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European Journal of Echocardiography 2007 8(2):116-121; doi:10.1016/j.euje.2006.02.007
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Copyright © 2006, The European Society of Cardiology

Evaluation of mitral valve area by the proximal isovelocity surface area method in mitral stenosis: Could it be simplified?

David Messika-Zeitoun*, Agnes Cachier, Eric Brochet, Bertrand Cormier, Bernard Iung and Alec Vahanian

Cardiovascular Division, AP-HP, Bichat Hospital, 46 rue Henri Huchard, 75018 Paris, France

Received 5 October 2005; received in revised form 21 February 2006; accepted after revision 28 February 2006.

* Corresponding author. Tel.: +31 1 40 25 66 01; fax: +31 1 40 25 67 32. david.messika-zeitoun{at}bch.ap-hop-paris.fr


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conclusion and clinical...
 References
 
Aim One limitation for a wider use of the proximal isovelocity surface area method (PISA) for the evaluation of the mitral valve area (MVA) in patients with mitral stenosis (MS) is the requirement of an angle correction factor (angle {alpha} between the mitral leaflets) which cannot be obtained using the machine's built-in software and requires a manual measurement. The aim of the present study was to evaluate if the use of a fixed angle could provide an acceptable MVA estimation.

Methods and results In 48 patients (53±14years, 75% female and 32% atrial fibrillation), MVA was prospectively measured by planimetry (MVA2D) and PISA (PISAmes). The angle {alpha} was manually measured on paper prints using a protractor.

MVA2D was 1.38±0.56cm2 [0.5–2.40]. PISAmes ({alpha}=104±13° inter-quartiles 90–115) was 1.34±0.64cm2 [0.31–2.95] and did not differ from and correlated well with MVA2D (P=0.25; r=0.93, P<0.0001). MVA estimated using the PISA method and a fixed angle value from 90 to 110 (MVA{alpha}=90 to MVA{alpha}=110) progressively increased from 1.20±0.66 to 1.48±0.81cm2. Only MVA{alpha}=100 (1.34±0.74cm2) did not differ from and correlated well with both MVA2D and PISAmes (both P>0.35 and r>0.90, P<0.0001).

Conclusion The angle formed by the mitral leaflet only slightly changes in between patients and use of a fixed angle value of 100° provides an accurate estimation of the MVA by the PISA method in patients with MS. This simplification would facilitate and extend the use of the PISA as an additional method for the assessment of MS severity in routine practice.

Keywords: Mitral stenosis; Echocardiography; Proximal isovelocity surface area method (PISA)


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conclusion and clinical...
 References
 
Management of mitral stenosis (MS) relies on accurate assessment of the mitral valve orifice area (MVA). As cardiac catheterization is mostly used to perform percutaneous mitral commissurotomy (PMC), for clinical decision-making MVA is most often non-invasively measured by Doppler echocardiography. Several methods such as the pressure half-time method, the continuity equation and planimetry can be used1–5 but all have potential intrinsic limitations, and additional methods are desirable.

The proximal isovelocity surface area (PISA) method is based on the continuity principle6,7 and assumes that blood flow converging toward a flat orifice forms hemispheric isovelocity shells. It has been shown that the PISA method is accurate and reproducible.5,6,8–10 This method is attractive in MS since the proximal convergence method can be easily visualized and it may be the only method available.6,8 Despite these theoretical advantages, in contrast to valvular regurgitation,11 the PISA method is seldom used in routine practice for the assessment of MS severity. One reason may be that this method is reputed to be technically demanding and time-consuming since it requires several measurements, in particular an angle correction. As a result of leaflets doming in MS, only a fraction of a hemisphere crosses the orifice and an angle correction factor {alpha} need to be considered (funnel angle formed by the mitral leaflets, Fig. 1). This angle cannot be obtained using machine's built-in software and requires a manual measurement using a protractor.


Figure 1
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  		Figure 1 The proximal flow convergence region. r, radius of the flow convergence (distance from the orifice to the color boundary). {alpha}, angle between the valve leaflets. The aliasing velocity is indicated is the upper right corner.

 
We hypothesized that this angle only changes slightly between patients with MS and that a fixed angle value could provide acceptable MVA estimation. Therefore, the aim of the present study was, in patients with MS, to compare MVA calculated with the measured angle {alpha} and MVA estimated with various fixed angles using planimetry as reference method.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conclusion and clinical...
 References
 
Patients
We prospectively evaluated by echocardiography 48 consecutive patients with a wide range of MS severity between February and July 2005. There were no exclusion criteria.

