DETERMINATION OF THE MOST APPROPRIATE VELOCITY THRESHOLD FOR APPLYINGHEMISPHERIC FLOW CONVERGENCE EQUATIONS TO CALCULATE FLOW-RATE - SELECTED ACCORDING TO THE TRANSORIFICE PRESSURE-GRADIENT - DIGITAL-COMPUTERANALYSIS OF THE DOPPLER COLOR-FLOW CONVERGENCE REGION

Background. While flow convergence methods have been promising for cal culating volume flows from color Doppler images, it appears that the v elocity threshold used and the transorifice pressure gradient dramatic ally influence the accuracy of application of the simple hemispheric f low convergence equation for calculation of flow rate. The present in vitro study was performed to determine whether the value of velocity t hreshold at which the shape of proximal isovelocity surface best fits given shape assumptions with different orifice sizes and flow rates is predictable as a function independent of orifice size from clinically measurable peak velocity or transorifice pressure gradient informatio n.Methods and Results. In an in vitro model built to facilitate ultras ound imaging, steady flow was driven through circular discrete orifice s with diameters of 3.8, 5.5, and 10 mm. Flow rates ranged from 2.88 t o 8.28 L/min with corresponding driving pressure gradients from 14 to 263 mm Hg. At each flow rate, Doppler color-encoded M-mode images thro ugh the center of the flow convergence region were obtained and transf erred into the microcomputer (Macintosh IIci) in their original digita l format. Then, the continuous wave Doppler traces of maximal velocity through the orifice were derived for the calculation of driving press ure gradient. Direct numerical spatial velocity measurements were obta ined from the digital color encoded M-mode velocities with computer so ftware. For each flow rate, we could calculate flow volume from any nu mber of velocity distance combinations with a number of assumptions an d use the results to assess, expected flow convergence shape based on a priori knowledge of the progression from oblate hemispheroid to hemi sphere to prolate hemispheroid changes observed previously. Our result s showed that for a given ratio of calculated flow rate to actual flow rate (0.7 and 1), the velocity threshold that could be used for the c alculation of flow rate with a hemispheric flow convergence equation c orrelated well with the pressure gradient for a given orifice size, an d the differences in velocity threshold that could be used this way am ong different orifice sizes once they were adjusted for the covariate pressure gradients were not statistically significant (P=.79 for ratio =0.7, and P=.81 for ratio=1). Conclusions. Our present study provides an orifice size-independent quantitative method that can be used to se lect the most suitable velocity threshold for applying a simple hemisp heric flow convergence equation based on clinically predictable pressu re gradients ranging from 40 to 200 mm Hg, and it offers a correction factor that can be applied to the hemispheric flow convergence equatio n when the pressure gradient is less than 40 mm Hg.