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Doppler color mapping of the proximal flow convergence region: A new quantitative physiologic tool
JACC Vol . 18, No . 3 September 1991 833-6
833
Editorial Comment
Doppler Color Mapping of the Proximal Flow Convergence Region : A New Quantitative Physiologic Tool* ROBERT A. LEVINE, MD, FACC Boston, Massachusetts
The diagnostic value of echocardiography has grown in proportion to its quantitative applications . The quantitative potential of two-dimensional echocardiography was high lighted by its ability to provide a direct measure of valve area in mitral stenosis (1), a measure that remains the most reliable noninvasive standard (2). The diagnostic value of Doppler echocardiography was dramatically enhanced by applying the simplified Bernoulli collation to Doppler flow velocities to provide a noninvasive measure of intracardiac pressure gradients based on fundamental physical principles (3) . Doppler color flow mapping, the most recent addition to the ultrasound techniques, clearly provides a convenient qualitative demonstration of regurgitant and shunt lesions; it is reasonable to ask whether it can provide additional quantitative information for use in clinical applications. The first such proposed application was the semiquanlitmive estimation of valvular regurgitation from jet size (4,5). While currently the best available measure in routine practice, the limitations of this method include the dependence of jet size on driving pressure, receiving chamber environment, instrument settings and orifice geometry (a determinant of proximal jet dimensions) (6-11). The assessment of shunt lesions from jet size is even more severely limited by such factors: the jet emerging from a ventricular septal defect, for example, rapidly interacts with right ventricular outflow and restraining walls . These limitations have motivated the search for truly quantitative measures of regurgitant and shunt flow based on the application of fluid mechanical principles to velocities extracted from the Doppler flow map .
Proximal flow convergence . One approach that uses this velocity information has been suggested by the observation that Doppler color flow maps reveal accelerating flow patterns proximal to the restrictive orifices of stenotic, regurgitant and shunt lesions (12-16) . It has been proposed that these laminar patterns can provide a measure of orifice flow unaffected by the complexities of the turbulent jet (13,1618) . This measure is based on the conservation of mass, and assumes that fluid converges uniformly and radially toward an orifice, forming concentric isovelocity layers . For orifices that are small relative to the region of acceleration, these isovelocity surfaces are hemispheric (19-22) . The color flow map allows us to measure the radius (r) of such a surface as the distance from the orifice at which the velocity of accelerating flow first aliases, or changes color . The flow through this isovelocity surface can then be calculated as the area of the surface (2arr2) times the aliasing velocity. For regurgitant
orifices, this equals orifice flow by conservation of mass because all how through the isovelocity surface must pass throat,', the orifice . Initial studies . Recent reports (13,16-24) have demonstrated the validity of this concept by computational modeling, in vitro studies and canine models of mitral regurgitation. Whi1N there is no quantitative standard for regurgitation in patients, the concept has been validated in patients with mitral stenosis, in whom it correctly predicts mural valve area by a continuity calculation (area = peak volume flow rate by the proximal convergence method/peak velocity by continuous wave Doppler echocardiography) (25). Shunt flow . The study by Moises et al. (26) in this issue of the Journal reports the first application of the flow convergence principle to assess shunt flow across a ventricular septal defect . An instantaneous flow rate derived from a systolic frame of the Doppler flow map was correlated with time-averaged shunt flow determined by oximetrv is i8 patients and by pulsed Doppler ultrasound in a separate group of 8 patients (shunt flow = difference between integrated milral inflow and aortic outflow) . The correlation was good within each group, implying thet a relation exists between the visualized flow convergence region and the volume of flow across the defect . Correlations were also good when, instead of using the calculated hemispheric isovelocity surface area (2xrr2), the area within the first alias on the nro-dimensional image was substituted ; this is rea-
-Editorial, published iu Journal f the American College f Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology .
