A 23 year old male with hypertrophic cardiomyopathy, was implanted with a Subcutaneous Implantable Cardioverter Defibrillator (S-ICD) (Cameron Health SQ-RX 1010). Four years later the patient developed symptomatic bifascicular block and a dual chamber pacemaker (PPM) (Biotronik Epyra-6) was implanted. To highlight the potential challenge of interactions between an S-ICD and atrial pacing. N/A The PPM was programmed DDD with a lower rate of 60 bpm and an upper rate of 150 bpm. The S-ICD conditional and shock zones were set to 250 bpm, in the Secondary vector. Primary and Alternate vectors were not viable options due to oversensing of the patient's intrinsic rhythm and smart pass filtering unavailable on this device model. Two months post PPM insertion, an untreated episode was detected by the S-ICD displaying intermittent triple counting of the P, R and T-wave (figure 1). Consequently simultaneous interrogation of the PPM and S-ICD was performed. Upon conducting the atrial threshold test in AAI mode, the S-ICD P-wave oversensing was replicated (figure 2). The paced P-wave amplitude was similar to that of the intrinsic R-wave (figure 1 circled in red) resulting in sensing of both components. S-ICD sensing utilises auto gain control whereby the average amplitude of the last two sensed signals is taken and the decay to sensing floor begins at 75% of this calculated amplitude. As the interval between sensed beats decreases, the shorter the refractory period and more aggressive the decay to ensure appropriate sensing of small amplitude signals, typically seen during ventricular fibrillation. A fortuitous ventricular ectopic (figure 1 highlighted in green) resets the sensing profile as the amplitude is significantly greater and momentarily avoids further oversensing and inappropriate therapy. Following this episode, the PPM was reprogrammed with a reduced lower rate limit of 40 bpm and atrial auto-capture turned off allowing intrinsic P-wave sensing. No further untreated episodes due to oversensing were seen for the remainder of the S-ICD battery longevity of approximately 2 years. It is important to screen all possible paced morphologies from both the atrial and ventricular chambers to prevent S-ICD oversensing when concurrent devices are in situ.View Large Image Figure ViewerDownload Hi-res image Download (PPT)