To quantify the pulmonary systolic pressure, it is necessary to estimate right atrial systolic pressure. How is this done?

Echocardiographers use the size of the right atrium, the tricuspid regurgitation peak velocity, the size and respiratory variation of the IVC, and the characteristics of the systolic hepatic venous flow to estimate right atrial pressure, which may vary from 5 to 20 mm Hg. If most of these parameters are normal, right atrial systolic pressure is assumed to be 5 mm Hg.