Asian Cardiovasc Thorac Ann 1999;7:190-194
© 1999 Asia Publishing EXchange Pte Ltd
Inaccuracy of Prediction of Mitral Valve Prosthesis Size
Probal Ghosh, FRCS, FETCS,
Narendra Bhonsle, MS
Department of Cardiovascular & Thoracic Surgery
Sanjay Gandhi Post-Graduate Institute of Medical Sciences
Lucknow, India
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For reprint information contact: Probal Ghosh, FRCS, FETCS Tel: 972 3 964 7843 Fax: 972 7 640 9966 email: probalg{at}hotmail.com Harav Zinger 8, Rishon Le Zion 75255, Israel.
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Abstract
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Several formulae related to body surface area have been proposed to predict normal mitral annular size. This study examined the applicability of such formulae in predicting the size of mitral valve prostheses. Developmental criteria and echocardiographic parameters were used retrospectively to determine mitral annular size in 92 consecutive patients undergoing mitral valve replacement. Discrepancies were noted among the different predicted annular diameters. Moreover, predicted diameters differed from the diameters of the implanted prostheses. These differences were not influenced by type of prosthesis, type of lesion, technique of surgery, or sex of the patient. A greater degree of disparity was noted for larger body surface areas with all prediction formulae. However, valve size index and valve area index remained incremental with increasing prosthesis size. It was concluded that prediction formulae for mitral annular size based on body surface area do not help the surgeon to predetermine the size of mitral valve prosthesis that may be implanted at operation.
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Introduction
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Several morphometric studies have established normal human heart valve dimensions in adults, children, and infants. Some have been derived from autopsy data.1–7 Others have used angiocardiographic or echocardiographic parameters.8–14 Most studies have correlated valve dimensions to body surface area (BSA) and some have indicated a correlation with height, weight, or age. In mitral valve replacement, the native annular size is assessed and the corresponding size of prosthetic valve is selected to achieve maximal hemodynamic performance. We studied the efficacy and applicability of BSA-related formulae for mitral valve dimensions to predetermine the size of prosthetic valve that may be implanted at surgery.
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Patients and Methods
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Ninety-two consecutive patients undergoing isolated mitral valve replacement with Medtronic-Hall (Medtronic, Inc., Minneapolis, MN, USA) or Sorin Carbocast (Sorin SPA, Saluggia, Italy) prostheses from January 1996 through March 1997, were studied retrospectively. Demographic data including height, weight, age, sex, BSA, detailed medical history, and operative findings were obtained. There were 49 males and 43 females. BSA ranged from 0.8 to 1.88 m2 (mean, 1.38 ± 0.17 m2). Age ranged from 10 to 59 years (mean, 32.33 ± 12.29 years). All lesions were rheumatic in origin. Dominant stenosis was present in 58 and dominant regurgitation was present in 34. Conventional mitral valve replacement with excision of the entire mitral valve complex was performed in 42 patients, the posterior leaflet with the subvalvar apparatus was preserved in 50. The major orifice of the implanted prosthesis was oriented anteriorly. Medtronic-Hall prostheses (nos. 25–18, 27–23, and 29–11) were implanted in 52 patients and Sorin Carbocast prostheses (nos. 25–12, 27–26, and 29–2) were used in 40 patients.
The normal mitral annulus diameter was calculated by the autopsy-derived nomogram of Rowlatt and colleagues2 and the angiography-derived formula of Kishimoto and colleagues12 (mitral annulus diameter in mm = 24.3 x BSA0.44).15 The echocardiography-derived formula of King and colleagues14 was used to determine antero-posterior (AP) and lateral diameters: mitral annular anteroposterior diameter (a) = 23.9 + 8.56 x log BSA; mitral annular lateral diameter (b) = 32.3 + 12.47 x log BSA. The ellipsoid perimeter of the mitral valve annulus was then calculated using the formula: 2
x 
[(a2 + b2)/2]. This was then used to calculate the circularized orifice diameter. The derived diameter was obtained by deducting 4 mm from the circularized orifice diameter. This was considered relevant for prediction because implantation of a mitral valve prosthesis circularizes the annulus and requires leaving behind a skirt of leaflet tissue. Similarly, the mitral valve circumference-derived diameter was obtained using the Mayo Clinic data.6,7 The diameter of the prosthesis actually implanted was then correlated with each of these formula-predicted mitral valve annular diameters. Moreover, valve size index was determined (in mm•m–2) for each patient. Valve area index was calculated from the geometric prosthetic orifice area (in cm2, from the manufacturers data) divided by BSA (in m2).
All values are given as mean ± standard deviation. The statistical analyses, including multivariate regression analysis, were performed using SPSS-PC Plus statistical software (SPSS, Inc., Chicago, IL, USA). The differences between sizes of valves or predicted sizes were assessed by the Student t test. A p value of less than 0.05 was regarded as significant.
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Results
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The predicted diameters of the mitral valves for each group (receiving 25, 27, or 29-mm prostheses of both brands) according to Rowlatt's and Kishimoto's formulae, King's AP diameter, and the derived diameters from King's formula and the Mayo Clinic data are detailed in Table 1
. Each group was further subdivided according to sex, technique of surgery, type of lesion, and type of implanted prosthesis.
