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Ann Thorac Surg 1998;65:485-490
© 1998 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Cosgrove-Edwards Mitral Ring Dynamics Measured With Transesophageal Three-Dimensional Echocardiography

Department of Cardiopulmonary Surgery, Thoraxcenter, Dijkzigt University Hospital Rotterdam, Rotterdam, the Netherlands
Department of Cardiology, Thoraxcenter, Dijkzigt University Hospital Rotterdam, Rotterdam, the Netherlands

Accepted for publication August 22, 1997.

Dr Van Herwerden, Department of Cardiopulmonary Surgery, Thoraxcenter Bd 156, University Hospital Dijkzigt, Dr Molewaterplein 40, 3015 GD Rotterdam, the Netherlands (e-mail: vanherwerden@thch.azr.nl).

	Abstract

Top Abstract Introduction Patients and Methods Results Comment References

 
Background. Flexible rings have been introduced for improved mitral valve annuloplasty. These rings allow systolic-diastolic variation of both the shape and the area of the valve orifice, mimicking the normal dynamics of the mitral valve ring. In humans, information on the functional behavior of the Cosgrove-Edwards ring during the cardiac cycle is limited at present.

Methods. We used transesophageal three-dimensional echocardiography to analyze mitral valve rings in 19 consecutive patients who underwent annuloplasty because of severe (grade III to IV) mitral regurgitation. Fifteen patients received a Cosgrove-Edwards ring and 4 received a Carpentier ring. The acquisition for three-dimensional reconstruction was performed using the transesophageal rotational technique, immediately after operation. Horizontal cross-sections through the mitral valve ring were selected from the data sets for measurement of the dimensions and surface area of the mitral valve orifice at end-systole and end-diastole. Measurements of the flexible Cosgrove-Edwards ring and the rigid Carpentier ring were compared.

Results. Adequate images for measurements were obtained in 17 of 19 patients. The end-systolic orifice area of the Cosgrove-Edwards ring was 4.21 ± 1.50 cm² (mean ± standard deviation) and the end-diastolic area was 4.81 ± 1.56 cm² (p < 0.0001). No significant change in the orifice area of the Carpentier ring was observed.

Conclusions. Three-dimensional transesophageal echocardiography allows the functional assessment in vivo of mitral valve annuloplasty rings. The Cosgrove-Edwards ring maintains its flexibility early after implantation and demonstrates significant systolic-diastolic changes in the orifice area during the cardiac cycle.

	Introduction

Top Abstract Introduction Patients and Methods Results Comment References

 
Mitral valve reconstruction is now a common surgical procedure for the treatment of mitral regurgitation. Most surgeons consider the use of an annuloplasty ring an essential part of basic repair techniques for obtaining good early and late results [1] [2].

Better understanding of the functional anatomy of both the normal and the dilated mitral annulus has led to the development of a variety of annuloplasty rings that have different shapes and flexibility and are made of different materials [3] [4] [5] [6] [7] [8] [9]. Recently, the Cosgrove-Edwards flexible annuloplasty ring was introduced [10] to preserve the physiologic function of the annulus. It differs from other flexible rings by its shape, which is that of a C rather than a circle.

The function of the normal mitral annulus and some types of annuloplasty rings have been studied by different methods under experimental conditions [3] [11] and by echocardiography in patients [4] [12]. Because of the difficulty of appreciating the mitral annulus in its third dimension with two-dimensional echocardiography, three-dimensional techniques would offer advantages [13] [14] [15].

In this study, multiplane transesophageal echocardiographic (TEE) images of implanted flexible Cosgrove-Edwards and rigid Carpentier rings were acquired and analyzed in the immediate postoperative period with a three-dimensional reconstruction system to study the systolic-diastolic changes in the shape and area of the mitral orifice at the level of the ring.

	Patients and Methods

Top Abstract Introduction Patients and Methods Results Comment References

 
Patients
Nineteen consecutive patients (8 men and 11 women with a mean age of 60 years and a range of 32 to 75 years) were studied immediately after mitral valve repair: 15 received a Cosgrove-Edwards ring (ranging in size from 32 to 38) and 4 received a Carpentier ring (ranging in size from 28 to 34). Eleven patients were in sinus rhythm and 8 were in atrial fibrillation. All patients were operated on for symptomatic mitral insufficiency (grade III to IV on left ventricular angiography).

