10 ECG Changes in Myocardial Infarction / Ischemia

ECG findings in ischemia and infarction span from the earliest minutes of coronary occlusion through the chronic healed phase. Here are the key 10 changes:

1. Hyperacute T Waves (earliest change — minutes)

Tall, broad, peaked T waves appear in the leads overlying the ischemic territory. This is often the very first ECG sign of STEMI, preceding ST elevation. The T wave is abnormally large and "tent-like."

"Note the broad, tall T waves in leads V3 and V4... These are the hyperacute T waves of early ST segment elevation myocardial infarction." — Rosen's Emergency Medicine

2. ST-Segment Elevation (minutes to hours — transmural ischemia)

ST elevation ≥1 mm in ≥2 contiguous leads defines STEMI. The morphology is typically flat, horizontal, or convex (tombstone-like) — distinct from the concave elevation of pericarditis or benign early repolarization.

Transmural ischemia from epicardial coronary occlusion (thrombosis or vasospasm)
Indicates the "culprit" territory (anterior, inferior, lateral, posterior)

3. Reciprocal ST Depression (simultaneous with ST elevation)

ST depression in leads on the opposite side of the infarcting territory. Examples:

Inferior STEMI (II, III, aVF elevation) → reciprocal ST depression in aVL
Anterior STEMI (V1–V4 elevation) → reciprocal depression in inferior leads (II, III, aVF)
Posterior MI → ST depression in V1–V3 (representing posterior ST elevation "mirrored")

Reciprocal changes increase specificity, and correlate with larger infarct size and worse outcomes. — Rosen's Emergency Medicine

4. ST-Segment Depression (subendocardial ischemia / NSTEMI)

Horizontal or downsloping ST depression indicates subendocardial ischemia. Seen in:

Unstable angina and NSTEMI
Demand ischemia (exercise, tachycardia)
As a reciprocal change in STEMI

Upsloping depression is less specific for ischemia. — Miller's Anesthesia

5. T-Wave Inversion (hours after onset → subacute phase)

Symmetrical, narrow T-wave inversions in the ischemic leads. The Wellens syndrome is a critical subtype:

Type I: Deep symmetrical T-wave inversions in anterior precordial leads (V2–V3)
Type II: Biphasic T waves in anterior leads
Both indicate critical LAD stenosis with high risk of progressing to anterior STEMI

"The T wave inversions of ACS are classically narrow and symmetrical." — Rosen's Emergency Medicine

6. Q-Wave Formation (hours to days — transmural necrosis)

Pathological Q waves (≥40 ms wide, ≥25% depth of R wave) reflect loss of viable myocardium and depolarisation away from that territory. Development depends on:

Volume of infarcted tissue (more so than transmurality)
Reperfusion status — Q waves may be transient if perfusion is restored early

"Most patients initially presenting with ST-segment elevation ultimately evolve Q waves on the ECG." — Harrison's Principles of Internal Medicine

7. Loss of R-Wave Progression / Poor R-Wave Progression

Progressive decrease or absence of R waves across precordial leads V1–V4 in anterior MI. A QS complex (entirely negative deflection, no R wave) indicates full-thickness necrosis.

8. Biphasic T Waves

An intermediate finding between upright T waves and full inversion. Classically seen in Wellens Type II (anterior leads) — a warning sign of impending anterior STEMI due to LAD lesion.

9. PR-Segment Changes (less common in MI; important to distinguish from pericarditis)

In acute pericarditis (a mimic of MI): diffuse PR depression with PR elevation in aVR. In contrast, MI ECG does not show PR changes — this distinction helps differentiate the two.

10. Normalisation / Chronic ECG Pattern (weeks to months — healed MI)

After the acute phase, the ECG may evolve to:

Persistent Q waves (permanent marker of prior infarction)
T-wave normalisation (may re-invert or flatten)
ST elevation persisting >2 weeks suggests left ventricular aneurysm
Return toward baseline in successfully reperfused patients (Q waves may disappear)

Summary Table: Temporal Sequence of ECG Changes in STEMI

Time	ECG Finding
Minutes (earliest)	Hyperacute tall T waves
Minutes–hours	ST elevation (convex/flat)
Simultaneous	Reciprocal ST depression
Hours	T-wave inversion begins
Hours–days	Pathological Q waves
Hours–days	Loss of R-wave progression / QS complex
Days–weeks	Deep symmetrical T-wave inversions (subacute)
Weeks–months	T-wave normalisation
>2 weeks	Persistent ST elevation → LV aneurysm
Chronic	Fixed Q waves (permanent scar)

ECG Territory Localisation

Territory	Elevated Leads	Artery
Anterior	V1–V4	LAD
Inferior	II, III, aVF	RCA or LCx
Lateral	I, aVL, V5–V6	LCx
Posterior	ST ↓ V1–V3 (mirror)	RCA/LCx
RV	V4R (right-sided leads)	Proximal RCA

Key ECG Images

Serial ECG evolution — anterior STEMI showing QS complexes and T-wave inversions

Serial ECG showing anterior STEMI evolution: QS complexes (V2–V5) with deep symmetrical T-wave inversions — classic subacute phase post-LAD occlusion

Inferior STEMI ECG progression over time

Serial inferior leads (II, III, aVF) showing baseline → acute STEMI (ST elevation + Q waves) → subacute/chronic phase (T-wave flattening)

Sources: Rosen's Emergency Medicine, 10e | Harrison's Principles of Internal Medicine, 22e | Fuster and Hurst's The Heart, 15e | Miller's Anesthesia, 10e

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