Extracardiac oversensing

A VT zone is programmed starting at 180/min with a persistence of 12 cycles and a VF zone starting at 230/min with a fast VT section of 230 to 255/min with a burst followed by shocks, and a VF section with maximum shock directly.

→ Figure 1

→ Figure 2

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Your turn

What is the most probable diagnosis ?

1. VT which the shock did not terminate
2. RV lead fracture
3. 50 Hz
4. Diaphragmatic myopotentials
5. External interference

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Interpretation

1. Using the EGM « Zoom » function to amplify and expand the tracing make EGMs much easier to interpret.
2. It is clear that the ventricular detections with very short RR intervals do not correspond to any ventriculogram on the ventricular EGM. It is therefore oversensing.
3. Many RR intervals have a duration of 125 ms which is the value of the post-ventricular ventricular blanking. Such values are not physiological but correspond either to external interference, a 50 Hz current, myopotentials, a lead facture, or a connection fault.
4. Analysis of the morphology of the oversensing on the EGM is therefore essential. By expanding the Case, we see that the line is thick on the ventricular EGM, but also to a certain degree on the atrial signal. This phenomenon can correspond to an external interference, a 50 Hz current, diaphragmatic-type myopotentials, although likely not pectoral, but certainly not to a lead fracture or a lack of connection which would cause chaotic potentials of large amplitude. In addition, the manipulation of the defibrillator case through the skin, which could not be aggressive given the very recent implantation procedure, does not induce any aberrant signal on the EGM in real-time. The circumstances surrounding of occurrence of the shock do not evoke exposure to external interference or a 50 Hz current. Hence, myopotentials? Counter movements of the arm, which are also painful, do not give rise to any anomaly.
5. On the other hand, the intermittent and sometimes regular nature of the appearance of the aberrant signals and the reproduction of the phenomenon on both leads through deep breathing during the consultation signal the diagnosis of myopotentials of diaphragmatic origin! A control image reveals that the ventricular lead is in its proper position, placed along the anterior aspect of the RV, while the atrial lead has moved to the anterior and lateral side of the RA, next to the right hemidiaphragm! Hence the reason for the observed artefacts in the form of a thick line appearing on the atrial EGM when the patient performs deep respiratory movements, an event reproducible in consultation. The atrial signals are, however, not of sufficient amplitude to be detected.

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We can also observe that the charging of the capacitors was not homogeneous, the process being interrupting upon the appearance of slow cycles causing the absence of majority. The charging of the capacitors resumes as soon as the majority becomes VF again. At the end of the charge, the next cycle being in the VF zone, the shock is delivered. However, in this instance, a slow ventricular tachycardia was induced, detected outside of the VT or VF zone, such that the episode terminates due to the diagnosis of « Slow Rhythm », while the tachycardia continues. We do not know how long the latter persisted. The patient states he remained asymptomatic.

Correct answer – n° 4