There is not yet a completely satisfactory Doppler method for determining the severity of valvular regurgitation. Recently, interest has focused on the so-called "flow convergence region." Color Doppler provides a longitudinal velocity profile of the flow convergence region proximal to restrictive orifices. With respect to determination of regurgitant flow rate, we studied the influence of orifice flow rate and orifice size on the proximal velocity profile. In a phantom model, flow across circular orifices was studied. The distance r (v) of discrete velocities detected proximal to the orifice was measured along the flow center line. Velocity profiles were established. (1) Proximal isovelocity surfaces: The radius r (v) also represented the central radius of the proximal isovelocity surfaces. Increases in flow resulted in larger central radii r (v). Application of the continuity equation and analysis of the resulting values yielded a radius and an orifice size dependence of the geometric isovelocity surface shape. Therefore, their surface area and flow rate could not be calculated from these axial measurements alone. (2) Proximal velocity profiles: The resulting hyperbolic velocity profile curves (x axis = velocity [v], y axis = distance [r (v)]) are shifted rightward by increases in flow. In contrast, increases in orifice size make the curve steeper, but large r (v) are not affected. This differential influence of flow rate and orifice size allows orifice size independent determination of flow rate. A nomogram is presented as one possible method for flow determination.