An Electrocardiogram (EKG/ECG) measures the heart’s electrical activity and can record abnormal heart episodes.  An EKG is usually done along with a medical history and physical examination, lab tests, and a chest X-Ray.
If you do not have an episode of irregular heart rhythm while at the doctor’s office, your doctor will probably as you to wear a portable EKG to record your heart rhythm on a continuous basis.  This is referred to by several names, including ambulatory electrocardiogram, ambulatory ECG, holter monitoring, 24-hour EKF, or cardiac event monitoring.  This will allow your heart rhythm to be recorded while you are having an episode.