Three different electrophysiological conditions can lead to T wave oversensing due to spontaneous ventricular activity:

1. delayed T wave: this is a typical pattern in patients with long QT syndrome in whom repolarization is delayed; the T wave occurs when the ventricular sensitivity is at its maximum level. In this type of channelopathy, there is also a risk of dynamic changes in the duration of the QT interval induced by catecholamines, as well as of the morphology and amplitude of the T wave thereby increasing the risk of inappropriate therapies during exertion;
2. large amplitude T wave and normal R wave: this pattern is preferentially observed in patients with 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 (acute alcoholism;
3. small R wave amplitude which was the case in this patient: when the R wave is of low amplitude, the probability of T wave oversensing increases (adjustment of the sensitivity level as a function of the amplitude of the preceding signal).

As a result, the device rapidly reaches high sensitivity levels which favor T wave oversensing particularly during exertion (possible decrease in the R wave amplitude and slight increase in T wave amplitude). A sudden and rapid decrease in R wave amplitude after implantation may be a manifestation of lead micro-dislodgement. A small amplitude R wave can also be observed in patients with right ventricular arrhythmogenic dysplasia, Brugada syndrome, cardiac sarcoidosis or dilated cardiomyopathy altering the right ventricle. This limits the possibility of correcting this oversensing by reducing ventricular sensitivity which could lead to an undersensing of a VF. These situations are therefore difficult and often only lead to programming compromises. It is possible to modify various settings separately (these settings can be accessed by a code known to Biotronik^TM employees) or to program an enhanced suppression of the T wave. Two parameters are thus modified relative to standard programming: the upper threshold is increased to 75% of the R wave amplitude (versus 50% with standard programming) and the high-pass filter is increased to 32 Hz to filter the T wave (« softer » or lower-frequency signal than an R wave). This option is hardly feasible in this patient given the risk of undersensing.

This type of tracing has nowadays become exceptionally scarce. Indeed, starting with the Lumax 740 platform, various modifications have been made to the Biotronik^TM devices, considerably reducing the incidence of T wave oversensing. Compared to the Lumax 540 platform and prior models, different elements of the ventricular sensing process have been modified:

1. the analog-to-digital converter has been modified with integration of a 10-bit parallel converter allowing better adjustment to rapid signals and better signal amplitude resolution; this new converter also reduces energy consumption and potentially prolongs the life of the device;
2. the filters have also been modified, the low-pass filter is no longer programmable and the high-pass 2 filter no longer exists;
3. for the old platforms, the incoming signal was systematically rectified (a negative signal resulted in a signal of the same amplitude but with a positive deflection), which is no longer the case starting with the Lumax 740 platform. When a signal reaches the sensing threshold, the device opens a 110 ms sensing window; during this interval, the device searches for the largest absolute value (positive or negative) of the signal amplitude which then corresponds to the value of the measured R wave; this value has consequently been prolonged from 80 to 110 ms to allow a better assessment of long-duration signals. After this 110 ms window, the sensing threshold is set at 50% (modifiable) of this measurement as in previous generations; the starting value can be much higher than in the previous platforms, the R wave can be measured up to a value of 25 mV (which considerably reduces the risk of T wave oversensing). The threshold remains fixed at 50% for 240 ms; the threshold subsequently decreases after 350 ms (110 + 240 ms) to 25% (modifiable) of the amplitude; on previous platforms, this duration was 360 ms; the ensuing decrement is 12.5% every 156 ms; this decrement is relatively large so as to reach maximum sensing values to accurately sense a VF episode despite a potentially high initial value; the values of 12.5% and 156 ms are not modifiable; 4) the sensing circuit after ventricular pacing has also been modified; after ventricular pacing, the device analyzes the amplitude of the evoked response for 110 ms without possibility of double sensing during a period of 120 ms (modification relative to the previous platforms or this blanking was programmed to Auto); there is an absolute blanking of 19.5 ms following the stimulus to avoid sensing of the spike; the behavior is thereafter identical to that occurring after ventricular sensing.