Mitral valve repair with the Colvin-Galloway Future Band

Heart Valve Biomechanics and Underlying Mechanobiology

Heart valves control unidirectional blood flow within the heart during the cardiac cycle. They have a remarkable ability to withstand the demanding mechanical environment of the heart, achieving lifetime durability by processes involving the ongoing remodeling of the extracellular matrix. The focus of this review is on heart valve functional physiology, with insights into the link between disease-induced alterations in valve geometry, tissue stress, and the subsequent cell mechanobiological responses and tissue remodeling. We begin with an overview of the fundamentals of heart valve physiology and the characteristics and functions of valve interstitial cells (VICs). We then provide an overview of current experimental and computational approaches that connect VIC mechanobiological response to organ- and tissue-level deformations and improve our understanding of the underlying functional physiology of heart valves. We conclude with a summary of future trends and offer an outlook for the future of heart valve mechanobiology, specifically, multiscale modeling approaches, and the potential directions and possible challenges of research development. © 2016 American Physiological Society. Compr Physiol 6:1743-1780, 2016.

Mitral Valve Annuloplasty Rings: Review of Literature and Comparison of Functional Outcome and Ventricular Dimensions

In the past decades, more than 40 mitral valve annuloplasty rings of various shapes and consistency were marketed for mitral regurgitation (MR), although the effect of ring type on clinical outcome remains unclear. Our objective was to review the literature and apply a simplification method to make rings of different shapes and rigidity more comparable. We studied relevant literature from MEDLINE and EMBASE databases related to clinical studies as well as animal and finite element models. Annuloplasty rings were clustered into 3 groups as follows: rigid (R), flexible (F), and semirigid (S). Only clinical articles regarding degenerative (DEG) or ischemic/dilated cardiomyopathy (ICM) MR were included and stratified into these groups. A total of 37 rings were clustered into R, F, and S subgroups. Clinical studies with a mean follow-up of less than 1 year and a reported mean etiology of valve incompetence of less than 60% were excluded from the analysis. Forty-one publications were included. Preimplant and postimplant end points were New York Heart Association class, left ventricular ejection fraction (LVEF), left ventricular end-systolic dimension (LVESD), and left ventricular end-diastolic dimension (LVEDD). Statistical analysis included paired-samples t test and analysis of variance with post hoc Bonferroni correction. P < 0.05 indicated statistical difference. Mean ± SD follow-up was 38.6 ± 27 and 29.7 ± 13.2 months for DEG and ICM, respectively. In DEG, LVEF remained unchanged, and LVESD decreased in all subgroups. In our analysis, LVEDD decreased only in F and R, and S did not change; however, the 4 individual studies showed a significant decline. In ICM, New York Heart Association class improved in all subgroups, and LVEF increased. Moreover, LVESD and LVEDD decreased only in F and S; R was underpowered (1 study). No statistical difference among R, F, and S in either ICM or DEG could be detected for all end points. Overall, owing to underpowered data sets derived from limited available publications, major statistical differences in clinical outcome between ring types could not be substantiated. Essential end points such as recurrent MR and survival were incomparable. In conclusion, ring morphology and consistency do not seem to play a major clinical role in mitral valve repair based on the present literature. Hence, until demonstrated otherwise, surgeons may choose their ring upon their judgment, tailored to specific patient needs.

An inverse modeling approach for stress estimation in mitral valve anterior leaflet valvuloplasty for in-vivo valvular biomaterial assessment

