electrocardiography

Electrocardiography is a noninvasive technique that records the electrical activity of the heart over time using surface electrodes placed on the chest and limbs. An electrocardiogram, or ECG, represents the summed electrical activity of cardiac depolarization and repolarization and is used to assess heart rate, rhythm, and conduction.

A standard 12-lead ECG provides multiple views of the heart's electrical activity. Three limb leads form Einthoven's triangle (I, II, III); augmented limb leads (aVR, aVL, aVF) complement chest leads (V1-V6). This arrangement helps detect regional problems, arrhythmias, conduction delays, and myocardial injury.

Key waveforms include the P wave (atrial depolarization), the PR interval, the QRS complex (ventricular depolarization), the ST segment, and the T wave (ventricular repolarization). Measurements such as heart rate, PR interval, QRS duration, and QT interval aid interpretation.

Clinical uses include evaluation of chest pain, suspected arrhythmias, syncope, preoperative risk assessment, and monitoring during acute illness or anesthesia. Ambulatory monitoring (Holter, event recorders) and stress testing extend ECG assessment beyond a single resting trace.

ECG interpretation depends on pattern recognition and clinical context. Limitations include sensitivity and specificity for certain conditions, artifacts from movement or poor contact, and the fact that ECG reflects electrical rather than mechanical function. It complements imaging and laboratory data.

Willem Einthoven developed the first practical electrocardiograph in the early 20th century; his work earned the Nobel Prize in Physiology or Medicine in 1924.