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Introduction Impaired left atrial (LA) function is associated with adverse outcomes in aortic stenosis (AS) and decreased LA strain precedes volume derangement. LA stiffness index (LASI) combines LA filling pressure and strain and is elevated and associated with symptoms and adverse outcomes in those with asymptomatic severe AS. We aimed to (i) comprehensively assess the LA, utilising novel cardiac magnetic resonance (CMR)-derived LASI, in patients with asymptomatic and symptomatic AS, and healthy controls (HC) and (ii) explore associations with LASI in asymptomatic AS.Methods In this analysis of three prospectively recruited cohorts, patients with symptomatic severe AS, asymptomatic moderate to severe AS, and HC underwent CMR. Medis Suite MR v3.1 was used to calculate LA strain and volumes during reservoir, conduit and booster pump phases. LASI was calculated as pulmonary capillary wedge pressure (PCWP)/LA reservoir strain, where PCWP = 6.1352+(0.07204xLA maximum volume) + (0.02256xleft ventricular mass). LA parameters were compared using ANOVA, and linear regression analysed associations with LASI.Results We recruited 174 patients with asymptomatic moderate-severe AS, 78 patients with symptomatic severe AS and 21 HC. Due to artefacts during strain/volume assessment, LASI was calculable in 142 and 71 patients with asymptomatic and symptomatic AS respectively.The groups were matched for age and sex (table 1). There were no differences in LA volumes or strain between the asymptomatic AS cohort and HC. However, LASI was higher in asymptomatic AS. Almost all LA parameters were significantly deranged in symptomatic AS compared to HC, with lower strains and higher volumes and LASI. When comparing symptomatic with asymptomatic AS, a similar pattern was seen. A graded increase in LA volumes and LASI and decrease in strain were noted from HC to asymptomatic to symptomatic AS. (Table 1).On univariable analysis, age, Log(NT-proBNP), E/A ratio, extracellular volume fraction and late gadolinium enhancement (LGE) presence were directly correlated with LASI in the asymptomatic AS cohort (table 2). LGE presence and Log(NT-proBNP) remained independently associated with LASI after adjustment for age and sex.Abstract 1-021 Table 1Baseline characteristics, strain, volumetric and LASI comparison Variable Healthy controls (n=21) Asymptomatic AS Patients (n = 174) Symptomatic AS patients (n=78) Age years68(9)66(13)67(12) Male sex, n (%)15(71)133(76)58(74) BMI kg/m2 26.7(3.6)b 28.0(4.2)c 29.6(5.1) AVAi (cm2/m2)1.74(0.4)a, b 0.6(0.1)c 0.4(0.1) LAS reservoir%32.1(9.2)b 30.4(9.1)c 24.5(9.0) LAS BP%17.1(6.4) 16.2(5.6)14.3(6.0) LAS conduit%15.1(4.5)b 14.2(6.2)c 11.2(4.8) LAV max ml71.6(17.6)b 79.9(23.2)c 90.4(29.8) LAV min ml32.4(10.8)b 39.4(16.4)c 50.7(24.6) PCWP mmHg 13.2(1.3)a, b 14.5(2.0)c 16.0(2.5) LASI 0.44(0.1)a, b 0.54(0.2)c 0.8(0.7) Significant difference (p<0.05) between:a. HC and asymptomatic ASb. HC and symptomatic ASc. Asymptomatic and symptomatic ASAbstract 1-021 Table 2Associations of LASI in those with asymptomatic AS Univariable associations Multivariable associations Variable Estimate (95% CI ) p-value Estimate (95% CI ) p-value Age0.006 (0.003, 0.01)<0.0010.003 (-4.4 x105, 0.01) 0.053Sex0.018 (-0.07, 0.11)0.6820.042 (-0.04, 0.12)0.304Log (NT-proBNP)0.05 (0.03, 0.07)<0.0010.031 (0.01, 0.05)0.003*E/A ratio0.14 (0.01, 0.28)0.048 ECV*0.03 (0.01, 0.05)0.005 AVAi-0.20 (-0.46, 0.07)0.145 MPG0.002 (-0.001, 0.005)0.171 LGE presence0.14 (0.07, 0.21)<0.0010.11 (0.05, 0.18)0.001**ECV was not significant on multivariable analysiConclusions Patients with asymptomatic and symptomatic AS demonstrated increased CMR-measured LASI compared to HC, despite LA volumetric and strain parameters being similar between asymptomatic patients and HC. In asymptomatic AS, LASI was independently associated with LGE, a marker of myocardial fibrosis. LASI may serve as a marker of subclinical dysfunction and fibrosis in asymptomatic patients with AS and should be studied further as a risk stratification tool.