Estimation of regional tissue stresses in the functioning heart valve remains an important goal in our understanding of normal valve function and in developing novel engineered tissue strategies for valvular repair and replacement. Methods to accurately estimate regional tissue stresses are thus needed for this purpose, and in particular to develop accurate, statistically informed means to validate computational models of valve function. Moreover, there exists no currently accepted method to evaluate engineered heart valve tissues and replacement heart valve biomaterials undergoing valvular stresses in blood contact. While we have utilized mitral valve anterior leaflet valvuloplasty as an experimental approach to address this limitation, robust computational techniques to estimate implant stresses are required. In the present study, we developed a novel numerical analysis approach for estimation of the in-vivo stresses of the central region of the mitral valve anterior leaflet (MVAL) delimited by a sonocrystal transducer array. The in-vivo material properties of the MVAL were simulated using an inverse FE modeling approach based on three pseudo-hyperelastic constitutive models: the neo-Hookean, exponential-type isotropic, and full collagen-fiber mapped transversely isotropic models. A series of numerical replications with varying structural configurations were developed by incorporating measured statistical variations in MVAL local preferred fiber directions and fiber splay. These model replications were then used to investigate how known variations in the valve tissue microstructure influence the estimated ROI stresses and its variation at each time point during a cardiac cycle. Simulations were also able to include estimates of the variation in tissue stresses for an individual specimen dataset over the cardiac cycle. Of the three material models, the transversely anisotropic model produced the most accurate results, with ROI averaged stresses at the fully-loaded state of 432.6±46.5 kPa and 241.4±40.5 kPa in the radial and circumferential directions, respectively. We conclude that the present approach can provide robust instantaneous mean and variation estimates of tissue stresses of the central regions of the MVAL.

In-vivo analysis of selectively flexible mitral annuloplasty rings using three-dimensional echocardiography

BACKGROUND

Selectively flexible rings, Colvin-Galloway (CG) Future and Carpentier-Edwards (CE) Physio II, are used for annuloplasty during mitral valve repair to facilitate dynamic annular motion while preventing annular dilation. In this study, we assessed the extent and nature of the flexibility of these rings in vivo, which has not been objectively demonstrated.

METHODS

Three-dimensional transesophageal echocardiography was used intraoperatively to acquire data regarding dynamic motion of mitral annuli and annuloplasty rings in 33 patients undergoing mitral repair (15 CG Future and 18 CE Physio II) and in 15 control patients. Data were analyzed to assess the dynamic changes in annular geometry after implantation of selectively flexible rings.

RESULTS

After annuloplasty, there was an immediate and significant decrease in annular displacement (p < 0.001) and annular displacement velocity (p < 0.01). Dynamic change in multiple variables including anteroposterior diameter (p < 0.001) and annular area (p < 0.001) was also significantly depressed. In comparison with normal mitral valves, partially flexible rings allowed limited dynamic motion: percentage changes in anteroposterior diameter (p < 0.001), anterolateral posteromedial diameter (p < 0.001), and total circumference (p < 0.001) were significantly lower. Compared with each other, the two rings resulted in similar changes in anterior annulus length (p = 0.93), posterior annular length (p = 0.82), and annular area (p = 0.31).

CONCLUSIONS

Mitral annular dynamics were uniformly depressed after implantation of these rings. Selective flexibility could not be demonstrated in vivo using echocardiographic data.

A novel shape memory alloy annuloplasty ring for minimally invasive surgery: design, fabrication, and evaluation

A novel annuloplasty ring with a shape memory alloy core has been developed to facilitate minimally invasive mitral valve repair. In its activated (austenitic) phase, this prototype ring has comparable mechanical properties to commercial semi-rigid rings. In its pre-activated (martensitic) phase, this ring is flexible enough to be introduced through an 8-mm trocar and easily manipulated with robotic instruments within the confines of a left atrial model. The core is constructed of 0.50 mm diameter NiTi, which is maintained below its martensitic transition temperature (24 °C) during deployment and suturing. After suturing, the ring is heated above its austenitic transition temperature (37 °C, normal human body temperature) enabling the NiTi core to attain its optimal geometry and stiffness characteristics indefinitely. This article summarizes the design, fabrication, and evaluation of this prototype ring. Experimental results suggest that the NiTi core ring could be a viable alternative to flexible bands in robot-assisted minimally invasive mitral valve repair.

Evaluation of a shape memory alloy reinforced annuloplasty band for minimally invasive mitral valve repair

PURPOSE

An in vitro study using explanted porcine hearts was conducted to evaluate a novel annuloplasty band, reinforced with a two-phase, shape memory alloy, designed specifically for minimally invasive mitral valve repair.

DESCRIPTION

In its rigid (austenitic) phase, this band provides the same mechanical properties as the commercial semi-rigid bands. In its compliant (martensitic) phase, this band is flexible enough to be introduced through an 8-mm trocar and is easily manipulated within the heart.