Echocardiographic examination
Comprehensive two-dimensional Doppler echocardiography
All patients underwent a complete two-dimensional and Doppler examination using commercially available systems (Sonos 7500 and iE33, Philips, Andover, MA). Planimetry (MVA2D) was performed by experienced observers in parasternal short-axis view, adjusting the probe to obtain the optimal mitral valve orifice. Mean transmitral pressure gradient and systolic pulmonary artery pressure (S-PAP) were also assessed. Mitral valve anatomy was classified in 3 groups12: flexible valves and mild subvalvular disease (chordae ≥10mm long) (group I), flexible valves and extensive subvalvular disease (chordae <10mm long) (group II) and calcified valves (group III). Degree of mitral regurgitation (MR) was semi-quantitatively graded from zero to four (11). Left ventricular (LV) dimension and function and left atrial (LA) diameter were measured as recommended by the American Society of Echocardiography.13

Determination of the MVA by the PISA method
The conceptual basis of the PISA method has been described elsewhere.14 The method is based on flow convergence analysis proximal to the stenotic orifice by shifting the colour-flow scale baseline upward to decrease the colour aliasing-velocity. With an hemispheric shape of the proximal isovelocity surface, the diastolic flow rate is calculated as


Formula

in which r (cm) is the maximal radius of the flow convergence region in early diastole measured on the centreline of the flow convergence region, Valias is the aliasing velocity, and {alpha}/180 is the correction factor accounting for the angle {alpha} between the mitral leaflets. MVA is then determined by dividing maximal diastolic flow rate (flowmitral [ml/s]) by peak continuous wave Doppler velocity of mitral inflow (Vmax [cm/s]).

Aliasing velocity was set for obtaining satisfactory red-blue boundary and adequate convergence shape. The angle {alpha} was manually measured on paper prints using a protractor. All these measurements were performed in apical four-chamber view. Three to five beats were analysed and averaged. PISA calculations were not performed until the end of the examination to avoid potential bias as much as possible.

Statistics
Quantitative variables were expressed as mean±SD. Comparisons of MVA obtained by the PISA method and the planimetry were analysed using paired t-tests, Pearson's correlation and Bland–Altman analysis.15 Statistical significance was defined with P<0.05.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conclusion and clinical...
 References
 
Population
Forty-eight patients were enrolled in the present study. Mean age was 53±14years, 75% were female and 32% were in atrial fibrillation (AF). Mean MVA2D was 1.38±0.56cm2 (range 0.5–2.40), mean transmitral gradient 8±5mmHg (range 3–31) and mean S-PAP 46±13mmHg (range 25–85). Twenty-five patients (52%) had severe MS (MVA2D <1.5cm2). Twenty-seven patients were in anatomic group II (56%) and 21 in group III (44%). None were in group I. An MR grade ≥2/4 was present in 10 patients (21%).

MVA assessment by the PISA method
Measured angle
By the PISA method, MVA (PISAmes) was 1.34±0.64cm2 (range 0.31–2.95) using a measured angle of 104±13° (median 103, inter-quartiles 90–115). No difference with the planimetry was observed (P=0.25), mean difference was small (0.18±0.16cm2) and correlation excellent (r=0.93, P<0.0001).

Fixed angle
MVA was then estimated using the PISA method and a 5° angle increase from 90 to 110 (MVA{alpha}=90 to MVA{alpha}=110). other parameters remaining the same as those used for PISAmes. MVA progressively increased from 1.20±0.66 to 1.48±0.81cm2. Only MVA{alpha}=100 (1.34±0.74cm2) did not differ from and correlated well with both MVA2D and PISAmes (both P>0.35 and r>0.90, P<0.0001) (Figs. 2A and 3A). Quality control plots showed no trend for under- or overestimation overall (Figs. 2B and 3B) but a difference of ≥0.5cm2 was observed in four patients with mild MS (patients plotted in right upper corner of Fig. 2B). Mean difference with MVA2D was slightly higher than the mean difference between PISAmes and MVA2D (0.24±0.23 vs. 0.18±0.16cm2, P=0.02). Results are summarized in Table 1.


Figure 2
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  		Figure 2 (A) Correlation between mitral valve area assessed by the PISA method (MVA-PISA) using a fixed angle correction of 100° and by planimetry (MVA2D) (r=0.91, P<0.0001). (B) Quality control plots using Altman and Bland analysis for the two methods.

 


Figure 3
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  		Figure 3 (A) Correlation between mitral valve area assessed by the PISA method (MVA-PISA) using a fixed correction angle of 100° and MVA also measured by the PISA method but with the exact angle measured using a protractor (r=0.97, P<0.0001). (B) Quality control plots using Altman–Bland analysis for the two methods.

