Clinical meaning
The 12-lead electrocardiogram records cardiac electrical activity from 12 different perspectives using 10 electrodes (4 limb, 6 precordial). The cardiac conduction system originates at the sinoatrial (SA) node (primary pacemaker, intrinsic rate 60-100 bpm), propagates through the atrial myocardium producing the P wave, reaches the atrioventricular (AV) node where physiological delay occurs (PR interval 0.12-0.20 seconds), then rapidly conducts through the bundle of His, right and left bundle branches (left dividing into left anterior and left posterior fascicles), and Purkinje fibers to depolarize the ventricular myocardium (QRS complex <0.12 seconds). The ST segment represents the plateau phase of the ventricular action potential (phase 2), and the T wave represents ventricular repolarization. Each lead provides a unique electrical viewpoint: leads II, III, aVF view the inferior wall (right coronary artery territory); leads I, aVL, V5-V6 view the lateral wall (left circumflex territory); leads V1-V4 view the anterior wall (left anterior descending territory). Right bundle branch block (RBBB) shows a wide QRS (>0.12s) with an RSR' pattern in V1-V2 ('rabbit ears'), while left bundle branch block (LBBB) shows a wide QRS with...