## Pathophysiology of ST Elevation ### Normal Ventricular Repolarization During normal repolarization, the epicardium repolarizes before the endocardium, creating a current flow from endocardium (negative) to epicardium (positive). This produces a **positive T wave** on the ECG. ### Transmural Myocardial Injury **Key Point:** ST elevation occurs when there is **transmural (full-thickness) myocardial ischemia/injury**. The ischemic zone becomes electrically inert and cannot repolarize normally. **High-Yield:** In acute transmural MI: 1. The ischemic zone loses its normal resting potential 2. The border zone between normal and ischemic tissue generates an **injury current** 3. This current flows from the **ischemic epicardium (negative) to the healthy endocardium (positive)** 4. The recording electrode "sees" this as **positive voltage → ST elevation** ### Clinical Correlation The patient's ECG shows: - **ST elevation in II, III, aVF** → **Inferior wall MI** (RCA territory) - **Reciprocal ST depression in I, aVL** → Confirms inferior MI - **Normal PR and QRS intervals** → No conduction delay ### The Injury Current Mechanism ```mermaid flowchart TD A[Acute Transmural MI] --> B[Ischemic zone loses resting potential] B --> C[Epicardium becomes electrically negative] C --> D[Healthy endocardium remains positive] D --> E[Injury current: Epi→Endo] E --> F[ECG electrode records positive deflection] F --> G[ST Elevation]:::outcome ``` **Clinical Pearl:** ST elevation indicates **acute, ongoing myocardial injury** and is the hallmark of STEMI, requiring urgent reperfusion therapy (PCI or thrombolysis). [cite:Harrison 21e Ch 297] 
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