Interpretation

EGM n°1

1. One must first begin by reading the markers, which reveal the functioning of the prosthesis as well as the interpretation of the events.
2. There is sensing of the P-waves with a PP interval of approximately 780 ms (diamonds). The first is a double of 780, at 1540 ms, suggesting that a P wave was not sensed, as is the case at the end of the EGM (the P wave falls in the post-ventricular refractory period).
3. On the ventricular side, the double counting of R waves appears obvious with a short Vs-Vs interval of the order of 160ms (red stars), and another Vs-Vs interval of 300-350ms. The short RR couples correspond to ventricular electrograms that are faster than the P-wave markers, and it is not possible to find an association between them. These ventricular electrograms correspond to undetected visible events on the atrial channel, which are far-field signals of R-waves in the atrium (arrows). A crosstalk cannot be evoked as long as there is no sensing of these events.
4. On the atrial channel, electrograms featuring a fast slope are superimposed and represent sinus signals (diamonds). At the end of the tracing, the last two sinus events are not recorded because they appear in the post-ventricular atrial blanking.
5. In conclusion, this is an authentic ventricular burst with dissociation from sinus rhythm, accompanied by double R-wave counting. The small fast signal following the large ventricular signal is probably the far-field LV depolarisation signal (triangles).

EGM n°2

The second short burst observed has a similar presentation.

1. In this instance, there is also double ventricular counting, although the morphology of the ventriculograms is completely different, with a first portion seemingly inverted compared to the morphology of those of the first burst.
2. Far-field signals are observed on the atrial channel (arrows), although there is a much larger number of atrial signals (diamonds) superimposed on these far-field signals.
3. A very visible atrial burst can be seen with its very fast slope signals. Not all are detected, roughly one out of two, falling in the post-ventricular atrial blanking.

Comments

1. In both cases, double counting can be observed: in the first case, during a ventricular burst; in the second, during an atrial burst and therefore also on QRS complexes normally conducted by the nodo-Hissian pathway.
2. If the bursts become sustained tachycardias, the risk is that, in the first case, a simple ventricular tachycardia, even if slow, will be interpreted as a supraventricular tachycardia (annotation n°4) because of the stability criterion that will not be fulfilled (alternating slower cycle followed by a faster cycle). There is therefore a risk of no treatment. However, in the second case, the absence of therapy would be dictated by the same cause: ventricular rhythm identified as unstable. In the event where the majority coupling intervals fall in the VF zone as a result of ventricular rates being faster, the rhythm would then be interpreted as a fast tachycardia or even VF, leading to treatment.
3. Resolution of this problem is difficult because on Microport defibrillators, it is not possible to change the value of the post-ventricular ventricular blanking that would solve the problem. But even if this were the case, it would mean a prolongation of its duration and therefore a lower detection capacity of fast ventricular rhythms. It may be possible to set a higher programmed ventricular sensitivity in the case of ventricular oversensing below 1 mV. But the risk of under-detection of VF will be increased. If this solution is applied, it is advisable to perform an induction/termination of VF to verify that fibrillation is correctly detected with reduced ventricular sensitivity. The last solution is a change in positioning of the ventricular lead. Indeed, it should be remembered that the RV lead is implanted in the necrotic zone which perhaps explains the fact that potentials of sufficient amplitude are detected (the triangles in the figures) after the post-ventricular ventricular blanking period due to slow local conduction.

Message

1. When implanting a right ventricular defibrillation lead, it is important to collect and analyse the RV signal to ensure that there is no R-wave double sensing or T-wave oversensing, in which case, an immediate repositioning of the lead is necessary.
2. To ensure the positioning of the shock electrode completely within the RV, connect the cathode of the test cable to the distal contact of the lead, and the anode to the n°3 ring of the lead, corresponding to the intraventricular shock electrode, so as to ensure the absence of sensing of atrial signals (i.e. if the lead is too far back toward the tricuspid valve). In this latter case, the relocation of the lead is imperative!