Patient implanted with a dual-chamber ICD (Virtuoso DR) for ischemic cardiomyopathy and hospitalized for syncope and for electrical shock. This tracing allows emphasizing the risks associated with T-wave oversensing.
On the graph, there is a sudden acceleration of the ventricular rhythm with intervals detected at the limit between the FVT zone and VF zone; a burst is delivered with acceleration of the intervals in the VF zone; a maximum electrical shock terminates the arrhythmia.
This tracing shows the potential pro-arrhythmogenic effect of an electrical shock or an anti-tachycardia pacing sequence delivered following an oversensing of the T-wave. The electrical shock is synchronized either on the R-wave or the T-wave. Similarly, as in this patient, the coupling of the first stimulus of an anti-tachycardia pacing burst is calculated by the device based on an erroneous heart rate which can lead to pacing at the peak of the T-wave, the vulnerable ventricular period. In this example, the burst accelerates and disrupts the heart rhythm into an extremely fast polymorphic arrhythmia. Patients with T-wave oversensing and high defibrillation threshold are particularly at risk. Indeed, as explained previously, in the presence of T-wave oversensing, the probability that the shock is delivered on the T-wave and hence during the vulnerable ventricular period is 50%. If the defibrillation threshold is high and approaches the maximum capabilities of the device, the upper vulnerability value is also high. A shock delivered to the T-wave has therefore 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). This implies that any oversensing of the T-wave should be considered as an emergency, since a modification of the programming is required to avoid the occurrence of inappropriate therapies.
