Background. Assessment of pulmonary vascular resistance (PVR) is essential in the diagnosis and evaluating the treatment response in pulmonary arterial hypertension (PAH). While invasive right heart catheterization remains the standard method to measure PVR, noninvasive measurements of PVR by echocardiography have been tried. Estimation of PVR depends on estimation of pulmonary arterial pressure and cardiac output. Previous method using the ratio of peak tricuspid regurgitant velocity (TRV) to the right ventricular outflow tract time-velocity integral (TVI_rvot) was reported as a reliable method to determine PVR. However, this method has been reported unreliable in patients with high PVR to evaluate the disease progression in severe PAH patients because of anatomical modifications of the right heart structures. Objective. We hypothesized that estimation of PVR using left ventricular outflow tract time-velocity integral (TVI_lvot) and TRV is superior method to estimation using TVI_rvot and TRV in severe PAH patients with high PVR. Methods. 21 Patients with PAH who underwent right heart catheterization were prospectively recruited. All patients underwent echocardiography within a week of the right heart catheterization. Correlation between invasive PVR with each of TRV/TVI_lvot or TRV/TVI_rvot ratio was analyzed. Results. Mean PVR measured by catheterization was 20 짹 14 Wu. Correlation coefficients between invasive PVR and TRV/TVI_lvot then TRV/TVI_rvot were 0.623 and 0.654, respectively. Two new equations were found by linear regression: [ PVR = (TRV/TVI_lvot) x 82 – 5.0; p<0.001, R2=0.561 ], [ PVR = (TRV/TVI_rvot) x 44 + 2.8; p=0.001, R2=0.427 ]. In patients with severe PAH with PVR more than 20 Wu, TRV/TVI_lvot was well correlated with invasive PVR while TRV/TVI_rvot was not correlated. (p=0.007, correlation coefficient=0.817 vs. p=0.378) Conclusion. Non-invasive estimation of PVR is possible using Doppler echocardiography, however proper method should be selected when considering the limitation of Doppler measurement associated with the anatomical variation of right ventriclular structure. In patients with severe PAH, estimation of PVR by TRV/TVI_lvot is better method than method by TRV/TVI_rvot and it can possibly overcome the limitation of measurement of TVI_rvot.