Electrocardiography
method to record the electrical activity of the heart through passive electrodes placed over the skin.
Electrocardiography is the process of using an electrocardiograph (a device) to produce an electrocardiogram (a recording, often called an ECG or EKG) that shows a line graph of the heart's electrical activity through repeated cardiac cycles. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including:
Cardiac rhythm disturbances, such as atrial fibrillation and ventricular tachycardia
Inadequate coronary artery blood flow, such as myocardial ischemia and myocardial infarction
electrolyte disturbances, such as hypokalemia
Traditionally, "ECG" refers to a 12-lead ECG taken while lying down as discussed below.
However, other devices can record the electrical activity of the heart, such as a Holter monitor or some models of smartwatch.
ECG signals can also be recorded in other contexts with other devices.
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.
There are three main components to an ECG:
The P wave, which represents depolarization of the atria
The QRS complex, which represents depolarization of the ventricles
The T wave, which represents repolarization of the ventricles
During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial node, spreads throughout the atrium, and 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.
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