DIAGNOSTIC PROCEDURES << BACK Electrophysiology Testing An electrophysiology test may be needed to evaluate fainting, an irregular heartbeat, a very fast or very slow heartbeat, to check the heart before and after a pacemaker or a defibrillator is implanted in the body. An EP test involves placing wire electrodes within the heart to determine the characteristics of heart arrhythmias. The cardiologist inserts a catheter through a small incision in a groin vein after cleaning the site and numbing it with a local anesthetic. This catheter is equipped with an electrode connected to electrocardiographic monitors, then carefully threaded into the heart using an X-ray imaging technique called fluoroscopy. Electrodes are placed in the heart to measure electrical activity along the heart's conduction system and within heart muscle cells themselves.