ANALYSIS OF THE EFFECT OF FLOW-RATE ON THE DOPPLER CONTINUITY EQUATION FOR STENOTIC ORIFICE AREA CALCULATIONS - A NUMERICAL STUDY

Background-Flow-rate dependencies of the Doppler continuity equation a re addressed in this study. Methods and Results-By use of computationa l fluid dynamic (CFD) software with turbulence modeling, three-dimensi onal axisymmetric models of round stenotic orifices were created. Flow simulations were run for various orifice area sizes (0.785, 1.13, 1.7 6, and 3.14 cm(2)) and flow rates (0.37 to 25.0 L/min). Reynolds numbe rs ranged from 100 to 8000. Once adequate convergence was obtained wit h each simulation, the location of the vena contracta was determined. For each run, maximum and average velocities across the cross section of the vena contracta were tabulated and vena contracta cross-sectiona l area (effective orifice area) determined. The difference between the maximum velocity and the average velocity at the vena contracta was s mallest at high-flow states, with more of a difference at low-flow sta tes. At lower-flow states, the velocity vector profile at the vena con tracta was parabolic, whereas at high-flow states, the profile became more flattened. Also, the effective orifice area (vena contracta cross -sectional area) varied with flow rate. At moderate-flow states, the e ffective orifice area reached a minimum and expanded at low-and high-f low states, remaining relatively constant at high-flow states. Conclus ions-We have shown that significant differences exist between the maxi mum velocity and the average velocity at the vena contracta at low flo w rates. A likely explanation for this is that viscous effects cause l ower velocities at the edges of the vena contracta at low flow rates, resulting in a parabolic profile. At higher-flow states, inertial forc es overcome viscous drag,causing a flatter profile. Effective orifice area itself varies with flow rate as well, with the smallest areas see n at moderate-flow states. These flow-dependent factors lead to flow r ate-dependent errors in the Doppler continuity equation. Our results h ave strong relevance to clinical measurements of stenotic valve areas by use of the Doppler continuity equation under varying cardiac output conditions.