From the Cardiac Ultrasound Laboratory, Massachusetts General Has. petal and the Department or Medicine. Harvard Medical School . Boston. Massachusetts . This work was supported in pan by Grant HL-38176 from the National Heart. Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland and by a grant from the American Heart Association, Dallas, Texas. Dr. Levine is an Established Invevigator of the American
Heart Association. with funds conrributed . in par[, by its Massachusetts Aatlinte, Needham, Massachusetts . Address for reprinn : Robert A . Lenin . MD, Cardiac Ultrasound Laboratory. Phillips House 8, Massachusetts General Hospital, Boston, Masso-
chose tts 02114. n1991 by the
American CoOege of Cardiology
sonable because images were selected to have a roughly semicircular flow convergence region, which should have an area of rer2 12. Limitations . These initial promising results establish a correlation between an instantaneous flow rate at some point during systole and a mean flow rate averaged over the cardiac cycle ; they do not, however, validate the accuracy of the flow rate calculation . Further work is therefore required to determine whether the calculated flow rate actually agrees with a suitable standard and can be integrated over time to predict shunt flow accurately (27,28) . 0735-10971911$3 .50
934
LEVINE EDITORIAL COMMENT
Such work is necessary to address a basic question regarding application of the proposed method to ventricular septa) defects . The flow convergence principle requires that all flow crossing the aliasing boundary must enter the orifice . This contiuaiiv of flow auto not strierfv hold in the case of a veuteirtrlar .sepial defect, particularly the membranous types that were predominantly studied . Blood entering the flow convergence region, which can occupy a substantial portion of the left ventricular outflow tract, can leave by way of the aorta as well as through the septa) defect (Fig. I). To the extent that this flow is perceived by the Doppler instrument at the site where the aliasing radius is measured, the flow rate calculated by squaring that radius will overestimate shunt flow. This overestimation will occur whenever left ventricular outflow has a component that is not strictly perpendicular to the ultrasound beam at the point where the radius is measured . If this component is directed toward the t ransducer . it will augment the apparent velocity of flow toward the defect. Therefore, for a given aliasing velocity, the distance from the defect orifice at which flow first aliases will be greater in the presence of left ventricular outflow than in its absence (Fig. IQ . (This is not unreasonable because left ventricular outflow moves anteriorly toward the aorta as it passes over the anterior mitral leaflet [Fig . IBI.) For example, for an aliasing velocity of 54 curls, as in Figure I of Moises et al . (26), with a peak outflow velocity of 80 to 100 cents making an angle of as little as 10° toward the Doppler beam, shunt flow rate will in principle be overestimated by 35% to 46% (Fig. l, legend). This overestimation will increase as the aliasing velocity decreases relative to the outflow velocity . If this interaction were present, a plot of calculated versus actual shunt flow would extrapolate back to a y intercept >0 (predicting shunt flow in the absence of actual shunt), reflecting the superimposed left ventricular outflow . This is . in fact, suggested by Figures 4 and 6 of Moises et al . (26), in which measurable shunt flow is predicted by the flow convergence calculation in the absence of flow by either oximetry or pulsed Doppler ultrasound . Of course, that may simply reflect limitations of the comparative techniques, and this question will require further study . The same reasoning as in Figure I would suggest that, at peak systole, left ventricular outflow would alter not only the measured radius of the flow convergence region but also its overall shape, with the isovelocity contour extending apically into the left ventricle and receding from the aorta (Fig . IC). A hemispheric shape, then, would most likely be observed in late systole, when aortic outflow is decreasing while a gradient persists across the defect, This is consistent with the observation of Moises et al . (26) that the largest hemispheric images measured were obtained primarily near the peak or downslope of the electrocardiographic (ECG) T wave, although the limited temporal resolution of the color flow system could also create a delay between flow and ECG events . These issues can be explored further by triggering
JACC Vol . 18, No. 3 Seprembe, 1991 :833-6
A
NO OUTFLOW VSD Ff..
ao LV
ewnm.
Cnmponeras Ponalleltoeearn
Perceised Velocity
C RESULT INCREASED RADIUS AT S4M£ ALIASING VELOLYTY van Flow t Ao .-
LV
--fs+emir a
.~ LA
Figure 1 . A, Schematic of shunt flow through a membranous ventricular septa) defect (VSD) in the idealized case of no left ventricular (LV) outflow . A particular isovelocity shell defined by an aliasing velocity of 54 curls, as in Figure I of Moises el al ., is indicated . All flow entering the shell passes through the defect . B . Superimposed left ventricular outflow can pass through the aliasing boundary without entering the defect. The component of outflow velocity parallel to the Doppler beam (inset) will increase the apparent velocity of flow toward the defect at the point where the radius of the flow convergence region is measured (along the central axis of flow) . For example, an outflow velocity of 100 curls that makes an angle of 10' toward the beam will augment the recorded flow by (100 cmls)(sin loo ) = 17 calls, so that the velocity at the surface defined in panel A is increased from 54 to 71 cmls at the point shown . C, Therefore, the aliasing border (velocity = 54 curls) is displaced farther from the defect as shown (solid curve), and r, the axial distance from the defect to the aliasing
boundary, is increased . In that case, the actual shunt flow rate can be calculated as 2vr3 x (54 - 17) curls, where subtracting 17 cns/s corrects for the superimposed left ventricular outflow that does not cross the defect. If this correction is not employed, the
calculated shunt flow rate, 2vra x 54 curls, will overestimate the actual value by a factor of 541(54 - 17) = 1 .46 . Ao = aorta; LA = left atrium .
