During an autopsy of a 62-year-old man who died of acute myocardial infarction, the pathologist examines the coronary arteries. Which of the following is the most common site of coronary artery occlusion in acute myocardial infarction?

Explanation

## Most Common Site of Coronary Artery Occlusion in Acute MI **Key Point:** The left anterior descending (LAD) artery is the most common site of acute myocardial infarction, accounting for approximately 40–50% of all acute MIs. ### Anatomical Basis for LAD Predominance 1. **Vessel characteristics:** - Longest coronary artery - Supplies the largest territory of myocardium - Highest blood flow demand - Increased shear stress on vessel wall 2. **Hemodynamic factors:** - Greater turbulent flow at bifurcations - Higher pressure gradients - Increased plaque formation risk 3. **Plaque vulnerability:** - LAD has the highest frequency of vulnerable plaques - More prone to rupture due to mechanical stress ### Frequency of Coronary Artery Occlusion in Acute MI | Coronary Artery | Frequency | Myocardial Territory | Clinical Features | |-----------------|-----------|----------------------|-------------------| | Left anterior descending (LAD) | 40–50% | Anterior wall, anterior septum, apex | Anterior wall MI, ST elevation in V1–V4 | | Right coronary artery (RCA) | 30–40% | Inferior wall, right ventricle | Inferior wall MI, ST elevation in II, III, aVF | | Left circumflex (LCx) | 15–20% | Lateral wall, posterolateral | Lateral wall MI, ST elevation in V5–V6, I, aVL | | Left main coronary artery | < 5% | Extensive anterior and lateral | Cardiogenic shock, high mortality | **High-Yield:** LAD occlusion is the most common cause of acute MI and typically results in anterior wall infarction with the largest infarct size due to the extensive territory supplied. **Clinical Pearl:** LAD occlusion in the proximal segment (before the first diagonal branch) results in the largest infarct territory and highest mortality rate among acute MIs. ### Myocardial Distribution by Coronary Artery ```mermaid flowchart TD A[Acute Myocardial Infarction]:::outcome --> B{Which coronary artery?}:::decision B -->|LAD 40-50%| C[Anterior wall MI]:::outcome C --> D[ST elevation V1-V4]:::action B -->|RCA 30-40%| E[Inferior wall MI]:::outcome E --> F[ST elevation II, III, aVF]:::action B -->|LCx 15-20%| G[Lateral wall MI]:::outcome G --> H[ST elevation V5-V6, I, aVL]:::action B -->|Left main < 5%| I[Extensive anterior + lateral]:::urgent I --> J[Cardiogenic shock]:::urgent ``` **Mnemonic:** **LAD-FIRST** = LAD is First in frequency → Anterior wall → Largest territory → Diagnostic ST changes in V1–V4 → Infarct size often large → Right ventricle usually spared → Septum involved → Tachycardia common ### Reason for LAD Predominance **Mechanical stress hypothesis:** The LAD experiences the highest hemodynamic stress due to its length, branching pattern, and the large volume of blood it carries. This chronic stress promotes atherosclerotic plaque formation and rupture at specific sites (proximal LAD, after the first diagonal branch).