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Eur J Cardiothorac Surg 2008;33:1049-1054. doi:10.1016/j.ejcts.2008.03.040
Copyright © 2008, European Association for Cardio-thoracic Surgery. Published by Elsevier. All rights reserved.
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Effect of local annular interventions on annular and left ventricular geometry

Tomasz A. Timeka, David Liangb, George T. Daughtersa,c, Neil B. Ingels, Jr.a,c, D. Craig Millera,*

a Department of Cardiothoracic Surgery, Stanford University School of Medicine, Stanford, CA, United States
b Division of Cardiovascular Medicine, Stanford University School of Medicine, Stanford, CA, United States
c Laboratory of Cardiovascular Physiology and Biophysics, Research Institute of the Palo Alto Medical Foundation, Palo Alto, CA, United States

Received 9 December 2007; received in revised form 9 March 2008; accepted 13 March 2008.

* Corresponding author. Address: Department of Cardiothoracic Surgery, Falk Cardiovascular Research Center, Stanford University School of Medicine, Stanford, CA 94305-5247, United States. Tel.: +1 650 725 3826; fax: +1 650 725 3846. (Email: dcm{at}stanford.edu).


   Abstract
Top
 Abstract
1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 References
 
Objective: Etiology-specific annular interventions and annuloplasty rings are now commercially available for the treatment of different types of mitral regurgitation; however, knowledge concerning the effects of local annular alterations on annular and left ventricular (LV) geometry is limited. Methods: Seven adult sheep underwent implantation of eight radiopaque markers around the mitral annulus (MA) and eight markers on the LV (four each on two levels: basal and apical), and one on each papillary muscle tip. Trans-annular septal-lateral (SL) sutures were placed between the corresponding markers on the septal and lateral annulus at valve center (CENT) and near anterior (ACOM) and posterior (PCOM) commissures and externalized. Hemodynamic parameters and 4D marker coordinates were measured before and during SL annular cinching (‘SLAC’; suture tightening 3–5 mm for 20 s) at each suture location. Mitral annular SL diameter, annular area (MAA), and distance from the mid-septal annulus to the LV markers and papillary muscle tips were determined from marker coordinates every 17 ms. Results: End-systolic MAA decreased from 5.93 ± 1.27 to 5.23 ± 1.29* cm2, 5.98 ± 1.16 to 5.33 ± 1.31* cm2, and 6.30 ± 1.65 to 5.61 ± 1.37* cm2 for SLACACOM, SLACCENT, and SLACPCOM, respectively (* p < 0.05 vs pre-cinching). Each SLAC intervention reduced the SL diameter at all three locations, while both SLACACOM and SLACCENT affected ventricular geometry, and SLACPCOM only slightly altered valvular–subvalvular distance. Only SLACCENT altered papillary muscle position. Conclusions: Local annular SL reduction influences remote annular SL dimensions and affects LV geometry. The effect of local annular interventions on global annular geometry and LV remodeling should be considered in surgical or interventional approaches to mitral regurgitation and the design of new annular prostheses as well as supra-annular and sub-annular catheter interventions.

Key Words: Mitral valve • Mitral valve repair • Mitral annulus


   1. Introduction
Top
 Abstract
 1. Introduction
2. Materials and methods
 3. Results
 4. Discussion
 References
 
Ring annuloplasty has become the central component of modern mitral valvuloplasty techniques for most etiologies of mitral insufficiency [1]. Repair of ischemic [2,3] and function [4] mitral regurgitation, in particular, almost universally involves implantation of a ring. Recently, ring undersizing has been introduced by Bolling et al. [5] in an attempt to alter ventricular geometry along with annular remodeling and hence treat the ‘ventricular disease’ that constitutes functional and ischemic mitral regurgitation. Ring annuloplasty, however, has not shown to be perfect for these difficult patients since recurrent MR rates of up to 30% at 6 months postoperatively have been reported [6]. Experimental [7–9] and clinical [10,11] studies have shed more light on the specific geometric perturbations in the annulus and subvalvular apparatus associated with IMR and FMR, thus allowing a more rational approach to ring design. Etiology-specific annuloplasty rings are now commercially available for both IMR [12] and FMR [13] focusing on rectifying pathological geometry at the annular and subvalvular levels. The Edwards IMR ETlogix annuloplasty ring designed specifically for ischemic mitral regurgitation associated with Type IIIb leaflet motion asymmetrically reduces the posterior annulus near the posterior commissure (segments P2–P3) to alleviate posterior leaflet tethering while disproportionally reducing the septal-lateral (SL) annular diameter to augment leaflet coaptation [12]. The Edwards Geoform ring focuses on maximal reduction of the SL annular diameter and pulling up the displaced posteromedial papillary muscle to correct central FMR [13]. Although such custom ring designs are attractive, clinical data confirming their efficacy are lacking and the precise influence of these annular prostheses on ventricular shape is unknown because the effect of local annular interventions on subvalvular geometry has not been fully elucidated. We utilized myocardial marker technology and three trans-annular mitral sutures to study the effect of local annular interventions on annular and subvalvular geometry to develop a functional map of the valvular complex that may further the development of annular interventions and prostheses for IMR and FMR.