JACC Vol . 18, No . 3 September 1991 :8A-6
color flow acquisitions to various times within the cycle and examining patients with a slower heart rate . Further studies will also be required if an ernpiric correlation is to be used to predict shunt flow from instantaneous flow rate ; this is the case for several reasons : I) The slopes of the observed correlations are different depending on whether integrated shunt flow was determined by oximetry or pulsed Doppler ultrasound (Fig . 4 and 6 of Moises et al . [26]), and the two sets of points are not superimposable . 2) Further, while the calculated instantaneous (presumed peak) flow rate always exceeded the mean flow by catheterization as expected (Table I of Moises et ai . [261), it aas nearly equal to or less than the mean in three of the eight patients studied by pulsed Doppler ultrasound (Table 2 of the study of Moises et al . [261) . This could, for example, reflect variation in the time within the cardiac cycle at which the flow convergence measurement was made or variability of the comparative technique . Despite these limitations, however, the calculated instantaneous flow rates could separate patients with a pulmonary to systemic shunt flow ratio >2 :1 (by catheterization) from those with a smaller shunt . Two additional factors should be kept in mind when applying this method : I) The assumption of hemispheric isovelocity contours will hold best for small or restrictive defects but may require modification for larger defects with a diameter comparable to the distance from the orifice to the aliasing boundary (19,21,22,29) . In such cases, the isovelocity contours flatten out near the orifice and a hemispheric formula underestimates flow (19) . (In the extreme, calculated flow = 0 when the aliasing boundary is at the orifice .) Further, the ability to display hemispheric contours farther from the orifice by lowering the aliasing velocity may be limited by the finite size of the left ventricular outflow tract. (Of course, in such cases, the imaging study alone might be able to demonstrate the large defect size .) 2) The calculation requires modification to account for nonplanar surface geometry surrounding the orifice (25,30) ; for example, a membranous septal aneurysm may restrict the flow convergence region to less than a hemisphere. Conclusions . The proximal flow convergence technique promises to be an important quantitative application of Doppler flow mapping. It is pc tentially capable of providing direct measures of regurgitant and shunt flow, and is sufficiently simple and rapid to be incorporated into routine clinical practice . Further validation, however, is required before its full quantitative potential can be realized . Ongoing studies will also be able to take advantage of recent advances in our understanding, including the need to correct for the effects of orifice size and nonplanar surface geometry (19,25,30), and will be facilitated by using digitally extracted flow maps to automate the flow convergence calculation (31) .
We thank Arthur E . Weyman, MD for reviewing the manuscript and Sharon Tremer for expert seorerarial assistance.
LEVINE EDITORIAL COMMENT
835
References I . Hears WL . Griffith Jht. hirchaelis LL . Mdntash CL . Morrow AG . Epaein SE . Measurement of mural orifice area in patients with initial valve disc ,e by real it.,. two-dimensmnal echrcardiugraphy . Circulaon 1973 :51.827-31 . 2 . Thomas JD . Wilkins GT. Choong CYP. ei vi. Inaccuracy of milral pressure halftime immediately after percuraneous milral valvotomy . Circulation 1999 :78 :980-93 . 3 . HatIo L, Brahakk A . Tromsdal A. Angels, . B. Noninvasive assessment of pressure drop in milral slenosis by Doppler ultrasound . Br Heart 1 1978'-90 :131-40. 4. hfryetake K . Okommo ht- Kinoshita N . et al. Semiquanriturive grading of erity of milral regurgitation by real-rime two-dimensional echocardiegraphy Doppler flow imaging technique. l Am Coll Cardioi 1986:7 :82-9. 5. He]mcke F, Noeda NC, fishing MC . et a1. Color Doppler assessment of mirralregurgitation with onhegonal planes. Circulation 1987 ;75 :175-97 . 6 . Bolger AF . Eigler NL, Pfaff JM, Reener Ki . Maurer G . Computer analysis of Doppler polar Pro, mapping images for quanlilati,e -smeet of in vitro fluid jets. J Am Coll Candid 1988:12 :450-7. 7 . SitepsarIA .Valdes-CruzLM.SahnDi,hturfloA.TamuuT .ChungKJ . Dapplo, color flow mapping of simulated in vitro regurgitant jets: evaluarian of the effects of orifice size and hemodynamic variables. J Am Coll Cardial 1989:13 :1195-107. 8. Cape EG. Skoufis EG. Weyman AE. Yoganathan AP. Levine RA . A new method fur, noninvasive quantification of valvular regurgitation based os rvatinn of momentum: in vitro salutation, Circulation 1989 :79:134353, 9. Thomas JD . Liu C-M- Flachskampf FA, O'Shea Its . Davidof R . Weyman AE . Quantification ofjet flow by momentum analysis : an in vitro color Doppler flow study . Circulation 1990:81:247-59. 