(Circulation. 1997;96:3409-3415.)
© 1997 American Heart Association, Inc.
Articles |
Grading of Mitral Regurgitation by Quantitative Doppler Echocardiography
Calibration by Left Ventricular Angiography in Routine Clinical Practice
From the Division of Cardiovascular Diseases and Internal Medicine (K.S.D., M.E.-S., R.A.N., J.B.S., A.J.T.) and the Section of Biostatistics (K.R.B.), Mayo Clinic and Mayo Foundation, Rochester, Minn.
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Abstract |
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Background Quantitative Doppler echocardiography and proximal flow convergence methods are validated techniques for quantifying mitral regurgitation. However, the clinical interpretation of the values calculated is hindered by the absence of calibration of ranges of severity in large numbers of patients.
Methods and Results In 180 consecutive patients (men, 62%; mean age±SD, 66±11 years), the results of Doppler quantification of isolated mitral regurgitation were calibrated by use of left ventricular angiographic grading performed within 3 months in routine practice and without intervening events. The thresholds of the quantitative variables corresponding to the angiographic grades were identified by maximizing the sum of sensitivity and specificity and minimizing their difference. The mitral regurgitation grade by angiography was 2.7±1.3. The mean value and correlation with angiographic grades for effective regurgitant orifice were 43±37 mm and r=.79 (P<.0001); for regurgitant volume, 62±45 mL and r=.80 (P<.0001); and for regurgitant fraction, 45±17% and r=.78 (P<.0001). Despite some overlap, differences between mitral regurgitation grades were all significant (all P<.05). The thresholds for severe mitral regurgitation (grade 4) were 60 mL, 50%, and 40 mm2 for regurgitant volume, regurgitant fraction, and orifice, respectively.
Conclusions In routine practice in large numbers of patients in a clinical laboratory, Doppler echocardiographic quantification of mitral regurgitation shows highly significant correlation with qualitative angiographic grades. Despite an expected overlap between classes, the calibration by angiography of grading ranges for the quantitative variables provides a framework for their interpretation and allows the definition in clinical practice of thresholds for severe mitral regurgitation.
Key Words: angiography • echocardiography • mitral valve • regurgitation
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Introduction |
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In patients with mitral regurgitation, the degree of regurgitation is a major determinant of the natural history. Patients with mild mitral regurgitation have excellent survival,1 whereas those with severe regurgitation observed medically experience excess mortality and high morbidity.2 Surgery, especially repair,3 can treat mitral regurgitation successfully; however, it carries a small but definite risk4 and therefore should be reserved for patients with a severe degree of mitral regurgitation. Consequently, determining the degree of regurgitation is a crucial part of the clinical evaluation of patients with mitral regurgitation.4 5 Although color flow Doppler echocardiography is very sensitive, it is limited by several pitfalls in assessing the degree of mitral regurgitation.6 7 8
To define the degree of regurgitation quantitatively, new methods9 10 and new concepts11 12 using Doppler echocardiography have allowed the measurement of RVol, RF,13 14 15 and ERO, a measure of lesion severity.11 12 These methods have been validated and their accuracy verified in confirmatory studies.11 12 13 14 15 They can be used alternatively or in combination to define the degree of mitral regurgitation as values of RVol, RF, and ERO. However, because of their more recent application, a calibration and clinical perception of the values of these quantitative variables—particularly the thresholds corresponding to severe mitral regurgitation—have not been fully developed. Preliminary values of these thresholds based on the physiological consequences of regurgitation have been suggested,12 but because of the limitations of pilot studies (ie, small numbers and possible selection bias), the calibration of a gradation framework in routine practice is warranted.
Because qualitative angiography has long been used in routine practice to assess mitral regurgitation,16 17 a clinical perception of the grades of mitral regurgitation, which is widely shared and comprehended, has formed despite the pitfalls of the method.18 Therefore, notwithstanding these pitfalls and the fact that angiographic grading is not a gold standard, it is a unique method to calibrate newer quantitative methods and to translate the perception of angiography to quantitative variables. Performed in routine practice, to avoid selection biases, this calibration should allow development of a framework for interpreting quantitative measurements of mitral regurgitation.
