Electrocardiography (not to be confused with echocardiography – see below) is a non-invasive test that measures the electrical activity of the heart externally recorded via skin electrodes. These are small sticky patches that are put onto the arms, legs and chest. Electrical impulses generated in the heart are conducted through body fluids to the skin. The ECGs are often referred to by the number of leads used, for example, a 3-lead, 5-lead or 12-lead ECG. A 12-lead ECG is more often used as a stand-alone test that is printed out. The 3-lead or 5-lead ECGs are more often viewed on a monitoring device such as those used in ambulances or when in theatre for monitoring during an operation.

An ECG is the best way to diagnose and measure abnormal heart rhythms, particularly in the case of damage to conductive tissue or electrolyte imbalance. It can help detect if a person is having a heart attack (myocardial infarction) or if they have had one in the past. In the case of an MI (myocardial infarction), an ECG can help detect if a certain part of the heart muscle has been damaged.