EXPERIMENTAL STUDY
Interaliasing distance of the flow convergence surface for determining mitral regurgitant volume: a validation study in a chronic animal model
Marta Sitges, MD*,
Michael Jones, MD ,
Takahiro Shiota, PhD, FACC*,
David L. Prior, PhD*,
Jian Xin Qin, MD*,
Hiroyuki Tsujino, MS*,
Fabrice Bauer, MD*,
Yong Jin Kim, MD*,
Dimitri Deserranno, BSME*,
Neil L. Greenberg, PhD*,
Lisa A. Cardon, RDCS*,
Arthur D. Zetts,
Mario J. Garcia, MD, FACC* and
James D. Thomas, MD, FACC*
* Cardiovascular Imaging Center, Department of Cardiology, The Cleveland Clinic Foundation, Cleveland, Ohio, USA
National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland, USA
Manuscript received December 31, 2000;
revised manuscript received May 10, 2001,
accepted June 25, 2001.
Reprint requests and correspondence: Dr. Takahiro Shiota, Department of Cardiology/Desk F-15. The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, Ohio 44195
shiotat{at}ccf.org
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Abstract
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OBJECTIVES
We aimed to validate a new flow convergence (FC) method that eliminated the need to locate the regurgitant orifice and that could be performed semiautomatedly.
BACKGROUND
Complex and time-consuming features of previously validated color Doppler methods for determining mitral regurgitant volume (MRV) have prevented their widespread clinical use.
METHODS
Thirty-nine different hemodynamic conditions in 12 sheep with surgically created flail leaflets inducing chronic mitral regurgitation were studied with two-dimensional (2D) echocardiography. Color Doppler M-mode images along the centerline of the accelerating flow towards the mitral regurgitation orifice were obtained. The distance between the two first aliasing boundaries (interaliasing distance [IAD]) was measured and the FC radius was mathematically derived according to the continuity equation (Rcalc = IAD/(1 –   ), v1 and v2 being the aliasing velocities). The conventional 2D FC radius was also measured (Rmeas). Mitral regurgitant volume was then calculated according to the FC method using both Rcalc and Rmeas. Aortic and mitral electromagnetic (EM) flow probes and meters were balanced against each other to determine the reference standard MRV.
RESULTS
Mitral regurgitant volume calculated from Rcalc and Rmeas correlated well with EM-MRV (y = 0.83x + 5.17, r = 0.90 and y = 1.04x + 0.91, r = 0.91, respectively, p < 0.001 for both). However, both methods resulted in slight overestimation of EM-MRV ( was 3.3 ± 2.1 ml for Rcalc and 1.3 ± 2.3 ml for Rmeas).
CONCLUSIONS
Good correlation was observed between MRV derived from Rcalc (IAD method) and EM-MRV, similar to that observed with Rmeas (conventional FC method) and EM-MRV. The Rcalc using the IAD method has an advantage over conventional Rmeas in that it does not require spatial localization of the regurgitant orifice and can be performed semiautomatedly.
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Abbreviations and Acronyms
| | CFD | = computational fluid dynamics | | CW | = continuous wave | | FC | = flow convergence | | IAD | = interaliasing distance | | MRV | = mitral regurgitant volume | | Rcalc | = calculated radius | | Rmeas | = measured radius | | RF | = regurgitant fraction | | ROA | = regurgitant orifice area | | 2D | = two-dimensional | | VTI | = velocity-time integral |
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Accurate assessment of valvular regurgitation severity is of paramount importance, particularly mitral regurgitation, in which early diagnosis and correct timing for surgical intervention become essential (1,2). Despite evidence showing the accuracy of different quantitative methods (3–6), the proper quantification of mitral regurgitation remains an unresolved issue in the setting of a busy echocardiography laboratory. Complex and time-consuming features of pre-
viously validated methods for determining the severity of regurgitant valvular lesions have limited their widespread clinical use.
The flow convergence (FC) or proximal isovelocity surface area method is one of the most popular methods, especially when simplified approaches are applied (7,8). This method is available in any cardiac ultrasound system equipped with color Doppler technology and has been proved to be accurate for quantifying regurgitant lesions in both in vitro (9) and in vivo models (10,11). However, besides other limitations, the correct localization of the regurgitant orifice is an important source of error (12).
