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 = AV block 5. QRS Duration: <0.12 sec normal. Wide QRS = bundle branch block or ventricular origin 6. QRS Axis: Normal -30° to +90°. LAD or RAD suggests pathology 7. QRS Morphology: Q waves (necrosis), R wave progression V1-V6, bundle branch block patterns 8. ST Segment: Elevation (STEMI, pericarditis) or Depression (ischemia, reciprocal changes) 9. T Waves: Inversion (ischemia, strain), peaked (hyperkalemia), flattened (hypokalemia) 10. QT Interval: Corrected QTc <0.44 sec (male), <0.46 sec (female). Prolonged QTc = Torsades risk 11. U Waves: May indicate hypokalemia 12. Compare with prior ECGs: New changes vs chronic findings