How the S-ICD (subcutaneous implantable cardiac defibrillator) senses cardiac signals to minimize cardiac over-sensing and maximize rhythm discrimination.

Transvenous Implantable Cardioverter Defibrillator (TV-ICD) and subcutaneous ICD (S-ICD) devices are available for patients at risk of sudden cardiac death (SCD). Both devices sense ventricular arrhythmias with near 100% sensitivity (Gold et al., 2012) and have similar rates of inappropriate shocks (IAS) (2.5-4.8%). IAS due to supraventricular tachycardias (SVT) is higher for TV-ICDs and IAS due to cardiac over-sensing is higher for S-ICDs (Basu-Ray et al., 2017). The S-ICD's sensing signal is morphologically rich, resembling the surface electrocardiogram (ECG) signal. As such, correlation waveform analysis (CWA) is used by the S-ICD to compare the morphology of two signals to minimize S-ICD cardiac over-sensing and rhythm discrimination errors. Early experience showed that the S-ICD had a comparable overall incidence of IAS compared to TV-ICDs. Progressive improvements in S-ICD algorithms and programming strategies have resulted in overall IAS rate reduction to 4.3% (Theuns et al., 2018), substantiating the S-ICD as a viable choice for patients at risk of SCD.