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Introduction The arrhythmic risk stratification of patients with repaired Tetralogy of Fallot (TOF) is still controversial. The performance of an electrophysiologic (EP) study before pulmonary valve replacement (PVR), regardless of patient's risk factors, is an extended practice in some centers that is not recommended in current guidelines. The aim of our study was to explore the differences in ventricular tachycardia (VT) inducibility in patients with TOF during programmed ventricular stimulation (PVS) depending on the clinical indication. Methods All patients with repaired TOF who underwent an EP study with PVS between January 2001 and October 2020 were included. EP studies performed in the context of ventricular or supraventricular tachycardia ablations that had been previously diagnosed were excluded. We defined two clinical scenarios for performing the EP study: pre-PVR (performed previous to pulmonary valve replacement) or diagnostic EP study (performed due to high risk symptoms which included palpitations, syncope or presyncope). Baseline clinical information, electrocardiogram, echocardiogram and cardiac MRI parameters were retrospective recorded. Results A total of 139 EP studies with PVS were included; 87 in the pre-PVR group and 52 in the diagnostic EP study group. There was a greater incidence of palpitations, syncope and presyncope in the “Diagnostic EP study” group. Moreover, there were statistical significant differences in right ventricle dimensions and function between groups. The repair surgical approach was similar in both groups. It was detected a statistical significant difference in VT induction between the pre-PVR group and the diagnostic indicated group (16,1% vs 34,6%, p=0,012). Conclusions Differences in VT induction are observed during PVS performing in TOF patients depending on the clinical indication. Symptomatology is an important parameter that must be taken into account in order to decide whether to perform an EP study in this population. Funding Acknowledgement Type of funding sources: None.

Background In the last years a substantial effort has been made to improve the understanding of the sex-differences in cardiovascular disease. However, no studies have examined differences in presentation and outcomes between men and women presenting with syncope and bundle branch block (BBB). Aim To determinate if there are sex-specific differences in the characteristics and outcomes of syncope in patients with BBB Methods Cohort study carried out in a tertiary hospital that is a reference center for syncope from January 2008 to February 2021. Patients (p.) with BBB and syncope of unknown origin after the initial evaluation without direct indication of an ICD were included. They were managed according to the current ESC guidelines. All patients underwent to an EPS and an ILR was implanted if it was not diagnostic. Results 374p. were included (75±1 y. o; 135 (36%) female). Mean follow up was 2.3±1.6 y.o. No differences in baseline characteristics where found comparing both groups, except that LBBB was more prevalent in female and RBBB in male (Figure 1A). EPS and ILR diagnostic yield was 44%/44% respectively in females and 50% /40% in males (p=ns). However, basal HV interval in EPS was significant shorter in females (Figure 2A). In the multivariate analyses female sex was associated with a significant lower risk of AV block (Figure 1B), and with a trend of less need of a pacemaker implantation (PM) (53% in females, 60% in males) (Log-rank 0.1) (Figure 2B). No significant differences in recurrences neither in mortality rate were found. Conclusions Female patients with syncope and BBB have lower risk of AV block compared to males, and only half of them required a pacemaker implantation. A strategy of direct PM implantation should be avoided, specially in woman. Funding Acknowledgement Type of funding sources: None. Figure 1Figure 2

Introduction The electrophysiologic (EP) evaluation with programmed electrical stimulation (PVS) is generally recommended in patients with repaired Tetralogy of Fallot and additional risk factors for sudden cardiac death. Nevertheless, different PVS protocols have been described. The aim of our study was to evaluate the differences in ventricular tachycardia (VT) inducibility of patients with TOF after the implementation of a standard PVS protocol in the EP laboratory of a Congenital Heart Disease reference center. Methods All patients with repaired TOF who underwent an EP study with PVS between January 2001 and October 2020 were included. The new standardized PVS protocol was performed in 2 ventricular sites (apex and outflow tract) with 3 drive trains (cycle lengths 400, 500 and 600ms) and up to 3 extrastimuli. In absence of VT induction, the protocol was repeated under isoprenaline infusion. This new protocol was implemented since January 2012. Non protocolized PVS studies before 2012 were defined as “Non-standardized”. Baseline clinical information about symptoms and previous arrhythmias was recorded as well as electrocardiogram, echocardiogram and cardiac MRI parameters. Finally, the follow-up events (ICD implantation, sudden cardiac death, global mortality, arrythmias and ICD therapies) were also retrospective recorded. Results A total of 154 EP studies with PVS were performed in 128 patients with repaired TOF. 31 of them were performed before the 1st January 2012 (non-standardized PVS) and 112 were performed with the new standardized protocol. The median follow-up was 6,5 years. Both groups had similar baseline characteristics except LVEF and RVEF, that were lower in the “Non-standardized PVS” group. There were no differences between the ventricular tachycardia inducibility of both protocols (22,3% vs 33,3%; p=0,162). The risk factors for VT inducibility were the QRS length (184,46ms vs 169,34 ms; p=0,038), the RVEF (36,25% vs 43,79; p=0,0007), the presence of ventricular ectopia (VE) (38,5% vs 20,0%; p=0,024) and previous VT (35,9% vs 13,9%; p=0,003). VT induction during EP study was related with ICD implantation (71,8% vs 21,7%, p≤0,001), VT (30,8% vs 20%, p Conclusions The implementation of a standardized and more complete PVS protocol in patients with repaired TOF has not shown differences in the experience of our center. The risk factors for VT inducibility were the QRS length, the RVEF, the presence of ventricular ectopia and previous VT, which have also been reported as risk factors for sudden cardiac death in previous studies. The presence of VT induction entailed more ICD implantation and more arrythmias at follow-up. Funding Acknowledgement Type of funding sources: None.

