(Circulation. 1996;94:119-121.)
© 1996 American Heart Association, Inc.
Articles |
Noninvasive Quantification of Valvular Regurgitation
Getting to the Core of the Matter
the Department of Internal Medicine, Division of Cardiology, University of Texas Southwestern Medical Center (Dallas)
Correspondence to Paul A. Grayburn, MD, Division of Cardiology, UT Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, TX 75235-9047. E-mail grayburn@ryburn.swmed.edu.
Key Words: Editorials • echocardiography • magnetic resonance imaging • valves • regurgitation
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Introduction |
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 Top Introduction Conservation of Mass: The...Conservation of MomentumThe Jet Laminar Core:...Potential ProblemsConclusionsReferences |
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Initial attempts to quantify the severity of valvular regurgitation have been focused on imaging the regurgitant jet in the downstream chamber. Such methods, although intuitively appealing, are not based on solid physical principles and have not been proven to be accurate. Angiography, which for years was considered the gold standard, is invasive, only semiquantitative, and subject to a number of technical limitations.1 Likewise, subjective visual assessment of the downstream jet with the use of Doppler color flow mapping is semiquantitative at best and is affected by several hemodynamic and technical variables.2 3 4 5 Accordingly, a great deal of effort has centered on finding an accurate quantitative method of evaluating valvular regurgitation. Many of these efforts have been founded on sound physical principles but have been difficult to implement due to inherent weaknesses in the imaging methodology.
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Conservation of Mass: The Continuity Equation |
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TopIntroductionConservation of Mass: The...Conservation of MomentumThe Jet Laminar Core:...Potential ProblemsConclusionsReferences |
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The oldest principle used to quantify valvular regurgitation is the continuity equation, which is based on the principle of conservation of mass. According to the continuity equation, forward flow across each of the heart valves should be equal in the absence of an intracardiac shunt or valvular regurgitation. In the catheterization laboratory, left-sided valvular regurgitation is calculated as the difference between left ventriculographic stroke volume and forward stroke volume according to either the Fick or the thermodilution method.1 Unfortunately, angiographic stroke volumes may be affected by arrhythmias or failure to fully opacify the entire left ventricle due to improper catheter position or an insufficient amount of contrast agent. Furthermore, Fick outputs are less accurate at high cardiac outputs where the measured arteriovenous oxygen saturation difference is small. Conversely, thermodilution outputs are less accurate at low cardiac outputs or in the presence of tricuspid regurgitation. The net result of these technical problems is that a calculated regurgitant fraction as high as 20% can be seen in patients without any valvular regurgitation.
A similar application of the continuity equation can be performed with quantitative Doppler techniques in which left-sided regurgitant volume is calculated as the difference between Doppler-derived flow across the aortic and mitral valves.5 6 Again, this method is theoretically sound but has technical limitations. Measurement of the valve annulus can be difficult in patients with suboptimal echocardiographic images, prosthetic heart valves, or certain anatomic variants such as mitral annular calcification or asymmetric septal hypertrophy. The accuracy of pulsed Doppler velocity measurements can be affected by the angle dependence, failure to properly locate the sample volume at the valve annulus, or aliasing. Nevertheless, a "false" regurgitant fraction of <10% is typical in patients without valvular regurgitation, suggesting that this method may be superior to the catheterization method.
Recently, Hundley et al7 used MRI to calculate mitral regurgitant volume with the use of the continuity equation. Forward stroke volume in the aorta was calculated with a velocity-encoded phase-difference technique to determine flow volume in the aorta. Total left ventricular stroke volume was derived from the left ventricular end-diastolic and end-systolic frames on gradient-echo imaging from multiple short-axis slices. Regurgitant volume and regurgitant fraction by this technique correlated closely with invasive measurements at near-simultaneous catheterization. Even if precise measurements of aortic and mitral stroke volumes were present with MRI or quantitative Doppler, this application of the continuity equation has one major flaw. It measures total regurgitant volume and therefore cannot be used to distinguish the amount of mitral regurgitation from aortic regurgitation in patients with both lesions.
