ECG

An ECG records voltage over time from 12 electrical viewpoints using 10 electrodes: four limb electrodes and six precordial electrodes. It shows electrical activation and recovery; it does not directly measure coronary blood flow, myocardial contractility, or tissue perfusion. A normal tracing therefore cannot, by itself, exclude evolving ischemia. Normal activation follows a predictable route. The sinoatrial (SA) node initiates the impulse, which spreads across the atrial myocardium and produces the P wave. The impulse then reaches the atrioventricular (AV) node, where conduction slows briefly. That delay allows the ventricles time to fill before ventricular depolarisation travels through the His bundle, right and left bundle branches, fascicles, and Purkinje fibres. The resulting ventricular depolarisation is the QRS complex. Ventricular recovery produces the T wave. The intervals reveal where conduction is delayed. The PR interval normally measures 120–200 ms. A PR interval longer than 200 ms, with every atrial impulse still conducted, is first-degree AV block. A QRS duration of 120 ms or more is wide and suggests delayed intraventricular conduction; a complete right- or left-bundle branch block requires the corresponding...