## Distinguishing Proximal vs. Distal LAD Occlusion ### Anatomical Basis The left anterior descending artery supplies the anterior wall of the left ventricle and the first diagonal branch (D1) supplies the anterolateral wall. Proximal LAD occlusion (before D1) causes extensive anterior and anterolateral infarction, whereas distal LAD occlusion (after D1) spares the anterolateral territory. ### Key ECG Discriminator **Key Point:** ST elevation in lead aVL is the most specific finding for proximal LAD occlusion because aVL views the anterolateral wall, which is supplied by the first diagonal branch. When the LAD is occluded proximal to D1, both the LAD and D1 territories are infarcted, producing ST elevation in aVL. Distal LAD occlusion preserves D1 perfusion, so aVL typically remains isoelectric or shows minimal change. ### Comparison Table | Feature | Proximal LAD Occlusion (before D1) | Distal LAD Occlusion (after D1) | | --- | --- | --- | | **ST elevation in V1–V4** | Present | Present | | **ST elevation in aVL** | **Present (≥1 mm)** | **Absent or minimal** | | **Reciprocal ST depression in II, III, aVF** | Present | Present | | **Territory affected** | Anterior + anterolateral | Anterior only | | **Risk of cardiogenic shock** | Higher (larger infarct) | Lower | ### Clinical Pearl **Clinical Pearl:** Proximal LAD occlusion is a high-risk presentation because it affects a larger myocardial territory and carries higher risk of mechanical complications (papillary muscle rupture, ventricular septal defect) and cardiogenic shock. Recognition of aVL involvement helps identify these high-risk patients early. ### High-Yield Summary **High-Yield:** In anterior STEMI, always check lead aVL. Its presence indicates proximal LAD occlusion and extensive infarction; its absence suggests distal LAD occlusion with preserved collateral flow to the anterolateral wall via D1. [cite:Harrison 21e Ch 297] 
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