Annuloplasty Ring Characteristics
The Cosgrove-Edwards Annuloplasty System (Baxter Healthcare Corporation, Edwards CVS Division, Irvine, CA) is composed of a silicone rubber band impregnated with barium sulfate to enable radiographic visualization, and it is wrapped with polyester velour cloth, sewn together with a single seam. It is provided in multiple lengths to accommodate different posterior leaflet dimensions. It is C-shaped, with no anterior portion, to preserve the natural shape of the anterior mitral annulus in this area [10].

The Carpentier ring (Baxter Healthcare Corporation) is composed of a titanium alloy and the suture border is composed of silicone rubber, covered by polyester. The ring is elliptic, or kidney bean–shaped, and has an aperture at its anterior part [16].

Echocardiography
Two-Dimensional Imaging and Three-Dimensional Reconstruction
At the Thoraxcenter, intraoperative TEE is performed routinely in all patients who undergo mitral valve repair before and after extracorporeal circulation to assess the functional result. Standard TEE examination procedures are followed [17].

For this study, a multiplane TEE probe (Varioplane, 5 MHz, 64 elements; Oldelft, Delft, the Netherlands) was used and interfaced with a Toshiba SSH-140A echo system (Toshiba Corp, Otawara-Shi, Japan), and the video output was connected to the three-dimensional reconstruction system (Echoscan; TomTec GmbH, Munich, Germany). The imaging transducer was positioned at the midesophageal level to acquire cross-sectional images of the mitral valve annulus region after extracorporeal circulation. The gain setting was adjusted for optimal visualization of the prosthetic ring. A complete scan of the mitral valve annulus was performed to ensure the position of the transducer and the axis of rotation, which allowed visualization of the ring in the center of the scanned volume during acquisition.

Acquisition of the basic images was controlled by the steering logic system that activated a motor and rotated the transducer in steps of 2 degrees. Appropriate RR intervals and respiratory phases were selected. At each step, during one cardiac cycle, a cross-section was sampled, within the electrocardiographic and respiratory parameters, at 25 frames per second. After recording 90 cardiac cycles, for an arch of 180 degrees, the cross-sections were digitized and stored in the computer memory. Afterward, in an off-line procedure, the acquired cardiac images were formatted in their correct sequence, according to their electrocardiographic phase, in volumetric data sets [18].

Image Display and Reconstruction
From the volumetric data set, any desired cross-section of a structure could be computed and displayed in a dynamic two-dimensional format (anyplane mode). This allowed unlimited cut planes independent of any ultrasonic window. Moreover, from a selected cut plane, new cut planes parallel to it, in a manner similar to computed tomography or magnetic resonance imaging, could be reconstructed and displayed in motion. Rotation angle sliders were used to adjust the selected image in the volumetric data set: alpha rotated the cut plane counterclockwise around the center of rotation in the reference image, beta rotated it around the displayed cut line in the reference image, and gamma rotated it around the perpendicular third axis [18].

To analyze the dimensions and surface areas of the rings, optimal short axis views of the mitral valve region were reconstructed using anyplane and paraplane modes. Guided by the orientation on basic images data, a horizontal cross-section of the ring was obtained by rotating the cut plane around the x and z axes of the three-dimensional coordinate system with the use of alpha and beta rotational angle sliders (anyplane function), and with the paraplane function, the cut plane was positioned at the level of the ring and adapted for each phase of the cardiac cycle (Fig 1). Finally, the ring was displayed in its horizontal cross-section as it is viewed from an atrial perspective (Fig 2).

View larger version (64K): [in this window] [in a new window]   Spatial assessment of the cut plane during the cardiac cycle. (A) Reference image from the volumetric data set representing a longitudinal view of the mitral valve region in end-systole. The cut plane (white bar line) to obtain the horizontal cross-section has been positioned at the level of the annulus. (B) In end-diastole, there is an upward motion of the annulus. Therefore, the cut plane has been readjusted with the use of alpha () and beta (ß) angle sliders. (C) The adjusted cut plane in the end-diastolic position.

