False diagnosis of bradycardia or asystole due to sensing failure

The automatic recording of episodes of bradycardia or tachycardia allows increasing the diagnostic yield of the implantable loop recorder. The asymptomatic and symptomatic episodes in patients occurring in patients without the ability to actively trigger the recording are thus memorized. However, an important limitation of this function is the frequent spurious recording of false arrhythmias such as bradycardia due to undersensing of ventricular signals or tachycardia due to sensing of noise or other intra-cardiac signals such as P waves or T waves or double counting of the R wave. This implies that all the episodes stored in the memory need to be verified. The recording of spurious arrhythmia episodes is frequent since found in over half of the implanted patients.

It is possible to modify the programming in order to avoid the iterative recording of false bradycardia episodes. Four programming options are available with different filter and target threshold values ​​(standard, variable amplitude, T wave suppression, premature ventricular contraction detection):

1. the high-pass filter: the higher the value, the more the signal corresponding to the baseline but also to the T wave is filtered (this value is programmed at 18 Hz for the T wave suppression option and 10 for the 3 others);
2. the initial threshold: when a signal is sensed, the sensitivity level adapts to the amplitude of this signal and increases progressively during the interval; depending on the choice between the 4 configurations, the initial threshold varies between 100% (T wave suppression, the device is poorly sensitive at the beginning of the interval in order to avoid sensing of the T wave) and 62.5% (sensing of extrasystoles, device highly sensitive at the beginning of the interval to allow the sensing of a premature interval of moderate amplitude);
3. the target sensing threshold: the objective is to modify the level of sensitivity at the moment when the next signal occurs (if ventricular rhythm is stable); depending on the 4 options, this value varies between 25 (sensing of extrasystoles) and 50% (standard programming); the lower the value, the greater the probability of sensing even when the amplitude of the intervals is highly variable (as in the last two patients with premature ventricular contractions).