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Ann Thorac Surg 2008;86:1473-1478. doi:10.1016/j.athoracsur.2008.07.073
© 2008 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Value of Augmented Reality-Enhanced Transesophageal Echocardiography (TEE) for Determining Optimal Annuloplasty Ring Size During Mitral Valve Repair

Joerg Ender, MDa,*, Jasmina Koncar-Zeh, MDa, Chirojit Mukherjee, MDa, Stephan Jacobs, MDb, Michael A. Borger, MD, PhDb, Christoph Viola, BSc, Michael Gessat, BSd, Jens Fassl, MDa, Friedrich W. Mohr, MD, PhDb, Volkmar Falk, MD, PhDb

a Department of Anesthesiology and Intensive Care Medicine II, Leipzig Heart Center, University of Leipzig, Leipzig, Germany
b Department of Cardiac Surgery, Leipzig Heart Center, University of Leipzig, Leipzig, Germany
d Innovation Center for Computer Assisted Surgery, University of Leipzig, Leipzig, Germany
c TomTec, Munich, Germany

Accepted for publication July 14, 2008.

* Address correspondence to Dr Ender, Department of Anesthesiology and Intensive Care Medicine II, Leipzig Heart Center, University of Leipzig, Struempellstr 39, Leipzig, 04289, Germany (Email: joerg.ender{at}medizin.uni-leipzig.de).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Background: Mitral valve (MV) annuloplasty is an integral part of MV repair, but sizing under direct vision is occasionally challenging. Furthermore, traditional sizing is not possible for percutaneous MV repair techniques. This study compared augmented reality-enhanced three-dimensional (3D) transesophageal echocardiography (TEE) for determining MV annuloplasty size with conventional surgical sizing.

Methods: In patients undergoing elective MV repair, a 3D MV reconstruction was performed using TEE. Modified 4D valve assessment software was used to create 3D computer-aided design models of standard annuloplasty rings (28 to 36 mm), which were stored in a digital database. These virtual 3D annuloplasty ring templates were superimposed on the preoperative 3D TEE reconstructions of the MV, and results were compared with conventional sizing under direct vision. A post hoc validation of the 3D models was performed using the implanted rings as a control. The echocardiographer was blinded to the implanted ring size.

Results: The study included 50 patients. The correlation between the selected 3D annuloplasty ring template and the implanted annuloplasty ring size was 0.83. Thirty ring templates (60%) were the same size as the implanted annuloplasty ring, 19 templates (38%) differed by ±2 mm in size, and 1 template differed by +4 mm. Postoperatively, the validation protocol revealed a correlation of 0.94 between the size of the ring templates and the implanted annuloplasty prostheses.

Conclusions: Augmented reality-enhanced TEE for determining optimal annuloplasty ring size during MV repair correlates well with conventional surgical sizing and may facilitate future percutaneous MV repair techniques.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
The mitral valve (MV) annulus is a dynamic, saddle-shaped structure that has variations in size of 15% to 35% during the cardiac cycle [1]. Remodeling of the MV annulus by ring annuloplasty is considered an essential component of an effective, durable MV repair. Several alternatives for MV annuloplasty rings are available, including rigid vs flexible, complete vs partial, and planar-shaped vs saddle-shaped rings. Although the debate on the best type of ring is likely to continue for the foreseeable future, there is general agreement that correct sizing is of major importance when performing an effective MV repair.

Mitral annuloplasty ring sizing is usually performed using commercial sizer models during cardioplegic cardiac arrest on the unloaded and nonbeating heart. Sizing is based on the inter-trigonal distance or the height of the anterior mitral leaflet, with the latter usually being considered the most important criterion. Accurate sizing of the mitral annulus may occasionally be challenging in patients in whom good exposure of the MV is difficult to obtain.

The use of intraoperative transesophageal echocardiography (TEE) is a class I indication to detect the underlying MV pathology and to grade the severity of mitral regurgitation during MV repair [2]. Three-dimensional (3D) reconstruction of TEE images can be used to accurately define all of the components of the MV, including the mitral annulus. The value of echocardiography for determining optimal ring annuloplasty size has been recently reported, and echocardiographic dimensions of the anterior MV leaflet have been shown to correlate well with in vivo sizing [3].

