Clinical meaning
The electrocardiogram records the electrical activity of the heart through surface electrodes, with each waveform reflecting specific phases of the cardiac cycle at the cellular level. The P wave represents atrial depolarization initiated by the sinoatrial node, the PR interval reflects conduction through the AV node (normal 0.12-0.20 seconds), the QRS complex represents ventricular depolarization via the His-Purkinje system (normal less than 0.12 seconds), and the T wave represents ventricular repolarization. ST segment changes carry critical diagnostic significance: ST elevation indicates transmural myocardial injury (STEMI) with contiguous lead patterns localizing the affected coronary artery territory, while ST depression suggests subendocardial ischemia or reciprocal changes. The nurse interprets ECG rhythm systematically (rate, rhythm, P waves, PR interval, QRS duration), recognizes life-threatening dysrhythmias (ventricular fibrillation, pulseless ventricular tachycardia, third-degree heart block, torsades de pointes), identifies acute STEMI patterns requiring emergent cardiac catheterization, and communicates findings using precise electrocardiographic terminology.