Therefore, our prospective experience with both Doppler echocardiographic quantification and angiographic assessment of mitral regurgitation obtained in routine practice was used to determine (1) thresholds of quantitative variables corresponding best to each angiographic grade and (2) a definition, based on these criteria, of severe mitral regurgitation as encountered in clinical practice.
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Methods |
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Study Patients
The inclusion criteria were that (1) patients had isolated, pure mitral regurgitation; (2) they underwent routine transthoracic echocardiographic quantification of mitral regurgitation by either quantitative Doppler or the flow-convergence method or both between September 1990 and March 1996; and (3) they had left ventricular angiography and quantitative Doppler echocardiography within 3 months of each other. Excluded were patients who (1) developed new cardiac events or underwent a therapeutic cardiac intervention such as valve repair or replacement, bypass surgery, or percutaneous angioplasty between the echocardiographic examination and catheterization; (2) had associated aortic valve disease, mitral stenosis, or constrictive pericarditis; or (3) had a change in systolic blood pressure
30 mm Hg
between the two studies.
Echocardiographic Analysis
A comprehensive Doppler echocardiographic
examination was performed and analyzed as described
previously.19 20 21 The mechanism of mitral
regurgitation was determined on the basis of the
two-dimensional appearance of the left ventricle, subvalvular
apparatus, and valve leaflets and the dimension of the
mitral annulus. Organic mitral regurgitation was
characterized by intrinsic valvular disease, and
ischemic/functional mitral regurgitation was
characterized by normal valves, enlarged annulus, and global or
regional left ventricular dysfunction. Quantification of
mitral regurgitation was performed by two methods.
Quantitative Doppler method. As previously described,15 20 two-dimensional and Doppler echocardiography were used to make annular cross-sectional area and TVI measurements to calculate the mitral and aortic stroke volumes. Next, the Doppler RVol, RF, and ERO were calculated as follows: RVol=(mitral-aortic) stroke volume; RF=RVol/mitral stroke volume; and ERO=RVol/regurgitant TVI, where regurgitant TVI is the TVI (ie, stroke distance in centimeters) of the mitral regurgitant jet obtained by continuous-wave Doppler.
Proximal isovelocity area method. This method used
the proximal flow convergence22 23 24 and was performed as
previously described24 : regurgitant
flow=2
xr2xVr, where r is radius of
proximal flow convergence and Vr is aliasing velocity;
ERO=regurgitant flow/peak regurgitant velocity; RVol=EROxregurgitant
TVI; and RF=RVol/[(regurgitant+aortic) stroke volumes].
Final quantitative values. The final quantitative values of RVol, RF, and ERO were either the mean of those calculated by the two methods or the values calculated by the method used. Both methods were used in 40 patients, quantitative Doppler was used exclusively in 85, and proximal flow convergence was used exclusively in 55.
Angiographic Assessment
In each patient, left ventriculography was performed in a
biplane 30° right and 60° left anterior oblique projection with
50 mL of iopamidol (100% concentrated) injected over 3 to 4 seconds
and recorded on a 35-mm cinefilm at 60 frames per second. The
angiographic severity of mitral regurgitation was
graded according to a historically accepted grading
scheme16 17 in four grades (1, 2, 3, and 4, from mild to
severe). The left ventricular angiogram was interpreted at
the time of catheterization by an experienced
angiographer who reviewed the 35-mm film without knowledge that the
data would be analyzed for the present study.
Statistical Methods
Group results were reported as mean±SD for continuous
variables and as percentages for categorical variables. Group
comparison relied on the standard t test or, for multiple
group comparisons, on ANOVA. The relation between angiographic severity
of mitral regurgitation and RVol, RF, and ERO was
examined by use of the Spearman rank-order correlation.
