Estimation of Right Ventricular / Pulmonary Artery Pressure by Doppler
Estimation of right ventricular / pulmonary artery systolic pressure is done by Doppler echocardiographic measurement of the tricuspid regurgitation jet velocity. From the jet velocity, the gradient is calculated with the formula:
Pressure gradient (∆P) = 4V2, where V is the velocity. (Modified Bernoulli equation). Modified Bernoulli equation takes into account only the velocity distal to the orifice.
The original Bernoulli equation is ∆P = P2-P1 = 4(V22 – V12). V1: Proximal velocity; V2: Distal velocity. When the proximal velocity is small, its square is smaller and can be ignored, leading to the modified equation. In this case the proximal velocity is the blood flow velocity on the right ventricular aspect of tricuspid valve in systole which is minimal.
Once the tricuspid regurgitation (TR) jet gradient is estimated, adding the expected right atrial pressure (nominal value taken as 10 mm Hg) gives the right ventricular systolic pressure (RVSP). If the right atrial pressure is elevated, a clinical estimate of right atrial pressure from the measured jugular venous pressure may be added to the gradient to get a better estimate of RVSP. Diameter of the inferior vena cava measured from the subcostal view can also be used to get an estimate of the right atrial pressure. When the inferior vena cava is plethoric (dilated with poor inspiratory collapse), a simple method is to add 20 mm Hg to the TR gradient to get the RVSP, though various complex algorithms have been published for this purpose.
When there is no obstruction at the pulmonary valve, RVSP gives a good estimate of the pulmonary artery systolic pressure as the gradient at the pulmonary valve is minimal. Estimates of the RVSP can be erroneous if a good envelope of the TR jet is not obtained. Incomplete jets lead to underestimation of RVSP. When there is cycle to cycle variation in jet gradient, an average of multiple values may be taken. TR is jet usually evaluated in the apical four chamber view.