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Introduction: Implantable cardioverter-defibrillators (ICDs) are susceptible to oversensing of extracardiac signals, also known as electromagnetic interference (EMI). We report a case of an unusual source of electrical interference of only the high voltage (HV) impedance measurement in the Teligen™ ICD (Boston Scientific, St. Paul, MN, USA) caused by electrical interference from an electrosurgical generator with an electrocautery patch located in close proximity to the ICD pulse generator. Method and Results: A patient underwent an uneventful implant of a Boston Scientific Teligen™ 100 ICD. Once the device was inserted in a pocket, interrogation of the device repeatedly demonstrated HV electrode impedance measurements between <20 and 40Ω and noise only on the HV electrode. A new lead and generator were implanted without a change in the interrogation results. The erroneous measurements of HV impedance were caused by a combination of the close proximity of the electrocautery patch to the ICD generator. The continuous low-amplitude current emitted by the contact quality monitoring system of the electrosurgical cautery generator interfered with an equally weak current delivered through the lead by the device to measurement HV impedance. The Medtronic Virtuoso™ (Medtronic Inc., Minneapolis, MN, USA) ICD and the St. Jude Medical Current™ DR (St. Jude Medical, St. Paul, MN, USA) ICD were not affected by the patch due to greater magnitude of current delivered by the device to measure HV electrode impedance. Conclusion: It is important that the operator must be aware of any potential source of EMI, as it may affect the device and require immediate troubleshooting. Failure to recognize this interaction may result in inappropriate and unnecessary pulse generator replacement. (PACE 2010; 994–998) [ABSTRACT FROM AUTHOR]