Image-guided selection of the correct prosthetic valve size is already a clinical reality in patients undergoing transapical and transfemoral aortic valve implantation procedures [4]. Percutaneous MV repair procedures are already under clinical investigation, with several new devices in the preclinical stages [5]. Conventional sizing of the MV annulus with surgical exposure and sizing tools will not be possible for such procedures. Although current percutaneous MV repair devices do not require specific sizing of the MV annulus, it is likely that future generations will be tailored to the patient's anatomy.

We therefore investigated the utility of augmented reality techniques to superimpose 3D computer-aided design (CAD) annuloplasty ring models on 3D echo reconstructions of the MV and its annulus, and compared the results with the gold standard of conventional surgical sizing during MV repair.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Acknowledgments
 References
 
Patients undergoing isolated, elective, minimally invasive MV repair through a right lateral minithoracotomy between January and October 2007 were eligible to participate in the study. The local Ethics Committee approved the research protocol. Written informed consent was obtained for 55 patients (44 men, 11 women). Five patients had to be excluded because the implanted MV annuloplasty ring size (ie, sizes 26, 38, or 40) was not available as a computerized model, resulting in a final sample size of 50.

After the induction of anesthesia, a 3D reconstruction of the MV was performed preoperatively using standard TEE equipment (Sonos 5500, Philips, Eindhoven, The Netherlands) and specialized 3D reconstruction software (4D-Valve Assessment, TomTec; Munich, Germany). The reconstruction software automatically created 2D video loops in 5° increments (range, 0° to 180°) in the midesophageal 4-chamber view. These 2D loops were digitally stored, and 3D reconstruction was subsequently performed (Fig 1).


Figure 1
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  		Fig 1. Three-dimensional reconstruction of the mitral valve.

 
The commercially available 3D reconstruction software was modified specifically for this study to allow for the integration of 3D computer models of standard MV annuloplasty rings. The CAD models of different sized Carpentier-Edwards Physio annuloplasty rings (Edwards Lifesciences, Irvine, CA) were generated using computed tomography (Brilliance, Philips; Fig 2). Geometries of the artificial rings were defined by triangular meshes stored in a polygon (PLY) file format. The mitral annulus was represented by a 3D cubic spline curve that was defined by a set of points. With the help of Ring Tool, an automated ring adjustment algorithm that was developed within the 4D-Valve Assessment software, the ring models could be superimposed on the 3D reconstruction of the MV (Fig 3). The entire process, which is described in more detail subsequently, took approximately 15 minutes.


Figure 2
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  		Fig 2. Computer-aided design model of a Carpentier-Edwards Physio ring size 34 (Edwards Lifesciences, Irvine, CA).

 

Figure 3
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  		Fig 3. Superimposed computer-aided design model of the Carpentier-Edwards Physio ring (Edwards Lifesciences, Irvine, CA) over the three-dimensional reconstructed mitral valve.

 
The first step of the ring adjustment algorithm was to translate the annuloplasty ring to match its center of gravity within that of the annulus. The center of gravity was determined by averaging over the set of points defining the respective geometry. In the next step, the principal axes of inertia were calculated for the ring and for the mitral annulus. The inertial tensors were determined and underwent a singular value decomposition to get the tensor matrix's eigenvalues and eigenvectors. In this context the eigenvalues corresponded to the principal moments of inertia and the eigenvectors to the principal axes of inertia [6].

Because of the asymmetric shapes of both the annulus and the ring model, the orientation of some principal axes of inertia were flipped to get the same relative orientation for both geometries. This was accomplished by some geometric assumptions exploiting the saddle shape of the ring and annulus. Next, the principal axes were sorted by descending eigenvalues, and the annuloplasty ring was rotated to match the ring's axes with those of the mitral annulus. Because the annuloplasty ring cannot lie directly inside the annulus in vivo, the last step was to shift the ring by a small, predetermined distance along its main principle axis toward the center of the left atrium. If the resulting adjustment was unsatisfactory, the Ring Tool offered the opportunity to arbitrarily orient the computerized ring model directly in space or accordingly in any 2D planar reconstruction throughout the ultrasound volume.

A validation protocol was performed at the end of the MV repair, after the annuloplasty ring was implanted. A second 3D reconstruction was performed, and the echo investigator was blinded to the actual size of the implanted ring. The modified 4D-Valve Assessment software was used to determine the size of the annuloplasty ring by superimposing the computerized ring models over the actual implanted ring. The implanted annuloplasty ring was visualized either in a 3D reconstruction (Fig 4) or in three 2D planes that were determined from the 3D model (Fig 5). The size of the selected computerized annuloplasty ring was then compared with the size of the actual implanted annuloplasty ring.


