Patient implanted with a single-chamber ICD (Protecta XT VR) for ischemic cardiomyopathy; this tracing illustrates the characteristic pattern of the graph during an oversensing of the T-wave.
The graph shows a railroad track pattern potentially corresponding to the oversensing of a physiological cardiac signal (T-wave, P-wave, R-wave double counting); in this tracing, there is a relatively short interval and a longer interval; this oversensing occurs in a setting of sinus tachycardia (100 beats/minute).
- the EGM shows 2 alternating signals of different morphology, a high frequency "sharp" signal corresponding to the QRS complex and a "smooth" low frequency signal corresponding to the T-wave; there are also two alternating intervals: a short interval (250 ms) and a longer interval (360 ms).
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 pattern of alternating 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 cycle duration (RT intervals and TR intervals) is usually pronounced for slow rates (short RT intervals and long TR intervals) although often less during exertion (RT and TR intervals generally equivalent) or for patients with a long QT syndrome.
Three different electrophysiological conditions can lead to T-wave oversensing during spontaneous ventricular beat:
- delayed T-wave: this pattern is typical in patients with long QT syndrome in whom repolarization is delayed; the T-wave occurs when ventricular sensitivity is at its maximal level. In this type of channelopathy, there is also a risk of dynamic changes in the duration of the QT interval induced by catecholamines, but also in the morphology and amplitude of the T-wave thus increasing the risk of inappropriate therapies during exercise;
- high amplitude T-wave and normal R-wave: this pattern is preferentially observed in hypertrophic cardiomyopathy, short QT syndrome, certain forms of long QT syndrome, certain metabolic abnormalities (hyperkalemia, hyperglycemia) as well as certain reversible causes of repolarization abnormalities (alcohol intoxication);
- small R-wave (most frequent case): when the R-wave is of low amplitude, the probability of oversensing of the T-wave increases (adaptation of the sensitivity level as a function of the amplitude of the preceding signal). When the R-wave is of low amplitude, the device rapidly reaches high levels of sensitivity which is conducive to the oversensing of the T-wave particularly during exertion (possible decrease in the amplitude of the R-wave and slight increase of the amplitude of the T-wave). A sudden and rapid deterioration of the amplitude of the R-wave after implantation may be a manifestation of lead microdislodgement. A low-amplitude R-wave can also be observed in patients with right ventricular arrhythmogenic dysplasia, Brugada syndrome, cardiac sarcoidosis, or dilated cardiomyopathy involving the right ventricle. The reprogramming options are limited in this setting, any reduction in ventricular sensitivity being associated with a major risk of undersensing of a VF.