Limitations inherent to the simplified Bernoulli equation explain the inaccuracy of Doppler echocardiographic estimates of pulmonary artery pressures in patients with pulmonary hypertension.

I would like to congratulate Rich and colleagues, 1 who, in their recent article in CHEST (May 2011), showed clinically relevant discrepancies between Doppler-estimated and invasively measured pulmonary artery systolic pressure (PASP) in . 50% of measurements, discrepancies not resolved by simultaneous measurement of the two entities. Imprecise right atrial pressure estimation, suboptimal alignment between the Doppler beam and the regurgitant jet, and the presence of severe tricuspid valve regurgitation were suggested as the causes of the discrepancy. However, even when beam alignment and right atrial pressure estimation are correct, signifi cant discrepancies between the two measurements should be expected because of limitations inherent to the simplifi ed Bernoulli equation. 2 Indeed, the simplifi ed Bernoulli equation assumes negligible viscous and inertial forces and complete conversion of potential energy (the pressure in the right ventricle) into kinetic energy (that of the regurgitant jet). However, as highlighted in a recent mock circuit experiment, 2