Cardiac electrophysiology is the study of elucidating, diagnosing, and treating the electrical activities of the heart. It is usually used to describe studies of such phenomena by invasive (intracardiac) catheter recording of spontaneous activity as well as of cardiac responses to programmed electrical stimulation (PES). It typically measures the response of the injured or cardiomyopathic myocardium to PES on specific pharmacological regimens in order to assess the likelihood that the regimen will successfully prevent potentially fatal Sustained Ventricular Tachycardia (VT) or Ventricular Fibrillation VF (VF) in the future. Sometimes a series of EPS drug trials must be conducted to enable the cardiologist to select the one regimen for long-term treatment that best prevents or slows the development of VT or VF following PES. Such studies may also be conducted in the presence of a newly implanted or newly replaced cardiac pacemaker or AICD.