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Background: The most frequent cause of functional tricuspid regurgitation (TR) is pulmonary hypertension. However, moderate or severe TR is often not explained by significant systolic pulmonary arterial pressure (sPAP) elevation, and its mechanism and determinants remain elusive.Methods: Consecutive patients analyzed in our laboratory with TR and sPAP ≥ 35 mmHg on Doppler evaluation or any degree of RV dilatation or dysfunction were included. Pulmonary hemodynamics and right ventricular (RV) features were analyzed by Doppler echocardiography, including annular tricuspid size, tricuspid tethering distance and pulmonary vascular resistance (PVR) measured as TR velocity divided by RV outflow tract flow (VTI). We excluded patients with organic tricuspid valve disease, pacemaker carriers, primary RV dilatation or dysfunction, severe tricuspid valve leaflet malcoaptation and significant pulmonary hypertension, defined as sPAP ≥ 50 mmHg.Results: We analyzed 160 patients (age 71,8±11,3 years-old, 50,6% men, atrial fibrillation in 31,2%). Mild TR was present in 107 patients (66,9%), moderate TR in 43 (26,9%) and severe TR in 10 (6,2%). In univariate analysis, greater degrees of TR correlated with age (r=0,17), female gender (r=0,18), left atrial size (r=0,23), RV dilatation (r=0,42), tricuspid annular size (r=0,42), tricuspid tethering distance (r=0,23), sPAP (r=0,35), systolic RV dysfunction (r=-0,26), PVR (r=0,42) and atrial fibrillation (r=0,48) (all with p<0,05). The multivariate analysis revealed female sex, tricuspid annular size, tricuspid tethering distance, PVR and atrial fibrillation as independent predictors of greater degrees of TR. 90% of patients with severe TR had significant afterload increase by PVR (defined as TR velocity/VTI quotient >1,6).Conclusions: Functional TR in the absence of significant sPAP elevation relates to tricuspid annular dilatation and valve tethering, and atrial fibrillation and female gender appear as factors associated with greater degrees of TR. Non-invasive PVR is an independent predictor of TR in this population, suggesting that this more accurate measure may unmask inapparent significant RV afterload.