This patient presented oversensing of myopotentials originating from the diaphragm. A deep inspiration enabled replicating the phenomenon.
Two types of myopotentials can be oversensed by an ICD:

1. diaphragmatic myopotentials: the use of a high self-adjusting sensitivity allows optimizing the quality of the sensing of the low-voltage VF signals, but also increases the risk of oversensing of diaphragmatic myopotentials at the end of diastole when the sensitivity reaches its maximum. Diaphragmatic myopotential oversensing is rare but has been increasingly observed in patients implanted with an integrated bipolar lead positioned at the apex of the right ventricle. Permanent ventricular pacing is associated with an increased risk of oversensing of these myopotentials since, after pacing, the time spent at maximum sensitivity is prolonged especially at slow heart rates. An integrated bipolar lead favors the phenomenon due to a wider sensing antenna. Diaphragmatic myopotentials correspond to low-amplitude, high-frequency signals, most often detected exclusively on the sensing channel (absent on the far-field channel). The two main characteristics of this type of signal are that their amplitude varies with the respiratory cycle and that can be replicated by specific maneuvers (deep inspiration, Vasalva, forced cough). Oversensing occurs initially at the end of diastole when sensitivity is maximal. Sensing of the true R wave (of high amplitude) modifies the sensitivity level and interrupts, at least temporarily, the oversensing of these small signals, which explains why prolonged oversensing only occurs in pacemaker-dependent patients (absence of spontaneous R wave, sensitivity level permanently at maximum). Oversensing may be avoided by reducing the sensitivity level with the need to verify the accurate sensing of VF signals. In paced patients, an increase in the minimal pacing rate may also have a favorable effect. In some instances, it may be necessary to implant a new defibrillation (DF4 system) or pacing (DF1 system) lead at a remote distance from the diaphragm (septum or infundibulum).
2. pectoral myopotentials: in an ICD, the pulse generator being positioned in the pocket near the pectoral muscles (and thus not part of the sensing circuit), the pectoral myopotentials should therefore not generate oversensing. The amplitude of these myopotentials is greater when recorded at the level of the far-field channel which includes the pulse generator (sensing between the right ventricular coil and the generator). On the other hand, if there is an insulation break (typically an erosion leading to a current leak) at the level of the pocket portion of the lead (friction between the pulse generator and the lead), then the sensing (near-field) channel may oversense the pectoral myopotentials, which can lead to pacing inhibition and/or the occurrence of inappropriate therapies. Analysis of the EGMs in this setting reveals the presence of very rapid non-physiological (high-frequency) signals. Oversensing can be replicated by isometric movements of the arm ipsilateral to the generator or by manipulation of the lead in the pocket. When there is suspicion of pectoral myopotential oversensing, a chest X-ray must be performed along with complete control verification of the device (impedance values, pacing and sensing thresholds). The presence of an abnormally low impedance value (pacing and/or defibrillation) or a sudden decrease in this value is suggestive of an insulation break. In very rare cases, myopotential oversensing can be observed when a DF1 system has been implanted with inversion of the pins in the connector.