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Introduction Cardiac computed tomography (CT) use has progressively increased as the preferred initial test to rule out coronary artery disease (CAD) when clinical likelihood is low. Coronary artery calcium (CAC) detected by CT is a well-established marker for cardiovascular risk. However, it is not recommended for diagnosis of obstructive CAD. Absence of CAC, defined as an Agatston score of zero, has been associated to good prognosis despite underestimation of non-calcified plaques. Purpose To evaluate whether zero CAC score could help ruling out obstructive CAD in a safely manner. Methods Observational study based on a prospective database of patients (pts) referred to cardiac CT between 2017 and 2019. Pts with an Agatston score of zero were selected. Results We included 176 pts with zero CAC score and non-invasive coronary angiography performed. The median duration of follow-up was 23.9 months. Baseline characteristics of the population are shown in Table 1. In 117 pts (66.5%), cardiac CT was indicated as part of their chest pain evaluation. Mean age was 57.2 years old, 68.2% were women and only and 9.4% were active smokers. Normal coronary arteries were found in 173 pts (98.3%). Obstructive CAD, defined as ≥50% luminal diameter stenosis of a major vessel, was present in 1/176 (0.6%); while non-obstructive atherosclerotic plaques were found in 2 pts (1.1%). During follow-up, one patient died of out-of-hospital cardiac arrest. None either suffered from myocardial infarction or needed coronary revascularization. Conclusions In our cohort, a zero CAC score detected by cardiac CT rules out obstructive coronary artery disease in 98.3%, with only 1.7% of non-calcified atherosclerosis plaques and 0.6% of major adverse events. Although further research on this topic is needed, these results support the fact that non-invasive coronary angiography could be avoided in patients with low clinical likelihood of CAD and zero CAC score, facilitating the management of the increasing demand for coronary CT and reduction of radiation dose. Funding Acknowledgement Type of funding source: None

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Background: In patients (pts) with sick sinus syndrome (SSS), right ventricular apical (RVA) pacing increased the risk of developing atrial fibrillation (AF). However, the mechanism of proarrhythmic effect of RVA pacing remains unclear. Methods: We performed detailed echocardiograhic examination with Tissue Doppler Imaging in 60 pts with SSS (mean age 73A9 years, 42 F) who implanted with DDD pacemakers during atrial and ventricular pacing with atrioventricular interval programmed at 120-150 mesc (ApVp mode) and AAI mode with (ApVs mode) at 70 bpm. Echo measurements were taken after 15 mins of pacing in each mode. The myocardial atrial contraction velocity was measured at annulus of right free wall (Ra), septal (Sa) and lateral free wall (La) respectively. Results: As expected, the AV interval was significantly shorter (118A25 vs.163A45 ms, P=0.002), and QRS duration was longer (146A33 vs.97A26 ms, P,0.001) during ApVp mode as compared with ApVs mode. Although there was no significant difference in left ventricular ejection fraction, left atrial (LA) ejection fraction (50A14 vs.55A14%, P=0.005), LA active emptying fraction (32A17 vs.37A16%, P=0.018) and LA filling fraction (43A13 vs. 48A13%, P=0.007) were all significant improved by 18%, 54% and 18%, respectively during ApVs mode as compared with ApVp mode. Furthermore, atrial myocardial contraction velocities among Ra (14.0A3.8 vs.15.2A4.6cm/s, P=0.026), Sa (7.8A2.6 vs. 8.8A2.8cm/s, P=0.001), and La (8.9A3.2 vs.9.7A2.7cm/s, P=0.020) were also significantly increased during ApVs mode by 12%, 19% and 21%, respectively as compared with ApVp mode (Figure). Conclusions: In pts with SSS, avoidance of RVA pacing during ApVs mode improves LA haemodynamic and mechanical function, which might contribute to a lower risk of development of AF after pacemaker implantation. P773Qualitative and quantitative assessment of 3 novel post-processing methods for enhancing echocardiographic images. Echocardiography, while a prevalent tool for assessing cardiac morphology and function, suffers from a range of artefacts that reduce its diagnostic value. This work qualitatively and quantitatively evaluates 3 novel post-processing methods for enhancing echocardiographic images. Data enhancement is achieved by utilising multiple partially decorrelated instances of a cardiac cycle acquired through a single acoustic window. Such information has until now been largely disregarded during data post-processing. Moreover, unlike past approaches, data enhancement is achieved without filtering out information based on static or adaptive selection criteria. Qualitative assessment using 32 clinical datasets demonstrated (i) suppression of cavity noise, (ii) increase in tissue/cavity contrast, and (iii) visual enhancement of tissue structures previously masked-out by various artefacts (Figure 1). The effect of each post-processing method on the diagnostic value of cardiac ultrasound data was quantitatively assessed by examining the repeatability coefficient variations (via Bland-Altman plots) in clini...