SecureSense™ algorithm and P wave oversensing

Patient

This 65-year-old man received a Abbott Ellipse™ VR defibrillator; an episode of noise on the ventricular lead was stored in the memories.

Trace

Tracing 5A

VS² markers on the discrimination channel indicate that the noise counter was activated and had been previously incremented;
oversensing, on the bipolar channel, of a signal preceding the QRS complex, probably corresponding to P waves. Absence of oversensing on the discrimination channel. Alternans between a) non-classified cycle (the instantaneous interval is long though, on average, the interval is in a zone of tachycardia) and b) F-classified cycle (the instantaneous interval and the averaged interval are both in the tachycardia zone);
2 VS-classified cycles (insufficient number of cycles to fill the counter of return to sinus rhythm);
another F-classified cycle; the VF counter continues to increase (it is at 9 with this cycle); in view of the absence of short cycles on the discrimination channel, the noise counter increases in parallel;
the VF counter is at 12 (full); the noise counter is ≥10; therefore, RV lead noise is diagnosed and the therapies are inhibited;

Patient

This 69-year-old man received a Abbott Unify Assura™ triple chamber ICD for management of a dilated cardiomyopathy with non-specific bundle branch block; episode of VF diagnosed and stored in memory.

Tracing 5B

the VS² markers present on the discrimination channel indicate that the noise counter had been activated and previously incremented; atrial sensing and biventricular stimulation; oversensing present on the discrimination channel and probably corresponding to P waves; no oversensing noted on the bipolar channel;
oversensing present on the bipolar channel as well as on the channel of atrial activity discrimination; stimulation is absent and spontaneous ventricular activity present (with a decrease in the percentage of biventricular stimulation) following that oversensing;
alternans of (-) and (F) cycles; gradual increment of the VF counter;
the VF counter is filled; the noise counter is frequently reset to 0 (alternans between long and short cycles on the discrimination channel; reset to 0 after 2 short cycles even when non-consecutive); since the noise counter is below 10, the therapies are not inhibited;
ATP during the charge (burst + charge of the capacitors);
persistence of oversensing on the discrimination channel and intermittent oversensing on the bipolar channel;
return to sinus rhythm diagnosed after 3 VS-classified cycles (intermittent oversensing).

Comments

Oversensing of atrial depolarization (P wave) by the RV lead is rare and is observed mainly in recipients of integrated bipolar leads. In patients in sinus rhythm, the RV lead senses both the atrial and ventricular depolarization since the PR interval is longer than the post-sensed ventricular blanking period. In patients presenting with complete AV block, P wave oversensing may inhibit ventricular pacing and cause asystole. Likewise, oversensing of atrial depolarization during flutter or atrial tachycardia may cause both inappropriate therapies and asystole if the patient is pacemaker-dependent.

Oversensing of the atrial signal occurs preferentially in 2 circumstances: 1) in case of RV lead dislodgement at the AV junction (coincides with a fall in R wave amplitude), or 2) when an integrated bipolar lead is implanted near the tricuspid ring, with the distal coil straddling the valve (coinciding with a preserved R wave amplitude). This observation is more likely in patients whose cardiac chambers are small, such as children or patients presenting with hypertrophic cardiomyopathy.

Atrial oversensing may also occur under less common circumstances:

the RV lead has inadvertently been placed in the coronary sinus;
an insulation breakdown has occurred at the level of the atrial segment of the lead, causing oversensing of the atrial activity;
interaction between the atrial and the RV lead, with the leads in contact with each other and causing a signal at the time of atrial systole;

While, as discussed previously, the SecureSense™ algorithm can virtually eliminate the therapies triggered by T wave oversensing, it is less effective against P wave oversensing. If the discrimination channel is set between the coil and the can, oversensing on this channel is more likely (coil in the atrium if the lead is placed near the tricuspid valve) than on the bipolar channel (both electrodes in the ventricle). These 2 tracings illustrate 2 cases of figure:

on the first tracing, oversensing is limited to the bipolar channel: the SecureSense™ algorithm prevents one or more inappropriate therapies, which would have been inevitable;
on the second tracing, oversensing is incessant on the discrimination channel and intermittent on the bipolar channel. When the VF counter is filled, a therapy is inappropriately delivered;

By changing the discrimination channel to program the detection between the distal electrode and the pulse generator (tip to can), the risk of oversensing on this channel could be lowered, though would not eliminate oversensing on the bipolar channel. In both cases, repositioning of the high-voltage lead must be considered, while verifying during the implant that the distal coil is indeed completely contained in the RV chamber.

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