Erroneous diagnosis of VT due to T-wave oversensing
A 54-year old man suffering from ischemic cardiomyopathy with a left ventricular ejection fraction of 56% presented after 2 syncopal episodes. After undergoing negative electrophysiological studies and programmed ventricular stimulation, the patient underwent implantation of a Reveal DX.
In absence of recurrent syncope, the implantable Holter recorded 2 FVT and 1 VT episode.
- sinus rhythm;
- railroad-track appearance probably indicative of a supernumerary cardiac signal; the actual ECG recording corresponds to an FVT episode;
- no oversensing in presence of high-amplitude ventricular electrograms;
- oversensing of the T-wave when the R-wave amplitude decreased; whereas the T-wave amplitude remained fixed throughout the recording, the R-wave amplitude varied prominently; the railroad-track appearance of the graph is explained by the alternating incidence of the 2 signals (short RT, long TR);
- a FVT episode was detected.
This episode displays characteristics often observed in presence of T-wave oversensing:
- it occurred during exercise, which is often associated with a decrease in R-wave amplitude while the T-wave remains stable or increases;
- T-wave oversensing is promoted by a decrease in R-wave amplitude. This tracing distinctly shows that oversensing does not occur when the R-wave is of high amplitude. Indeed, the sensing threshold adapts according to the previous R-wave. Thus, for a T-wave of the same amplitude, the likelihood of oversensing increases as the previous R-wave amplitude decreases;
- the railroad-track appearance on the graph is a characteristic finding due to the sensing of a supernumerary cardiac signal (double counting of the R-wave, oversensing of P or T-wave) with alternating incidence and morphology of the 2 signals.
The programmable settings that can prevent this oversensing are a) the refractory period, b) the duration of threshold stability before its decrease which, in these patients, must be increased and 3) the sensitivity.