53-year-old man implanted with a Lumax 340 VR-T single-chamber ICD for Brugada syndrome with syncope; electric shock received during an exertion; event report (yellow color) in the setting of a classified VF.
Telecardiology tracing: 3 channels are available; the markers with the time intervals, the shock channel (FF: far field) between the ventricular lead coil and the pulse generator, the right ventricular sensing channel (RV).
Programmer tracing (different episode): the 3 channels are the same as for the telecardiology tracing.
This patient presented several episodes of T wave oversensing upon exertion with delivery of shock or interruption of the charge. T wave oversensing currently remains a significant problem in the management of ICD-implanted patients since it can be accompanied by the occurrence of inappropriate therapies particularly during exertion (when RT and TR intervals correspond to the VF zone due to sinus tachycardia). T wave oversensing is associated with a typical alternating pattern between 2 morphologically different signals, namely a high frequency signal (R wave) and a low frequency signal (T wave). For each cardiac cycle, the device counts the R wave and the T wave as a second additional signal resulting in a doubling of the heart rate. The alternating interval duration (RT intervals and TR intervals) is usually pronounced for slow rates (short RT intervals and long RT intervals) although often less during exertion (RT and RT intervals roughly equivalent) or for patients with long QT syndrome.
Any oversensing of the T wave should be considered as an emergency, with mandatory modification of the programming to avoid the occurrence of inappropriate therapies. An electrical shock or an antitachycardia pacing sequence delivered as a result of T wave oversensing may be accompanied by a proarrhythmogenic effect with the risk of inducing polymorphic ventricular arrhythmia. Indeed, the electric shock synchronizes either on the R wave or on the T wave, with a 50% probability that the shock is delivered on the T wave, hence during the vulnerable ventricular period. Patients with T wave oversensing and high defibrillation threshold are particularly at risk. If the defibrillation threshold is high and approaches the maximum capabilities of the device, the upper vulnerability value is also high. A shock delivered on the T wave therefore has a very high probability of inducing ventricular fibrillation (concept of upper limit of vulnerability) which will subsequently be very difficult to terminate even with a maximum defibrillation shock (high defibrillation threshold).
