Electrocardiogram (ECG/EKG): recording the heart's electrical activity

Basic physiology and waveform components

Each heartbeat is initiated and coordinated by electrical impulses. A typical ECG tracing shows a sequence of waveforms and intervals: the P wave (atrial depolarization), the QRS complex (ventricular depolarization) and the T wave (ventricular repolarization). Intervals such as the PR and QT intervals reflect conduction times and ventricular recovery. Interpreting these features allows clinicians to infer heart rate, rhythm regularity, conduction delays and patterns that suggest myocardial injury or electrolyte disturbance.

Leads, electrodes and placement

The recording is made by placing surface electrodes on the chest and limbs after minor skin preparation at standardized locations. A standard 12-lead ECG combines signals from multiple electrodes to provide different electrical views of the heart, enabling assessment of specific regions such as the anterior, lateral and inferior walls. Correct electrode placement and good skin contact are important to reduce artifact and improve diagnostic accuracy. Guidance on electrode setup and placement is commonly available in technical and training materials.

Types of ECG and common procedures

Routine resting ECGs are brief and usually performed while the patient is supine. Ambulatory monitors (Holter) record continuously for 24–48 hours or longer to capture intermittent symptoms. Event recorders and implantable loop recorders can track rare events. Exercise stress ECGs evaluate exertional symptoms and can suggest exercise-induced ischemia. Continuous bedside monitoring is used in acute care settings to detect life-threatening arrhythmias.

Clinical uses

• Assessing heart rate and diagnosing arrhythmias or abnormal rhythms.
• Detecting evidence of acute or prior myocardial infarction and ischemia.
• Monitoring drug effects, electrolyte disturbances and conduction system disease.
• Evaluating the function and interaction of implanted devices such as an artificial pacemaker.
• Screening for chamber enlargement, conduction blocks and some inherited electrical disorders.

History and technology

The practical development of clinical ECG recording is closely associated with Willem Einthoven, who described and refined early instruments and introduced systematic tracings. Over time, large analog machines were replaced by compact digital electrocardiographs, automated analysis software and portable monitoring systems, increasing accessibility and enabling long-term ambulatory recordings.

Limitations, artifacts and interpretation

An ECG is a sensitive but not perfectly specific test: findings must be interpreted in the clinical context. A normal ECG does not exclude heart disease, and transient abnormalities may normalize between episodes. Common sources of error include poor electrode contact, patient movement, electrical interference and lead misplacement. Skilled interpretation and, when appropriate, supplementary tests such as imaging, laboratory studies or electrophysiology are used to confirm or refine diagnoses.

Practical notes and patient experience

The procedure is painless for most people. Preparations typically include removing jewelry and exposing the chest. Results from a resting ECG are usually available immediately and discussed with the patient or team. For practical guides and further technical detail, see resources on Einthoven and history at Einthoven and history, electrode setup at electrode guidance, skin preparation at skin-electrode interface, rhythm classification at arrhythmia resources, and pacemaker patterns at pacemaker monitoring.