Figure 4
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  		Fig 4. Superimposed computer-aided design model over the actual implanted ring in a three-dimensional reconstruction.

 

Figure 5
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  		Fig 5. Superimposed computer-aided design (CAD) model over the actual implanted annuloplasty ring in three two-dimensional (2D) views based on 3D reconstruction. The CAD model in 2D views is seen as yellow circles.

 
Continuous variables are expressed as mean ± standard deviation and categoric variables as proportions. Statistical analyses were performed with SPSS software (SPSS Inc, Chicago, IL). Significance testing was performed with the Pearson test.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Acknowledgments
 References
 
The average patient age was 56.1 ± 9.2 years. Preoperative MR grade was 3.5 ± 0.5, and New York Heart Association functional class was 2.9 ± 0.8.

Successful minimally invasive MV repair with implantation of an annuloplasty ring was performed in all 50 patients. Isolated ring implantation was performed in 4 patients (8%), and the remainder required concomitant MV leaflet or chordal repair, or both. In 40 patients (80%), MV prolapse was corrected by creating artificial chords, whereas 6 patients (12%) required partial leaflet resection.

Operative mortality was 0%, and no strokes occurred. Residual mitral regurgitation after MV repair was none in 33 patients (66%), trivial in 10 (20%), mild in 3 (6%), and mild to moderate in 4 (8%). None of the patients required reoperation for residual mitral regurgitation. All patients were discharged between postoperative day 9 and 12.

The correlation between the suggested ring size from the preoperative 3D reconstruction and the actual implanted annuloplasty ring was 0.83 (p < 0.01; Table 1). The correlation between the implanted ring size and the postoperative ring tool assessment was 0.94 (p < 0.01; Table 2). Image quality was judged to be excellent in 7 patients (14%), good in 30 (60%), and poor in 13 (26%).


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  		Table 1 Differences in Annuloplasty Ring Sizes Before Implantation
 

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  		Table 2 Differences in Annuloplasty Ring Sizes After Implantation
 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Acknowledgments
 References
 
The low perioperative risk and the recently published results of conservative therapy in patients with asymptomatic severe mitral regurgitation [7] has led experts to suggest that the threshold for surgical intervention should be lowered for MV repairs [5]. If an operation is to be undertaken in asymptomatic patients, however, it is important to ensure a high success rate of MV repair because MV replacement is associated with worse long-term outcomes [8, 9]. The implantation of an annuloplasty ring has been long recognized as an essential component of a durable MV repair [10–12]. The annuloplasty ring reinforces suture lines, improves leaflet coaptation, and prevents further annular dilation.

Different methods of intraoperative annuloplasty ring sizing have been suggested, including measuring the distance between the medial and lateral trigones, assessing the size of the anterior MV leaflet, and simple "eyeballing." Although sizing of the anterior MV leaflet with commercially available sizers is the most commonly used method, it is subject to observer interpretation and may be more difficult for surgeons with limited MV repair experience, particularly if exposure of the MV is suboptimal. Distortion of the mitral annulus during exposure of the MV may also complicate the decision-making process. In addition, conventional ring sizing is performed during nonbeating heart conditions on the cardioplegic, arrested heart, which may be less physiologic than during normal systole and diastole. The mitral annulus is a very dynamic structure, with variations in surface area of 15% to 35% during the cardiac cycle [1].

A mismatch in annular ring sizing may lead to either residual mitral regurgitation if the annuloplasty ring is too large, or mitral stenosis if the ring is too small. Ring oversizing can cause postoperative mitral regurgitation in the early postoperative period if there is decreased leaflet coaptation or in the late postoperative period if the annuloplasty ring dehisces as a result of excessive tension on the fixation sutures. Ring undersizing may also lead to early mitral regurgitation by systolic anterior motion of the anterior leaflet, with subsequent left ventricular outflow tract obstruction. The systolic anterior motion phenomenon tends to occur in patients with a reduced distance between the interventricular septum and leaflet coaptation as well as in patients with a relatively large posterior leaflet [13–15].