View larger version (319K): [in this window] [in a new window]   Horizontal cross-sections of the mitral valve rings. (A) Horizontal cross-section of the Carpentier mitral valve annuloplasty ring. Note the kidney bean shape. (B) Horizontal cross-section of the Cosgrove-Edwards mitral valve annuloplasty ring. Note the C shape without the anterior portion. (A = anterior; L = lateral; M = medial; P = posterior.)

View larger version (156K): [in this window] [in a new window]   Horizontal cross-section of a Cosgrove-Edwards mitral valve annuloplasty ring demonstrating the measured dimensions. The area of the ring (surface within the solid line), the anteroposterior diameter of the ring (interrupted line AP), the lateromedial diameter of the ring (interrupted line LM), the shortest distance between the tips of the ring (solid line AB), and the length of the ring.

For measurements, we used the inner border of the prosthetic ring echoes. Measurements were made three times per phase and a mean value was calculated. All measurements were repeated on the stored patient material for internal validation after 1 to 2 months. All values are reported as means plus or minus the standard deviation. The paired, two-tailed t test was used to analyze the grouped data. Statistical significance was defined as p less than 0.01.

	Results

Top Abstract Introduction Patients and Methods Results Comment References

 
Feasibility
It was possible to acquire and reconstruct the data sets of the 19 patients. Visualization of the Cosgrove-Edwards ring was good in 6 patients, adequate in 7, and poor in 2. The images of the Carpentier ring were good in 2 patients and adequate in 2. The shape of the ring was assessed in all the good and adequate images (17 of 19 patients): the Cosgrove-Edwards ring was C- or U-shaped and the Carpentier ring was D-shaped. The resolution of the echoes was insufficient to visualize the anterior aperture of the Carpentier ring (Fig 2).

Cosgrove-Edwards Ring
The mean internal area of the Cosgrove-Edwards ring was 4.21 ± 1.50 cm² during systole and 4.81 ± 1.56 cm² during diastole. The percentage reduction of the internal area was 12.47% (p < 0.0001). There was no statistically significant difference in the percentage reduction of the internal area of the ring between patients in atrial fibrillation and patients in sinus rhythm. The mean circumference of the ring was 8.35 ± 1.39 cm in systole and 8.74 ± 1.38 cm in diastole (p = 0.0008).

The anteroposterior diameter of the ring was 1.92 ± 0.27 cm in systole and 2.05 ± 0.22 cm in diastole (p < 0.01). The transverse diameter of the ring was 2.45 ± 0.48 cm in systole and 2.50 ± 0.57 cm in diastole. The distance between the tips of the ring, corresponding to the intercommissural area, did not change during the cardiac cycle; it was 2.12 ± 0.42 cm in systole and 2.27 ± 0.52 cm in diastole (Table 1).

View larger version (10K): [in this window] [in a new window]   Correlation between standard (manufacturer) and measured lengths in 13 Cosgrove-Edwards rings during end-diastole (r = 0.95). Note the uniform underestimation of the measurements.

	Comment

Top Abstract Introduction Patients and Methods Results Comment References

 
Our results show that the surface area of the flexible Cosgrove-Edwards mitral ring changes by approximately 13% during the cardiac cycle. The anteroposterior diameter decreases in systole, whereas the transverse diameter does not change. These changes may be explained by the particular material from which the ring is made, which allows it to adapt to the motion of the native annulus after implantation. Not unexpectedly, the Carpentier ring does not show any change in surface area [11] [12].

The mitral annulus is a virtual structure that is delimited anteriorly by the right and left fibrous trigones, and is interconnected with the posterior wall of the aorta to form a "curtain" of fibrous tissue between the aortic valve and the anterior leaflet of the mitral valve that is referred to as the mitral-aortic fibrous continuity. The posterior part of the annulus is ill-defined and without true anatomic substance [19] [20] [21].