For each quantitative variable (RVol, RF, and ERO), thresholds optimally separating the continuous quantitative variables in correspondence with the angiographic grades were determined sequentially for three levels, that is, for separation of grade 1 from 2, grades 1 and 2 from grades 3 and 4, and grade 3 from 4. The thresholds that best separated these angiographic grades were determined by testing the whole range of each variable by increments of 5 units (ie, 5 mL of RVol, 5% of RF, 5 mm2 of ERO) and determining for each level the sensitivity, specificity, negative and positive predictive values, odds ratio, and negative and positive likelihood ratios.25 The candidate thresholds that were considered to best separate the continuous quantitative variables in correspondence with the angiographic grades were those with the highest sum of sensitivity and specificity and the lowest value of their difference. If more than one value met these criteria, the final choice was made by prioritizing sensitivity. However, the results of alternative choices of interest were also presented. ROC curves were calculated for the diagnosis of grade 3 or 4 mitral regurgitation for the three quantitative variables, and the areas under the curves, which represent the diagnostic value of the quantitative variables, were compared by the paired t test. To determine the range of uncertainty of the thresholds between grades, the analysis was repeated with logistic regression between the angiographic grade (dependent, classified as grade 1 versus 2 to 4, 1 to 2 versus 3 to 4, and 1 to 3 versus 4) and quantitative degree of mitral regurgitation (independent), and the thresholds for each grade with their standard errors (by ANCOVA) were determined. All probability values were two-tailed, and a value of P<.05 was considered significant.
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Results |
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Patient Characteristics
The 180 patients (112 men and 68 women) included were 26 to 89 years old (mean, 66±11 years). The mechanism of mitral regurgitation was ischemic/functional (n=84) or organic (n=96). Of these patients, 133 were in sinus rhythm, 39 in atrial fibrillation, and 8 in paced rhythm. The absolute value of the delay between the echocardiographic study and angiography was 34±8 days. Systolic blood pressure was 130±20 mm Hg at echocardiography and 134±22 mm Hg at angiography (P<.001). The difference was small but, because of the large number of patients, reached statistical significance. The RVol, RF, and ERO in the overall population are presented in Table 1
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Calibration of Echocardiographic Quantification
With Angiographic Grades
The mitral regurgitation grade by angiography was
2.7±1.3. The distribution of patients among the four angiographic
grades and the values of RVol, RF, and ERO for each angiographic grade
are summarized in Table 1
and demonstrate significant differences
between grades (overall P<.001, P<.001, and
P<.001 for RVol, RF, and ERO, respectively; all-group
comparison, P<.05). Significant correlation was found
between angiographic grades and ERO (r=.79,
P<.0001) (Fig 1), RVol
(r=.80, P<.0001) (Fig 2), and RF (r=.78,
P<.0001) (Fig 3). There were
notable overlaps between grades (Figs 1 through 3), as expected, but
despite this, the differences between each angiographic grade in terms
of RVol, RF, and ERO were significant (Table 1, all P<.05).
Possible threshold values for ERO, RVol, and RF corresponding to each
angiographic grade are listed in Table 2,
with the descriptors of their diagnostic values. The
thresholds that were finally selected as best separating the grades at
each level are in boldface in Table 2 and summarized in Table 3. The superimposition of the selected
thresholds on the scatterplots is shown in Figs 1B, 2B, and 3B for ERO,
RVol, and RF, respectively. The area under the ROC curves for the
diagnosis of grade 3 to 4 mitral regurgitation was 0.93
for RVol, which was not significantly different from 0.92 for ERO
(P=.08) and 0.90 for RF (P=.08) (Fig 4). The use of logistic regression to
determine the thresholds for the four grades yielded results similar to
those of the analysis based on the diagnostic value
and, most importantly, demonstrated that despite the overlap, the
standard errors of the thresholds were narrow (for RVol, 28±2.2,
47±2.7, and 62±3.4 mL; for RF, 31±3.4%, 42±1.4%, and 49±1.4%;
for ERO, 17±2, 31±2.2, and 43±2.7 mm2 for the
thresholds of grades 2, 3, and 4, respectively). No interaction was
found between the type of regurgitation (organic or
ischemic/functional) and the thresholds defined
(P=.08, .63, and .19, respectively, for RVol, RF, and
ERO).
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Discussion |
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The present study calibrated quantitative echocardiographic measures of degree of mitral regurgitation using angiographic grades and (1) defined ranges of the quantitative indexes corresponding with satisfactory diagnostic value to angiographic grades and (2) determined that thresholds for severe (grade 4) mitral regurgitation are RVol
60 mL per beat, RF 50%, and
ERO 40 mm2.
Importance of Grading Mitral Regurgitation
The degree of mitral regurgitation is a
determinant of outcome in terms of both morbidity and mortality.
