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Background A major predictor of eligibility of subcutaneous implantable cardiac defibrillators (S-ICD) is the T:R ratio. The eligibility cut-off of the T:R ratio incorporates a safety margin to accommodate for fluctuations of ECG signal amplitudes. We introduce a deep learning-based tool that accurately measures the degree of T:R ratio fluctuations and explore its role in S-ICD screening. Methods Patients were fitted with Holters for 24 h to record their S-ICD vectors. Our tool was used to assess the T:R ratio over the duration of the recordings. Multiple T:R ratio cut-off values were applied, identifying patients at high risk of T-wave oversensing (TWO) at each of the proposed values. The purpose of our study is to identify the ratio that recognises patients at high risk of TWO while not inappropriately excluding true S-ICD candidates. Results Thirty-seven patients (age 54.5 + / − 21.3 years, 64.8% male) were recruited. Fourteen patients had heart-failure, 7 hypertrophic cardiomyopathy, 7 had normal hearts, 6 had congenital heart disease, and 3 had prior inappropriate S-ICD shocks due to TWO. 54% of patients passed the screening at a T: R of 1:3. All patients passed the screening at a T: R of 1:1. The only subgroup to wholly pass the screening utilising all the proposed ratios are the participants with normal hearts. Conclusion We propose adopting prolonged screening to select patients eligible for S-ICD with low probability of TWO and inappropriate shocks. The appropriate T:R ratio likely lies between 1:3 and 1:1. Further studies are required to identify the optimal screening thresholds.

Background Patients with an existing subcutaneous implantable cardiac defibrillator (S-ICD) may develop a pacing indication. When transvenous pacing is not feasible, combining an S-ICD and a leadless pacemaker (LP) can be a reasonable option. There are reports of concomitant use of both devices. However, the effect of pacing on the S-ICD sensing is not well studied. We hypothesise that pacing changes R and T-wave amplitudes, causing changes in R:T ratios as perceived by a S-ICD, increasing the risk for T wave oversensing (TWO) during paced rhythm with a subsequent risk of inappropriate shocks. Methods This is a prospective study in patients undergoing electrophysiological studies. Participants were fitted with a Holter®, and the leads were placed to correspond to the vectors of an S-ICD. The right ventricle was paced at four positions for 10 beats each at 8 mA/2 ms. The Holter® traces were analysed, using two-way analysis of variance (ANOVA) to assess the effect of pacing on the R:T ratio. Results Forty-seven patients (age 56.02 ± 16.02, 72% male) were enrolled (81% structurally normal heart, 15% dilated cardiomyopathy, 2% ischaemic cardiomyopathy, and 2% adult congenital heart disease). Age, sex, and aetiology had no effect on the R:T ratio. Pacing caused significant changes in the R:T ratio. There was no significant difference in the R:T ratios between the pacing sites (p Conclusions Pacing alters the R:T ratio significantly in most patients, theoretically increasing the risk for TWO and inappropriate shocks. Tailored programming for both devices is important for concomitant use of LPs and S-ICDs.

Introduction: Patients with an S-ICD may develop a pacing indication. When transvenous pacing is not feasible, the combination of an S-ICD and a leadless pacemaker (LP) might be a sound option. There are increased reports of concomitant use of LPs and S-ICDs. However, the effect of pacing on the S-ICD sensing is not well studied. Hypothesis: We hypothesize that pacing changes R and T waves morphology and amplitudes, causing changes in R:T ratios perceived by an S-ICD, increasing the risk for T wave oversensing (TWO) during paced rhythm with a subsequent risk of inappropriate shocks. Methods: We ran a prospective study in patients undergoing an electrophysiological study on clinical grounds. Participants were fitted with a Holter®, the leads of which placed to correspond to the three vectors of an S-ICD. We used a standard catheter to pace the right ventricle at four positions (figure 1) for 10 beats each at 8mA/2ms. We analysed the Holter® traces and used two-way ANOVA to assess the effect of pacing on the R:T ratio. Results: A total of 47 patients (mean age 56.02 ± 16.02,72% male) were enrolled (81% structurally normal heart, 15% DCM, 2% ICM, and 2% congenital heart disease). Age, sex, and aetiology had no effect on the R:T ratio. Pacing caused significant decrease in the R/T ratio, but there was no significant difference in the R/T ratio between different pacing sites. (P Conclusions: Pacing- regardless of position- causes the S-ICD perceived R:T ratio to fall below the eligibility screening threshold in most patients, increasing the risk of TWO and inappropriate shocks. Caution needs to be exercised and tailored programming of both devices is key for concomitant use of LPs and S-ICDs.