This Article Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Grayburn, P. A. Articles by Peshock, R. M. Search for Related Content PubMed PubMed Citation Articles by Grayburn, P. A. Articles by Peshock, R. M. Pubmed/NCBI databases Medline Plus Health Information Heart Valve Diseases

(Circulation. 1996;94:119-121.)
© 1996 American Heart Association, Inc.

Articles

Noninvasive Quantification of Valvular Regurgitation

Getting to the Core of the Matter

Paul A. Grayburn, MD; Ronald M. Peshock, MD

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

	Introduction

Top Introduction Conservation of Mass: The... Conservation of Momentum The Jet Laminar Core:... Potential Problems Conclusions References

 
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.

	Conservation of Mass: The Continuity Equation

Top Introduction Conservation of Mass: The... Conservation of Momentum The Jet Laminar Core:... Potential Problems Conclusions References

 
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 2r², 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.

	Conservation of Momentum

Top Introduction Conservation of Mass: The... Conservation of Momentum The Jet Laminar Core:... Potential Problems Conclusions References

 
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:

(E1)

where M is momentum, v is velocity, and r is the radial distance from the jet centerline to the point at which each velocity was measured. Because momentum equals flow multiplied by velocity, regurgitant flow could be calculated by dividing downstream jet momentum by the orifice velocity determined with continuous wave Doppler. Although this method is theoretically sound for free jets, most regurgitant jets in clinical practice are constrained by adjoining walls, valves, or counterflows, and the method does not work well for bounded jets.¹³

	The Jet Laminar Core: A New Approach?

Top Introduction Conservation of Mass: The... Conservation of Momentum The Jet Laminar Core:... Potential Problems Conclusions References

 
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.

	Potential Problems

Top Introduction Conservation of Mass: The... Conservation of Momentum The Jet Laminar Core:... Potential Problems Conclusions References

 
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.

	Conclusions

Top Introduction Conservation of Mass: The... Conservation of Momentum The Jet Laminar Core:... Potential Problems Conclusions References

 
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.

	References

Top Introduction Conservation of Mass: The... Conservation of Momentum The Jet Laminar Core:... Potential Problems Conclusions References

 

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.[Abstract/Free Full Text]
   
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]
   
6. Enriquez-Sarano M, Bailey KR, Seward JB, Tajik AJ, Krohn MJ, Mays JM. Quantitative Doppler assessment of valvular regurgitation. Circulation.. 1993;87:841-848.[Abstract/Free Full Text]
   
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.[Abstract/Free Full Text]
   
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]
   
9. 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.[Abstract/Free Full Text]
   
10. Vandervoort PM, Thoreau DH, Rivera JM, Levine RA, Weyman AE, Thomas JD. Automated flow rate calculations based on digital analysis of flow convergence proximal to regurgitant orifices. J Am Coll Cardiol.. 1993;22:535-541.[Abstract]
    
11. 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.[Abstract/Free Full Text]
    
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.[Abstract/Free Full Text]
    
13. Grayburn PA, Cigarroa CG, Willett DL, Brickner ME. Quantitative assessment of simulated regurgitant flow using direct digital acquisition of Doppler color flow images. Echocardiography. In press.
    
14. Diebold B, Delouche A, Delouche P, Guglielmi J-P, Dumee P, Herment A. In vitro flow mapping of regurgitant jets: systematic description of free jet with laser Doppler velocimetry. Circulation.. 1996;94:000-000.
    
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]
    
16. Bohs LN, Trahey GE. A novel method for angle independent ultrasonic imaging of blood flow and tissue motion. IEEE Trans BME.. 1991;38:280-286.
    
17. Bohs LN, Friemel BH, McDermott BA, Trahey GE. Real-time system for angle-independent ultrasound of blood flow in two dimensions. Radiology.. 1993;186:259-261.[Abstract/Free Full Text]
    
18. Bohs LN, Friemel BH, Trahey GE. Experimental velocity profiles and volumetric flow via two-dimensional speckle tracking. Ultrasound Med Biol.. 1995;21:885-898.[Medline] [Order article via Infotrieve]
    

This article has been cited by other articles:

				J. Hung, J. Solis, J. L. Guerrero, G. J.C. Braithwaite, O. K. Muratoglu, M. Chaput, L. Fernandez-Friera, M. D. Handschumacher, V. J. Wedeen, S. Houser, et al. A Novel Approach for Reducing Ischemic Mitral Regurgitation by Injection of a Polymer to Reverse Remodel and Reposition Displaced Papillary Muscles Circulation, September 30, 2008; 118(14_suppl_1): S263 - S269. [Abstract] [Full Text] [PDF]

				J. Hung, M. Chaput, J. L. Guerrero, M. D. Handschumacher, L. Papakostas, S. Sullivan, J. Solis, and R. A. Levine Persistent Reduction of Ischemic Mitral Regurgitation by Papillary Muscle Repositioning: Structural Stabilization of the Papillary Muscle Ventricular Wall Complex Circulation, September 11, 2007; 116(11_suppl): I-259 - I-263. [Abstract] [Full Text] [PDF]

				J. Zamorano, L. P. de Isla, L. Oliveros, C. Almeria, J. L. Rodrigo, A. Aubele, J. Banchs, and C. Macaya Prognostic influence of mitral regurgitation prior to a first myocardial infarction Eur. Heart J., February 2, 2005; 26(4): 343 - 349. [Abstract] [Full Text] [PDF]

				J. Hung, J. L. Guerrero, M. D. Handschumacher, G. Supple, S. Sullivan, and R. A. Levine Reverse Ventricular Remodeling Reduces Ischemic Mitral Regurgitation: Echo-Guided Device Application in the Beating Heart Circulation, November 12, 2002; 106(20): 2594 - 2600. [Abstract] [Full Text] [PDF]

This Article Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Grayburn, P. A. Articles by Peshock, R. M. Search for Related Content PubMed PubMed Citation Articles by Grayburn, P. A. Articles by Peshock, R. M. Pubmed/NCBI databases Medline Plus Health Information Heart Valve Diseases