   2. Materials and methods
Top
 Abstract
 1. Introduction
 2. Materials and methods
3. Results
 4. Discussion
 References
 
2.1 Surgical preparation
The surgical protocol for this group of animals has been described in a previous report [14], and the current study presents additional analysis of that data set. While on cardiopulmonary bypass, eight myocardial markers were inserted subepicardially on the left ventricle at two equatorial levels (basal and apical) with an additional marker at the left ventricular (LV) apex. Subsequently, a marker was placed on each papillary muscle tip, and eight markers were sewn around the mitral annulus. (Fig. 1 ) A single 3-0 polypropylene suture was anchored with a pledget at the mid-septal annulus (annular ‘saddle horn’) and externalized through the mid-lateral annulus (SLACCENT) to a tourniquet on the epicardial surface. Two other similar sutures (without pledgets) were anchored at the right and left fibrous trigones and externalized through their corresponding opposite portions of the lateral annulus near the anterior (SLACACOM) and posterior (SLACPCOM) commissures, respectively (Figs. 2 and 3 ). Subsequently, the heart was defibrillated, the animal weaned from cardiopulmonary bypass, and transferred immediately to the experimental animal catheterization laboratory where the animals were studied intubated, open-chest, and anesthetized with ketamine (1–4 mg/kg/h IV infusion) and diazepam (5 mg IV bolus as needed). Intravenous esmolol infusion (20–50 µg/kg/min) was utilized to minimize reflex sympathetic responses. Simultaneous biplane videofluoroscopy, hemodynamic data recordings, and transesophageal color Doppler echocardiography were performed before and during each sepal-lateral annular cinching intervention (SLAC; each approximately 3–5 mm suture tightening for 15–20 s). A 5 min recovery period followed each intervention.


Figure 1
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  		Fig. 1. Schematic of the left ventricle illustrating the implanted marker array used in the experiment. Left ventricular and annular markers are shown as filled circles while papillary muscle tip markers are shown as filled squares; APM: anterior papillary muscle tip, PPM: posterior papillary muscle tip.

 

Figure 2
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  		Fig. 2. Diagram of the mitral valve with a schematic of the three SLAC sutures externalized to the epicardial surface through the lateral annulus; ACOM: anterior commissure, PCOM: posterior commissure, AML: anterior mitral leaflet, PML: posterior mitral leaflet, RFT: right fibrous trigone, LFT: left fibrous trigone, AV: aortic valve. Arrow indicates the direction of suture cinching from the lateral toward the septal annulus.

 

Figure 3
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  		Fig. 3. Intra-operative left atrial view of the mitral valve and the SLAC sutures in one of the study animals.

 
All animals received humane care in compliance with the Principles of Laboratory Animal Care formulated by the National Society for Medical Research and the Guide for Care and Use of Laboratory Animals prepared by the National Academy of Sciences and published by the National Institutes of Health (DHEW NIHG publication 85-23, revised 1985). This study was approved by the Stanford Medical Center Laboratory Research Animal Review committee and conducted according to Stanford University policy.

2.2 Data acquisition and analysis
Data acquisition, digital transformation, and 3D reconstruction were performed as previously described [15]. Two to three consecutive steady state beats before and during each SL annular cinching at valve center or near the anterior or the posterior commissure (designated as ‘SLACCENT’, ‘SLACACOM’, and ‘SLACPCOM’, respectively) were recorded for each animal. End-systole (ES) was defined as the frame containing the peak rate of fall of LV pressure (–dP/dt) and end-diastole (ED) as the videofluoroscopic frame containing the peak of the ECG R-wave. Instantaneous LV volume was computed from the epicardial LV markers using a space filling multiple tetrahedral volume method. The degree of mitral regurgitation (MR) was assessed with transesophageal echocardiography.

2.3 Annular and subvalvular geometry
Mitral annular area (MAA) was computed from 3D coordinates of the eight markers sutured to the mitral annulus by first defining an annular centroid and then dividing the annulus into eight triangular areas which were summed to yield total annular area. Mitral SL annular diameter was calculated at the three SLAC suture locations as the distance in 3D space between the corresponding marker pairs across the septal and lateral annulus. To assess the effect of annular cinching interventions on subvalvular geometry, the distance from the anterior (APM) and posterior (PPM) papillary muscle tips to mid-septal annular ‘saddle horn’ was calculated before and after each SLAC intervention. The distance from the anterior, lateral, and posterior LV markers at basal and apical levels to the annular saddle horn was also calculated.