10 . Taylor AL, Eichham EJ . Brickwr E . Eberhaet RC. Graybum PB. Aortic valve morphology : on important in vitro determinant of proximal regurgitation jet width by Doppler color how mapping . J .Am Con Cordial 199(h16:405-1'-. 11, Cape EG . Yoganalhan AP, Weyman AE . Levine RA. Adjacent solid boundaries alter the size of regurgitant jets on Doppler color flow maps . J Am Cult Cordial 1991 :17:1094-102 . 12 . Simpson IA, Sahn DJ. Valdes-Cruz LM. Chung KL Sherman FS, Swe an RE. Color Doppler flow mapping in patients with cocamilation of the aorta : new observations and improved evaluation with color flow diameter and proximal acceleration as predictors of severity . Circulation 1988;77 :716-44. Il. Bargiggia G, Recusani F . Yoganathae A P. et al . Color Sow Doppler quantitation or regurgitant flow rate using the flow convergence region proximal to the orifice of a regurgitant jet labsB) . Circulation 1988: 781suppl 11) :11-609. 14. Okamoto M, Tsubokura T. Nakagawa H, et al . The suction signal detected by color Doppler echocardiography in patients with etitml regurgitation: its clinical significance. J Cardial 198&18:739-46 . 15 . Appleton CP. Hatle LK. Nellessen U, Scheitiger I. Popp RL. Flow velocity acceleration in the left ventricle : a useful Doppler echoeardiographic sign of hemodynamically significant mind regurgitation . 1 Am Sac Echo 1990 ;3:3145 . 16. Ulsunomiya T, Ogawa T, Tang HA . Henry WL. Gardin JM . Doppler color flow mapping of the "proximal isovekmity surface arei' : a new method for measuring volume blood flow across an orifice labsln .I Am Call Cardial 1989;13 :225A. 17 . Recusani F, Bargiggia GS, Yoganathan AP. e1 al . A new method far quantification of regurgitant go, rate using color flow imaging lithe flow convergence region proximal to a discrete orifice : an in vitro study . Circulation 1991;83 :594-604 . 18 . Ulsunomiya T. 0gawa T, Doshi R, el al . Doppler color flow "proximal ovelocity surface area" method for estimaling volume flow rate : effects of orifice shape vad machine factors. J Am Call Cardio11991:17:1103-11. 19 . RodriguezL,Anconinal,HarriganP .otatNyquisrlimit andodficearea independently affect the accuracy of proximal isovelocity surface area estimation of flow tale : an in vitro study labetr). J Am Call Cardial 19941Y109A. 20 . pool L, Flachskarepf FA, Abascal VIA . Levine RA, Harrigan P . Thomas JD . Regurgitaat flow rate calculated by proximal isovelocity
836
LEVINE EEITORIAL COMMENT
surface area is independent of orifce shupc labor) . Circulation 1989 : 801suppiID :11-570, 21 . Moines VA . Chao K . Shandas R . et al. Effects oforifce size and shape on flaw ram estimated from flow convergence region imaged by color Doppler flow mapping proximal to restrictive orifices : an in vitro study (abor) . 3 Am Coll Cardiol 1990;15;109A . 22 . Utttteottriya 1, Q000 M, Rojon C, Ford D. GroAn 3M. ERrol offowrato,
orifce size and abasing velocity on volume calculation using Doppler color proximal isovelocily surface area method lebstel . J Am call Cardiol 1990115:89A . 23 . Raduguen L. Vlahakes Gl. Yoganathan AP,. Guerre JL. Weymen AE, Levine RA. Quantification of regurgilant flow rate using the proximal flaw vergenca method : in vivo validation )abstr) . Circulation 1989R0(suppl 11111-571 . 24, Value, -Cruz L, Recusaoi F, Shandas R . et al . Accuracy of now cancer. gone methods for calaularing mitral regoegiranl 1flow : vnlidulion in an annual model (abstr) . 1 Am Call Cannot 199015 : 10A . 25 . Rod, igare L, Morterrosn V, Mueller L, el a1 . Validation ofa new method for valve area calculation using the proximal isovelocily surface area in patients with mitral slenosis )abate) . I Am Call Cardinl 1990;15 :109A, 26 . Moines VA, Mociel DC . Hornberger LK. et al, A new method for
IACC Vol . 18. No 3 September 1991:833-6
noninvasive estimarion of ventricular scrawl defect shunt flaw by Doppler color flow mapping : imaging of the laminar flow convergence region on the left septa) surface . 1 Am Call Cardiol 1991 ;18:824-32, 27. Cape ELI . Yoganathan AP, Rodriguez L, Weymen AE . Levine RA . The proximal flow convergence method can be extended to calculate regurgitant stroke volume : in vitro application of the color Doppler M •mode )abstr). l Am Coll Cordial 1990:15 :109A . 28. Utsunomiya T . Nguyen D, Doshi R, Pawl D, Gardin 1M, Regurgilant volume estimation in mitral regurgitation by color Doppler using the "proximal isovelocily surface area'' method (abstr) . Circulation 1989: 80(suppl 111:11-577 . 29, Shandas R, Lee Y . Golebiovski P, Mrosko B, Saha DJ. Flow convergence calculation of flow rate through no minimally restrictive oriflcev In-oiler mtdirr, t;bstr!. 1 Am Coll Cardiol 1991 :17:359A . 30, Levine RA, Rodrigucz L, Cape EG, et al . The proximal flow convergence method for calculating orifice now rate requires correction for surrounding leaflet geometry labor). J Am Coll Cardiol 1991 :37:359 A . 31 . Thoreau DH, Rodriguez LL, Weymen AE, Thomas JD, Digital mapping of proximal acceleration with computerized analysis to determine the effective orifice center and flow rate (abstrl . J Am Coll Cardiol 1991 :17: 201A .
