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Here is a comprehensive guide to ECG changes in Myocardial Infarction (STEMI/NSTEMI), drawn from Harrison's Principles of Internal Medicine (22nd ed.) and Goldman-Cecil Medicine.
ECG Changes in Myocardial Infarction
The Underlying Mechanism
Acute ischemia causes a current of injury - a voltage gradient between normal and ischemic myocardial zones that manifests as ST-segment deviation on the surface ECG.
Diagram - Current of Injury:
- A (Subendocardial ischemia): ST vector directed inward → overlying leads show ST depression
- B (Transmural/epicardial ischemia): ST vector directed outward → overlying leads show ST elevation
STEMI - ECG Sequence of Changes
The ECG evolves in a predictable sequence:
| Time | ECG Finding |
|---|---|
| Minutes (earliest) | Hyperacute T waves - tall, broad, peaked T waves |
| Hours | ST elevation in leads overlying the infarcted zone |
| Hours - Days | T-wave inversions develop in same leads |
| Hours - Days | Q waves form (indicates transmural necrosis) |
| Weeks - months | ST returns to baseline; Q waves may persist permanently |
Lead Localization of Infarct Territory
| Leads with ST elevation | Territory | Artery |
|---|---|---|
| V1-V4 | Anterior/septal | LAD |
| V4-V6, I, aVL | Lateral | LCx or diagonal |
| II, III, aVF | Inferior | RCA (or LCx) |
| V1-V3 (ST depression = reciprocal) | Posterior | RCA or LCx |
| Right-sided leads (V3R-V4R) | Right ventricle | Proximal RCA |
ECG Sequence - Anterior STEMI (top row = acute, bottom row = evolving):
Acute anterior STEMI shows ST elevation in leads I, aVL, V2, V4, V6 with reciprocal ST depression in leads II, III, aVF. In the evolving phase, deep Q waves and T-wave inversions develop in the precordial leads.
ECG Sequence - Inferior STEMI (top row = acute, bottom row = evolving):
Acute inferior STEMI shows ST elevation in II, III, aVF with reciprocal ST depression in the precordial leads (V1-V6).
Reciprocal Changes
Reciprocal ST depression in leads opposite the infarct zone is an important clue:
- Anterior STEMI → reciprocal ST depression in II, III, aVF
- Inferior STEMI → reciprocal ST depression in V1-V6
- Posterior MI → ST depression in V1-V3 is itself the "equivalent" of ST elevation (no direct overlying leads)
NSTEMI / Non-ST Elevation ACS
- ECG shows ST depression and/or T-wave inversions (no ST elevation)
- Indicates subendocardial ischemia with incomplete coronary occlusion
- A minority of NSTEMI patients eventually develop Q waves ("non-Q-wave MI")
- ST depression in multiple leads with ST elevation only in aVR suggests left main or proximal LAD occlusion
Q Waves
- Pathological Q waves develop when transmural necrosis destroys depolarization forces
- Indicate established (old) MI if seen without acute ST changes
- Loss of posterior/lateral forces may show as reciprocal tall R waves in V1-V2 (posterior MI)
- Important: Q waves can also occur in non-MI conditions - LVH, LBBB, hypertrophic cardiomyopathy, WPW
Key Differential Diagnoses for ST Elevation
Other causes of ST elevation that must be distinguished from STEMI:
- Acute pericarditis (diffuse ST elevation, saddle-shaped, ST elevation in all leads except aVR)
- Left bundle branch block (LBBB)
- Left ventricular hypertrophy
- Brugada pattern (V1-V3)
- Takotsubo syndrome
- Early repolarization (benign variant)
- Acute pulmonary embolism
- Hyperkalemia / hypercalcemia
Key clinical pearl: STEMI is defined by ST elevation in 2 or more contiguous leads (≥2 mm in V1-V3, ≥1 mm elsewhere) and requires emergency reperfusion (primary PCI or thrombolytics) - the ECG is the gating criterion for this decision.
Sources: Harrison's Principles of Internal Medicine, 22nd ed., Ch. 247; Goldman-Cecil Medicine, Ch. 56
Ecg diagram explain with different cases on ECG paper
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That covers 8 clinical cases with ECG paper diagrams and explanations. Here's a quick summary of what was covered:
| Case | Condition | Key ECG Finding |
|---|---|---|
| 1 | Normal Sinus Rhythm | Regular P-QRS-T, rate 60-100, PR 0.12-0.20 sec |
| 2 | Anterior STEMI | ST elevation V1-V4, reciprocal depression in II, III, aVF |
| 3 | Posterior MI | ST depression V1-V3, tall R waves (mirror image) |
| 4 | MI Evolution Over Time | Normal → ST elevation → T inversion → Q wave |
| 5 | Atrial Fibrillation | No P waves, irregularly irregular QRS |
| 6 | Heart Blocks (1st/2nd/3rd) | PR changes → dropped QRS → complete dissociation |
| 7 | Hyperkalemia | Peaked T → no P → wide QRS → sine wave |
| 8 | Current of Injury (mechanism) | Subendocardial = ST depression; Transmural = ST elevation |
Would you like me to go deeper on any specific case - for example, how to calculate the cardiac axis, bundle branch blocks (LBBB/RBBB), or a specific arrhythmia like VT/VF?
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