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Background: Patients with hypertrophic cardiomyopathy (HCM) and left atrial (LA) enlargement have increased mortality and morbidity, including a high risk of atrial fibrillation (AF). Although LA size may reflect chronic changes due to diastolic dysfunction (DD) the relationship between LA size and left ventricular filling pressures is modest in HCM patients.Purpose: To identify the independent correlates of LA volume and the relationship between LA dilation and new onset atrial fibrillation in patients with HCM.Methods: We prospectively enrolled 37 consecutive patients with HCM (51±16 years, 18 men) and sinus rhythm. Indexed LA volume (LAVi) was determined using the area length method. Left ventricular (LV) filling pressures were assessed using average E/' ratio. DD has been graded according to the ASE/EAE recommendations. Global longitudinal LV strain (GLS) was assessed from the apical views using a commercially available 2D Strain software. Basal and apical LV rotation and backrotation rate, peak systolic LV torsion and peak untwisting rate have been also measured. Time intervals from peak R wave (ECG) to each of them were measured and normalized to the RR interval. Mitral regurgitation (MR) severity was graded as mild, moderate or severe. A subgroup of 20 patients has been followed up for 1 year for the occurrence of paroxysmal/persistent AF.Results: All patients had preserved LV ejection fraction (65±7%), LV hypertrophy (187±64 g/m2) and enlarged LA (67±34 ml/m2). LV outflow tract obstruction was present in 20 patients. LAVi correlated significantly with LVmass (r=0.64, p<0.001), GLS (r=0.54, p=0.02) and time to LV peak untwisting velocity (r=0.35, p=0.04). In patients with moderate to severe MR (n=24), LAVi was significantly higher than in patients with mild MR (75±38 vs 50±13 ml/m2, p=0.03). LAVi was not significantly related to E/E', DD degree or dynamic LV outflow tract gradient (p>0.2 for all). In multivariate regression analysis LVmass (β=0.71, p<0.001) and time to LV peak untwisting velocity (β=0.32, p=0.01) have been independently correlated with LAVi (r2=0.65, p<0.01). Moreover, LAVi emerged as the only parameter that was significantly different between patients with (n=8) and without AF during follow up (86±31 vs 61±20 ml/m2, p=0.04).Conclusions: In patients with HCM, increased LV mass and delayed LV untwisting are independently related to LA enlargement. The degree of LA dilation is related to the occurrence of paroxysmal/persistent AF.