833
Editorial Comment
Doppler Color Mapping of the Proximal Flow Convergence Region : A New Quantitative Physiologic Tool* ROBERT A. LEVINE, MD, FACC Boston, Massachusetts
The diagnostic value of echocardiography has grown in proportion to its quantitative applications . The quantitative potential of two-dimensional echocardiography was high lighted by its ability to provide a direct measure of valve area in mitral stenosis (1), a measure that remains the most reliable noninvasive standard (2). The diagnostic value of Doppler echocardiography was dramatically enhanced by applying the simplified Bernoulli collation to Doppler flow velocities to provide a noninvasive measure of intracardiac pressure gradients based on fundamental physical principles (3) . Doppler color flow mapping, the most recent addition to the ultrasound techniques, clearly provides a convenient qualitative demonstration of regurgitant and shunt lesions; it is reasonable to ask whether it can provide additional quantitative information for use in clinical applications. The first such proposed application was the semiquanlitmive estimation of valvular regurgitation from jet size (4,5). While currently the best available measure in routine practice, the limitations of this method include the dependence of jet size on driving pressure, receiving chamber environment, instrument settings and orifice geometry (a determinant of proximal jet dimensions) (6-11). The assessment of shunt lesions from jet size is even more severely limited by such factors: the jet emerging from a ventricular septal defect, for example, rapidly interacts with right ventricular outflow and restraining walls . These limitations have motivated the search for truly quantitative measures of regurgitant and shunt flow based on the application of fluid mechanical principles to velocities extracted from the Doppler flow map .
Proximal flow convergence . One approach that uses this velocity information has been suggested by the observation that Doppler color flow maps reveal accelerating flow patterns proximal to the restrictive orifices of stenotic, regurgitant and shunt lesions (12-16) . It has been proposed that these laminar patterns can provide a measure of orifice flow unaffected by the complexities of the turbulent jet (13,1618) . This measure is based on the conservation of mass, and assumes that fluid converges uniformly and radially toward an orifice, forming concentric isovelocity layers . For orifices that are small relative to the region of acceleration, these isovelocity surfaces are hemispheric (19-22) . The color flow map allows us to measure the radius (r) of such a surface as the distance from the orifice at which the velocity of accelerating flow first aliases, or changes color . The flow through this isovelocity surface can then be calculated as the area of the surface (2arr2) times the aliasing velocity. For regurgitant
orifices, this equals orifice flow by conservation of mass because all how through the isovelocity surface must pass throat,', the orifice . Initial studies . Recent reports (13,16-24) have demonstrated the validity of this concept by computational modeling, in vitro studies and canine models of mitral regurgitation. Whi1N there is no quantitative standard for regurgitation in patients, the concept has been validated in patients with mitral stenosis, in whom it correctly predicts mural valve area by a continuity calculation (area = peak volume flow rate by the proximal convergence method/peak velocity by continuous wave Doppler echocardiography) (25). Shunt flow . The study by Moises et al. (26) in this issue of the Journal reports the first application of the flow convergence principle to assess shunt flow across a ventricular septal defect . An instantaneous flow rate derived from a systolic frame of the Doppler flow map was correlated with time-averaged shunt flow determined by oximetrv is i8 patients and by pulsed Doppler ultrasound in a separate group of 8 patients (shunt flow = difference between integrated milral inflow and aortic outflow) . The correlation was good within each group, implying thet a relation exists between the visualized flow convergence region and the volume of flow across the defect . Correlations were also good when, instead of using the calculated hemispheric isovelocity surface area (2xrr2), the area within the first alias on the nro-dimensional image was substituted ; this is rea-
-Editorial, published iu Journal f the American College f Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology .
