Electrical failure of an ICD lead due to a presumed insulation defect only diagnosed by a maximum output shock

A 55-year-old male patient presented after a single shock caused by oversensing of isolated nonphysiologic signals on both the distal HV and pace-sense channels. No other abnormalities were found. He subsequently returned complaining of device "vibration" and his St. Jude implantable defibrillator (ICD; St. Jude Medical, St. Paul, MN, USA) was found to be in VVI backup mode and could not be interrogated. Direct testing in the electrophysiology lab showed normal lead impedances and thresholds with an inability to reproduce the abnormal signals. Detailed cine fluoroscopy of the leads found no abnormalities. A new ICD was connected and successfully delivered a 20-joule shock but failed to deliver a maximum output (39-joule) shock. The new ICD was again found to be in backup mode. A new Endotak Reliance G lead (Boston Scientific, Natick, MA, USA) was implanted and a maximum-output shock was successful using a new Fortify DR ICD. This case likely represents a Durata lead insulation defect in the form of an inside-out abrasion under the distal HV coil. Increased awareness of this defect is warranted, particularly since routine interrogation and submaximum-output shocks may fail to detect the problem.