Discussion
Device interrogation showed stable overall capture thresholds and
impedances. Lead dislocation was excluded by means of chest X-ray
showing normal position of atrial and ventricular leads, fully similar
to previous X-rays. These findings support the hypothesis that the found
interference signal was of external origin.
Upon closer examination of the ICD-interrogation showed a few instances
of different interpretations of the interference signal by the ICD
(figure 1).
The main concerns that remained were:
- (Near-)Syncope caused by asystole / bradycardia due to inhibition of
pacing
- Oversensing induced inappropriate tachy therapy or even shocks
The chosen solution was programming separate levels of sensitivity for
the detection of bradycardia and tachycardia providing different
responses to the same noise signal. The sensitivity threshold for
bradycardia detection was elevated to a level above the amplitude of the
recurrent noise signal (i.e. 1mV). This significantly reduces the risk
inappropriate inhibition of ventricular pacing resulting in asystole and
danger of fainting. In addition, the sensitivity threshold for
tachycardia detection was kept below the amplitude of the noise signal.
By doing so the noise could still be monitored for change in amplitude
or duration. And raising the sensitivity threshold for detecting
tachycardia would also have introduced the danger of undersensing
ventricular arrhythmias, especially ventricular fibrillation.
In this particular case, the SecureSense⢠algorithm was successful in
distinguishing noise from true high rate ventricular episodes and
thereby provided the ability to withhold tachycardia therapy in the
presence of the recurrent interference signal. To further reduce the
risk of inappropriate shocks the incount for ventricular tachycardia was
increased to 50 intervals.