Funding Acknowledgements Type of funding sources: Public hospital(s). Main funding source(s): Hospital Universitari Vall d"Hebron - Vall d"Hebron Institut de Recerca. CIBERCV BACKGROUND Patients with structural heart disease (SHD) or bundle branch block (BBB) are at high risk of having an arrhythmic syncope (AS). In patients with recurrent syncope episodes (RSE) complete work-up including electrophysiological study (EPS) and/or implantation of a loop recorder (ILR) is recommended, however patients with their first episode may also be at risk of an AS. AIM To determinate if the patients with SHD or BBB with a single syncope episode (SSE) are at high risk of having an AS and compare it with patients with recurrent episodes (RSE). METHODS Cohort study carried out in a tertiary hospital that is a reference centre for syncope. From January 2008 to August 2020 patients with SHD or BBB with syncope of unknown origin after the initial evaluation and without direct indication of an ICD were included. They were managed according the current ESC guidelines. RESULTS 417patients were included (74 ±11 y. o; 39% female). Mean follow up was 2.5 ±1 y. 223 patients were diagnosed from an AS [113 (53%) in SSE group and 110 (54%) in RSE group, p = 0.9], 210 were due to bradyarrhythmia (AV block or sinus arrest). No differences in baseline characteristics where found comparing both groups, except that BBB was slightly more prevalent in patients with RSE (81% vs 90%, p = 0.01) (FIGURE- PANEL A). Risk of AS was 53% in patients with SE and 54% (p = 0.9). RSE were not associated with an increased risk of AS in univariate analyses (OR 1.01 IC95% 0.7-1.5) neither in the multivariate (FIGURE- PANEL B). EPS and ILR diagnostic yield was 45% / 32% respectively in SSE group and 44% / 33% in RSE group (p = 0.8) (FIGURE - PANEL C). After appropriate treatment, recurrence syncope rate was 10% in SSE group and 9% in RSE group (p = 0.8). No significant differences in mortality rate were found. CONCLUSIONS Patients with SHD or BBB and single syncope episode are at a high risk of having AS, and similar to those with RSE. EPS and ILR offer a similar diagnostic yield in both groups. Complete syncope work-up must be recommended in these patients despite having had only a single episode. Abstract Figure

Introduction Patients with repaired tetralogy of Fallot (rTF) and severe pulmonary regurgitation frequently progress to dilation and dysfunction of the right ventricle (RV). It has been documented in the literature that there is a correlation between the duration of the QRS in the surface electrocardiogram and the hemodynamic parameters of the RV of these patients, suggesting the presence of a mechanical-electrical interaction. Purpose To determine if there is an association between the contraction delay in certain areas of the RV measured in M-mode echocardiography and the delay in electrical activation measured in the electroanatomic map (EAM) of RV in patients with rTF. Methods Unicentric and observational study of all patients with rTF undergoing EAM, echocardiography with study of RV asynchrony and cardiac magnetic resonance imaging (MRI). Activation delay in the antero-basal area and in the RV outflow tract (RVOT) in the EAM were both analysed (Figure 1A). The shortening delay in the same areas in M-mode echocardiography was also evaluated (Figure 1B, C). MRI data regarding volume and ejection fraction was also collected. Results 64 patients were included (36.7±10.6 years, 65% men). The mean total activation time of the RV (RV-TAT) was 127.3±42.4 ms. Activation mapping showed a recurrent pattern with beginning in the interventricular septum and ending in RV antero-basal region and/or RVOT. A linear positive correlation was observed between RV-TAT and the activation delay in both regions analysed (ρ=0.60 and ρ=0.52, respectively; p Conclusions There is a mechanical-electrical interaction in the RV of patients with rTF, with a negative correlation between the activation delay and RVEF and between mechanical and electrical activation delay in specific anatomical regions (regional mechanical-electrical interaction). These results may guide future studies on resynchronization in this heart disease. Figure 1. EAM and echocardiographic measures Funding Acknowledgement Type of funding source: None

