The electrocardiogram (ECG) records cardiac electrical activity through surface electrodes, with each waveform component representing specific phases of the cardiac cycle: the P wave (atrial depolarization), PR interval (AV conduction time, normal 0.12-0.20 seconds), QRS complex (ventricular depolarization, normal < 0.12 seconds), ST segment (early ventricular repolarization), and T wave (ventricular repolarization). ST elevation in contiguous leads indicates acute transmural myocardial injury (STEMI) requiring emergent reperfusion therapy, while ST depression and T wave inversion suggest subendocardial ischemia. Hemodynamic monitoring through arterial lines provides continuous blood pressure measurement and waveform analysis, while central venous pressure (CVP) monitoring reflects right atrial pressure and volume status. The nurse must systematically analyze rate, rhythm, axis, intervals, and ST-T wave changes on every 12-lead ECG.