We report a case of oversensed atrial flutter with a cycle length of 213–225 ms (panel A), terminated by an implantable cardioverter defibrillator (ICD) shock in a patient with ischaemic cardiomyopathy and complete atrio-ventricular block. The intracardiac tracings, in panel A represent, respectively, from the top to the bottom, the atrial electrograms, the ventricular electrograms, the leadless electrocariogram, and the ICD markers. Due to the proximity of the right ventricular lead to the tricuspid valve and the right atrium, as shown in panels B and C, this atrial arrhythmia is oversensed on the ventricular lead as ventricular fibrillation (VF) and causes indirectly asystole, since the patient is pace-dependent. Of note, as evidenced at the bottom of the tracing, ICD annotations testify the activation of the atrial tachycardia response (ATR) algorithm, which collects atrial signals within a temporal window of interest (ATR-Dur) and switch the device in a VVI fallback mode (ATR-FB) when a sufficient programmed number of cycles satisfy the criteria of an ‘atrial tachycardia’.
The ICD shock consequently is simultaneously inappropriate, because VF is false, and ‘appropriate’ because restores correct pacing and cardiac contraction. It is relevant to remember that post-ventricular-atrial-refractory-period (PVARP) is prolonged automatically after the ICD shock (as shown by the marker PVP—PVARP prolongation—in panel A), to reduce the probability of pacemaker -mediated tachycardias in this time frame, when the device switches from a VDI pacing mode (typically activated during ventricular tachycardia recognition and accumulation of charge in the ICD capacitors) to a DDD post-shock pacing mode. High output ventricular pacing after an ICD shock is usually delivered for seconds/minutes to avoid bradycardia and potentially re-induction of ventricular arrhythmias.
In summary, this case should remind the importance of avoiding significant far-field signals on atrial and ventricular leads at the time of implantation, to limit the probability of oversensing of the device and consequent malfunctions.
Conflict of interest: none declared.
