Clinical application in routine practice of the proximal flow convergence method to calculate the mitral surface area in mitral valve stenosis
Ahmed Bennis
1, Abdennasser Drighil
1, Christophe Tribouilloy
2, Asmaa Drighil
3&
Nacer Chraibi
11
Center of Cardiology, CHU Ibn Rochd, Casablanca, Morocco;
2Centre Hospitalier Universitaire d’Amiens, France;
3University Hassan II, LPSCM, Casablanca, Morroco
Received 12 February 2001; accepted in revised form 29 January 2002
Key words: echocardiography, mitral valve stenosis, proximal isovelocity surface area, valvular heart disease
Abstract
Background: Two-dimensional (2D) echocardiography planimetry, the Doppler pression half-time (PHT), and the continuity equation methods were used to estimate mitral valve area (MVA) in patients with mitral stenosis (MS). Recently, the proximal isovelocity surface area (PISA) method has been shown to be accurate for calculating MVA. The purpose of this study is (1) to compare in a large non-selected popu- lation the accuracy of the PISA and planimetry methods for echocardiographic estimation of MVA; (2) to determine the effect of atrial fibrillation (AF), Wilkins score, associated mitral regurgitation (MR), aortic regurgitation (AR), and of commissural calcifications on the accuracy of the PISA method. Methods: One hundred and eight consecutive patients with rheumatic MS were studied (76 females and 32 males; mean age: 36 ± 12 years); 64 were in sinus rhythm; 51 had associated MR and 46 had AR. By the PISA method, MVA was calculated assuming a uniform radius flow convergence region along a hemispherical surface.
Results: The mean value of 2D MVA was 1.32 ± 0.59 cm
2(0.4–3.1 cm
2) and that of PISA MVA 1.33 ± 0.62 cm
2(0.38–3 cm
2). MVA calculated using the PISA method correlated well with 2D MVA (r ¼ 0.93, y ¼ 0.97x + 0.04, p < 0.0001, SEE ¼ 0.21 cm
2). The correlation was also good in patients with AF(r ¼ 0.93, y ¼ 0.99x + 0.03, p < 0.0001, SEE ¼ 0.21 cm
2), with MR (r ¼ 0.94, y ¼ 1.014x + 0.003, p < 0.0001, SEE ¼ 0.19 cm
2), with AR (r ¼ 0.93, y ¼ 0.90x + 0.11, p < 0.0001, SEE ¼ 0.2 cm
2), when Wilkins score was >8 (r ¼ 0.92, y ¼ 0.96x + 0.06, p < 0.0001, SEE ¼ 0.19 cm
2), and in patients with commissural calcifications (r ¼ 0.90, y ¼ 0.88x + 0.009, p < 0.0001, SEE ¼ 0.20 cm
2). Conclusion: Our study shows that in routine practice, MVA calculated by the PISA method correlated well with the area obtained by planimetry even in the presence of commissural calcifications, associated MR, AR, AFand of high Wilkins score. Therefore, the PISA method provides a reliable measurement of the MVA in MS under different anatomic and clinical conditions and may be a useful alternative method for calculating MVA.
Abbreviations: AF– atrial fibrillation; AR – aortic regurgitation; 2D – two-dimensional; FCR – flow conver-
gence region; MR – mitral regurgitation; MS – mitral stenosis; MVA – mitral valve area; PHT – pressure half-
time; PISA – proximal isovelocity surface area; SR – sinus rhythm
Introduction
Mitral valve area (MVA) has been calculated using two-dimensional (2D) echocardiographic plani- metry, pressure half-time (PHT), and continuity equation methods in patients with mitral stenosis (MS) [1–7]. 2D echocardiography has been shown to provide reliable non-invasive measurement of MVA in patients with MS. Accordingly, it became the standard non-invasive method for evaluating MVA [1–3]. However, it may be difficult to cal- culate the MVA in patients with irregular mitral valve orifice and/or with severe calcification using 2D echocardiographic planimetry [2–4]. PHT is reported to be inaccurate for calculating MVA in patients with aortic regurgitation (AR) and af- fected by left ventricular stiffness [7, 8]. Moreover, the PHT is inaccurate for measuring MVA after percutaneous mitral valvulotomy. More recently, the proximal isovelocity surface area (PISA) method has been proposed as a new method for calculating the MVA [9–11]. However, the impact of atrial fibrillation (AF), associated mitral re- gurgitation (MR), associated AR, Wilkins score, and of commissural calcifications on the MVA calculated by the PISA method has not been studied in a large population of patients with MS.
The purpose of the present study is (1) to evaluate the accuracy of the PISA method for evaluating the MVA in routine practice in patients with MS using 2D echocardiography planimetry as refer- ence method, and (2) to assess the impact of as- sociated MR and AR and of different anatomic and rhythm conditions on the PISA method.
