Circulation, Vol 88, 1699-1708, Copyright © 1993 by American Heart Association
YB Deng, T Shiota, R Shandas, J Zhang and DJ Sahn
BACKGROUND: While flow convergence methods have been promising for
calculating volume flows from color Doppler images, it appears that the
velocity threshold used and the transorifice pressure gradient dramatically
influence the accuracy of application of the simple hemispheric flow
convergence equation for calculation of flow rate. The present in vitro
study was performed to determine whether the value of velocity threshold 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 information. METHODS AND
RESULTS. In an in vitro model built to facilitate ultrasound imaging,
steady flow was driven through circular discrete orifices with diameters of
3.8, 5.5, and 10 mm. Flow rates ranged from 2.88 to 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 through the center of the flow
convergence region were obtained and transferred into the microcomputer
(Macintosh IIci) in their original digital format. Then, the continuous
wave Doppler traces of maximal velocity through the orifice were derived
for the calculation of driving pressure gradient. Direct numerical spatial
velocity measurements were obtained from the digital color encoded M- mode
velocities with computer software. For each flow rate, we could calculate
flow volume from any number of velocity distance combinations with a number
of assumptions and use the results to assess expected flow convergence
shape based on a priori knowledge of the progression from oblate
hemispheroid to hemisphere to prolate hemispheroid changes observed
previously. Our results 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 calculation of flow rate with a hemispheric flow convergence
equation correlated well with the pressure gradient for a given orifice
size, and the differences in velocity threshold that could be used this way
among 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 select
the most suitable velocity threshold for applying a simple hemispheric flow
convergence equation based on clinically predictable pressure gradients
ranging from 40 to 200 mm Hg, and it offers a correction factor that can be
applied to the hemispheric flow convergence equation when the pressure
gradient is less than 40 mm Hg.
ARTICLES
Determination of the most appropriate velocity threshold for applying hemispheric flow convergence equations to calculate flow rate: selected according to the transorifice pressure gradient. Digital computer analysis of the Doppler color flow convergence region
Clinical Care Center for Congenital Heart Disease, Oregon Health Sciences University, Portland 97201.
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