An ECG or EKG is a non-invasive test that measures the electrical signals in your heart. During the test, several electrodes are placed on your chest, arms and legs. The electrodes are connected by wires to a machine that turns the heart’s signals and displays it as waves on a monitor or on paper. The test allows the doctor to check your heart’s rhythm, diagnose a heart attack, and check on anything abnormal, such as thickened heart muscle. Any change in an ECG can be a sign of many heart-related conditions.