Late sensing due to extremely delayed right ventricular activation in arrhythmogenic right ventricular cardiomyopathy

A patient with arrhythmogenic right ventricular cardiomyopathy/dysplasia, implanted with a dual-chamber implantable cardioverter-defibrillator programmed in DDD mode, showed an unexpected ventricular sensing dysfunction: despite a very long (320 ms) programmed atrioventricular (AV) interval, ventricular stimuli were delivered in the ST segment after each spontaneous conducted QRS complex. This suggested the presence of ventricular undersensing. When, however, the system was programmed in VVI mode, spontaneous QRS complexes were normally sensed, although electrogram (ECM) analysis revealed that ventricular sensing occurred 160 ms after the beginning of QRS complex. A new ventricular lead was then implanted in the outflow tract of the right ventricle, resulting in normal ventricular sensing function. At the time of intervention, the amplitude of the spontaneous ventricular signal recorded from the old ventricular lead was 2.8 mV.In this patient, no true undersensing occurred although ventricular stimuli were delivered 140 ms after the beginning of spontaneous QRS complexes when the system was in DDD mode: the cause of the apparent pacemaker malfunction was the extremely prolonged ventricular depolarization. Due to fibrofatty muscle replacement, the depolarization wavefront reached the ventricular muscle surrounding the pacemaker lead with such a delay that at the end of the programmed sensed AV interval (320 ms) the ventricular EGM had not yet attained the sensing threshold. This apparent undersensing should, thus, be defined as "late sensing."