The mitral annulus is not a rigid circumferential fibrous ring, but has been shown to have a complex movement in three-dimensional space during the cardiac cycle [3] [11] [14]. It has been shown that both the shape and the size of the mitral annulus vary during the cardiac cycle: the surface area gradually increases during diastole to a maximum in late diastole and then decreases to a minimum in middle to late systole [4] [11]. The degree of this variation in normal native valves ranges from 20% to 35%, depending on the loading conditions [11]. This change in surface area is still present, for instance, in the rheumatic annulus, although it is of smaller magnitude [22]. The shape of the normal native annulus is more circular during diastole and more elliptic during systole [11].

Over the years, different types of mitral annuloplasty rings have been used. The Carpentier ring is rigid and remodels the mitral annulus to the size and shape of a "normal" mitral annulus [23]. The Duran ring is flexible and reduces the annulus to the size of the chosen ring and preserves changes in area of approximately 10% to 14% during the cardiac cycle [11] [12]. The Cosgrove-Edwards mitral annuloplasty ring is highly flexible and produces a measured plication of the posterior annulus. Preliminary data in 5 patients suggested that the annular area decreased by 7% in systole immediately after operation and by 19% after 1 year [10]. This study used a three-dimensional echocardiographic technique, which allowed optimal analysis of the surface area of the ring, and the results showed a change of approximately 15%. These values are consistent with current knowledge of annular motion: because the anteroposterior diameter decreases in systole while the transverse diameter remains constant, the change in shape during the cardiac cycle after the implantation of a Cosgrove-Edwards ring mimics the natural changes in annular shape.

Three-dimensional TEE echocardiography allows the reconstruction, by spatial arrangements, of real horizontal cross-sections of the rings in every phase of the cardiac cycle. Thus, measurements can be made without the need for geometric assumptions or the application of mathematic formulas as is required with other echocardiographic techniques [12].

Echocardiography has expanded our knowledge of the morphology and function of normal and pathologic mitral valves [4] [14] [22]. Conventional transthoracic two-dimensional echocardiography allows us to visualize the mitral valve and to measure the size of the annulus, but different windows are needed because of the lack of planarity of the structure. Two-dimensional TEE overcomes this limitation, allowing continuous visualization of the mitral valve region throughout a 180-degree arc from a fixed transducer position [24]. This still requires the use of geometric assumptions about the shape of the mitral valve and mathematic algorithms to calculate its surface area [12]. Three-dimensional echocardiography, in its actual state, has been shown to be able to reconstruct unique views of the mitral valve apparatus [18] [25] [26] [27]. This study shows its feasibility in assessing the real shape of the prosthetic rings and in providing measurements of their areas or diameters by readjustment of the cut plane throughout the cardiac cycle, without the previously mentioned assumptions.

Making quantitative measurements with three-dimensional echocardiography is extremely time-consuming. An obvious limitation of the method is the complete visualization of nonplanar cardiac structures such as the mitral valve annulus. Consistent measurements of these structures are hampered further by their complex movements in three-dimensional space during the cardiac cycle. This may lead to nonhomogeneous resolution inside the three-dimensional data set from which the measurements are derived. It is important to note that the measurements made in this study were taken from the two-dimensional display of the volumetric data and not from the volume-rendered display. Indeed, measurements made on the basis of the volume-rendered display are not accurate at this stage of development. In this respect, the software for three-dimensional imaging may be subject to further improvement.

The clinical relevance of our findings is limited mainly by the current understanding of the influence of the rigidity of the mitral annulus on the function of the left ventricle. There is contradictory evidence from experimental animal studies on this subject [3] [9] [11]. Clinical studies of adequate power that compare rigid and flexible rings are lacking and load-independent parameters of ventricular function are difficult to obtain. Concern regarding the development of rigidity as a result of fibrotic ingrowth over time still exists. Intuitively, it seems desirable in mitral valve reconstruction to restore or preserve annular sphincteric action together with valve competence and left ventricular dimensions [5] [10]. From this perspective, other parameters of clinical performance, such as rate of reoperation and late outcome, from clinics with no connection to the proponent of a particular type of ring may provide an important contribution to the future practice of valve reconstruction.

Three-dimensional echocardiography is an improved tool for analysis of the complex shape and motion of the mitral annulus in the normal and diseased heart. Using this method, the Cosgrove-Edwards ring seems to comply with the expectations regarding changes in the area and shape of the mitral valve during the cardiac cycle shortly after implantation.

	References

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