EVALUATION

In its rigid phase, the prototype band displayed similar mechanical properties to commercially available semi-rigid rings. Dynamic flow testing demonstrated no statistical differences in the reduction of mitral valve regurgitation. In its flexible phase, the band was easily deployed through an 8-mm trocar, robotically manipulated and sutured into place.

CONCLUSIONS

Experimental results suggest that the shape memory alloy reinforced band could be a viable alternative to flexible and semi-rigid bands in minimally invasive mitral valve repair.

In vivo dynamic deformation of the mitral valve annulus

Though mitral valve (MV) repair surgical procedures have increased in the United States [Gammie, J. S., et al. Ann. Thorac. Surg. 87(5):1431-1437, 2009; Nowicki, E. R., et al. Am. Heart J. 145(6):1058-1062, 2003], studies suggest that altering MV stress states may have an effect on tissue homeostasis, which could impact the long-term outcome [Accola, K. D., et al. Ann. Thorac. Surg. 79(4):1276-1283, 2005; Fasol, R., et al. Ann. Thorac. Surg. 77(6):1985-1988, 2004; Flameng, W., P. Herijgers, and K. Bogaerts. Circulation 107(12):1609-1613, 2003; Gillinov, A. M., et al. Ann. Thorac. Surg. 69(3):717-721, 2000]. Improved computational modeling that incorporates structural and geometrical data as well as cellular components has the potential to predict such changes; however, the absence of important boundary condition information limits current efforts. In this study, novel high definition in vivo annular kinematic data collected from surgically implanted sonocrystals in sheep was fit to a contiguous 3D spline based on quintic-order hermite shape functions with C(2) continuity. From the interpolated displacements, the annular axial strain and strain rate, bending, and twist along the entire annulus were calculated over the cardiac cycle. Axial strain was shown to be regionally and temporally variant with minimum and maximum values of -10 and 4%, respectively, observed. Similarly, regionally and temporally variant strain rate values, up to 100%/s contraction and 120%/s elongation, were observed. Both annular bend and twist data showed little deviation from unity with limited regional variations, indicating that most of the energy for deformation was associated with annular axial strain. The regionally and temporally variant strain/strain rate behavior of the annulus are related to the varied fibrous-muscle structure and contractile behavior of the annulus and surrounding ventricular structures, although specific details are still unavailable. With the high resolution shape and displacement information described in this work, high fidelity boundary conditions can be prescribed in future MV finite element models, leading to new insights into MV function and strategies for repair.

Ischaemic mitral regurgitation and the role of surgical intervention

The correct management of ischaemic mitral regurgitation (IMR) in patients presenting for coronary artery bypass grafting (CABG) remains contentious. Although it is generally agreed that severe MR in patients presenting for CABG should be corrected and that mild MR can be left alone, there is no consensus as to the correct treatment for those patients with moderate MR undergoing CABG. Further, there has never been a prospective randomised trial looking at this issue. The data from retrospective reviews is conflicting. The available evidence clearly shows an increase in cardiac morbidity in those patients undergoing revascularisation who are left with significant MR. However, those retrospective studies comparing patients who have a mitral valve procedure with their CABG to those who undergo CABG alone have not consistently shown a benefit. This may be due to the retrospective nature of these reports. But it may also be due to the type of repairs being performed which may not be adequately addressing the specific pathology involved. The aim of this paper therefore is to evaluate the available evidence to help guide decision making in the treatment of ischaemic MR. The underlying pathophysiology of this condition is outlined and the various types of repair being advocated are reviewed. Although there is no clear answer to the question of optimal management of IMR at present, what is clear is that a prospective trial is urgently needed to guide us with the treatment of this clinical problem.

Clinical outcomes of mitral valve repair with the Colvin-Galloway Future Band: a single-center experience

OBJECTIVES

In 2001, a semirigid band (Colvin-Galloway Future Band, Medtronic, Inc., Minneapolis, Minnesota, USA) for mitral valve repair came onto the market. We report our experience of the correction of all types of mitral regurgitation using this device.

