Components of the ECG pattern in myocardial infarction
Myocardial infarction on an ECG represents a combination of three processes: ischemia, injury, and necrosis. Each produces its own electrocardiographic equivalent, and their combination forms the characteristic pattern of infarction.
- Transmural ischemia — deep negative ("coronary") T wave.
- Transmural injury — pronounced ST-segment elevation, often in the shape of a "coved" or domed ST-T segment (monophasic curve).
- Transmural necrosis — QS complex (absence of the R wave).
- Nontransmural necrosis — pathological Q wave (greater than 1/4 of the R-wave amplitude and wider than 0.03 s).
Stages of myocardial infarction
The classical Russian classification distinguishes four stages, differing in timing and combination of ECG signs. In practice, the stage is determined primarily by the medical history (duration of pain syndrome, time from disease onset), and the ECG serves as confirmation.
- Hyperacute stage (up to 3 days). An injury zone and a small necrosis zone are formed. The ECG registers ST-segment elevation and the beginning of a pathological Q wave formation. A tall "coronary" T wave often precedes the elevation.
- Acute stage (up to 3 weeks). The injury zone decreases, an ischemia zone appears at the periphery, and necrosis stabilizes. The ECG shows a pathological Q wave, persistent ST elevation, and a negative coronary T wave.
- Subacute stage (up to 3 months). Injury disappears, while necrosis and ischemia remain. The ST segment returns to the baseline, the pathological Q wave persists, and the coronary T wave reaches its maximum expression.
- Scar stage. The necrotic tissue is replaced by a connective tissue scar, which is electrically inactive. The ECG retains only the pathological Q wave (or QS) — a lifelong sign of prior myocardial infarction.
Non-Q-wave infarction and reciprocal changes
In non-Q-wave infarction, the focus of necrosis is located within the thickness of the myocardium, the surrounding tissue undergoes compensatory activation, and a pathological Q wave does not form. Diagnosis relies on clinical presentation (typical pain syndrome), elevated cardiospecific enzymes (troponins), and indirect ECG signs: ST elevation or depression, T-wave inversion, and decreased R-wave amplitude.
ST-segment depression during infarction is often a mirror (reciprocal) reflection of elevation in opposite leads. For example, depression in anterior leads corresponds to elevation in posterior leads (V7–V9).
Signs of myocardial infarction on ECG — Summary
- Pathological Q wave or QS complex.
- Decreased R-wave amplitude over the affected area.
- ST-segment elevation (or depression as a reciprocal sign).
- Negative coronary T wave (symmetric, peaked).
- Possible conduction disturbances — left bundle branch block, which is easily confused with infarction; medical history is essential for differential diagnosis.
Topical diagnostics: Localization of infarction
The localization of the infarction is determined by the leads in which ischemic-necrotic changes are recorded. Lead orientation corresponds to Einthoven's triangle and the chest lead system:
- I, aVL — anterolateral wall of the left ventricle.
- II, III, aVF — inferior (posterior-diaphragmatic) wall.
- V1–V2 — interventricular septum and right heart chambers.
- V3–V4 — anterior wall and apex.
- V5–V6 — lateral wall of the left ventricle.
- V7–V9 (additional leads) — posterobasal regions.
Typical localizations: anteroseptoapical infarction (V1–V4), posterobasal (V7–V9 with reciprocal depression in V1–V3), posterolateral and anterolateral (V5–V6 combined with I and aVL or II, III, aVF). Involvement of a large number of leads is referred to as extensive myocardial infarction.
Practical notes
The ECG picture of infarction is rarely "textbook": stages overlap, signs may be blunted, and bundle branch blocks mask typical changes. Therefore, diagnosis is always based on correlating the ECG, clinical presentation, and laboratory markers. Precise localization of infarction by ECG has limited practical value and requires significant experience in analyzing real tracings.
Key points
- Myocardial infarction on ECG is a combination of ischemia, injury, and necrosis.
- Signs of transmural processes: coronary T wave, ST elevation, QS complex.
- Four stages — hyperacute, acute, subacute, and scar — differ in timing and combination of signs.
- Pathological Q wave persists for life as a marker of a scar.
- Non-Q-wave infarction is diagnosed by ST elevation/depression, T inversion, and troponins.
- ST depression is often a reciprocal reflection of elevation in opposite leads.
- Localization is determined by lead groups: anterior, septal, lateral, inferior, posterobasal.
- Definitive diagnosis requires correlation of the ECG with clinical and laboratory data.