Patients with mild mitral regurgitation do not usually
develop left ventricular remodeling,12 26
whereas left ventricular dysfunction is a frequent and
serious complication of severe mitral
regurgitation.4 5 Moreover, in patients
with predominantly mild regurgitation, survival is
excellent,1 whereas in those with predominantly severe
regurgitation, excess mortality and high morbidity are
noted.2 Surgical correction of mitral
regurgitation is highly successful3 ;
however, the operative risk of valve repair or replacement, despite
recent improvements, is still not negligible.3 4 27
Therefore, surgery, particularly for patients with no or minimal
symptoms,28 should be considered mostly for patients with
well-documented severe mitral regurgitation.
Methods of Grading Mitral Regurgitation
Semiquantitative methods for the assessment of mitral
regurgitation often show disagreement,29
and all these techniques have important theoretical and practical
limitations. Semiquantification by the
echocardiographic color flow-jet area method has
pitfalls in that it tends to underestimate the severity of eccentric
jets6 7 and to overestimate the severity of central
jets.8 Quantitative methods allow the measurement of RVol
and RF to assess volume overload and also the measurement of ERO area,
a surrogate of lesion severity.11 12 Because quantitative
methods have only recently been introduced into routine
practice,9 10 13 15 a clinical comprehension and
perception of the values calculated have not yet developed. Calibration
of quantitative indexes was tentatively defined by determining
thresholds associated with marked degrees of left
ventricular enlargement and elevation of pulmonary
artery pressure,12 14 but full documentation of a
gradation framework is warranted.
Left ventricular angiography can define the degree of mitral regurgitation based on the retrograde opacification of the left atrium,16 17 but it is limited by its invasive nature and small inherent risk30 31 and thus cannot be used for regular follow-up. Another limitation is the frequent disagreement with the findings of other methods,29 and even in simultaneous studies, only modest correlations with quantitative indexes have been obtained.18 32 33 Therefore, the capacity of left ventricular angiography as a gold standard and accurate reference method in validation studies is questionable. However, one of its major merits is the historic use, which has created a clinical perception of the semiquantitative grading scale that is used to report angiographic degrees of mitral regurgitation.16 17 29 Therefore, left ventricular angiography is an essential tool for calibration of quantitative methods to define corresponding grading frameworks34 and to translate the perception of angiography to values reported by quantitative methods.
Pilot studies on new quantitative methods did not define ranges of degree of mitral regurgitation, for several reasons. First, the studies consisted of small populations of patients.12 13 15 35 36 Second, the pitfalls of the quantitative methods were not fully determined in the initial pilot studies, precluding meaningful comparison with angiography,9 10 37 but more recent studies have defined the field of applicability of the quantitative methods.12 14 22 38 Third, restrictive inclusion criteria of pilot studies23 24 35 36 necessary for validation of quantitative methods may induce a selection bias,39 40 leading to results that may not reflect the framework of gradation in routine practice. Therefore, calibration of quantitative measurements of mitral regurgitation using angiographic grades obtained in routine practice, as reported for the first time here, is essential. By design, without restrictive inclusion, a notable overlap between grades is expected.18 41 42 Despite this expected overlap, adequate correlations were found. Also, all angiographic grades showed significant differences in RVol, RF, and ERO, demonstrating that a statistical (in addition to visual) separation between grades is present. Furthermore, the diagnostic values of the thresholds defined in this study were good, allowing these thresholds to be used for the interpretation of quantitative data in routine clinical practice. Of note, regurgitation may not always be graded similarly by ERO and by RVol or RF. For any given ERO, higher afterload may result in higher RVol.43 Nevertheless, the present framework should allow enhanced comprehension of the results of quantitative methods and, in particular, of the role of lesion severity11 12 and loading conditions26 43 in the overall severity of mitral regurgitation.
What Is Severe Mitral Regurgitation?
The ROC curves in Fig 4 suggest that the quantitative methods are
an excellent test to distinguish grade 1 to 2 from grade 3 to 4. This
implies that several candidate threshold values can be identified that
have both good sensitivity and good specificity to separate between
grades, as reported in Table 2. For example, for the threshold
separating grade 3 from grade 4, an ERO of 50 mm2
prioritized specificity but had a low sensitivity, and a threshold of
40 mm2 prioritized sensitivity with some loss in
specificity. The final choice is to be made by each physician depending
on the goals defined for routine clinical practice. The boldface values
in Table 2 represent our final choice, based mostly on
prioritizing sensitivity for severe degrees of mitral
regurgitation.
