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Ann Thorac Surg 2009;88:740-744. doi:10.1016/j.athoracsur.2009.04.069
© 2009 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Impact of Prosthetic Mitral Rings on Aortomitral Apparatus Function: A Cardiac Magnetic Resonance Imaging Study

Philippe Primo Caimmi, MD, PhDa,*, Marco Diterlizzi, MDb, Elena Grossini, MD, PhDc, Emmanouil Ioannis Kapetanakis, MS, MDd, Matteo Gavinelli, MDa, Alessandro Carriero, MD, PhDb, Giovanni Vacca, MD, PhDc

a Department of Cardiac Surgery, Medical School, University of Eastern Piedmont "A. Avogadro," Novara, Piedmont, Italy
b Department of Radiology, Medical School, University of Eastern Piedmont "A. Avogadro," Novara, Piedmont, Italy
c Clinical and Experimental Medicine, Azienda Ospedaliera Universitaria "Maggiore della Carità," Medical School, University of Eastern Piedmont "A. Avogadro," Novara, Piedmont, Italy
d Department of Cardiothoracic Surgery, King's College Hospital, London, United Kingdom

Accepted for publication April 16, 2009.

* Address correspondence to Dr Caimmi, Baluardo M. D'Azeglio no. 5, Novara, 28100, Italy (Email: philippe.caimmi{at}med.unipmn.it).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 References
 
Background: The circumference of the aortic annulus adjusts proportionally with changes in left ventricular volume. These dimensional changes in the aortic annulus improve the left ventricular outflow tract (LVOT) hemodynamics and enhance the anterior mitral valve leaflet (AML) movement. In this study, we investigated the impact of circumferential and partial circumference prosthetic mitral rings on aortomitral apparatus function.

Methods: Forty patients who underwent coronary artery bypass graft surgery and restrictive annuloplasty of the mitral valve annulus through either a partial circumference flexible ring (group A = 20 patients) or a circumferential rigid ring (group B = 20 patients) were evaluated using cardiac magnetic resonance imaging. Imaging was performed at the end of a 2-year follow-up period. Variations in LVOT diameter, transmitral valve gradient, and effective mitral valve area were measured and compared.

Results: Mean variation in LVOT diameter was significantly higher in group A compared with group B (12.7% ± 4% versus 3.6% ± 5%, p = 0.0005). Transmitral valve gradient was higher in group B than in group A (6.2 ± 3 mm Hg versus 4.6 ± 2 mm Hg, p = 0.007), whereas effective mitral valve area was larger in group A than group B (3.9 ± 4 cm2 versus 3.1 ± 6 cm2, p = 0.009). The long-axis cardiac magnetic resonance imaging of patients in group B demonstrated that movement at the base of the AML was hindered with the AML pivotal point appearing to shift posteriorly.

Conclusions: This study demonstrated that the use of circumferential annular rings significantly impairs overall aortomitral apparatus function by reducing outflow diameter and AML movement.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 References
 
Experimental studies have demonstrated that the motion of the aortomitral membrane is interrelated to the changes in the aortic and mitral valve orifice areas during the cardiac cycle. The membrane moves toward the aortic orifice in diastole, allowing for maximal dilatation of the mitral valve annuli, whereas movement of the membrane in the opposite direction occurs during systole, allowing for proper dilatation of the aortic valve. Therefore, the perimeter of the aortic annulus varies proportionally to left ventricular (LV) volume changes: it reaches its largest measure at end systole and smallest at end diastole. These dimensional changes in the aortic annulus improve left ventricular outflow tract (LVOT) hemodynamics and facilitate the movement of the anterior mitral leaflet (AML) [1–8].

The gold standard of mitral valve repair is the annuloplasty procedure with the use of prosthetic rings. The two main types of prosthetic mitral rings currently in clinical use are the complete and partial circumference rings [9]. Extensive dilative disease of the mitral annulus represents the primary surgical criterion for selecting a circumferential ring, and partial circumference rings are utilized in isolated leaflet prolapse without extensive annular dilatation [10, 11].

However, the impact of these different kinds of rings on mitral-aortic function is poorly understood [12]. Through this study, we investigated the effect of complete and partial circumference annuloplasty rings on aortomitral apparatus functional variables through the use of cardiac magnetic resonance imaging (cMRI).


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 References
 
Forty patients undergoing elective, uncomplicated, combined coronary artery bypass graft (CABG) and mitral valve annuloplasty procedures were included in the study. Ethical approval for the study was sought and obtained from our institution's research committee. Individual patient consent was waived by the committee. In 20 patients (group A), a partial circumference flexible ring (Sovering Miniband; Sorin, Saluggia, Italy), size either 40 mm or 50 mm, was used to achieve annular reduction; and in a further group of 20 patients (group B), a rigid circumferential ring (Saddle Ring; St. Jude, St. Paul, MN) size range of 24 mm to 34 mm, was utilized.

