What is it?

Electrocardiography (ECG or EKG) is the process of recording the electrical activity of the heart over a period of time using electrodes placed on the skin. These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle’s electrophysiologic pattern of depolarizing and repolarizing during each heartbeat. It is a very commonly performed cardiology test.

In a conventional 12-lead ECG, ten electrodes are placed on the patient’s limbs and on the surface of the chest. The overall magnitude of the heart’s electrical potential is then measured from twelve different angles (“leads”) and is recorded over a period of time (usually ten seconds). In this way, the overall magnitude and direction of the heart’s electrical depolarization is captured at each moment throughout the cardiac cycle. The graph of voltage versus time produced by this noninvasive medical procedure is referred to as an electrocardiogram.

During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventricular node down into the bundle of His and into the Purkinje fibers, spreading down and to the left throughout the ventricles. This orderly pattern of depolarization gives rise to the characteristic ECG tracing. To the trained clinician, an ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system. Among other things, an ECG can be used to measure the rate and rhythm of heartbeats, the size and position of the heart chambers, the presence of any damage to the heart’s muscle cells or conduction system, the effects of cardiac drugs, and the function of implanted pacemakers.

Medical Uses

 The overall goal of performing electrocardiography is to obtain information about the structure and function of the heart. Medical uses for this information are varied and generally relate to having a need for knowledge of the structure and/or function. Some indications for performing electrocardiography include:

• Suspected myocardial infarction (heart attack) or new chest pain
• Suspected pulmonary embolism or new shortness of breath
• A third heart sound, fourth heart sound, a cardiac murmur or other findings to suggest structural heart disease
• Perceived cardiac dysrhythmias either by pulse or palpitations
• Monitoring of known cardiac dysrhythmias
• Fainting or collapse
• Seizures
• Monitoring the effects of a heart medication (e.g. drug-induced QT prolongation)
• Assessing severity of electrolyte abnormalities, such as hyperkalemia
• Hypertrophic cardiomyopathy screening in adolescents as part of a sports physical out of concern for sudden cardiac death (varies by country)
• Perioperative monitoring in which any form of anesthesia is involved (e.g. monitored anesthesia care, general anesthesia); typically both intraoperative and postoperative
• As a part of a pre-operative assessment some time before a surgical procedure (especially for those with known cardiovascular disease or who are undergoing invasive or cardiac, vascular or pulmonary procedures, or who will receive general anesthesia)
• Cardiac stress testing
• Computed tomography angiography (CTA) and Magnetic resonance angiography (MRA) of the heart (ECG is used to “gate” the scanning so that the anatomical position of the heart is steady)
• Biotelemetry of patients for any of the above reasons and such monitoring can include internal and external defibrillators and pacemakers