 


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  		Table 1 MVA assessment by the PISA method

 
Agreement between MVA{alpha}=100 and MVA2D for the diagnosis of severe MS (MVA2D <1.5cm2) was good (kappa=0.75) and sensitivity, specificity, positive predictive and negative predictive values were respectively 100%, 74%, 81% and 100%. Five patients were classified as having severe MS by the PISA method with {alpha}=100 and non-severe by planimetry. This discrepancy was mainly encountered in patients with MS of borderline severity and these patients were also misclassified using the PISA method with a measured angle. No patients with non-severe MS by the PISA method were misclassified.


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conclusion and clinical...
 References
 
Main results
In the present study, (1) we confirm that the PISA method is an accurate method for MVA assessment in patients with MS and (2) we show that MVA estimated using this method and a fixed angle value of 100° accounting for the angle formed by the mitral leaflets, does not differ from and correlates well with MVA assessed by both the planimetry and the PISA method with the exact angle measured using a protractor. Therefore, estimated PISA is an acceptable method for the assessment of MS severity and this simplification should facilitate and extend the use of the PISA for the assessment of MS severity in routine practice.

The angle correction factor for MVA assessment by the PISA method in MS
The PISA method has been widely validated for mitral and aortic valve regurgitation.14,16,17 This method is routinely used in many Echocardiography laboratories and grading of the severity of valvular regurgitation is mainly based on this quantitative evaluation.11 Multiple studies have also validated the PISA method for MVA assessment in mitral stenosis under various clinical conditions (rhythm, associated aortic or mitral regurgitation, severity of anatomic lesions, etc.)6,8–10,18 but its use remains infrequent in routine practice.

MVA calculation by the PISA method in MS requires a correction factor accounting for the angle formed by the mitral leaflet. Because of the funnel shape of the mitral valve in MS, only a fraction of a hemisphere crosses the orifice. Usually, a plane-angle correction factor (angle measured in one dimension only; Fig. 1) is used instead of a solid-angle correction factor, which accounts for the three-dimensional shape of the mitral valve.8 Despite this geometric approximation, MVA assessment by the PISA method still requires a two-dimensional angle measurement. This angle cannot be obtained using the machine's built-in software and needs to be measured manually using a protractor. This limitation may explain, at least partially, why use of the PISA method remains limited in MS.

In the present study, mean angle was 104±13° with a narrow range (80–135; inter-quartiles 90–115°). These values are similar to those reported in the literature.9,19 Because of this narrow range, we hypothesized that a fixed angle correction could provide an acceptable MVA estimation avoiding the use of a protractor. Using a value of 100°, MVA did not differ from and correlated well with planimetry and MVA calculated by the PISA method with the exact angle measured using a protractor. Mean difference was small and misclassification mainly observed in patients with MS of borderline severity. Of note, using 105°, MVA was slightly different from PISAmes but closer to MVA2D. Therefore, for routine practice, an acceptable estimation of the MVA can be obtained using a fixed angle of 100°.

Using the PISA method (with measured or fixed angle), five patients were classified as having severe MS whereas it was non-severe by planimetry. This discrepancy may be related to the fact that both methodologies do not exactly measure the same things. The PISA method provides an assessment of the physiologic or effective orifice area at the vena contracta, as opposed to the anatomic orifice area measured by planimetry, which is generally somewhat larger.20

Study limitations
No other methods for MVA assessment were considered. We recently underlined the limitations of the pressure half-time method5 and some patients were examined immediately after percutaneous mitral commissurotomy. The continuity equation is invalidated by the commonly associated aortic or mitral regurgitation. However, our aim was not to validate the PISA method (done in numerous previous studies and nevertheless confirmed in the present study) but to demonstrate that the use of a fixed angle provides similar results to a measured angle.

It has been previously shown that the shape of the flow convergence region changes with the aliasing velocity.21 We used a mean aliasing velocity of 28±3cm/s, a value slightly higher than is recommended10 but consistent with our previous work18 and the reports of others.8,19 The aliasing was chosen to obtain an adequate convergence shape and a satisfactory red-blue boundary. Our experience is that use of lower aliasing velocity results in inadequate flow convergence shape, ambiguous surface contour and therefore difficulty (or impossibility) of getting a reliable radius.

Finally, we are certainly not implying that angle measurement is useless (mean difference between MVA{alpha}=100 and MVA2D was indeed slightly higher than the mean difference between PISAmes and MVA2D), but highlight that its absence does not preclude the use of the PISA method for MS severity assessment. In the future, direct angle measurement may be usefully added to machines' built-in software.


   Conclusion and clinical implications
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion and clinical...
References
 
In the present study we demonstrated that the angle formed by the mitral leaflet changes only slightly between patients with MS and therefore the use of a fixed angle value of 100° provides an accurate MVA estimation in patients with MS. This simplification should facilitate and extend the use of the PISA as an additional method for the assessment of MS severity in routine practice.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion and clinical...
 References
 

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