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Electrophysiology study: (EP) An invasive test to determine risk for cardiac arrhythmia. Catheter electrodes are inserted into the patient's heart via veins in the groin and activated to induce arrhythmia. If an arrhythmia occurs, the test is deemed positive and the patient is defibrillated to return to the heartbeat to normal. A positive result points to treatment with an implantable cardioverter defibrillator, anti-arrhythmia drugs, or ablation.