Methods Patients
One hundred and twelve patients with typical rheumatic MS were screened prospectively in the echocardiographic laboratory by one of the in- vestigators. In four patients, planimetry was im- possible to perform because of poor echogenicity (n ¼ 2) and a shadowing effect of valve calcifica- tions (n ¼ 2). These four patients were excluded and 108 patients were finally included (mean age
was 36 ± 12 years (12–70)). There were 32 men and 76 women: 64 were in sinus rhythm (SR) and 44 in AF. Fifty-one patients had associated MR (32 mild, 19 moderate to severe) as determined by Doppler echocardiography [12]. Forty-six patients had AR.
PISA method
The principle of the PISA method has already been validated and described [9]. The PISA method was assumed to be hemispheric. There- fore, the flow rate, Q (ml/s), was calculated using the standard hemispheric equation Q ¼ 2pR
2a/
180 V
a, where V
ais the aliasing velocity (cm/s) at a radial distance from the MS and a/180 is a cor- rection factor accounting for the inflow angle formed by the mitral leaflets [9]. The MVA, A (cm
2) was calculated as A ¼ Q/V where V was the peak mitral velocity in protodiastole recorded by continuous wave doppler. The maximal radial distance R (in cm) was selected with the cine-loop function. The a angle and peak flow velocity were measured and averaged in five consecutive beats for patients in SR and 10 beats for patients in AF.
Doppler echocardiography
2D echocardiography, Doppler ultrasound and color flow mapping were performed using a Hewlett-Packard 1000 imager equipped with a 2.5 MHz phased array transducer and a standard velocity map.
The PISA in the flow convergence region (FCR)
close to the stenotic mitral orifice was observed in
diastole, from the apical four chamber view (Fig-
ure 1). From this window, the four-chamber view
provided the most consistent image of the largest
proximal convergence radius and allowed the
proximal flow to be viewed almost nearly parallel
to the ultrasound beam. This view was scanned to
image the largest proximal FCR, and the aliasing
velocity was reduced by shifting the color baseline
to maximize the area on the image. The gain setting
was optimized to eliminate random color in areas
without flow. The aliasing velocity was shifted at
21–29 cm/s to optimize the visualization of the
PISA and reduce the error in radius measurements.
The maximal radius, R (cm), of the PISA in a cardiac cycle was measured in protodiastole using the zoom and cine-loop functions. 2D echocardio- graphy mitral inflow velocities were measured from the apex by continuous wave Doppler, the mitral valve was scanned in the parasternal short axis view to image the smallest orifice area for plani- metry as previously described [1–4].
Observer variability
Two independent observers repeated the mea- surements the radius, angle, and peak flow velo- city. In 10 patients, interobserver variability was calculated as the standard deviation (SD) of the difference of their measurements. Similarly, one
observer repeated the measurements to determine interobserver variability. For the 10 patients in whom MVA was measured by a second observer, interobserver variability was low, with highly sig- nificant correlation between observers (p <
0.0001). The mean and the SD of differences be- tween measurements were 0.02 ± 0.11 cm
2(in- traobserver) and 0.03 ± 0.14 cm
2(interobserver) for the MVA PISA method, 0.03 ± 0.13 cm
2(intraobserver) and 0.05 ± 0.14 cm
2(interob- server) for the MVA planimetry method.
Statistical analysis
Results were expressed as the mean value ± SD for continuous variables and as percentages for
Figure 1. 2D echocardiography: apical four-chamber view of FCR proximal to mitral orifice.
categorical variables. The association between the PISA and reference method of measuring the MVA was analyzed with linear regression and the Bland and Altman method [13].
Results
Baseline characteristics
The mitral orifice was determined by 2D echo- cardiography in 108 patients of 112 patients (96%). The 108 patients (mean age 36 ± 12 years;
range 12–70 years) with adequate 2D planimetry constitutes our study group (32 men and 76 women). Of the 108 patients, 64 were in sinus rhythm and 44 in AF.
MVAs were determined in 108 patients by 2D transthoracic echocardiography (1.32 ± 0.59).
The PISA method was applied in all these 108 patients. The mean value of angles formed by mitral leaflets was 113 ± 7° (100°–140°), maximal initial velocity was 2.34 ± 0.55 cm/s (range 1.8–
3.77 cm/s), radius was 5.55 ± 1.24 cm (range 2.2–
9.9 cm), and V
awas 23 ± 3 cm/s (range 21–29 cm/s). The mean value of MVAs determined by FCR was 1.33 ± 0.62 cm
2(0.38–3 cm
2), and of MVA with 2D MVA was 1.32 ± 0.59 cm
2(0.4–
3.1 cm
2). The results of measurements of MVA in the entire population and in different subgroups are given in Table 1.
