A 58-year-old man presents to the emergency department with acute substernal chest pain radiating to the left arm, onset 6 hours ago. He has a history of hypertension and smoking. On examination, BP is 95/60 mmHg, HR 110/min, and JVP is elevated. Auscultation reveals a new holosystolic murmur at the apex. ECG shows ST elevation in leads II, III, and aVF. Echocardiography reveals severe mitral regurgitation with a flail posterior leaflet. What is the most likely pathological mechanism underlying the mitral regurgitation in this patient?

Explanation

## Pathological Mechanism of Acute Mitral Regurgitation in Inferior MI ### Clinical Context This patient has an acute inferior wall myocardial infarction (ST elevation in II, III, aVF) complicated by acute severe mitral regurgitation. The flail posterior leaflet on echo is the key finding. ### Anatomical Basis **Key Point:** The posteromedial papillary muscle receives its blood supply primarily from the right coronary artery (RCA). In inferior MI, RCA occlusion leads to ischemia and necrosis of this muscle. ### Pathological Sequence 1. RCA occlusion → inferior wall infarction 2. Posteromedial papillary muscle ischemia/necrosis 3. Papillary muscle rupture (partial or complete) 4. Loss of mechanical support for posterior mitral leaflet 5. Leaflet flails into left atrium during systole 6. Acute, severe mitral regurgitation ### Timing and Severity **High-Yield:** Papillary muscle rupture typically occurs 2–7 days post-MI, but can occur acutely (as in this case at 6 hours). Complete rupture is catastrophic and often fatal; partial rupture (as here, with flail leaflet) may be compatible with survival if managed urgently. ### Clinical Consequences - Acute pulmonary edema (elevated JVP, hypotension) - Cardiogenic shock (BP 95/60) - Holosystolic murmur (regurgitant jet throughout systole) - Echocardiographic hallmark: flail leaflet segment **Clinical Pearl:** Papillary muscle rupture is a mechanical complication of MI that requires urgent surgical intervention (mitral valve repair or replacement). Medical management alone is insufficient. ### Why This Diagnosis Over Others - **Acute onset** in the setting of acute MI rules out chronic processes (calcification, rheumatic disease) - **Flail leaflet** is pathognomonic for papillary muscle rupture - **Inferior MI location** with RCA territory infarction is the classic setting for posteromedial papillary muscle rupture - **Hemodynamic collapse** (hypotension, elevated JVP) reflects acute decompensation, not chronic valvular disease