From the Cardiac Ultrasound Laboratory, Massachusetts General Has. petal and the Department or Medicine. Harvard Medical School . Boston. Massachusetts . This work was supported in pan by Grant HL-38176 from the National Heart. Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland and by a grant from the American Heart Association, Dallas, Texas. Dr. Levine is an Established Invevigator of the American
Heart Association. with funds conrributed . in par[, by its Massachusetts Aatlinte, Needham, Massachusetts . Address for reprinn : Robert A . Lenin . MD, Cardiac Ultrasound Laboratory. Phillips House 8, Massachusetts General Hospital, Boston, Masso-
chose tts 02114. n1991 by the
American CoOege of Cardiology
sonable because images were selected to have a roughly semicircular flow convergence region, which should have an area of rer2 12. Limitations . These initial promising results establish a correlation between an instantaneous flow rate at some point during systole and a mean flow rate averaged over the cardiac cycle ; they do not, however, validate the accuracy of the flow rate calculation . Further work is therefore required to determine whether the calculated flow rate actually agrees with a suitable standard and can be integrated over time to predict shunt flow accurately (27,28) . 0735-10971911$3 .50
934
LEVINE EDITORIAL COMMENT
Such work is necessary to address a basic question regarding application of the proposed method to ventricular septa) defects . The flow convergence principle requires that all flow crossing the aliasing boundary must enter the orifice . This contiuaiiv of flow auto not strierfv hold in the case of a veuteirtrlar .sepial defect, particularly the membranous types that were predominantly studied . Blood entering the flow convergence region, which can occupy a substantial portion of the left ventricular outflow tract, can leave by way of the aorta as well as through the septa) defect (Fig. I). To the extent that this flow is perceived by the Doppler instrument at the site where the aliasing radius is measured, the flow rate calculated by squaring that radius will overestimate shunt flow. This overestimation will occur whenever left ventricular outflow has a component that is not strictly perpendicular to the ultrasound beam at the point where the radius is measured . If this component is directed toward the t ransducer . it will augment the apparent velocity of flow toward the defect. Therefore, for a given aliasing velocity, the distance from the defect orifice at which flow first aliases will be greater in the presence of left ventricular outflow than in its absence (Fig. IQ . (This is not unreasonable because left ventricular outflow moves anteriorly toward the aorta as it passes over the anterior mitral leaflet [Fig . IBI.) For example, for an aliasing velocity of 54 curls, as in Figure I of Moises et al . (26), with a peak outflow velocity of 80 to 100 cents making an angle of as little as 10° toward the Doppler beam, shunt flow rate will in principle be overestimated by 35% to 46% (Fig. l, legend). This overestimation will increase as the aliasing velocity decreases relative to the outflow velocity . If this interaction were present, a plot of calculated versus actual shunt flow would extrapolate back to a y intercept >0 (predicting shunt flow in the absence of actual shunt), reflecting the superimposed left ventricular outflow . This is . in fact, suggested by Figures 4 and 6 of Moises et al . (26), in which measurable shunt flow is predicted by the flow convergence calculation in the absence of flow by either oximetry or pulsed Doppler ultrasound . Of course, that may simply reflect limitations of the comparative techniques, and this question will require further study . The same reasoning as in Figure I would suggest that, at peak systole, left ventricular outflow would alter not only the measured radius of the flow convergence region but also its overall shape, with the isovelocity contour extending apically into the left ventricle and receding from the aorta (Fig . IC). A hemispheric shape, then, would most likely be observed in late systole, when aortic outflow is decreasing while a gradient persists across the defect, This is consistent with the observation of Moises et al . (26) that the largest hemispheric images measured were obtained primarily near the peak or downslope of the electrocardiographic (ECG) T wave, although the limited temporal resolution of the color flow system could also create a delay between flow and ECG events . These issues can be explored further by triggering
JACC Vol . 18, No. 3 Seprembe, 1991 :833-6
A
NO OUTFLOW VSD Ff..
ao LV
ewnm.
Cnmponeras Ponalleltoeearn
Perceised Velocity
C RESULT INCREASED RADIUS AT S4M£ ALIASING VELOLYTY van Flow t Ao .-
LV
--fs+emir a
.~ LA
Figure 1 . A, Schematic of shunt flow through a membranous ventricular septa) defect (VSD) in the idealized case of no left ventricular (LV) outflow . A particular isovelocity shell defined by an aliasing velocity of 54 curls, as in Figure I of Moises el al ., is indicated . All flow entering the shell passes through the defect . B . Superimposed left ventricular outflow can pass through the aliasing boundary without entering the defect. The component of outflow velocity parallel to the Doppler beam (inset) will increase the apparent velocity of flow toward the defect at the point where the radius of the flow convergence region is measured (along the central axis of flow) . For example, an outflow velocity of 100 curls that makes an angle of 10' toward the beam will augment the recorded flow by (100 cmls)(sin loo ) = 17 calls, so that the velocity at the surface defined in panel A is increased from 54 to 71 cmls at the point shown . C, Therefore, the aliasing border (velocity = 54 curls) is displaced farther from the defect as shown (solid curve), and r, the axial distance from the defect to the aliasing
boundary, is increased . In that case, the actual shunt flow rate can be calculated as 2vr3 x (54 - 17) curls, where subtracting 17 cns/s corrects for the superimposed left ventricular outflow that does not cross the defect. If this correction is not employed, the
calculated shunt flow rate, 2vra x 54 curls, will overestimate the actual value by a factor of 541(54 - 17) = 1 .46 . Ao = aorta; LA = left atrium .