Percutaneous (transapical and transfemoral) aortic valve implantation procedures are being performed without conventional sizing of the aortic annulus [4]. Because the valve and annulus are never exposed, image-guided methods are used to determine the correct size of the implanted device. Percutaneous MV repair procedures are already a clinical reality, and several new devices and procedures are in preclinical development [5]. Although device sizing is not yet an issue for leaflet-based procedures, annular-based devices will likely require some tailoring to the patient's MV anatomy. Because conventional surgical sizing is not possible for percutaneous procedures, alternative methods of device size selection will need to be developed.

Intraoperative TEE is known to be invaluable for supplying information on MV leaflet pathology and the mechanisms of mitral regurgitation during MV repairs [16, 17]. Previous investigators have attempted to perform sizing of the mitral annulus using echocardiographic measures only. Cook and colleagues [18] examined annuloplasty ring size selection based on intraoperative TEE in patients undergoing robotic MV repair, where insertion of a conventional annuloplasty-sizing device through a small, port access incision is a challenge [18]. These investigators observed postrepair systolic anterior motion in 3 of 11 patients (27%), leading them to conclude that isolated TEE sizing of the mitral annulus is inadequate. Choo and colleagues [19] attempted to determine the inter-trigonal distance based on measurement of the aortic valve diameter. This technique has not been adopted clinically, however, probably because of variability in aortic annular dimensions in patients with mitral regurgitation.

Although MV repair is the procedure of choice for patients with mitral prolapse, residual or recurrent mitral regurgitation occurs in 0.5% to 1.5% of patients per year [20]. We postulated that the use of reality-enhanced 3D echocardiography may allow surgeons to virtually select an annuloplasty ring size preoperatively and subsequently modify the size if it appears suboptimal during diastolic or systolic left ventricular function. Such a capability may improve ring-sizing techniques and outcomes for MV repair, particularly for surgeons with limited MV repair experience or during unusual cases where visualization of the MV apparatus is a challenge. Virtual sizing may also play an important role in patients undergoing percutaneous or robotic MV repair and may prove to be a valuable tool for resident education and training. We therefore studied the feasibility and reliability of reality-enhanced 3D echocardiographic ring sizing during minimally invasive MV repair.

We investigated modified TEE-guided sizing using computerized ring models that could be superimposed on 3D reconstructions of the mitral annulus. All patients received a Carpentier-Edwards Physio ring in the current study because it is our device of choice for MV repairs. We observed a good correlation between preoperative virtual annular sizing and the size of the actual implanted annuloplasty ring (r = 0.83). In addition, we observed very good correlation (r = 0.94) between the two techniques during our validation protocol, which was performed after annuloplasty ring implantation by an echocardiographer who was blinded to implanted ring size. We believe that the preoperative and validation protocol results provided evidence for the accuracy of the 3D reality-enhanced sizing technique.

The precise agreement of ring sizing by surgical and 3D TEE inspection in 30 of 50 patients may reflect that all MV repairs were performed by experienced surgeons in a high-volume center, where more than 350 MV repairs are performed each year. In 1 patient where the implanted ring size was 4 mm smaller than the TEE-guided ring, a large resection of the posterior mitral leaflet and the anterior mitral leaflet had been performed, leading to substantial undersizing. Some sort of calibration may therefore be required if an extensive leaflet resection is planned. The question of whether a difference of 2 mm in ring size between both methods is clinically important cannot be answered by this retrospective study. Poor image quality in 13 patients may explain some disparities. This problem is most likely to be solved in the near future, however, with the availability of real-time 3D TEE.

We conclude that reality-enhanced 3D TEE ring sizing with the Ring Tool software accurately predicts the size of the annuloplasty ring compared with the gold standard of conventional surgical sizing. The Ring Tool may be particularly helpful for surgeons with limited MV repair experience, for patients in whom exposure of the MV apparatus is very challenging, for surgical education, and for patients undergoing robotic MV repair. In addition, our technique may prove to be very useful in patients undergoing future annular-based percutaneous MV repair procedures.