There were incremental changes in BSA, valve area index, valve size index, and predicted diameters with increasing size of the implanted prostheses. However, these changes were not significant. The majority of patients (for all 3 sizes of valve) had a BSA in the range of 1.3 to 1.5 m2. King's predicted AP diameter was closest to the diameter of the implanted valve. Kishimoto's formula gave the next nearest prediction. There was no statistically significant difference between King's AP diameter and Kishimoto's predicted diameter. Generally, for any given BSA, the distribution of the sizes of implanted prostheses was similar (Figure 1). Thus, the prediction formulae diverged from the actual implanted size.
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Figure 1. Distribution of implanted prosthetic mitral valve sizes in relation to body surface area and the calculated relationships based on King's anteroposterior diameter, Kishimoto's formula, and Rowlatt's formula.
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The line of identity of the implanted valve size against King's predicted AP diameter, intersected the 27-mm valve recipient population midway (Figure 2). Most of the 25-mm valve recipients were above the line of identity, thus they received a smaller size of valve than the predicted diameter. Most of the 29-mm valve recipients were below the line of identity, thus receiving bigger valves than predicted. Such disparity was not influenced by the type of lesion, technique of surgery, sex, or brand of prosthesis.
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Figure 2. Relationship between implanted prosthetic mitral valve size and King's anteroposterior diameter. Conventional = conventional valve replacement operation, MH = Medtronic-Hall mitral valve prosthesis, modified = mitral valve replacement with preservation of the posterior leaflet and subvalvar apparatus, MR = dominant mitral regurgitation, MS = dominant mitral stenosis, Sorin = Sorin Carbocast mitral valve prosthesis.
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As a natural corollary, mitral valve area index and valve size index decreased for each size of implanted valve as Kishimoto's predicted diameter or King's AP diameter increased (Figure 3). However, the patients' clinical improvement was not influenced by changes in valve area index or valve size index. Multivariate regression analysis of 9 variables including height, weight, body surface area, body mass index, and the predicted diameters, did not identify any significant predictor of valve size (p = 0.11248 to 0.78301).
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Figure 3. Valve area indices of implanted mitral prostheses in relation to Kishimoto's predicted diameter.
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Discussion
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Gutgesell and colleagues13 showed a correlation between two-dimensional echocardiographic and autopsy measure-ments of atrioventricular valve orifice diameters. Shigenobu and colleagues11 claimed a good correlation in adults between the mitral annular diameter predicted from echocardiography in the long-axis view and the diameter of the prosthetic valve inserted at operation in 35 patients. However, a closer look at their data reveals a considerable overlap of 27-mm and 29-mm valve recipients in the same range of two-dimensional echocardiographic annulus diameter. Thus, specific prediction for the individual patient was difficult, as we also observed.
The formula of King and colleagues14 for mitral valve annulus AP diameter essentially predicts the same diameter estimated by Shigenobu and colleagues.11 As the standard error of the estimate in their equation is 1.8 mm, it virtually covers 3 sizes of valve prosthesis. This may explain the lack of prediction in individual patients, noted by us. This lack of specificity is further compounded by discrepancies between labeled and actual dimensions of prosthetic valves and sizers. Cochran and Kunzelman16 showed that the Medtronic-Hall mitral sizers are generally larger than their marked size by 0.49 ± 0.06 mm, whereas Medtronic-Hall mitral prostheses are 1.01 ± 0.07 mm larger than their marked sizes and thus larger than the corresponding sizers. Such industry-planned over-sizing may accommo-date increased orifice area but such differences between measured and actual diameter may have little or no clinical significance.
Matsuda and colleagues17 used Kishimoto's formula to determine normal mitral annular diameter and observed durable results after Paneth-Burr annuloplasty when annular size was adjusted to less than 90% of the normal mitral annular diameter. We observed an inadequate correlation of Kishimoto's formula for predicting annular diameter in individual cases. Westaby and colleagues15 found only congestive cardiac failure and sex to be significantly related to valve area. We did not notice any such correlation. Contrary to our findings, the mean observed annular diameter and the mean prosthetic size were larger in patients with mitral regurgitation in a study by Shigenobu and colleagues.11 However, annular dilatation is known to occur in mitral stenosis.18,19 This may explain the disparity in the observations of these two groups. Earlier, Hutchins and Araya4 also failed to find any significant difference in valve diameter in different conditions, with or without congestive heart failure.
Valve area indices in all patients in our study were greater than those observed by others.20 A review of recent literature did not show any other comparable study with tilting disc prostheses in the mitral position, such as the Medtronic-Hall or Sorin Carbocast. Interestingly, our earlier studies on small aortic roots had indicated a reliable correlation of BSA-based prediction formulae with the size of aortic valve implanted in individual patients.12,21,22 However, the specificity of BSA-based prediction formulae was found to be inadequate to help the surgeon predetermine the size of mitral valve prosthesis that may be implanted at operation.
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Acknowledgments
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We are grateful to Dr. S. K. Agarwal of Nagoya University for the statistical analyses.
Presented at the 13th Biennial Asian Congress of Cardiovascular and Thoracic Surgery, Sydney, Australia, October 13–15, 1997.
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Abstract
Introduction
