Pathophysiology
Clinical meaning
Advanced 12-lead ECG interpretation requires understanding the cardiac conduction system at the cellular level and correlating electrical patterns with anatomical regions of the heart. Cardiac Conduction Pathway: SA Node (intrinsic rate 60-100) → Atrial depolarization (P wave) → AV Node (0.12-0.20 sec delay = PR interval) → Bundle of His → Right and Left Bundle Branches → Purkinje Fibers → Ventricular depolarization (QRS complex, <0.12 sec) → Ventricular repolarization (T wave) 12-Lead Anatomical Correlation: • Inferior: Leads II, III, aVF → Right Coronary Artery (RCA) territory • Anterior: Leads V1-V4 → Left Anterior Descending (LAD) territory (the 'widow maker') • Lateral: Leads I, aVL, V5-V6 → Left Circumflex (LCx) territory • Septal: Leads V1-V2 → LAD septal perforators • Right Ventricular: V4R → RCA proximal • Posterior: Reciprocal changes in V1-V3 (ST depression, tall R waves) Systematic ECG Interpretation (12-Step Method): 1. Rate: Calculate (300 ÷ number of large boxes between R-R) 2. Rhythm: Regular or irregular? P waves present and consistent? 3. P Waves: Shape, direction, one P per QRS? 4. PR Interval: 0.12-0.20 sec (3-5 small boxes). Prolonged...