We propose a new method for quantifying valvular regurgitation based on the FC principle that eliminates the need to locate correctly the regurgitant orifice. Therefore, the aim of the present study was to validate this new methodology for determining mitral regurgitant volume (MRV) and orifice area in an animal experimental model with strictly quantified chronic mitral regurgitation. We also compared the accuracy of this new methodology with the previously validated conventional FC method.
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Methods
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Animal preparation.
Twelve sheep weighing 54.3 ± 12.2 kg were studied. Two or three secondary mitral chordae tendinae were surgically severed under direct vision using cardiopulmonary bypass to create mitral regurgitation. The preoperative, intraoperative and postoperative animal management and care were performed according to the Animal Care and Use Committee of the National Heart, Lung and Blood Institute and are extensively described elsewhere (13).
Six to eight months after the initial surgery, the sheep were again transferred to the laboratory where the present study was performed. Anesthesia was induced with pentobarbital 25 mg/kg and maintained using isoflurane 1% to 2%. Animals were intubated and mechanically ventilated. A left trans-sternal thoracotomy was performed, the pericardium was incised and the heart was suspended in a pericardial cradle.
Electromagnetic flow meters and experimental protocol.
Cardiopulmonary bypass was instituted and the left atrium was opened. An electromagnetic flow probe (model EP455c, Carolina Medical Electronics, Inc., King, North Carolina) was sutured above the mitral annulus, the left atrium was closed and cardiopulmonary bypass was suspended. Another electromagnetic flow probe (model EP455c, Carolina Medical Electronics, Inc.) was placed around the skeletonized ascending aorta. Both flow probes were connected to electromagnetic flow meters (model FM501, Carolina Medical Electronics, Inc.) and these were connected to a recorder (model ES 2000, Gould, Inc., Cleveland, Ohio). Calibration of the electromagnetic flow probes was performed as described elsewhere (14), according to the method proposed by Dent et al. (15). The difference between both stroke volumes obtained by the aortic and mitral flow probes and meters was used to determine the reference MRV. Regurgitant orifice area (ROA) was calculated by dividing the MRV by the velocity-time integral (VTI) of the regurgitant flow obtained from the continuous wave Doppler recordings. Regurgitant fraction (RF) was expressed as the percent of MRV divided by the total stroke volume at the mitral annulus level.
After baseline recordings, different degrees of mitral regurgitation were obtained by changing preload and afterload using blood infusion, angiotensin II and nitroprusside, as previously described (14). All measurements from the electromagnetic flow meters were obtained simultaneously with the echocardiographic recordings.
2D color Doppler echocardiography.
Two-dimensional (2D) color Doppler echocardiography was performed using a commercially available system (Power Vision, Toshiba Corp, Japan) from an epicardial window. Care was taken to adjust the color gain to avoid artifact from surrounding areas without flow. Color sector size was set at less than 45 degrees to maximize the frame rate. Aliasing velocities from 38 to 63 cm/s were selected to visualize the FC region from the apical view. When an optimal FC shape was obtained, the M-mode scan line was placed across it. Then, color Doppler M-mode images along the centerline of the accelerating flow towards the regurgitant orifice were obtained using the same Nyquist limit and magnified images using the system’s zoom function of the color Doppler M-mode spectrum were recorded. Continuous wave Doppler recordings of the regurgitant flow were also performed in order to measure peak regurgitant velocity and VTI. Data were digitally stored for later offline analysis.
Flow convergence method.
Visualization of the FC region was optimized by using the zoom-in function and adjusting the Nyquist velocity limit in order to obtain a hemispheric shape. The radius (Rmeas) between the first aliasing contour and the center of the regurgitant orifice was measured using the largest convergence image. Location of the regurgitant orifice was assessed under the guidance of gray-scale 2D images without color-encoded flow velocities. According to the conservation of mass principle applied to the FC phenomenon, MRV was calculated as follows:
where Rmeas is the radius measured from the first velocity contour to the regurgitant orifice, V equals to the first aliasing velocity, VTI is the velocity-time integral and Vmax is the peak regurgitant velocity. A previously validated correction factor ( /180), where equals the convergence angle between the two leaflets where the FC region is confined, was applied to avoid overestimation caused by proximal flow constraint in this particular flail leaflet animal model (9,16).
Then, ROA was derived as:
Interaliasing distance method.
The distance between the two first aliasing boundaries, that is, the interaliasing distance (IAD), of the flow convergence region was measured from the color Doppler M-mode images obtained along the centerline of the flow convergence region (Fig. 1). Then the FC radius was mathematically derived (Rcalc) according to the continuity equation as follows (Fig. 2):
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Figure 1 Example of a color Doppler M-mode tracing (left) obtained through the flow convergence region as seen by the two-dimensional color Doppler image (right). The distance between the two first aliasing boundaries (interaliasing distance [IAD]) shown by the two arrows, is measured and the radius of the flow convergence zone mathematically derived (see text).
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Figure 2 Rationale and mathematical derivation of the flow convergence radius (Rcalc) using the interaliasing distance (IAD).
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By continuity,
And if
Then, substituting R2 by R1 – IAD,
And solving for R1,
where R1 represents Rcalc. If baseline shift of the color Doppler velocity map is set at zero, then the second aliasing velocity (V2) is twice the first aliasing velocity (V1), so then,
And the equation can be simplified to:
Because a certain degree of variability in the regurgitant orifice area has been reported during systole (17,18), the interaliasing distance was measured at the time of the peak regurgitant velocity obtained from the CW Doppler spectrum recordings. This timing has been reported to be the most accurate for estimating regurgitant flow, especially in the presence of mitral prolapse (19).
Once Rcalc was derived using this methodology, MRV and ROA were estimated according to the FC method described above, being MRV = 2 Rcalc2·V·VTI/Vmax· (/180) and ROA = MRV/VTI. The same angle correction (/180) described for the conventional FC method in the previous paragraph was applied here to avoid overestimation by flow constraint when necessary.
Additionally, in order to assess the impact of translational cardiac movement on the M-mode scan-line, we measured, in 15 randomly selected conditions and using the same Nyquist limit, the IAD from the M-mode spectral display as described above and also from the color Doppler 2D zoomed images, where the distance from the first to the second aliasing boundaries was measured.
Intra- and interobserver variability.
To assess the effect of observer variability and the reproducibility of the IAD method, two independent observers analyzed 15 randomly selected hemodynamic conditions. Each observer measured the IAD from the color Doppler M-mode recordings and the conventional radius of the FC region (Rmeas) from the color Doppler 2D images. Measurements obtained from both observers were compared using linear regression analysis and variability was expressed as the mean percent error ± standard deviation. Additionally, interobserver variability was determined by one of the observers who measured the IAD and the FC radius at two different times.
Numerical modeling.
To prove the theoretical performance of the IAD, and to evaluate the impact of misleading orifice location on MRV quantification, we performed numerical modeling experiments. Computational fluid dynamics (CFD) simulations using commercially available software (Fluent Inc., Lebanon, New Hampshire) were used to obtain numerical approximations of flow acceleration proximal to a finite orifice, as previously described (20). A geometrical model consisting of two cylindrical tanks was used to model the left ventricle and the left atrium, with a circular orifice connecting both chambers. Calculations were based on an axisymmetric grid with increasing density near the orifice. Simulations were performed under steady flow conditions with a pressure drop across the orifice ranging from 70 to 120 mm Hg. The orifice diameter ranged from 2 to 8 mm. The software solved the Navier-Stokes equations of fluid flow throughout the flow domain until acceptable convergence criteria were reached (residuals  10–6). Flow convergence centerline velocity profiles extracted from CFD were exported to a personal computer equipped with customized software (LabVIEW, National Instruments, Austin, Texas) for further analysis.
Statistical analysis.
Data are expressed as mean ± standard deviation for descriptive statistics. Correlation between MRV and ROA estimated by both conventional FC and IAD methods and the ones obtained from the reference electromagnetic flow meters were analyzed by simple linear regression. Correlation coefficients were compared after Fisher’s z-transformation and a paired Student’s t test was used to compare the difference between methods. Agreement between different methods was evaluated using the Bland and Altmann analysis. A value of p < 0.05 was considered statistically significant.
