Charge circuit timeout: a sequence of events leading to failure of an implantable cardioverter-defibrillator to deliver therapy.

Since the advent of the implantable cardioverterdefibrillator (ICD) for primary prevention of sudden cardiac death, much attention has been given to the possibility of ICD or lead hardware failures resulting in inappropriate ICD function. Less attention has been given to the potential for software programming errors resulting in inappropriate device behavior. We report a case of a software programming error that led to a premature “end of service” alert that triggered a “lock-up” of the device antitachycardia therapies. Typically, the capacitor charge time for the Medtronic Secura ICD in anticipation of a full energy shock is about 7.7 to 9.2 seconds, depending on how close the device is to the elective replacement time and other factors.1 A charge circuit timeout indicates that at least 1 charging period exceeded 30 seconds. When 3 consecutive charging periods each exceed 30 seconds, the ICD charge circuit becomes inactive, and all automatic therapy functions and manual system tests become disabled except for emergency VVI pacing, all for the purpose of conserving battery voltage for pacemaker dependent patients when the generator reaches end of service.1 An alert is displayed to indicate that the time allotted to reach the programmed output energy (30 seconds) has been exceeded. A charge circuit timeout usually occurs when the battery voltage is depleted below the “elective replacement indicator.” In this specific instance, the charge circuit timeout occurred because of a glitch in the software. Three events occurred almost simultaneously, exposing the software programming error.2 These events occurred during an episode of tachycardia in the brief time span between the end of charging the capacitors and redetection/confirmation before delivery of a shock. The 3 events are: