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(Circulation. 1997;95:548-550.)
© 1997 American Heart Association, Inc.

Articles

How Leaky Is That Mitral Valve?

Simplified Doppler Methods to Measure Regurgitant Orifice Area

James D. Thomas, MD

the Cardiovascular Imaging Center, Department of Cardiology, Cleveland (Ohio) Clinic Foundation.

Correspondence to James D. Thomas, MD, Department of Cardiology, F-15, Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195. E-mail thomasj@cesmtp.ccf.org.

Key Words: mitral valve • regurgitation • echocardiography • hemodynamics • Editorials

	Introduction

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
Twenty years ago there was limited need for precise quantification of mitral regurgitation (MR). With valve replacement the only surgical option, leading cardiologists recommended that intervention be "considered only for patients who are in functional classes III and IV and do not respond to medical management,"¹ even though that strategy was associated with a 20% in-hospital mortality rate² and often led to severe postoperative left ventricular failure. With the improvement and widespread availability of valve repair surgery, cardiologists have been encouraged to refer patients for intervention earlier in the course of their disease,³ reflecting the low risk for this procedure (0% mortality in 595 primary, isolated mitral valve repairs over the past 4 years at the Cleveland Clinic). Today, patients may be operated on while still completely asymptomatic, with the magnitude of MR and the appearance of occult left ventricular dysfunction⁴ being the principal events triggering intervention. Thus, it is of paramount importance that MR be quantified accurately over time so that surgery may be timed appropriately.

	`Eyeball' Methods

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
Unfortunately, although a host of quantitative techniques derived from echocardiographic, angiographic, nuclear, and magnetic resonance data are available to characterize the severity of regurgitation, in routine clinical practice these are applied with surprising inconsistency. For the most part, regurgitation is assessed by the "eyeball" method, whereby an observer grades, in a categorical, semiquantitative sense, some imaging modality, typically color Doppler echocardiography⁵ or contrast ventriculography,⁶ and arrives at an interpretation of the MR as mild, moderate, or severe. For many clinical applications, this approach works surprisingly well. An experienced observer can inspect a color Doppler echocardiogram and accurately categorize the severity of regurgitation despite an abundance of research demonstrating that color Doppler jet area is exquisitely sensitive to driving pressure, chamber constraint, instrumentation factors, and left atrial size.^{7 8}

	Quantitative Methods

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
To go beyond this simple categorical assessment of the severity of mitral regurgitation, a variety of echocardiographic techniques are available. Volumetric methods have been validated for more than a decade and consist of measuring flow through the regurgitant valve and subtracting from this the net forward output of the heart.⁹ These stroke volumes may be obtained by pulsed Doppler measurement of flow across the valves or by assessment by two-dimensional echocardiography of the change from end systole to end diastole.¹⁰ Recently, it has become possible to automatically measure flow through the cardiac structures by automated integration of color Doppler velocities throughout the left ventricular outflow tract or mitral annulus.¹¹ Despite the demonstrated accuracy of these techniques, however, they are rarely applied in clinical practice, primarily because of the rigor with which they must be performed. Measurements must be obtained from multiple imaging windows, and an error in any of the measurements is propagated throughout the calculations. In particular, the need to subtract one stroke volume from another greatly amplifies the relative error in defining the regurgitant volume.

	Proximal Convergence Analysis

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
An alternative and potentially simpler quantitative approach is the proximal convergence method. In the region proximal to a regurgitant orifice, flow is laminar and accelerates smoothly, forming concentric shells of decreasing surface area and increasing velocity. For flow into a small orifice in a flat surface, theoretical, in vitro,¹² and clinical^{13 14} studies have shown these contours to be hemispheric, and so flow rate is given by 2r²v, where r is the distance to a contour of velocity v, typically defined by the change in color at the aliasing boundary. Dividing peak flow rate by the maximal velocity through the orifice (obtained by continuous-wave Doppler) yields the regurgitant orifice area (ROA), the actual size of the "hole" in the valve^{15 16} and the most fundamental quantitative parameter of severity of regurgitation. It is analogous to the stenotic orifice area, on which cardiologists have come to rely in aortic and mitral stenosis to indicate surgery. Surprisingly, however, ROA has failed to achieve widespread use, largely because it has previously been difficult to measure, but proximal convergence analysis has made this more feasible. An ROA <0.1 cm² is generally negligible (and corresponds to 1+ MR by jet area or angiography¹⁵ ), whereas those >0.3 cm² (roughly 3+) will impose a significant volume load on the heart and those >0.5 cm² (4+) will usually require surgery. Serial measurement of ROA can detect ongoing valve destruction by endocarditis or track the benefit of afterload reduction to improve MR due to ventricular dysfunction. ROA can also be used to derive a number of "traditional" indexes, such as regurgitant volume and regurgitant fraction.

The proximal convergence method does have a number of limitations, most of which relate to the geometry of the flow convergence region. Close to the orifice, isovelocity contours flatten out, and the hemispheric formula will predictably underestimate the true flow rate.¹⁷ Conversely, nearby walls can push isovelocity contours outward and cause flow overestimation.¹⁸ Fortunately, simple formulas have been validated for both of these geometric distortions, allowing the flow convergence method to be applied in most clinical situations. Other potential limitations, such as the impact of noncircular orifices and variable ROAs throughout the cardiac cycle,¹⁹ although of theoretical concern, seem not to affect the practical application of the flow convergence method.

Despite this validation, however, the flow convergence method likewise is rarely applied in routine clinical practice. This is an issue of time. In a busy echocardiography laboratory, particularly with the decreasing reimbursement over the past 5 years, technicians and echocardiographers are required to process more patients in any given day. There simply is not time to take the requisite measurements to fully implement either the volumetric or proximal convergence method. In an effort to increase the percentage of patients in our laboratory having mitral ROA quantified, we have recently instituted a simplified version of the proximal convergence formula. If we assume that the pressure difference between the left ventricle and the left atrium in systole is 100 mm Hg (producing a 5-m/s regurgitant jet), then if the aliasing velocity is set to 40 cm/s, the ROA can be calculated quite simply as r²/2, where r is the distance to the aliasing contour. With this approach, the ROA can be measured in the vast majority of patients in <1 minute of additional imaging time, resulting in much more frequent quantification of MR.

	Visualization of the Vena Contracta

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
Into this panoply of quantitative techniques comes the direct visualization of vena contracta jet width, reported in this issue of Circulation by Hall et al.²⁰ Building on the work of Mele et al,²¹ this approach is so simple and the results so encouraging that one may wonder why it has not previously emerged as the clinical standard for severity of regurgitation. With this technique, one visualizes the narrowest extent of the regurgitant jet, called the vena contracta for the slight reduction in cross-sectional area as the jet passes through the valve, and measures the diameter of this region in one or two dimensions. Hall et al report clinically useful, although not perfect, correlations between vena contracta width and ROA defined by volumetric Doppler methods.

Critical to understanding this method is an appreciation of how spatial resolution is determined in echocardiography. Resolution is most accurate in the axial direction (along a scan line); in two-dimensional (non-Doppler) echocardiography, the very short imaging pulse can allow structures to be localized within about one ultrasonic wavelength of their true location (<0.5 mm for a 3-MHz signal). Degradation of resolution is seen when one uses Doppler imaging, which must send a longer pulse of ultrasound in order to better define the frequency shift due to blood velocity. Also, lateral resolution is always poorer than axial resolution, for two reasons: (1) with the finite number of scan lines (100 in a 90° sector), scan lines are separated by more than 2 mm at 10-cm depth; and (2) the ultrasound beam itself spreads out significantly, so that multiple scan lines may intersect a single point in the imaging field. Thus, one would anticipate that vena contracta width obtained from the parasternal long-axis view (which uses axial resolution) would be superior to data obtained from the apical windows, for which lateral resolution must be used. Recent progress in echocardiographic instrumentation allows multiple scan lines to be analyzed simultaneously with preservation of phase information; this results in significantly improved lateral resolution, but for the time being, it should be anticipated that axial imaging of the vena contracta zone will be more reliable than use of lateral resolution.

	Limitations of the Present Study

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
There are some important limitations to the study by Hall et al that should be borne in mind as users attempt this method clinically. First, the method does not claim to be a direct measurement of the ROA. Because the regurgitant mitral orifice is often an irregularly shaped structure (the mitral closure line most closely resembling a smile), only short-axis imaging could capture its complex shape, and Hall et al noted that in the short axis, it is difficult to distinguish the narrowest flow zone corresponding to the vena contracta. Thus, we are left with an indirect guide to separate patients into mild and severe regurgitation, but leaving a large middle ground (with vena contracta widths between 0.3 and 0.5 cm) that shows considerable scatter in actual ROA and must be evaluated with other more quantitative methods. It should also be recognized that actual visualization of the vena contracta zone is not a trivial exercise but rather one that requires considerable practice before it can be used with confidence. Careful adjustment of color Doppler gain and the tissue priority algorithm must be used in conjunction with the zoom mode to distinguish the vena contracta zone from the proximal convergence zone and the rapidly expanding distal jet. Applying the methods of Hall et al, sonographers at the Cleveland Clinic Foundation have begun searching for the vena contracta on routine clinical patients. Our experience is that the vena contracta is measurable in most cases and generally corresponds to the ROA calculated by the proximal convergence method. One relevant limitation is that the vena contracta is critically matched to the shape of the often irregular regurgitant orifice, so multiple measurements are necessary to fully describe it. In contrast, the proximal convergence field blurs out details of the orifice shape, so that at a radius of 5 to 10 mm from the orifice, the flow field is fairly symmetrical even for quite irregular orifices, and ROA can be calculated from a single imaging vantage.

	Approach to the Patient With MR

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
How should the patient with asymptomatic severe MR be handled in 1997? For patients with a "fixed" regurgitant orifice (typically from ruptured chordae, rheumatic disease, or endocarditis), hypertension should be treated, but aggressive use of afterload reduction is discouraged, because this may mask the development of symptoms and has not been shown to prevent ventricular dysfunction. Progressive end-systolic ventricular enlargement (>40 to 45 mm) should prompt consideration of surgery. The response of the ventricle to isotonic stress has recently been studied by exercise echocardiography, with the postexercise end-systolic volume shown to be better than any resting index in predicting ventricular function after valve repair.⁴ A patient whose ventricle enlarges with exercise should therefore be considered for surgery. Finally, there may be patients whose MR is so severe that valve repair should be undertaken even if the ventricle responds normally to exercise. An ROA >0.5 cm² typically yields a regurgitant fraction >50%. With the mortality from valve repair surgery near zero (and the likelihood of repair rather than replacement is an important part of the equation), should such a patient be handled differently from an equally asymptomatic patient with an atrial septal defect causing a 2:1 shunt, whom most cardiologists would advise to have surgery? This is not the present approach, but by routinely quantifying ROA in patients with MR, we may learn that this is a reasonable course to take. Indeed, recent data indicate that minimally symptomatic patients with a flail mitral valve suffer excessive mortality (6.3% per year) when managed medically, with surgery reducing this rate by about 71%.²²

	Conclusions

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 
Hall et al are to be congratulated for having validated more completely a simple color Doppler approach to the quantification of MR. What this approach lacks in quantitative rigor, it certainly makes up in ease of application. The most sophisticated and accurate methodology is of no value if the implementation is so daunting that only a few research laboratories use it in selected studies. We have come a long way from the nihilistic era when patients with MR were left alone until they were too sick to benefit from surgery; as we move into an era in which proactive surgery is recommended to selected asymptomatic patients, routine quantification by rapid methods like vena contracta imaging or the simplified proximal convergence method will be critical to fine-tuning our approach to the patient with mitral regurgitation.

	Footnotes

 
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

	References

Top Introduction `Eyeball' Methods Quantitative Methods Proximal Convergence Analysis Visualization of the Vena... Limitations of the Present... Approach to the Patient... Conclusions References

 

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