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Results
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A total of 39 hemodynamic conditions were studied in 12 sheep. Data obtained from the electromagnetic flow probes and meters confirmed the presence of mitral regurgitation in all conditions, with MRV ranging from 2.0 to 27.0 ml (mean 10.4 ± 5.1 ml) (Table 1). Mean ROA was 9.6 ± 5.7 mm2, with areas ranging from 1.9 to 31.4 mm2. Regurgitant fraction ranged from 6% to 65% (mean 30 ± 12%). Accordingly, mitral regurgitation was severe (RF >55%) in three (8%) of the 39 studied hemodynamic conditions, moderate (RF 30% to 55%) in 18 (46%) and mild (RF <30%) in 18 (46%).
Rmeas ranged from 2.5 to 8.4 mm. MRV estimated by Rmeas from the conventional FC method showed a good correlation and agreement with those obtained by electromagnetic flow probes and meters (y = 1.04x + 0.91, r = 0.91, p < 0.001, = 1.3 ± 2.3 ml) (Fig. 3). Also, good correlation and agreement were seen between both techniques for determining ROA (y = 1.02x + 0.88, r = 0.93, p < 0.001, = 1.0 ± 2.2 mm2).
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Figure 3 Linear regression and analysis of agreement between mitral regurgitant stroke volume (MRV) derived from the measured radius (Rmeas) by using the conventional flow convergence (FC) method and electromagnetic (EM) flow probes and meters.
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Interaliasing distance ranged from 1.0 to 2.5 mm. Mitral regurgitant volume derived from Rcalc by the new IAD methodology also provided good correlation and agreement with MRV obtained by electromagnetic flow probes and meters (y = 0.83x + 5.17, r = 0.90, p < 0.001, = 3.3 ± 2.1 ml) (Fig. 4). Similarly, ROA estimates from the Rcalc showed good correlation and agreement with electromagnetically derived ROA values (y = 0.87x + 4.32, r = 0.94, p < 0.001, = 3.0 ± 1.9 mm2).
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Figure 4 Linear regression and analysis of agreement between mitral regurgitant stroke volume (MRV) derived from the calculated radius (Rcalc) by using the interaliasing distance (IAD) method and by electromagnetic (EM) flow probes and meters.
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Comparison of conventional FC and IAD methods.
Both Rmeas (conventional FC) and Rcalc (IAD) derived values showed similar correlation with the reference MRV (r = 0.91 and r = 0.90, respectively, p = NS) and ROA (r = 0.93 and r = 0.94, respectively, p = NS). Even though mean error was slightly higher for the IAD estimates of both MRV and ROA than for the conventional FC estimates (3.3 ± 2.1 vs. 1.3 ± 2.3 ml for MRV and 3.0 ± 1.9 vs. 1.0 ± 2.2 mm2 for ROA respectively, p < 0.05 for both), the error was small and of no hemodynamic or clinical significance for either method.
Comparison of M-mode and 2D for measuring IAD.
Good correlation was found between IAD measured from the color Doppler M-mode display and from the color Doppler 2D images (y = 0.88x + 0.03, r = 0.90, p < 0.01) with a tendency for larger IADs when obtained from the 2D images ( = 0.12 ± 0.31 mm).
Intra- and interobserver variability.
Measurements of the distance between the first and second aliasing boundaries (IAD) by two different observers showed good correlation and agreement (y = 0.81x + 0.02, r = 0.92, SEE = 0.010 mm, p < 0.001) as did the Rmeas measurement (y = 0.99x + 0.02, r = 0.92, SEE = 0.038 mm, p < 0.001). However, mean percent error was significantly smaller for the IAD measurements as compared to the conventional Rmeas (% = 1.7 ± 7.2% and 4.2 ± 7.5%, respectively, p < 0.05).
Reproducibility of repeated measurements by the same observer was also good for both the IAD (y = 1.02x – 0.004, r = 0.94, SEE = 0.009 mm, p < 0.001) and the Rmeas (y = 1.005x – 0.01, r = 0.95, SEE = 0.010 mm, p < 0.001). No significant difference was seen in the mean error between both methods (IAD % = 0.9 ± 6.3 and Rmeas % = 2.7 ± 6.5%, p = NS).
Numerical modeling: impact of misjudging orifice location on quantification on MRV.
