Noninvasive pulmonary artery systolic pressure (PASP) is calculated by summing the right ventricular systolic pressure obtained from Doppler velocity of regurgitant flow through the tricuspid valve and the right atrial (RA) pressure. The RA pressure is generally assumed from different formulas. An accurate RA pressure estimation will add precision to PASP calculation. One of the methods to estimate RA pressure is the inferior vena cava collapsibility index (IVCCI). In 45 patients referred for right heart catheterization, the authors tested a formula for the calculation of PASP based on the estimation of RA pressure from IVCCI and compared this method with two other formulas. The first method (method 1) assumed a constant RA pressure of 10 mm Hg irrespective of right ventricular pressure. The formula used was Doppler gradient + 10 (mm Hg). In the second method (method 2), a clinical estimate of RV pressure was made from the formula: right ventricular-right atrial Doppler gradient x 1.1 + 14. In the third method (method 3), the patients were classified into three groups on the basis of IVCCI: group A, IVCCI greater than 45%; group B, IVCCI between 35% and 45%; and group C, IVCCI less than 35%. The formula used was Doppler gradient + 6, 9, or 16 mm Hg in the presence of normal (group A), moderately reduced (group B), or markedly reduced (group 3) IVCCI. A good correlation between Doppler and catheter measurements of PASP was found for methods 1, 2, and 3, respectively (r=0.8933, SEE=6.4, r=0.8921, SEE=7.0, and r=0.8989, SEE=6.7). Correlation between invasive and noninvasive PASP was similar with the three methods, but correlation in method 2 was less satisfactory than with the other two methods. The mean difference between Doppler-derived and hemodynamic PASP was also high in method 2. In conclusion, the result of this study validates a relatively new, simple echo-Doppler formula for Doppler estimation of PASP based on a noninvasive evaluation of RA pressure through the IVCCI. However, this method is not better than the traditional method 1 for noninvasive PASP estimation.