Electrocardiogram: A recording of the electrical activity of the heart. Abbreviated ECG and EKG. An ECG is a simple, noninvasive procedure. Electrodes are placed on the skin of the chest and connected in a specific order to a machine that, when turned on, measures electrical activity all over the heart. Output usually appears on a long scroll of paper that displays a printed graph of activity on a computer screen. The initial diagnosis of heart attack is usually made through observation of a combination of clinical symptoms and characteristic ECG changes. An ECG can detect areas of muscle deprived of oxygen and/or dead tissue in the heart. If a medication is known to sometimes adversely affect heart function, a baseline ECG may be ordered before the patient starts taking the medicine, and follow-up testing may occur at regular intervals to look for any changes.