Nature of flow acceleration into a finite-sized orifice: Steady and pulsatile flow studies on the flow convergence region using simultaneous ultrasound Doppler flow mapping and laser Doppler velocimetry

Abstract

Objectives. This study investigated the proximal centerline flow convergence region simultaneously by both color Doppler and laser Doppler velocimetry. Background. Although numerous investigations have been performed to test the flow convergence method, to our knowledge there has yet been no experimental study using reference standard velocimetric techniques to define precisely the hydrodynamic factors involved in the accelerating flow region during steady and pulsatile flow. Methods. Using an in vitro model that allows velocity measurements by laser Doppler velocimetry with simultaneous comparison with color Doppler results, we studied the centerline flow acceleration region proximal to orifices of various sizes (0.08 to 2.0 cm^2). Results. Agreement between theory and experimental velocities was good for large flow rates through small orifices only, and only at distances >1.2 cm from the orifice. Changing the orifice shape from circular to slitlike produced no significant changes in velocity profiles. Constraining the proximal side walls caused a significant increase in proximal velocities at distances >0.7 cm for the largest orifice only (2.0 cm^2). Calculated flow rates agreed well with actual flow rates, with functional dependence on proximal distance and orifice size. Velocity profiles for pulsatile flow were similar to steady state flow profiles and could be integrated to calculate stroke volumes, which followed actual flow volumes well, although with general overestimation (y = 1.22x + 0.164, r = 0.92), most likely due to the use of all available proximal velocities. Conclusions. The accelerating proximal flow region responds to several hydrodynamic factors that can affect flow quantitation using the flow convergence method in the clinical situation.

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