2.4 Statistical analysis
All data are reported as mean plus or minus 1 standard deviation (±1SD). Hemodynamic and marker-derived data from consecutive steady-state beats from each heart were time-aligned at end-diastole. Marker data were calculated over twenty frames before and after end-diastole, thus allowing evaluation over a time period of 650 ms. The mean and SD for each variable at each sampling instant were computed for each condition. Measured values of annular and subvalvular geometry were compared at end-systole. The data obtained before and after cinching interventions were compared using Student's t-test for dependent observations.


   3. Results
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
4. Discussion
 References
 
The average sheep weight was 65 ± 5 kg. The mean cardiopulmonary bypass time was 83 ± 8 min, with an aortic cross-clamp time of 62 ± 7 min. Proper position of implanted myocardial markers was confirmed on post-mortem examination. No substantial degree of mitral regurgitation was observed during all SLAC interventions as assessed with transesophageal Doppler echocardiography. Group mean hemodynamic parameters for during each suture cinching intervention are summarized in Table 1 . SL cinching at each location did not affect left ventricular pressure or volume, and hence did not appear to have a deleterious effect on LV function.


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  		Table 1 Hemodynamics
 
The influence of local annular SL reduction on mitral annular area, perimeter, and SL size is presented in Table 2 . Mitral annular area and perimeter were reduced to similar degree by all three interventions. Each SLAC intervention significantly reduced annular SL diameter not only at the site of suture cinching but also at the other two remote sites. With SLACACOM, the SL diameter near the anterior commissure decreased by 37 ± 9% while the central and posterior commissure SL diameters decreased by 9 ± 4% and 3 ± 2%, respectively. Central suture tightening reduced annular SL diameter at valve center by 18 ± 8% while reducing the SL dimension near the ACOM and the PCOM by 14 ± 5% and 7 ± 5%, respectively. The degree of SL reduction of SLACPCOM at the site of suture cinching was somewhat lower than the other two interventions (14 ± 6%), but this intervention still resulted in a 12 ± 4% reduction of central SL diameter and 5 ± 4% decrease in SL near the anterior commissure. Thus, local SL reduction led to a decrease of SL annular size along the entire line of leaflet coaptation. Alterations in subvalvular geometry with each SLAC intervention are shown in Table 3 . Reduction of the distance from each papillary muscle tip to the mid-septal annulus was observed with central SLAC, but paracommissural SL reduction did not affect papillary muscle tip position relative to the saddlehorn. Similarly, central SL reduction had a greater effect on ventricular geometry at the basal and apical LV levels compared to the paracommissural interventions, yet SLACACOM reduced the distance from the anterior and lateral LV markers to the mid-septal annulus near the ventricular base. Surprisingly, SLACPCOM slightly increased the distance from the posterior LV marker to the annular saddle horn, possibly due to reduction of the annular distance along the orthogonal (SL) axis hence bulging the posterior basal LV marker away from the mid-septal annulus.


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  		Table 2 Annular geometry
 

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  		Table 3 Subvalvular geometry
 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
References
 
Suboptimal clinical results using undersized standard ring annuloplasty for ischemic and functional mitral regurgitation have spurred interest in developing specific etiology-based prostheses targeting specific geometric perturbations. This evolution has been driven by increased understanding of the pathophysiology of ischemic and functional mitral insufficiency, yet the effect of local annular remodeling on annular and ventricular geometry has not been elucidated. Our acute ovine study revealed that regional SL annular reduction not only decreased SL dimension across the entire line of leaflet coaptation, but also significantly influenced left ventricular geometry and papillary muscle tip position.

As ischemic and functional mitral regurgitation have been considered to be a ventricular disease [16], the concept of ventricular remodeling with an undersized ring annuloplasty has become central to the surgical treatment of these challenging patients. Reduction of annular SL size has become paramount in treating IMR [17,18] and FMR [16] to increase leaflet coaptation and simultaneously alter subvalvular geometry. Indeed, reverse LV remodeling has been reported clinically using this approach, yet high recurrent MR rates have been described [6] and survival benefit is questionable [19]. The recently introduced Geoform annuloplasty ring focuses on SL reduction while maintaining orifice area and remodels the shape of the posterior annulus to address the shortcomings of standard undersized ring annuloplasty. This prosthesis reduces the SL annular dimension by 41% centrally [13] which is more than double the 19% reduction achieved by our central SLAC. Thus, one may expect that influence on subvalvular geometry would be even more pronounced than that observed in the current experiment. Prior experiments from our laboratory have shown that reduction suture annuloplasty alters ventricular radius of curvature, [20] and the present study demonstrates that isolated central SL annular cinching can also affect ventricular geometry. We observed papillary muscle repositioning with this intervention, and it is not unreasonable to conjecture that such would also be the effect of the Geoform prosthesis consistent with improved coaptation borne out in mathematical modeling studies [13]. Undersized suture annuloplasty with 20% SL diameter reduction, however, has been associated with reduced anterobasal myocardial fiber shortening in experimental animals [21] suggesting a possible deleterious effect on local LV function. Similarly, isolated SL cinching of the central valve diameter reduces LV systolic wall thickening near the mitral annulus [22]. This was observed, however, with extreme reduction of the SL diameter of approximately 60%, while prior ovine studies suggest that SL reduction of only 22–26% is necessary to abolish acute [23] or chronic [17] ischemic mitral regurgitation. Whether the current degree of septal lateral reduction (maximum of 37% associated with SLACACOM) or that incorporated in the design of the Geoform ring is sufficient to perturb basal LV function remains speculative and requires further study.