All patients underwent cMRI without the use of enhancing contrast. A superconductive 1.5 Tesla MRI scanner (Intera Achieva; Philips, Netherlands) was utilized in the study. Balanced TFE T2w short-axis and two- and four-chamber long-axis cine-sequences were obtained. Pulse sequences utilized and image processing settings are presented in detail in Table 1. Transthoracic echocardiographic and cMRI studies were performed at the completion of 2 years of uneventful follow-up. Variations in LVOT diameter (% change), effective mitral valve area, and mean transmitral gradient in systole and in diastole were measured and compared between the two groups. The same radiologist and cardiac surgeon working in conjunction performed all observations and measurements.


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  		Table 1 Cardiac Magnetic Resonance Imaging Pulse Sequence Details and Image Processing Settings
 
All variables are reported as mean ± SD. The Student's t test was used for all univariable comparisons (Statview Software; SAS Institute, Cary, NC). A p value less or equal to 0.05 was considered significant.


   Results
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 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 References
 
There were no differences in preoperative or operative clinical characteristics between the two groups (Table 2). Similarly, no differences were demonstrated in preoperative or postoperative echocardiographic measurements (Table 3).


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  		Table 2 Preoperative and Operative Clinical Characteristics
 

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  		Table 3 Preoperative and 2-Year Follow-Up Echocardiography Measurements
 
Patients who received a circumferential ring (group B) demonstrated a significant reduction in aortic annular compliance as demonstrated by the change in LVOT diameters in systole and diastole. Mean LVOT diameter change in group B was 3.6% ± 5% compared with 12.7% ± 4% for group A (p = 0.0005; Table 4, Fig 1).


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  		Table 4 Follow-Up Cardiac Magnetic Resonance Imaging Measurements at 2 Years
 

Figure 1
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  		Fig 1. Preserved left ventricular outflow tract (LVOT) kinetics after mitral repair with a partial circumference ring: (A) diastole, (B) systole. Impaired LVOT kinetics after mitral repair with a circumferential ring: (C) diastole, (D) systole.

 
The transmitral mean gradient was also lower in group A compared with group B (4.6 ± 2 mm Hg versus 6.2 ± 3 mm Hg, p = 0.007; Table 4). In addition, the effective mitral orifice in patients with a circumferential ring was significantly smaller than in patients with a partial circumference ring (mitral valve area = 3.1 ± 6 cm2 versus 3.9 ± 4 cm2, p = 0.009; Table 4). The mean effective orifice area in patients who had circumferential rings implanted (group B) was also significantly smaller than the mean planimetric ring area (3.1 ± 6 cm2 versus 4.05 ± 3 cm2, p = 0.0001).

The short-axis cMRI images of the mitral valve of all patients in group B demonstrated a partial obstruction of the mitral valve orifice by the AML due to a significantly impaired opening movement (Fig 2A and B). This obstruction was not observed in group A (Fig 2C and D). The long-axis cMRI images of group B patients demonstrated that movement along the AML base is reduced, with the leaflet's pivotal point appearing to have shifted posteriorly (Fig 3B).


Figure 2
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  		Fig 2. Effective mitral valve area with a circumferential ring: (A) closed mitral valve, (B) open mitral valve. Arrow shows obstruction of the anterior mitral leaflet. Effective mitral valve area with a partial circumference ring: (C) closed mitral valve, (D) open mitral valve.

 

Figure 3
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  		Fig 3. Functional comparison between mitral repair with (A) partial circumference ring, (B) circumferential ring, and (C) healthy patient control. Arrow shows the posterior shift of the anterior mitral leaflet's pivotal point.

 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
References
 
The anatomical and functional interrelationship between the aortic and mitral valves is well known and established [1–8]. Anatomically, both valves are located at the base of the left ventricle, separated from each other by the aortomitral membrane (curtain) and anchored to the fibrous trigones. The upper portion of this membrane is part of the aortic annulus while the lower portion continues as the anterior mitral leaflet [1, 2]. It has been established that the reduction in the mitral orifice during systole is not only due to the reduction of the planar diameters of the valve but also to the displacement of the intertrigonal area toward the posterior mitral annulus and to the posterior displacement of the anterior part of the annulus toward the left atrium. The diastolic change of the mitral annulus is mainly caused by the increase in planar diameters but also by the displacement of the intertrigonal area toward the aortic orifice [12, 13]. The replacement or repair of a mitral valve with a rigid valve prosthesis or a rigid circumferential annuloplasty ring immobilizes the aortomitral membrane, thus causing significant modifications in functional mechanics, impairing the filling/emptying mechanisms of the left ventricle.