Table 3 summarizes these choices. The thresholds of RVol 60 mL/beat,
RF 50%, and ERO area 40 mm2 provide a relatively
high sensitivity for severe, grade 4 mitral
regurgitation. The corresponding thresholds for mitral
regurgitation of grade 3 or 4 are 45 mL/beat, 40%, and
30 mm2, respectively. These thresholds are close to
the ranges defined previously on the magnitude of the
pathophysiological consequences of mitral
regurgitation.12 They are also
consistent with the limited data previously available with
other quantitative methods.32 33 44 Ultimately, the exact
definition of severe mitral regurgitation should be
determined by the influence of the degree of volume overload (RVol and
RF) and lesion severity (ERO) on long-term outcome. However, follow-up
since the inception of the most applicable methods is limited and
cannot provide this crucial outcome information at the present
time.9 10 11 12 Therefore, the present calibration of
quantitative measures of the degree of mitral
regurgitation by angiography represents a
unique possibility of defining a framework of gradation and is
essential for the clinical use of quantitative data. Such a framework,
congruent with the long-used and well-perceived semiquantitative
gradation, should allow a better perception of the values calculated by
quantitative Doppler echocardiographic methods and
improve communication between physicians.29
Study Limitations
Not all patients in the present study had results of
quantification by two methods. However, the two Doppler
echocardiographic methods used to calculate ERO, RVol,
and RF show high correlations in the present study
(r=.94, P<.0001; r=.93,
P<.0001; r=.92, P<.0001,
respectively), as in previous studies,14 and no
significant difference in their results (P=.09,
P=.84, and P=.10, respectively). Also, the ROC
curves of RVol, RF, and ERO for the separation of grade 1 to 2 from
grade 3 to 4 did not differ between the two methods. Therefore, the
methods used do not represent a limitation but rather reflect
the use of quantification of regurgitation in routine
practice.
The patients included in the present study had chronic mitral regurgitation; therefore, the thresholds defined for severe mitral regurgitation apply primarily to chronic and not to acute mitral regurgitation. Future studies should analyze the pathophysiology of acute mitral regurgitation to better define specific diagnostic criteria.
A change in loading conditions between echocardiography and angiography could account for some of the misclassifications between quantification and angiographic grade observed in this study.43 Despite a significant difference between systolic blood pressure at the time of echocardiography and angiography, the absolute difference was small and the hemodynamic conditions were clinically similar. In routine practice, these tests are rarely performed simultaneously, and despite possible changes in loading conditions, the interpretation of the nonsimultaneous grading guides all clinical decisions. Therefore, the design of the present study is highly relevant to routine clinical practice.29
Furthermore, the aim of the present study was not to validate the quantitative methods, which have been endorsed in multiple studies from various medical centers,6 9 10 11 12 13 15 41 42 but rather to calibrate for the first time ranges of severity. Angiography may be conducive to misclassifications, which may partly explain the overlap observed,18 but these random misclassifications, which may reduce the coefficient of correlation, have little effect on the determination of the thresholds45 corresponding to the four grades and therefore do not hinder the present calibration. This is confirmed by the narrow standard error of the thresholds defined by the logistic regression, further emphasizing that the overlap does not represent a significant limitation of the present study.
Clinical Implications
Despite an overlap between angiographic grades, as expected in
routine practice, the diagnostic value of the quantitative
Doppler variables is excellent, as demonstrated by the ROC
curves. Therefore, in most cases, duplication of gradation does not
appear to be indispensable. The calibration of quantitative Doppler
echocardiographic measures of the degree of mitral
regurgitation by angiographic grading provides grading
ranges for the quantitative variables and should allow an improved
perception of the meaning and interpretation of the measured values. On
the basis of this calibration, patients with RVol 60 mL, RF 50%,
and ERO 40 mm2 are classified as having severe
mitral regurgitation.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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Dr Dujardin was supported by a fellowship from the Belgian American Educational Foundation.
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Footnotes |
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Reprint requests to Maurice Enriquez-Sarano, MD, Mayo Clinic, 200 First St SW, Rochester, MN 55905.
Received January 30, 1997; revision received May 29, 1997; accepted July 3, 1997.
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