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
Yb. Deng et al., 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, Circulation, 88(4), 1993, pp. 1699-1708
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.