A different application of the continuity equation involves imaging of the proximal isovelocity surface area (PISA) of a regurgitant jet.8 9 10 For jets emerging through small circular orifices in a flat plate, flow accelerates just proximal to the orifice, converging on the orifice in hemispheric shells of equal velocity. The surface area of a hemisphere is 2
r2, where r is the radius from the orifice center to the hemisphere. Because Doppler color flow signals tend to alias at relatively low velocities, a hemispheric red-blue aliasing line appears proximal to the regurgitant orifice, such that regurgitant flow can be calculated as 2r2V, where V is the aliasing velocity displayed on the instrument. Unlike the comparison of flow across different cardiac valves, the PISA technique has the advantage of being specific for the regurgitant jet being imaged. Unfortunately, the clinical application of this technique is limited by the fact that most regurgitant orifices in patients are neither flat nor circular, although correction factors for various valve geometries have been proposed.9 10 In addition, the alising region is not always hemispheric, and it is often difficult to precisely define its radius. Because any small error in measurement of r is squared, substantial errors in calculation of regurgitant flow can occur. A more general method using the control volume theory was recently described by Walker et al11 in which no geometrical assumptions are made about the region of flow convergence and MRI is used to measure the flow across any surface positioned on the proximal side of the orifice. Control volume methods could also be applied to the region of the jet laminar core.
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Conservation of Momentum |
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TopIntroductionConservation of Mass: The...Conservation of MomentumThe Jet Laminar Core:...Potential ProblemsConclusionsReferences |
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Thomas et al12 demonstrated that the momentum of an axisymmetric free jet could be calculated from a transverse velocity profile across the jet with the following formula:
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The Jet Laminar Core: A New Approach? |
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TopIntroductionConservation of Mass: The...Conservation of MomentumThe Jet Laminar Core:...Potential ProblemsConclusionsReferences |
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In this issue of Circulation, Diebold et al14 report the use of laser Doppler anemometry to characterize the central laminar core of pulsatile axisymmetric free jets. In theory, all fully developed turbulent jets exhibit a central core of laminar flow, the length (l) of which is proportional to the diameter (d) of the orifice (assuming a circular orifice) according to the formula l=kd, where k is an empiric constant. The laminar core is essentially a cyclinder of uniform diameter containing equal velocities throughout its volume. If an imaging technique were able to display the laminar core with an adequate spatial and velocity resolution, valvular regurgitation could be quantified in one of two ways. First, simple measurement of the length of the laminar core could be used to derive the diameter of the regurgitant orifice if the constant k is 4, as shown by Diebold et al.14 Although this should be applicable to free jets through circular orifices, it is not yet clear whether k=4 for noncircular orifices, eccentric jets, confined jets, and jets impinged by counterflows. Second, regurgitant volume could be precisely calculated as the product of the cross-sectional area of the laminar core multiplied by its velocity.
Unfortunately, current Doppler color flow imagers are incapable of displaying the laminar core of a regurgitant jet because aliasing occurs at velocities of 0.5 to 1.0 cm/s, far below the velocity of the laminar core. MRI is not limited by aliasing but has signal loss in voxels, which contain a wide range of velocities.15
To take advantage of the favorable physics of the laminar core of a regurgitant jet, new imaging modalities must be developed. It is theoretically possible to design an imaging system that can resolve the laminar core with sufficient spatial and temporal resolution. For example, a technique known as time-domain speckle tracking can be applied to ultrasound or radiofrequency data to accurately record velocity vectors without aliasing or angle dependency.16 Time-domain speckle tracking is based on detecting sum absolute differences in successive frames on a pixel-by-pixel basis. The feasibility of this method and its angle independence and resistance to aliasing have been demonstrated in prototype ultrasound systems.17 18 Such a technique, applied to either Doppler color flow or MR velocity mapping or a form of MRI bolus tracking, could display the laminar core of any intracardiac jet, giving its diameter and component velocities. This could allow calculation of the regurgitant volume for each specific jet in patients with multiple jets or in patients with combined aortic and mitral regurgitation. Furthermore, this technique could allow calculation of cardiac output or shunt flow across an atrial or a ventricular septal defect.
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Potential Problems |
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TopIntroductionConservation of Mass: The...Conservation of MomentumThe Jet Laminar Core:...Potential ProblemsConclusionsReferences |
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Although the study by Diebold et al14 suggests that the physics of the jet laminar core are favorable for quantification of valvular regurgitation, several factors remain uncertain. As acknowledged by the authors, only free jets through round orifices were studied, and the equations derived were applied only to peak flow conditions. Although pulsatile flow was used, it is not clear whether the flow profiles resembled mitral or aortic regurgitation. To determine whether the physics of the laminar core are stable under conditions of complex orifice geometry and impingement of the jet by adjacent cardiac structures or counterflows, further validation studies must be done in a physiological model.
