The methods used to detect and classify cardiac cycles differ completely between an intracardiac defibrillator and a subcutaneous defibrillator. Indeed, while the main purpose of a subcutaneous defibrillator is to correctly detect, without significant delay, an episode of ventricular arrhythmia that could compromise the patient’s survival, there is no absolute need to correctly classify each cycle as sinus rhythm or arrhythmia. Analysis of the tracings frequently reveals oversensed cycles, undersensed or incorrectly classified rapid cycles (e.g., ventricular extrasystoles classified as sinus rhythm), which could be problematic for an intracardiac defibrillator that provides pacemaker function requiring perfect detection of all cycles to avoid stimulation during vulnerable periods. The two priorities for a subcutaneous defibrillator are therefore to avoid inappropriate therapies due to oversensing of the T wave in sinus rhythm while maintaining the ability to detect rapid rhythms, which justifies the use of different sensitivity profiles depending on the heart rate. There are also a number of fundamental differences in the amplitude and shape of the signals detected by a subcutaneous defibrillator compared to the signals detected by an intracardiac defibrillator. These different elements explain certain differences in operation and programming:

• The slope and frequency of signals recorded by subcutaneous electrodes are lower in sinus rhythm, leading to the recording of “softer” QRS complexes that are less differentiated from the T wave or P wave. The high-pass filter value is lower in a subcutaneous defibrillator than in an intracardiac defibrillator (3 Hz vs. 14–20 Hz).
• The signals recorded by subcutaneous electrodes are broader than intracavitary signals, justifying the use of a longer post-detection refractory period to avoid double counting of the R wave. The duration of the refractory period varies according to the frequency of the detected cycles (shorter in the shock zone) so as not to limit the ability to detect very fast rhythms while minimizing the risk of oversensing in sinus rhythm. The refractory period value is not programmable (160 ms for the shock zone, 200 ms for cycles slower than the shock zone). – A subcutaneous defibrillator operates with a higher maximum sensitivity value than an intracardiac defibrillator (0.08 mV versus 0.3 to 0.6 mV). as the amplitude of intracardiac signals recorded by subcutaneous electrodes is generally lower (QRS complex amplitude of 0.3 to 3.6 mV in sinus rhythm); the maximum sensitivity level is not programmable (0.08 mV, 80 µV) and remains the same regardless of the frequency of the signals detected and certified.
• The signal-to-noise ratio, which corresponds to the difference between the amplitude of the signal to be detected (QRS complex) and the amplitude of the “noise” (P wave, T wave, myopotentials, electromagnetic interference), is lower; this is because, as explained above, the amplitude of the QRS complexes is lower. in addition, extracardiac signals have a higher amplitude, as the subcutaneous electrodes are much further apart than the bipolar electrodes of an endocavitary probe; proper functioning therefore requires the use of specific filters to reject high-frequency signals such as 50 or 60 Hz; Once the signal is detected, the certification phase classifies the cycle as certified or as noise/oversensing. Signals with too high a frequency (myopotentials, interference, probe fracture) are classified as noise and are excluded from the heart rate calculation. The signals then go through four additional certification steps to diagnose double counting of the R wave or oversensing of the T wave.
• Postural variations in the amplitude and appearance of QRS complexes are more pronounced than with an endocardial defibrillator, requiring the use of a corrective algorithm to mitigate these variations.
• Dynamic sensitivity is used to reduce the risk of T wave oversensing; once the signals have been filtered, the device uses an adaptive detection threshold based on the amplitude of the two previous certified cycles; the sensitivity level increases in parallel with the increase in heart rate to optimize detection capabilities during tachycardia. Different sensitivity profiles (refractory period, adaptation level, adaptation delay) are used depending on the frequency of the detected signals, the programming (single zone or two zones), and the similarity of the amplitude of the two previous certified signals. None of these parameters are programmable. When the rhythm is slow, the priority is to minimize the risk of T wave oversensing, so the sensitivity profile is adjusted (long refractory periods, high adaptation level, adaptation delay extended especially when the amplitude of the two previous certified complexes is different). In the shock zone, the main objective is to optimize the detection of possible ventricular fibrillation with rapid signals of variable amplitude, which requires the use of a shorter refractory period, a relatively low adaptation level, and a short adaptation delay.
• A subcutaneous defibrillator does not provide long-term pacemaker function, which considerably limits the constraints in terms of the need for correct cycle-to-cycle detection of all signals (no risk of stimulation during the vulnerable period). This allows the use of a system that averages over several cycles. During the decision phase, the device examines all certified cycles and continuously calculates a ventricular rate average over four certified RR cycles. For a cycle to be considered fast, the average of the four cycles must be fast.
• In addition to a shock zone, the device allows for a conditional shock zone using two morphology analysis algorithms and one QRS duration measurement algorithm to distinguish between ventricular arrhythmias and supraventricular arrhythmias. The existence of relatively distant detection electrodes allows for reliable analysis of signal morphology, albeit with greater postural variability. As explained above, programming a conditional shock zone not only introduces a discrimination zone but also modifies the detection profile.