Estimates of flow rate by the IAD method resulted in slight overestimation of the actual flow (Fig. 5, Rcalc[IAD], continuous line), whereas estimates by the conventional FC technique, when the orifice location was known (Fig. 5, Rmeas[FC], dashed line), led to a slight underestimation. This observation was especially apparent near the orifice. However, when the location of the orifice was not accurate, as may occur in clinical practice, errors in flow estimates by the FC method significantly increased. If the measurement of the radius was missed by –1 mm, underestimation of flow can increase up to  40% (Fig. 5, Rmeas[FC] <1 mm, open circle line), whereas an overestimation of the radius by 1 mm leads to an overestimation of the flow >20% (Fig. 5, Rmeas[FC] >1 mm, closed circle line). In contrast, flow estimates by the IAD method are not affected by errors in locating the orifice and therefore remain unchanged.
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Figure 5 Relative error ([calculated flow – actual flow)/actual flow] · 100) of both the interaliasing distance (Rcalc[IAD]) and conventional flow convergence (Rmeas[FC]) methods for estimating the flow rate depending on the distance to the orifice, which is expressed as normalized distance. The normalized distance equals to r/d, r being the radius of the flow convergence surface and d the diameter of the orifice (see text).
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Discussion
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In the present animal study, we have validated a new methodology for quantification of mitral regurgitation. The new IAD method provides good estimates of MRV and ROA when compared to data obtained from electromagnetic flow probes and meters.
The flow convergence principle has been largely applied to quantify a wide range of valvular regurgitant lesions (5,9,11,21), and its clinical usefulness has also been demonstrated (10,12). However, its applicability depends on the ability of the observer to locate correctly the regurgitant orifice. Because the flow convergence assumption is a hemispheric shape and therefore uses the squared radius to calculate regurgitant flows, small errors in the measurement of the FC radius can lead to larger errors in the estimation of regurgitant volumes and orifice areas. This has been clearly demonstrated in the present study with the numerical modeling.
Therefore, it is of paramount importance to locate accurately the actual orifice, which can be difficult in the daily routine practice of a busy echocardiography laboratory. In our study, we carefully examined both the 2D tissue gray-scale and the color Doppler 2D images. Consequently, our results using the conventional Rmeas for quantification of mitral regurgitation in this animal model were good and in accordance with other previously reported clinical studies (12,16).
The newly proposed IAD method to determine Rcalc has the great advantage of avoiding the need to locate the orifice and, therefore, not being affected by misjudgment of its location; this may provide the IAD method with more feasibility for its use, especially in the clinical setting. Even though adding the color M-mode scan along the centerline of the FC region in a diagnostic echocardiographic study may slightly increase its duration, it is easier to measure an M-mode distance with two delineated borders given by the two aliasing boundaries than to measure the distance between the zenith of the FC region and the site of the regurgitant orifice. This observation is supported by the smaller interobserver variability found in our study for the IAD measurement as compared to the conventional Rmeas.
Potential for automation.
Another advantage of this method is its potential for computerized automation. Automated measurement provides a simplified method for quantification, avoids losing most of the information available when only one point of the color Doppler M-mode spectrum is analyzed, and therefore narrows the range of random error. Previous reports have proposed different semiautomatic techniques to quantify mitral regurgitation based on the velocity information available in digital format for offline analysis. Different algorithms have been used to locate automatically the orifice, either based on centerline velocity profiles obtained by color Doppler M-mode tracings (14) or based on the computerized analysis of the 2D color Doppler velocity maps (22). However, these methods also have limitations for correct location of the regurgitant orifice, and their application to the clinical setting has not been fulfilled. More recently, approaches using the backscattered Doppler power at the vena contracta have been tested in vitro and in vivo (23); although accurate estimates of flow rates and regurgitant volumes were obtained, the complicated implementation of this technique requiring visualization and analysis of the vena contracta may limit its clinical use. Previous in vitro data from our laboratory have shown that an automated approach is feasible using the IAD technique in an in vitro model of valvular regurgitation (24).
Limitations of the IAD technique.
Several limitations of the IAD technique must be addressed. First, correct alignment along the centerline of the FC region is needed in order to avoid errors in estimation of the MRV; in the present study, despite the eccentric nature of the jet because of the flail mitral leaflet, proper alignment was achieved using an epicardial window. Nevertheless, correct alignment may not be achieved with transthoracic or even transesophageal echocardiography. Application to color Doppler imaging of the recently developed anatomic M-mode (25) may overcome this geometric limitation. Second, translational movements of the cardiac chambers during the cardiac cycle may affect the stability and accuracy of imaging along the real centerline of the FC region and therefore may lead to underestimation of regurgitant volumes. Hence, it is important to analyze the color Doppler M-mode spectra in concordance with the color Doppler 2D images, focusing on the correct position of the M-mode cursor. However, we did find a good correlation between the IAD measured from the M-mode and the 2D images, indicating a small impact of the translational cardiac movements on the accuracy of the new technique.
