STEMI: ECG Changes

Pathophysiology of ST Changes

Transmural (full-thickness) ischemia creates a current of injury between normal and ischemic myocardium. The ST vector shifts outward (toward the epicardium), producing ST elevation in leads overlying the affected zone and reciprocal ST depression in opposite leads.

Current of injury diagram — subendocardial vs transmural ischemia

(A) Subendocardial ischemia → ST depression in overlying leads. (B) Transmural (epicardial) ischemia → ST elevation in overlying leads — the STEMI pattern. — Harrison's Principles of Internal Medicine 22E, Fig. 247-11

Diagnostic ECG Criteria for STEMI

The standard criteria require new ST elevation at the J-point in ≥2 contiguous leads:

≥1 mm in all leads except V2–V3
≥2 mm in V2–V3 in men ≥40 years; ≥2.5 mm in men <40 years; ≥1.5 mm in women

Lead Localisation by Territory

Territory	ST Elevation Leads	Likely Vessel
Anteroseptal	V1, V2 (±V3)	LAD (proximal)
Anterior	V1–V4	LAD
Anterolateral	V1–V6, I, aVL	LAD (proximal) or LCx
Lateral	I, aVL	Diagonal / LCx
Inferior	II, III, aVF	RCA (80%) or LCx (20%)
Inferolateral	II, III, aVF, V5–V6	RCA or LCx
True posterior	Tall R in V1–V2 (R/S ≥1); ST depression V1–V3	LCx or posterior RCA
Right ventricular	II, III, aVF + V3R–V6R elevation	Proximal RCA

From Tintinalli's Emergency Medicine, Table 49-4

Clinical pearl: In inferior STEMI, always obtain right-sided lead V4R — ST elevation ≥1 mm in V4R is highly specific for right ventricular infarction.

Sequential ECG Evolution

STEMI produces characteristic time-dependent changes:

Phase	Timing	ECG Finding
Hyperacute	Minutes	Tall, peaked (hyperacute) T waves
Acute	Hours	Convex ("tombstone") ST elevation; ST merges with T wave
Early evolving	Hours–days	ST begins to fall; T-wave inversion develops
Late evolving	Days–weeks	Pathological Q waves form; T inversions deepen
Chronic/old MI	Weeks–months	Q waves persist; ST normalises; T may remain inverted or normalise

Key Individual ECG Findings

1. ST Elevation

Convex (coved) morphology is classic — the "tombstone" pattern
Must be measured at the J-point
Reciprocal ST depression in opposing leads confirms STEMI and indicates larger injury zone, more severe CAD, and higher mortality

2. Hyperacute T Waves

Broad-based, symmetrically tall T waves
Earliest sign — may precede ST elevation by minutes
Prominent in V2–V4 in anterior STEMI

3. Pathological Q Waves

Width ≥0.04 s (1 mm) or depth ≥25% of R wave
Indicate myocardial necrosis (not always transmural)
Develop within hours to days
May persist permanently or regress with smaller infarcts

4. T-Wave Inversions

Follow the ST elevation phase
Deep symmetric inversions in precordial leads (V1–V4 with or without I, aVL) = Wellens pattern → suggests high-grade LAD stenosis

Wellens T-wave sign — deep precordial T inversions

Deep T-wave inversions in V1–V6: Wellens sign, associated with critical LAD stenosis. — Harrison's Principles, Fig. 247-12

5. Reciprocal Changes

ST depression in leads "opposite" the infarct zone
Example: Inferior STEMI (II, III, aVF) → reciprocal ST depression in I and aVL
Example: Anterior STEMI → reciprocal ST depression in II, III, aVF
Their presence increases the probability of true STEMI and correlates with worse outcomes

Example ECGs

Anterolateral STEMI (LAD occlusion) — convex ST elevation V1–V6, I, aVL with reciprocal inferior depression:

Anterolateral STEMI ECG — ST elevation V1-V6 with reciprocal inferior changes

Inferior STEMI (RCA occlusion) — ST elevation II, III, aVF; reciprocal depression in I, aVL, V4–V6:

Inferior STEMI ECG — ST elevation in inferior leads with reciprocal lateral depression

STEMI Equivalents (No Classic STE)

These patterns should trigger the same urgent reperfusion response:

Pattern	Significance
New LBBB	Sgarbossa criteria to identify STEMI underneath
De Winter pattern	Upsloping ST depression + tall T waves in V1–V6 → proximal LAD occlusion
Posterior STEMI	ST depression V1–V3 + tall R waves (mirror image)
Right ventricular STEMI	ST elevation V3R–V6R
aVR elevation	ST elevation in aVR ≥1 mm with diffuse ST depression → left main or proximal LAD occlusion

ECG Limitations

A single normal ECG does not exclude STEMI — repeat at 15–30 min intervals if symptoms persist
LBBB, ventricular pacing, and WPW can mask ischemic changes
Posterior MI shows no direct ST elevation in standard leads — posterior leads (V7–V9) are required
<10% of patients with new LBBB actually have AMI

Sources: Harrison's Principles of Internal Medicine 22E; Tintinalli's Emergency Medicine, Comprehensive Study Guide

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