MVA determined by FCR correlated well with 2D (r ¼ 0.93, y ¼ 0.97x + 0.04, p < 0.0001, SEE ¼ 0.21 cm
2) (Figure 2). Quality control plots
Table 1.Quantitative measurements of MVA in the entire population and in different subgroups.
Mean ± SD Entire
population
AFScore >8 MR AR Calcification
n 108 44 62 51 46 39
MVA 2D (cm2) 1.32 ± 0.59 1.24 ± 0.56 1.12 ± 0.48 1.56 ± 0.57 1.33 ± 0.59 1.08 ± 0.48 MVA PISA (cm2) 1.33 ± 0.62 1.26 ± 0.59 1.14 ± 0.50 1.51 ± 0.6 1.33 ± 0.61 1.12 ± 0.49 Anglea(°) 113 ± 7 113 ± 6.40 113.95 ± 7.36 112.7 ± 5.7 113.89 ± 6.19 114.5 ± 8.3
Va(cm/s) 23 ± 3 23.5 ± 3 23.39 ± 2.95 23.35 ± 3.06 23.3 ± 3.10 23.4 ± 3.31
Radius (cm) 5.55 ± 1.24 5.40 ± 1.31 5.26 ± 1.17 5.97 ± 1.16 5.66 ± 1.35 5.39 ± 1.10 Vmax(cm/s) 2.34 ± 0.55 2.30 ± 0.58 2.43 ± 0.57 2.22 ± 0.53 2.34 ± 0.59 2.51 ± 0.59 2D – two dimensional; AF– atrial fibrillation; AR – aortic regurgitation; MR – mitral regurgitation; MVA – mitral valve area; PISA – proximal isovelocity surface area;Va– aliasing velocity.
Figure 2. Correlation between the PISA method and planime- try for calculation of the MVA (n¼108). Scatterplots of the difference (PISA minus planimetry,y-axis) to the average MVA (x-axis). The difference was not statistically different from 0.
using the Bland and Altman method indicated that there was no trend toward underestimation or overestimation. Good correlations (all p < 0.0001, all r 0.89) were found in the subgroups of pa- tients in SR or AF, with MR or AR, with Wilkins score >8, and with or without commissural calci- fications (Table 2 and Figure 3–7).
Discussion
This study reports good correlations and agree- ments between 2D planimetry and the PISA method for evaluation of MVA in 108 unselected patients with MS. Correlations are positive in the absence or the presence of (1) commissural calci- fications, (2) MR, (3) AR, (4) AF, (5) and high Wilkins score up to 8. Tendency to under or overestimate the MVA was globally moderate and few points came from intervals. It has been shown that conventional echocardiographic methods have various limitations in terms of MVA calcu- lations [3, 4]. The feasibility and accuracy of planimetry are reduced by valvular thickening calcification, chest deformity, and previous mitral valvulotomy procedures [3, 4]. PHT is affected by left ventricular preload, AR, and is inaccurate af- ter immediate balloon valvulotomy [3, 7, 8]. Con- versely, FCR appears to be relatively insensitive to differences in valvular regurgitation and orifice shape. In the presence of associated MR, the PISA method may be more accurate than the Gorlin formula, which tends to underestimate MVA in
Table 2.Correlation between planimetry and PISA method in the entire population and in different subgroups.
n r y¼ax+b SEE (cm2) p-Value
Total population 108 0.93 0.97x+ 0.04 0.21 <0.0001
AF44 0.93 0.99x+ 0.03 0.21 <0.0001
SR 64 0.93 0.89x+ 0.13 0.21 <0.0001
Wilkins score>8 62 0.92 0.96x+ 0.06 0.19 <0.0001
Wilkins score8 46 0.92 0.85x+ 0.22 0.23 <0.0001
Associated MR 51 0.94 1.014x+ 0.003 0.19 <0.0001
No MR 57 0.89 0.88x+ 0.13 0.23 <0.0001
Associated AR 46 0.93 0.90x+ 0.11 0.20 <0.0001
No AR 62 0.93 0.88x+ 0.13 0.21 <0.0001
Commissural calcification 39 0.90 0.88x+ 0.09 0.20 <0.0001
Without commissural calcification 69 0.94 0.88x+ 0.17 0.20 <0.0001
Figure 3. Correlation between the PISA method and planime- try for calculation of the MVA in patients with AF(n¼44).
Scatterplots of the difference (PISA minus planimetry, y-axis) to the average MVA (x-axis) in patients with AF. The difference was not statistically different from 0.
those patients [9]. Thus, the PISA method may provide a simple and useful alternative for calcu- lating the orifice area in MS [9].