The small size of the IAD itself can be a source of measurement inaccuracy owing to the finite resolution of even the most updated and current echocardiographic systems. That may have contributed to the slight overestimation observed in our study, especially for the smaller MRVs. We tried to overcome this problem by obtaining maximally magnified (zoomed) images of the color Doppler M-mode spectra across the FC zone and by averaging at least three measurements between the first and second aliasing boundaries. Semiautomated calculation of the IAD of a longer segment of the M-mode spectrum across the FC, averaging the distances of multiple points between the first and second aliasing boundaries, will provide a more accurate and higher resolution IAD measurement.
Finally, the IAD method relies on the assumption that all the isovelocity surfaces in the FC region are perfect hemispheres. However, it is known that because of the finite nature of the regurgitant orifice, flattening of the different isovelocity shells of the FC region occurs as they approach the regurgitant orifice (20). Progressive local flattening of the isovelocity shells proximal to the orifice leads to an increased area of the distal layers of the FC region and therefore to larger IADs (Fig. 6). Independently, distal enlargement is also produced by flow constraint (16). Larger sizes of the IAD will consequently result in overestimation in MRV and ROA values, as seen in our numerical modeling and animal study, where slight, but consistent overestimation of the reference values was found. We used a previously validated correction for the flow constraint (16), but local flattening of the isovelocity surfaces was not addressed in the present study. Further investigation is needed to predict and correct mathematically this flattening, improving the accuracy of the IAD method.
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Figure 6 Flow convergence (FC) region is theoretically composed of perfect hemispheres (left); however, because of the finite nature of the regurgitant orifice, progressive flattening of the isovelocity shells occurs as they approach the orifice (20) (right), leading to an increased distance between the first two aliasing boundaries (IAD).
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Conclusions.
In conclusion, the present study showed that correlation of MRV and ROA by the IAD method with data obtained from reliable reference standards, that is, electromagnetic flow probes and meters, was excellent in an animal model with chronic mitral regurgitation. This new method can accurately quantify the severity of mitral regurgitation and has an advantage over the conventional FC method in that it does not require location of the regurgitant orifice and can be performed semiautomatedly. The method validated in the present study appears to be promising for the routine quantification of mitral regurgitation in clinical practice.
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Acknowledgments
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We are indebted to the technical staff of the Laboratory of Animal Medicine and Surgery, NHLBI, for their assistance during the animal experiments.
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Footnotes
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Supported in part by grant 9951522V from the American Heart Association, Ohio Affiliate (Columbus, Ohio). Dr. Sitges was supported by a grant from the Spanish Government (BEFI-00/9279).
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References
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Three-Dimensional Echocardiography: The Benefits of the Additional Dimension
J. Am. Coll. Cardiol.,
November 21, 2006;
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2053 - 2069.
[Abstract]
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W. A. Zoghbi, M. Enriquez-Sarano, E. Foster, P. A. Grayburn, C. D. Kraft, R. A. Levine, P. Nihoyannopoulos, C. M. Otto, M. A. Quinones, H. Rakowski, et al.
American Society of Echocardiography: recommendations for evaluation of the severity of native valvular regurgitation with two-dimensional and Doppler echocardiography : A report from the American Society of Echocardiography's Nomenclature and Standards Committee and The Task Force on Valvular Regurgitation, developed in conjunction with the American College of Cardiology Echocardiography Committee, The Cardiac Imaging Committee, Council on Clinical Cardiology, The American Heart Association, and the European Society of Cardiology Working Group on Echocardiography, represented by:
Eur J Echocardiogr,
December 1, 2003;
4(4):
237 - 261.
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J. D Thomas
DOPPLER ECHOCARDIOGRAPHIC ASSESSMENT OF VALVAR REGURGITATION
Heart,
December 1, 2002;
88(6):
651 - 657.
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T Irvine, X K Li, D J Sahn, and A Kenny
Assessment of mitral regurgitation
Heart,
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iv11 - 19.
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Abstract
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