T wave oversensing

Patient

A 41-year-old man suffering from hypertrophic cardiomyopathy received an Atlas single chamber ICD for management of sustained VT. He was seen on a routine follow-up.

Main programmed parameters

VF zone at 222 bpm and VT zone at 182 bpm
12 cycles in each the VF and the VT zones were needed for the diagnosis
Maximum sensitivity programmed at 0.3 mV
VF zone: six 36-J shocks (maximum strength); VT zone: set on monitor
Effective discrimination in the VT zone
VVI pacing mode at 40 bpm; mode VVI post-shock pacing mode at 50 bpm

Trace

Narrative

Episode of VT falling in the monitor zone. For a diagnosis of VT, 2 out of 3 positive criteria were needed. Sudden onset and morphology were criteria in favor of VT, while stability favored SVT. VT was diagnosed though not treated, as it was being monitored.

Tracing 10a

High-amplitude ventricular EGM without T wave oversensing. The EGM channel corresponds to the ventricular pace/sense channel; the filtered and amplified signal was that analyzed by the device. The amplification (here 6.7 mm/mV) must be known in order to measure the amplitude of the various signals. The R wave is measured between the tallest positive and negative peaks of the ventricular EGM;
Lower-amplitude ventricular EGM with T wave oversensing (-). The T wave amplitude on this complex is approximately the same as that of the R wave;
Diagnosis of return of sinus rhythm after 5 consecutive VS or with intervening unclassified intervals; return to permanent VVI pacing mode;
VVI episode pacing after 4 T classified cycles following the return of sinus rhythm. It is noteworthy that the episode pacing mode is the same as the programmed permanent mode;
Repetitive oversensing of T wave and diagnosis of VT in the monitor zone (12 cycles were classified T without diagnosis of return of sinus rhythm). No therapy was delivered in that zone;
Wide variations in the R wave amplitude. Because of the higher amplitude of the ventricular EGM, the T wave was not oversensed; return of sinus rhythm was diagnosed.

Tracing 10b

The same episode and tracing are shown at an amplification of 1 mm/mV, which facilitates the measure of the various signals, revealing the very low amplitude of the ventricular EGM.

Comments

The systematic oversensing of different intracardiac signals causes the sensing of 2 signals of different morphology within a single cardiac cycle with alternating short and longer cycles. Oversensing of the T wave following a spontaneous ventricular event occurs preferentially in presence of a low-amplitude R wave. Sensitivity and gain are adjusted automatically, based on the amplitude of the R wave that was just sensed: when that amplitude is low, the likelihood is high of subsequently and rapidly reaching the highest sensitivity. Oversensing is facilitated by exercise, since effort may be associated with a decrease in the R wave and an increase in the T wave amplitudes. The episode that was detected fell in the VT monitor zone, avoiding the delivery of any inappropriate therapy. In this patient, any decrease in the ventricular sensitivity to prevent T wave oversensing would incur a serious risk of VF underdetection, as the amplitude of the ventricular EGM during sinus rhythm measured only 2.6 mV. Furthermore, as revealed by the tracing, sensing of the ventricular EGM was modified by exercise, with relatively wide amplitude variations from one ventricular complex to the other. However, the amplitude of ventricular EGM during sinus rhythm is not systematically correlated with their amplitude during VF.

Modifications in the adaptation level and delay completely eliminated oversensing of the T wave, including during a confirmatory exercise test. Remote monitoring enables an early diagnosis of this kind of asymptomatic episodes. The low quality of ventricular EGM sensing remains troublesome in this patient, and a search for an alternate sensing site must be strongly considered.

Pacing & Defibrillation

ECG

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