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Conclusions |
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TopIntroductionConservation of Mass: The...Conservation of MomentumThe Jet Laminar Core:...Potential ProblemsConclusionsReferences |
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For a decade, Doppler color flow mapping has been used to assess valvular regurgitation. Despite the existence of several different methods based on sound physical principles, the promise of a simple, accurate, noninvasive, quantitative marker of valvular regurgitation has not been achieved due to technical limitations of the current instrumentation. Although MRI techniques appear to be highly accurate, there are technical problems to overcome. Academia and industry should work together, focusing efforts on developing ultrasound and MRI techniques that can resolve the laminar core of the regurgitant jet without aliasing or angle dependency. Then, we can finally get to the core of the matter of quantifying valvular regurgitation accurately and noninvasively.
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References |
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TopIntroductionConservation of Mass: The...Conservation of MomentumThe Jet Laminar Core:...Potential ProblemsConclusionsReferences |
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1. Croft CH, Lipscomb K, Mathis K, Firth BG, Nicod P, Tilton G, Winniford MD, Hillis LD. Limitations of qualitative angiographic grading in aortic or mitral regurgitation. Am J Cardiol.. 1984;53:1593-1598.[Medline] [Order article via Infotrieve]
2. Spain MG, Smith MD, Grayburn PA, Harlamert EA, DeMaria AN. Quantitative assessment of mitral regurgitation by Doppler color flow mapping: angiographic and hemodynamic correlations. J Am Coll Cardiol.. 1989;13:585-590.[Abstract]
3. Sahn DJ. Instrumentation and physical factors related to visualization of stenotic and regurgitant jets by Doppler color flow mapping. J Am Coll Cardiol.. 1988;12:1354-1365.[Abstract]
4.
Chen C, Thomas JD, Anconina J, Harrigan P, Mueller L, Picard MH, Levine RA, Weyman AE. Impact of impinging wall jet on color Doppler quantification of mitral regurgitation. Circulation.. 1991;84:712-720.
5. Enriquez-Sarano M, Tajik AJ, Bailey KR, Seward JB. Color flow imaging compared with quantitative Doppler assessment of severity of mitral regurgitation: influence of eccentricity of jet and mechanism of regurgitation. J Am Coll Cardiol.. 1993;21:1211-1219.[Abstract]
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Enriquez-Sarano M, Bailey KR, Seward JB, Tajik AJ, Krohn MJ, Mays JM. Quantitative Doppler assessment of valvular regurgitation. Circulation.. 1993;87:841-848.
7.
Hundley WG, Li HF, Willard JE, Landau C, Lange RA, Meshack BM, Hillis LD, Peshock RM. Magnetic resonance imaging assessment of the severity of mitral regurgitation: comparison with invasive techniques. Circulation.. 1995;92:1151-1158.
8. Utsunomiya T, Ogawa T, Doshi R, Patel D, Quan M, Henry WL, Gardin JM. Doppler color flow `proximal isovelocity surface area' method for estimating volume flow rate: effects of orifice shape and machine factors. J Am Coll Cardiol.. 1991;17:1103-1111.[Abstract]
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Rodriguez L, Anconina J, Flaschkampf FA, Weyman AE, Levine RA, Thomas JD. Impact of finite orifice size on proximal flow convergence: implications for Doppler quantification of valvular regurgitation. Circ Res.. 1992;70:923-930.
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Walker PG, Oyre S, Pedersen EM, Houlind K, Guenet FSA, Yoganathan AP. A new control volume method for calculating valvular regurgitation. Circulation.. 1995;92:579-586.
12.
Thomas JD, Liu CM, Flaschkampf FA, O'Shea JP, Davidoff R, Weyman AE. Quantification of jet flow by momentum analysis: an in vitro color Doppler flow study. Circulation.. 1990;81:247-259.
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15. Simpson IA, Maciel BC, Moises V, Shandas R, Elias W, Valdes-Cruz L, Hesselink JR, Chung KJ, Sahn DJ. Cine magnetic resonance imaging and color Doppler flow mapping displays of flow velocity, spatial acceleration, and jet formation: a comparative in vitro study. Am Heart J.. 1993;126:1165-1174.[Medline] [Order article via Infotrieve]
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