JACC Vol . 18, No . 3 September 1991 :8A-6
color flow acquisitions to various times within the cycle and examining patients with a slower heart rate . Further studies will also be required if an ernpiric correlation is to be used to predict shunt flow from instantaneous flow rate ; this is the case for several reasons : I) The slopes of the observed correlations are different depending on whether integrated shunt flow was determined by oximetry or pulsed Doppler ultrasound (Fig . 4 and 6 of Moises et al . [26]), and the two sets of points are not superimposable . 2) Further, while the calculated instantaneous (presumed peak) flow rate always exceeded the mean flow by catheterization as expected (Table I of Moises et ai . [261), it aas nearly equal to or less than the mean in three of the eight patients studied by pulsed Doppler ultrasound (Table 2 of the study of Moises et al . [261) . This could, for example, reflect variation in the time within the cardiac cycle at which the flow convergence measurement was made or variability of the comparative technique . Despite these limitations, however, the calculated instantaneous flow rates could separate patients with a pulmonary to systemic shunt flow ratio >2 :1 (by catheterization) from those with a smaller shunt . Two additional factors should be kept in mind when applying this method : I) The assumption of hemispheric isovelocity contours will hold best for small or restrictive defects but may require modification for larger defects with a diameter comparable to the distance from the orifice to the aliasing boundary (19,21,22,29) . In such cases, the isovelocity contours flatten out near the orifice and a hemispheric formula underestimates flow (19) . (In the extreme, calculated flow = 0 when the aliasing boundary is at the orifice .) Further, the ability to display hemispheric contours farther from the orifice by lowering the aliasing velocity may be limited by the finite size of the left ventricular outflow tract. (Of course, in such cases, the imaging study alone might be able to demonstrate the large defect size .) 2) The calculation requires modification to account for nonplanar surface geometry surrounding the orifice (25,30) ; for example, a membranous septal aneurysm may restrict the flow convergence region to less than a hemisphere. Conclusions . The proximal flow convergence technique promises to be an important quantitative application of Doppler flow mapping. It is pc tentially capable of providing direct measures of regurgitant and shunt flow, and is sufficiently simple and rapid to be incorporated into routine clinical practice . Further validation, however, is required before its full quantitative potential can be realized . Ongoing studies will also be able to take advantage of recent advances in our understanding, including the need to correct for the effects of orifice size and nonplanar surface geometry (19,25,30), and will be facilitated by using digitally extracted flow maps to automate the flow convergence calculation (31) .
We thank Arthur E . Weyman, MD for reviewing the manuscript and Sharon Tremer for expert seorerarial assistance.
LEVINE EDITORIAL COMMENT
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References I . Hears WL . Griffith Jht. hirchaelis LL . Mdntash CL . Morrow AG . Epaein SE . Measurement of mural orifice area in patients with initial valve disc ,e by real it.,. two-dimensmnal echrcardiugraphy . Circulaon 1973 :51.827-31 . 2 . Thomas JD . Wilkins GT. Choong CYP. ei vi. Inaccuracy of milral pressure halftime immediately after percuraneous milral valvotomy . Circulation 1999 :78 :980-93 . 3 . HatIo L, Brahakk A . Tromsdal A. Angels, . B. Noninvasive assessment of pressure drop in milral slenosis by Doppler ultrasound . Br Heart 1 1978'-90 :131-40. 4. hfryetake K . Okommo ht- Kinoshita N . et al. Semiquanriturive grading of erity of milral regurgitation by real-rime two-dimensional echocardiegraphy Doppler flow imaging technique. l Am Coll Cardioi 1986:7 :82-9. 5. He]mcke F, Noeda NC, fishing MC . et a1. Color Doppler assessment of mirralregurgitation with onhegonal planes. Circulation 1987 ;75 :175-97 . 6 . Bolger AF . Eigler NL, Pfaff JM, Reener Ki . Maurer G . Computer analysis of Doppler polar Pro, mapping images for quanlilati,e -smeet of in vitro fluid jets. J Am Coll Candid 1988:12 :450-7. 7 . SitepsarIA .Valdes-CruzLM.SahnDi,hturfloA.TamuuT .ChungKJ . Dapplo, color flow mapping of simulated in vitro regurgitant jets: evaluarian of the effects of orifice size and hemodynamic variables. J Am Coll Cardial 1989:13 :1195-107. 