In this study, we investigated the impact of using a circumferential mitral ring on aortomitral function and compared it with a partial circumference ring. For this study, a three-dimensional rigid circumferential ring (Saddle Ring; St. Jude), and a flexible partial circumference ring (Sovering Miniband; Sorin) were selected as representative of the newest generation of mitral annuloplasty prostheses. Rigid partial circumference rings are not available, and flexible circumferential rings have an intermediate behavior that does not allow for an effective comparison. Our study groups were comparable both in baseline characteristics and by echocardiographic measurements made preoperatively and at the end of a 2-year follow-up period.

Our study indirectly confirms experimental data previously reported in the literature [11–13]. We observed a significant impairment in LVOT hemodynamics in patients who received circumferential rings compared with partial circumference rings, with a significant reduction in aortic annular compliance and flexibility. Based on previously reported results from the literature, we investigated variations in mitral valve area between our study groups [14–19]. We found that patients who received a circumferential annuloplasty band had a significantly smaller effective mitral area as compared with patients who had a partial circumference ring implanted. This finding is certainly due to the change in orientation and functional mechanics of the aortomitral membrane.

Furthermore, the average effective mitral valve area in patients with circumferential rings was significantly smaller compared with the reported average planimetric area of prostheses implanted in this group. This evidence suggests the existence of an additive mechanism of decreasing the effective valve area in these patients. The short-axis cMRI images of the mitral valve in the group of patients with circumferential rings showed that the AML partially obstructs the mitral valve orifice because the AML opening movement is significantly impaired. This impairment did not exist on patients who had flexible partial circumference rings utilized. We believe this is because, in those patients, the AML is not fixed by the annuloplasty band, thus allowing for movement of the aortomitral membrane during the cardiac cycle.

The long-axis cMRI images of patients with circumferential rings demonstrated that the impairment of the AML opening movement is caused by the blocking of the lower part of aortomitral membrane, which is in continuity with the base of the leaflet (Fig 3A,B,C). The insertion of the circumferential ring causes a posterior shift of the AML pivotal point and causes the decrease in its opening movement (Fig 4). We specifically observed that minimal displacement of the pivotal point has significant consequences on the valve's effective area because of the conical profile of the opened mitral valve produced by the impairment of the AML opening movement (Fig 4).


Figure 4
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  		Fig 4. Illustration of the changes in the mitral valve annulus: note that the annular diameter (a) of the circumferential ring (b) is higher than the effective orifice diameter (c) because of decrease of anterior mitral leaflet opening movement (d).

 
In conclusion, our study demonstrated that circumferential mitral valve rings can significantly alter LVOT hemodynamics and AML movement by blocking the lower part of the aortomitral membrane's physiologic movement. Furthermore, the implantation of circumferential rigid rings causes a shifting in the pivotal point of the AML toward the free margin of the leaflet, reducing even more the effective mitral valve area. This study contributes to the current knowledge of the impact of annuloplasty rings on mitral and aortic annuli dynamics and their interaction, thereby forming a rational basis for improving mitral annuloplasty ring designs.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 References
 

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  2. Anderson RH, Devine WA, Ho SY, Smith A, McKay R. The myth of the aortic annulus: the anatomy of the subaortic outflow tract Ann Thorac Surg 1991;52:640-646.[Abstract/Free Full Text]
  3. Pang DC, Choo SJ, Luo HH, Shomura Y, Daniel S, Nikolic S, et al. Significant increase of aortic root volume and commissural area occurs prior to aortic valve opening J Heart Valve Dis 2000;9:9-15.[Medline]
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  6. Van Renterghem RJ, Van Steenhoven AA, Arts T, Reneman RS. Deformation of the dog aortic valve during the cardiac cycle Eur J Physiol 1988;412:647-653.[Medline]
  7. Tsakiris AG, Von Bernuth G, Rastelli GS, Bourgeois MJ, Titus JL, Wood EG. Size and motion of the mitral valve annulus in anaesthetized intact dogs J Appl Physiol 1971;30:611-618.[Free Full Text]
  8. Ormiston JA, Shah P, Tei C, Wong M. Size and motion of the mitral valve annulus in man Circulation 1981;64:113-120.[Abstract/Free Full Text]
  9. Kumar N, Kumar M, Duran CMG. A revised terminology for recording surgical findings of the mitral valve J Heart Valve Dis 1995;4:70-75.[Medline]
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  11. Flachskampf FA, Chandra S, Gadipatti A, et al. Analysis of shape and motion of the mitral annulus in subjects with and without cardiomyopathy by echocardiographic 3-dimensional reconstruction Am Soc Echocardiogr 2000;13:277-287.
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