An electrocardiogram, also called an ECG or EKG, records the heart’s electrical activity, showing how fast it’s beating, whether your heartbeat is steady or irregular, and the strength and timing of electrical signals as they pass through each part of your heart.

How the test is performed. While you are lying down, the technologist will attach several small electrodes to your arms, legs, and chest. It may be necessary to shave or clip some hair so the electrodes will stick to the skin. The patches are connected by wires to a machine that turns the heart’s electrical signals into wavy lines, which will later be reviewed by your doctor.

How to prepare for the test. Make sure the technologist knows about all the medications you are taking, as some impact the test results.

How the test will feel. An ECG is painless. You usually need to remain still during the test and you may be asked to hold your breath for a few seconds as the test is being done. Any movement, including shivering, can alter the results, so it is important to be relaxed and relatively warm during an ECG recording.