: volume resuscitation, avoid preload reducers, emergent reperfusion.

References

Thygesen K, Alpert JS, Jaffe AS, et al. Fourth Universal Definition of Myocardial Infarction (2018). Circulation. 2018;138(20):e618-e651.
De Winter RJ, Verouden NJ, Wellens HJ, Wilde AA. A new ECG sign of proximal LAD occlusion. N Engl J Med. 2008;359(19):2071-2073.
Smith SW, Dodd KW, Henry TD, et al. Diagnosis of ST-Elevation Myocardial Infarction in the Presence of Left Bundle Branch Block. Ann Emerg Med. 2012;60(6):766-776.
Chou's Electrocardiography in Clinical Practice. 6th ed. Saunders; 2008.
ECG Library - LITFL - Life in the Fast Lane. https://litfl.com/ecg-library/

The ST Segment & T Wave