Factors affecting accuracy of in vitro valvar pressure gradient estimates by Doppler ultrasound

Doppler ultrasound is used successfully in clinical situations for noninvasive measurement of pressure changes across stenotic cardiac valves. However, situations that might lead to errors in measurement have not been identified. This study determines the effect of flow rate, viscosity, orifice shape and size on the calculation of Doppler transvalvar gradient. Pressure gradient is usually computed from the equation P1-P2 = 4 X Vmax2, where P1-P2 is the gradient and Vmax is the maximal jet velocity measured by Doppler ultrasound. An in vitro model was developed with interchangeable orifices that permitted the jet to be detected by an in-line Doppler transducer. The model allowed alteration of flow rates, viscosities and pressure gradients. When P1-P2 as predicted by Doppler was compared with that measured by manometers (PM), excellent correlations were obtained for triangular orifices of areas as small as 78.5 mm2 (r = 0.95) and for circular and elliptical orifices to as small as 50.2 mm2 (r = 0.99). For smaller orifices, P1-P2 correlated poorly with PM. Good correlation was found between P1-P2 and PM, with flow rates ranging from 0.7 to 8.4 liters/min (r = 0.97) with a 10-mm diameter circular orifice (area = 78.5 mm2). No observable differences were found in the accuracy of the equation between high and low flow rates. Viscosity had no effect on the accuracy of the P1-P2 comparison with PM over the range evaluated (1 to 10 cp). It is concluded that the modified Bernoulli Doppler gradient equation provides accurate results in the usual clinical situation when an orifice permits true jet formation.