   Acknowledgments
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
References
 
The Innovation Center Computer Assisted Surgery (ICCAS) at the Faculty of Medicine at the University of Leipzig is funded by the German Federal Ministry for Education and Research (BMBF) and the Saxon Ministry of Science and the Fine Arts (SMWK) in the scope of the initiative "Unternehmen Region" with the grant numbers 03 ZIK 031 and 03 ZIK 032.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 

  1. Timek TA, Miller DC. Experimental and clinical assessment of mitral annular area and dynamics: what are we actually measuring? Ann Thorac Surg 2001;72:966-974.[Abstract/Free Full Text]
  2. Thys D, Abel M, Bollen BA, et al. Practice guidelines for perioperative transesophageal echocardiography: a report by the American Society of Anesthesiologists and the society of cardiovascular anesthesiologists task force on transesophageal echocardiography Anesthesiology 1996;84:986-1006.[Medline]
  3. Brinster DR, Unic D, D'Ambra MN, Nathan N, Cohn LH. Midterm results of the edge-to-edge technique for complex mitral valve repair Ann Thorac Surg 2006;81:1612-1617.[Abstract/Free Full Text]
  4. Walther T, Falk V, Kempfert J, et al. Transapical minimally invasive aortic valve implantation: the inital 50 patients Eur Jl Cardiothor Surg 2008;33:933-938.
  5. Feldman T. Percutaneous mitral valve repair J Interv Cardiol 2007;20:495-508.[Medline]
  6. Abdi H. Eigen-decomposition: eigenvalues and eigenvectorsIn: Salkind NJ, editor. Encyclopedia of Measurement and Statistics. Sage; 2007. pp. 304-308.
  7. Sarano ME, Avierinos JF, Zeitoun DM, et al. Quantitative determinants of the outcome of asymptomatic mitral regurgitation N Engl J Med 2005;352:875-883.[Abstract/Free Full Text]
  8. Thourani VH, Weintraub WS, Guyton RA, et al. Outcomes and long-term survival for patients undergoing mitral valve repair versus replacement Circulation 2003;108:298-304.[Abstract/Free Full Text]
  9. Zhao L, Kolm P, Lewis C, et al. Comparison of recovery after mitral repair and replacement in patients with mitral valve regurgitation J Thorac Cardiovasc Surg 2007;133:1257-1263.[Abstract/Free Full Text]
  10. Carpentier A, Deloche A, Dauptain J, et al. A new reconstructive operation for correction of mitral and tricuspid insufficiency J Thorac Cardiovasc Surg 1971;61:1-13.[Medline]
  11. Lillehei CW, Gott VL, Dewall RA, Varco RL. Surgical correction of pure mitral insufficiency by annuloplasty under direct vision Lancet 1957;77:446-449.
  12. Meyer MA, von Segesser LK, Hurni M, Stumpe F, Eisa K, Ruchat P. Long-term outcome after mitral valve repair: a risk factor analysis Eur J Cardiothorac Surg 2007;32:301-307.[Abstract/Free Full Text]
  13. Grossi EA, Galloway AC, Kallenbach K, et al. Early results of posterior leaflet folding plasty for mitral valve reconstruction Ann Thorac Surg 1998;65:1057-1059.[Abstract/Free Full Text]
  14. Lee KS, Stewart WJ, Lever HM, et al. Mechanism of outflow tract obstruction causing failed mitral valve repair Circulation 1993;88:4-29.[Free Full Text]
  15. Maslow AD, Regan MM, Haering JM, et al. Echocardiographic predictors of left ventricular outflow tract obstruction and systolic anterior motion of the mitral valve after mitral valve reconstruction for myxomatous valve disease J Am Coll Cardiol 1999;34:2096-2104.[Abstract/Free Full Text]
  16. Cho L, Gillinov AM, Cosgrove 3rd DM, Griffin BP, Garcia MJ. Echocardiographic assessment of the mechanisms of correction of bileaflet prolapse causing mitral regurgitation with only posterior leaflet repair surgery Am J Cardiol 2000;86:1349-1351.[Medline]
  17. Freeman WK, Schaff HV, Khandheria BK, et al. Intraoperative evaluation of mitral valve regurgitation and repair by transesophageal echocardiography: incidence and significance of systolic anterior motion J Am Coll Cardiol 1992;20:599-609.[Abstract]
  18. Cook RC, Nifong LW, Lashley GG, et al. Echocardiographic measurements alone do not provide accurate non-invasive selection of annuloplasty band size for robotic mitral valve repair J Heart Valve Dis 2006;15:521-523.[Medline]
  19. Choo SJ, Olomon J, Bowles C, et al. An in vitro study of the correlation between aortic valve diameter and mitral intertrigonal distance: a simple method to select the correct mitral annuloplasty ring size J Heart Valve Dis 1998;7:593-597.[Medline]
  20. Suri RM, Schaff HV, Dearani JA, et al. Survival advantage and improved durability of mitral repair for leaflet prolapse subsets in the current era Ann Thorac Surg 2006;22:819-826.



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