Asian Cardiovasc Thorac Ann 1999;7:30-32
© 1999 Asia Publishing EXchange Pte Ltd
Late Effect of Mechanical Mitral Valve Diameter on Left Ventricular Function
Mert Kestelli, MD,
Rahmi Zeybek, MD,
Gökhan Önem, MD,
Ece Tonguç, MD,
Ahmet Baltalarli, MD,
Nursen Postaci, MD,
Riza Demir, MD,
Mansur Sagban, MD
Department of Cardiovascular Surgery  zmir State Hospital zmir, Turkey
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For reprint information contact: Mert Kestelli, MD Tel: 90 232 323 1265 Fax: 90 232 329 1398 email: ahmetbaltalarli{at}superonline.com 1730 Sok. No. 2/1, Karsiyaka, zmir, Turkey.
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Abstract
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During left ventricular ejection, the minor (transverse) axis of the inner wall shortens by 27% to 37%, while the major axis shortens by 9%. Therefore, shortening of the minor axis accounts for 85% to 90% of the stroke volume and the mitral annulus area alters to assist left ventricular contraction. Mitral valve prostheses with large diameters are preferred but these may lead to systolic malfunction by restricting minor-axis shortening. We studied echocardiographic data of patients who has received mechanical mitral valves with the same inner diameter and opening angle but with different outer diameters (29 mm or 31 mm). Although there was no difference preoperatively in ejection fractions in the two groups, the postoperative ejection fraction was significantly higher in patients with the smaller valve. This finding indicates that mitral valve replacement with a prosthesis of large external diameter caused a deterioration in left ventricular function.
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Introduction
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The mitral valve is an integral part of the left ventricle that plays an important role in left ventricular geometry and mechanics.1–6 The mitral annulus has a sphincter-like function and changes from a circular shape during diastole to become elliptical with a reduction in area of approximately 26% during systole.1,4,7 This change in size and shape is thought to be secondary to relaxation and contraction of the bulbospiral and sinospiral muscle bundles.7–9 During left ventricular ejection, the minor (transverse) axis of the inner wall shortens by 27% to 37%, while the major axis shortens by 9%. Therefore, shortening of the minor axis accounts for 85% to 90% of the stroke volume and the area of the mitral annulus changes to assist ventricular contraction.8 Mechanical prostheses with large diameters are preferred for mitral valve replacement to achieve a low diastolic valvular gradient. However, a large rigid prosthetic valve or annuloplasty ring may lead to systolic malfunction by restricting minor-axis shortening. This study was undertaken to compare postoperative echocardiographic data of patients who had received mechanical mitral valves with the same inner diameter and the same opening angle but different outer diameters (29 mm or 31 mm). Differences in left ventricular function were assessed in relation to the size of the annulus.
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Materials and Methods
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Between January 1994 and July 1994, 150 patients underwent mitral valve replacement at our institution. Twenty-seven of these patients received the Medtronic-Hall mechanical prostheses (Medtronic, Inc., Minneapolis, MN, USA) implanted with interrupted sutures. Fifteen of these patients were given 31-mm valves and 12 were given 29-mm valves. The size of the prosthesis was selected according to the size of the annulus after excision of the native valve. The clinical and echocardiographic data for both groups of patients are given in Table 1
. Five concomitant aortic valve replacements were carried out in each group.
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Table 1. Clinical Characteristics of 27 Patients Before Mitral Valve Replacement with Different Sizes of Medtronic-Hall Prostheses
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Surgery was performed through a median sternotomy in all patients using standard cardiopulmonary bypass with moderate hypothermia (28°C to 32°C). Topical cooling with ice slush and intermittent cold blood cardioplegia were employed for myocardial protection. The left atrium was opened posterior to the interatrial groove and complete excision of the native mitral valve was performed without chordal preservation. The prosthetic valves were implanted with interrupted sutures. Patients who were in atrial fibrillation underwent cardioversion but no patient was in sinus rhythm postoperatively.
Two-dimensional echocardiograms were obtained one week preoperatively and 25 months after the valve replacement procedure to assess left ventricular function and prosthetic valve motion. An Aloka SSD-118 echo-cardiography system (Aloka Corp., Tokyo, Japan) was used for these studies. Two-dimensional echocardiography was performed in all patients from the parasternal long-axis view at the level of the papillary muscles under basal conditions to evaluate systolic and diastolic diameters of the left ventricle. Ejection fractions were calculated from these data.
All data are presented as the mean ± standard deviation. Comparisons between the two groups of patients were performed using the Student t test for grouped data. The difference between groups was considered to be statistically significant when the value of p was less than 0.05.
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Results
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Valvular pathology among the two groups of patients is shown in Table 2. Five patients had aortic valve disease in the group implanted with the 31-mm mitral prostheses and all underwent concomitant aortic valve replacement. In the 29-mm valve group, 6 patients had aortic valve disease but only 4 underwent aortic valve replacement. In the preoperative assessment, there were no significant differences between the two groups of patients in terms of ejection fraction, left ventricular end-diastolic volume index, end-systolic volume index, or stroke volume index (Table 3). Although the mean age was younger and the body surface area was significantly larger in the 31-mm valve group, favoring these patients, the postoperative ejection fraction was significantly higher in the 29-mm group. The mean indices of left ventricular end-diastolic volume and end-systolic volume in the 31-mm valve group were higher than in the 29-mm group but these differences were not statistically significant (Table 3).
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Discussion
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These data indicate that the 29-mm Medtronic-Hall mechanical mitral valve prosthesis interferes less than the 31-mm size with the normal movement of the mitral annulus during the cardiac cycle. Ormiston and colleagues1 found that the mean reduction in mitral annulus area from diastole to systole was 26% in the normal population. The opening diameters of both sizes of the Medtronic-Hall mechanical prostheses are the same, thus the greater restriction of left ventricular systolic contraction by the 31-mm size is due to the larger outer diameter of the rigid prosthetic valve annulus.
Struber and colleagues10 reported that there was no hydrodynamic reason for using a valve larger than 25 mm in the mitral position for patients who exercised moderately. In 1976, Duran and Ubago11 demonstrated that there was no statistically significant difference in left ventricular function among patients with flexible or rigid annuloplasty rings although there was a significant difference in systolic shortening of the basal segment of the left ventricle measured by angiography. David and colleagues12 found that patients with a flexible annuloplasty ring had better left ventricular systolic function than patients with a rigid annuloplasty ring at 2 to 3 months after mitral valve reconstruction for chronic mitral regurgitation secondary to degenerative disease. This positive finding of differences between flexible and rigid rings is in agreement with our observations with mechanical mitral prostheses. Our finding of a statistically significant difference in left ventricular pump function suggests that the smaller 29-mm Medtronic-Hall mechanical mitral valve allows better left ventricular contraction by affording a greater degree of shortening of the minor axis and improving stroke volume.
We concluded that a prosthesis with a ratio of orifice area to body surface area of 3 cm2•m–2 is adequate and left ventricular function is less impaired with the smaller size valve. Thus, the external diameter of a prosthesis should be of secondary importance in size selection.
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References
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Abstract
Introduction