8. Cape EG. Skoufis EG. Weyman AE. Yoganathan AP. Levine RA . A new method fur, noninvasive quantification of valvular regurgitation based os rvatinn of momentum: in vitro salutation, Circulation 1989 :79:134353, 9. Thomas JD . Liu C-M- Flachskampf FA, O'Shea Its . Davidof R . Weyman AE . Quantification ofjet flow by momentum analysis : an in vitro color Doppler flow study . Circulation 1990:81:247-59. 10 . Taylor AL, Eichham EJ . Brickwr E . Eberhaet RC. Graybum PB. Aortic valve morphology : on important in vitro determinant of proximal regurgitation jet width by Doppler color how mapping . J .Am Con Cordial 199(h16:405-1'-. 11, Cape EG . Yoganalhan AP, Weyman AE . Levine RA. Adjacent solid boundaries alter the size of regurgitant jets on Doppler color flow maps . J Am Cult Cordial 1991 :17:1094-102 . 12 . Simpson IA, Sahn DJ. Valdes-Cruz LM. Chung KL Sherman FS, Swe an RE. Color Doppler flow mapping in patients with cocamilation of the aorta : new observations and improved evaluation with color flow diameter and proximal acceleration as predictors of severity . Circulation 1988;77 :716-44. Il. Bargiggia G, Recusani F . Yoganathae A P. et al . Color Sow Doppler quantitation or regurgitant flow rate using the flow convergence region proximal to the orifice of a regurgitant jet labsB) . Circulation 1988: 781suppl 11) :11-609. 14. Okamoto M, Tsubokura T. Nakagawa H, et al . The suction signal detected by color Doppler echocardiography in patients with etitml regurgitation: its clinical significance. J Cardial 198&18:739-46 . 15 . Appleton CP. Hatle LK. Nellessen U, Scheitiger I. Popp RL. Flow velocity acceleration in the left ventricle : a useful Doppler echoeardiographic sign of hemodynamically significant mind regurgitation . 1 Am Sac Echo 1990 ;3:3145 . 16. Ulsunomiya T, Ogawa T, Tang HA . Henry WL. Gardin JM . Doppler color flow mapping of the "proximal isovekmity surface arei' : a new method for measuring volume blood flow across an orifice labsln .I Am Call Cardial 1989;13 :225A. 17 . Recusani F, Bargiggia GS, Yoganathan AP. e1 al . A new method far quantification of regurgitant go, rate using color flow imaging lithe flow convergence region proximal to a discrete orifice : an in vitro study . Circulation 1991;83 :594-604 . 18 . Ulsunomiya T. 0gawa T, Doshi R, el al . Doppler color flow "proximal ovelocity surface area" method for estimaling volume flow rate : effects of orifice shape vad machine factors. J Am Call Cardio11991:17:1103-11. 19 . RodriguezL,Anconinal,HarriganP .otatNyquisrlimit andodficearea independently affect the accuracy of proximal isovelocity surface area estimation of flow tale : an in vitro study labetr). J Am Call Cardial 19941Y109A. 20 . pool L, Flachskarepf FA, Abascal VIA . Levine RA, Harrigan P . Thomas JD . Regurgitaat flow rate calculated by proximal isovelocity
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surface area is independent of orifce shupc labor) . Circulation 1989 : 801suppiID :11-570, 21 . Moines VA . Chao K . Shandas R . et al. Effects oforifce size and shape on flaw ram estimated from flow convergence region imaged by color Doppler flow mapping proximal to restrictive orifices : an in vitro study (abor) . 3 Am Coll Cardiol 1990;15;109A . 22 . Utttteottriya 1, Q000 M, Rojon C, Ford D. GroAn 3M. ERrol offowrato,
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noninvasive estimarion of ventricular scrawl defect shunt flaw by Doppler color flow mapping : imaging of the laminar flow convergence region on the left septa) surface . 1 Am Call Cardiol 1991 ;18:824-32, 27. Cape ELI . Yoganathan AP, Rodriguez L, Weymen AE . Levine RA . The proximal flow convergence method can be extended to calculate regurgitant stroke volume : in vitro application of the color Doppler M •mode )abstr). l Am Coll Cordial 1990:15 :109A . 28. Utsunomiya T . Nguyen D, Doshi R, Pawl D, Gardin 1M, Regurgilant volume estimation in mitral regurgitation by color Doppler using the "proximal isovelocily surface area'' method (abstr) . Circulation 1989: 80(suppl 111:11-577 . 29, Shandas R, Lee Y . Golebiovski P, Mrosko B, Saha DJ. Flow convergence calculation of flow rate through no minimally restrictive oriflcev In-oiler mtdirr, t;bstr!. 1 Am Coll Cardiol 1991 :17:359A . 30, Levine RA, Rodrigucz L, Cape EG, et al . The proximal flow convergence method for calculating orifice now rate requires correction for surrounding leaflet geometry labor). J Am Coll Cardiol 1991 :37:359 A . 31 . Thoreau DH, Rodriguez LL, Weymen AE, Thomas JD, Digital mapping of proximal acceleration with computerized analysis to determine the effective orifice center and flow rate (abstrl . J Am Coll Cardiol 1991 :17: 201A .