Ischemic mitral regurgitation after localized acute myocardial insult has been associated with local remodeling of the lateral LV wall and posterior papillary muscle [8] rather than the symmetrical LV dilatation that is observed with FMR [10]. The specific geometric perturbation is more limited to the posterior commissural portion of the lateral annulus and displacement of the posterior papillary muscle with resultant Type IIIb restricted leaflet motion and mitral insufficiency. The IMR ETlogix annuloplasty ring designed specifically to address these perturbations reduces the SL diameter near the posterior commissure by two ring sizes while reducing the central SL dimension by one size, and drops the height of the P2–P3 annular segment [12]. These design goals are akin to the posterior commissure SLAC used in our study, which reduced SL diameter both near the commissure and the valve center; however, with this intervention, neither posterior papillary tip nor ventricular wall position was brought closer to the annular saddle horn. This was perhaps due to the modest 14% reduction of the SL diameter near the posterior commissure as localized reduction of greater magnitude (37%) near the anterior commissure associated with SLACACOM repositioned the basal anterior and basal lateral LV markers closer to the mid-septal annulus. These findings suggest that localized paracommissural annular reduction can also affect ventricular geometry but the magnitude of diameter change must be greater to achieve a similar effect as a central intervention. Therefore, left ventricular remodeling using IMR ETlogix ring appears feasible and is supported by good short-term clinical outcomes even in patients with large leaflet tenting heights [12].

The goal of this experiment was to study the effect of annular interventions on annular and subvalvular geometry in hopes of developing a functional map of the valvular complex to guide annuloplasty ring design and evolution. As functional and ischemic mitral regurgitation can both be considered to be a ventricular disease, the conceptual leap of addressing a subvalvular problem with an annular solution would be more rational when we understand the mechanisms of linkage between local annular reduction and subvalvular LV remodeling. Our results support the notion that annular procedures can affect the subvalvular apparatus and even local interventions are sufficient to alter ventricular and papillary geometry. The juxtaposition of the current data with the design of commercially available IMR and FMR annuloplasty rings illustrates how mapping of annular and subvalvular interdependence could lead to more refined prosthetic ring design and potentially more clinical benefit.

4.1 Limitations
This study has several limitations, which must be considered in interpreting the data. The annular interventions were performed in normal healthy sheep without valvular pathology and therefore clinical extrapolation of the data is limited; however, our prior sheep studies have shown the efficacy of isolated annular SL reduction in abolishing acute [23] and chronic [17] ovine IMR. The degree of SL reduction was not the same with each SLAC step with the greatest relative reduction achieved with SLACACOM and least with SLACPCOM. This discrepancy may have been due to slippage of the cinched tourniquet during data acquisition or imprecise determination of cinched suture length during suture tightening. We conjectured that the lack of subvalvular effect of SLACPCOM was related to this suboptimal SL reduction as the other paracommissural intervention, SLACACOM, associated with a much greater diameter reduction did affect subvalvular geometry. Overall, however, the degree of SL reduction used in the study was within the range clinically achieved when using annuloplasty rings. There are interspecies differences between the human and the sheep mitral valve, [24] yet currently the ovine model of mitral pathophysiology appears to be favored [25].


   Acknowledgments
 
We appreciate the superb technical assistance provided by Mary K. Zasio, B.A., Carol W. Mead, B.A., and Maggie Brophy, A.S.


   Footnotes
 
{star} Presented at the 21st Annual Meeting of the European Association for Cardio-thoracic Surgery, Geneva, Switzerland, September 16–19, 2007.

{star}{star} This work was supported by grants HL-29589 and HL-67025 from the National Heart, Lung and Blood Institute. Dr Timek was supported by NHLBI INRSA grant HL-10452 and was also a recipient of the Thoracic Surgery Foundation Research Fellowship Award.


   References
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 References
 

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