Theoretically in FCR, the shape of the isove- locity surface depends on the aliasing velocity, pressure gradient between two chambers, flow velocity, and the orifice size [14, 15]. In some re- ports, hemiellipsoid or oblate hemispheric isove- locity surface represents MVA more appropriately [14–16]. There is a tendency to accept the fact that the hemielliptic model appears to correlate better with the actual flow rate and the hemispheric model underestimates the rate, but the hemispheric model is easier to apply and is recommended for
routine clinical practice [9, 17, 18]. It has been shown that low aliasing velocities allow an accurate calculation of volume flow rate with hemispheric model one radius because PISA shape is nearly hemispheric in that case [11–20]. Vandervoort et al.
[21] has shown in vitro that regurgitant flow rate and orifice area can be accurately estimated when aliasing velocity baseline is shifted <10% the peak transorifice velocity. Rifkin et al. [11] yielded
<12% error in MVA estimates by using an aliasing velocity between 18 and 30 cm/s. According to previous report by Deng et al. [20], aliasing of 21 cm/s provided the most accurate estimation of MVA. In the current study, aliasing velocities were
Figure 4. Correlation between the PISA method and planime- try for calculation of the MVA in patients with moderate to severe MR (n¼51). Scatterplots of the difference (PISA minus planimetry,y-axis) to the average MVA (x-axis) in patients with MR. The difference was not statistically different from 0.
Figure 5. Correlation between the PISA method and planime- try for calculation of the MVA in patients with AR (n¼51).
Scatterplots of the difference (PISA minus planimetry, y-axis) to the average MVA (x-axis) in patients with AR. The differ- ence was not statistically different from 0.
selected in the range of 21–29 cm/s to obtain ac- curate measurements on the appropriate image.
According to previous studies, estimation of MVA by the PISA method in AFwas as accurate as by planimetry [11, 22]. Our study indicates that the PISA method is accurate in assessing MVA in MS and is not influenced by significant associated MR or AR. Indeed, MVA by the PISA method is de- rived as the ratio between peak early transmitral flow rate and early diastolic peak velocity. Both these measurements are estimated in early diastole and, therefore, should be influenced by variations in transmitral gradient through diastole or by the rapid increase in left ventricular diastolic pressure caused by AR [20–22].
Limitations of this study
We did not determine the true feasibility of the measurement of MVA by the PISA method be- cause our patients were selected for the inability to acquire the 2D reference method. Our clinical impression is that this technique can be used in a large majority of patients because the MVA was calculated with the PISA method in all of the 108 patients with adequate 2D MVA planimetry. One of the limitations of this study is that both meth- ods were apparently performed by the same operator, during the same echocardiographic in- terrogation. Therefore, measurements were not obtained in a blinded fashion. Another limitation
Figure 6. Correlation between the PISA method and planime- try for calculation of the MVA in patients with Wilkins score
>8 (n¼62). Scatterplots of the difference (PISA minus plani-
metry,y-axis) to the average MVA (x-axis) in patients with Wilkins score>8. The difference was not statistically different from 0.
Figure 7. Correlation between the PISA method and planime- try for calculation of the MVA in patients with commissural calcification (n¼39). Scatterplots of the difference (PISA mi- nus planimetry,y-axis) to the average MVA (x-axis) in patients with commissural calcification. The difference was not statisti- cally different from 0.
is that the PISA results were compared only to 2D planimetry.
We used 2D planimetry as a reference method, rather than obtaining MVA obtained by cathe- terization. However, assessment of MVA by the Gorlin formula is limited both in the evaluation of the transmitral pressure gradient and of the car- diac output in patients with regurgitant lesions [23], whereas the MVA determined by 2D plani- metry is not affected by hemodynamic changes.
Although ideal quality images cannot always be obtained [24, 25], the 2D method has been shown to be accurate when compared to direct surgical examination. Moreover, high correlations between 2D planimetry and invasive reference method support the accuracy of this method [1–3]. There- fore, the reference method used in the current study does not represent a limitation.
Limitations related to the problem of calculat- ing the flow rate by the PISA method involve instrument limitations, angle dependence, poor spatial resolution and slow sampling speeds, with not accurate by reflect the acceleration and decel- eration events occurring in a pulsatile flow. How- ever, in MS, the flow rate varies slowly after its early diastolic peak, due to the diastolic gradient.
In the present study, the color angle was reduced to maximize the color flow imaging frame rate.
Clinical implications
This prospective study included 108 patients with MS and demonstrated that the PISA method can be used to measure the MVA in routine practice with a high accuracy. Our data show that the PISA method is reliable in the absence or presence of commissural calcifications, in the presence of as- sociated MR and associated AR, in AFand in the presence of high Wilkins score up to 8. Thus, multiple non-invasive Doppler echocardiographic methods in MS can be combined during the same examination to obtain an accurate measurement of the mitral area in most circumstances.
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Address for correspondence: Dr A. Bennis, Center of Cardiol- ogy, CHU Ibn Rochd, Casablanca, Morocco
Tel.: +212-28-26436; Fax: +212-22-74646