Introduction Patients with D-transposition of the great arteries (TGA) treated with Senning or Mustard surgery develop supraventricular tachycardias (SVTs) that require treatment with ablation. The use of multipoint, high-density, wavefront-activation-orientation independent mapping catheters hasn’t been reported in literature. Purpose To describe the feasibility of using a specific mapping catheter in SVTs in this set of patients. Methods Prospective observational study in patients with history of SVT and atrial switch surgery, that underwent EP study and electroanatomic (EA) mapping with a new 8Fr deflectable, multipoint (16 equidistant electrodes along 4 splines), with magnetic sensor, wavefront-activation-orientation independent catheter, in a third level hospital since April 2018 until May 2019, with medium-term follow-up. Results A total of 8 EPS (electrophysiologic studies) were performed in 7 patients (clinical data in Table 1). One patient had a tachycardia recurrence, accounting for a second EPS. The pulmonary venous atrium (PVA) was mapped in all procedures, whereas the systemic venous atrium (SVA) was mapped only in 75% of them, A total of 15 EA maps were obtained, with a ratio of 1.9 maps/patient, and an average of 20 375 ± 13 045 total points per patient. In all cases, PVA mapping was performed via retrograde transaortic approach, without transbaffle puncture. Tachycardia was induced in 5 out of 8 procedures, obtaining 6 different tachycardias (4 CTI dependent macro-reentry: 3 w/anticlockwise activation; 1 localized re-entry in SVA; 1 focal AT in SVA). No arrhythmia was induced in the other 3 procedures, however, in a patient with a previous CTI ablation, evidence of a gap in the ablation line on the voltage map was found. Ablation was performed with an irrigated, contact force, 3.5 mm catheter in 6 of the 8 procedures (75%). A retrograde transaortic approach was used in 3 of CTI dependent macro-reentries (75%). In one patient (who underwent two procedures) ablation was performed through a baffle leak. We report an acute success rate of ablation of 100%, and a recurrence rate of 20% on medium-term follow up. There were no major complications; nonetheless, one patient with history of morbid obesity had a minor vascular complication on the femoral puncture site treated medically. Conclusions it’s feasible and safe to use this new mapping catheter in patients with history of atrial switch surgery, both via retrograde and anterograde approach on the PVA and SVA respectively. The most frequent tachycardia observed in this study was anticlockwise CTI-dependent atrial flutter. Abstract Figure. Patient characteristics

INTRODUCTION Patients with D- transposition of the great arteries (TGA) treated with Senning or Mustard surgeries have several atrial scars that predispose them to develop atrial tachycardias (AT). Identification of scar zones and possible arrhythmic isthmus in voltage mapping will help to guide the ablation. AIM To describe the feasibility of using a specific mapping catheter to identify possible arrhythmic isthmus in this set of patients. METHODS Prospective observational study in patients with history of SVT and atrial switch surgery, that underwent electrophysiologic study (EP) and electroanatomic (EA) mapping with a new 8Fr deflectable, multipoint wavefront-activation-orientation independent Grid catheter, in a third level hospital since April 2018 until May 2019, with medium-term follow-up. RESULTS A total of 8 EPs were performed in 7 patients (3 (57%) Female, median age 35 ± 6,3 y.o.). Figure 1A shows the localization and percentage of scar identified in both atria. A total of 6 AT were induced. In this, an arrhythmogenic isthmus was identified and, in all patients, at least one non-arrhytmogenic isthmus was documented. Figure 1B shows anatomical and electrophysiological characteristics of the isthmus. Arrhythmogenic isthmus had slower conduction velocity than non-arrhytmogenic ( mean 0,44m/s (IQI 0,17-0,62) vs 1,05 m/s (IQI 0,86-1,39) p = 0.008) and fractionated potentials were detected more frequently (100% vs 50% p = 0.089) CONCLUSION EA mapping with a new a multipoint, high-definition, Grid Cather is feasible and allows the identification and electrophysiological characterization of arrhythmogenic and non-arrhytmogenic isthmus in patients with TGA treated with atrial switch surgery. Abstract Figure 1