METHODS

From August 2003 to December 2006, 140 patients (71% men, mean age 64 +/- 11 years) underwent valvuloplasty with this device for all types of mitral regurgitation: 94 (67%) degenerative, 34 (24%) postischemic, 11 (8%) dilative cardiomyopathy, and one (1%) postendocarditic. The patients underwent clinical and echocardiographic evaluation preoperatively, postoperatively before discharge, and after a median follow-up of 7 months (25th-75th percentile, 4-24 months).

RESULTS

Total mortality was 6.4% (nine out of 140 patients): four patients (2.8%) died within 30 days (early death) and five died subsequently (the cause was cardiac in one case). Predischarge echocardiographic examination revealed a reduction in mitral regurgitation of at least 2 degrees in 99.2% of patients and the absence of systolic anterior movement. At the last follow-up, we recorded an improvement in New York Heart Association functional class (2.4 +/- 0.9 vs. 1.1 +/- 0.4, P < 0.0001), a significant reduction in the degree of mitral regurgitation (3.5 +/- 0.9 vs. 0.9 +/- 0.5, P < 0.0001), an increase in ejection fraction (54 +/- 11 vs. 55 +/- 9, P = 0.09), and a significant reduction in end-diastolic diameter (59 +/- 6 vs. 55 +/- 6, P < 0.0001). Two patients were reoperated on for mitral valve replacement, and no postoperative thromboembolic events occurred.

CONCLUSION

Our experience shows that the Colvin-Galloway Future Band yields good results in mitral valvuloplasty for all types of mitral regurgitation. We are encouraged to continue using this device.

Description of regional mitral annular nonplanarity in healthy human subjects: a novel methodology

OBJECTIVE

Finite-element analysis demonstrates that the nonplanar shape of the mitral annulus diminishes mitral leaflet stress. It has therefore been postulated that repair with annuloplasty rings that maintain the nonplanar shape of the annulus could increase repair durability. Although the global nonplanarity of the mitral annulus has been adequately characterized, design of such a ring requires a quantitative description of regional annular geometry. By using real-time 3-dimensional echocardiography in conjunction with available image processing software, we developed a methodology for describing regional annular geometry and applied it to the characterization of the normal human mitral annulus.

METHODS

Five healthy volunteers underwent mitral valve imaging with real-time 3-dimensional echocardiography. Regional annular height was calculated at 36 evenly spaced intervals.

RESULTS

Maximal annular height/commissural width ratio was found to occur at the midpoint of the anterior annulus in all cases. These values averaged 26% +/- 3.1%, whereas those for the midposterior annulus averaged 18% +/- 3.0%. The average commissural width was 35.2 +/- 6.0 mm. Although substantial spatial heterogeneity was observed, regional annular height at a given rotational position was highly conserved among subjects when normalized to commissural width.

CONCLUSIONS

These quantitative imaging and analytic techniques demonstrate that the normal human mitral annulus is regionally heterogeneous in its nonplanarity, and they establish a means of describing annular geometry at a regional level. With wider application, these techniques may be used both to characterize pathologic annular geometry and to optimize the design of mitral valve annuloplasty devices.

Mitral valve annuloplasty with a semirigid annuloplasty band in ischemic mitral regurgitation: early results

OBJECTIVE

A trigone-to-trigone semirigid annuloplasty band (C-G Future Band, Medtronic, Inc., Minneapolis, Minnesota, USA) was introduced in 2001 for mitral valve repair. We report our early clinical and echocardiographic results with this new device to correct ischemic mitral regurgitation.

METHODS

Between January 2002 and December 2004, among 216 patients operated on for mitral regurgitation, 107 patients had a C-G Future Band annuloplasty and 85 consecutive patients (72.6% male; mean age 66.9 +/- 8.6 years) received this annuloplasty band to correct ischemic mitral regurgitation. Mean follow-up was 14.3 +/- 9.8 months (range 0.2-37 months). Clinical and echocardiographic assessment was accomplished preoperatively, postoperatively, at 6 and 12 months, and at two years.

RESULTS

Perioperative mortality was 3.7% (three in-hospital deaths), whereas overall survival at two years was 88.7 +/- 4.2%. Immediately after repair, echocardiographic mitral regurgitation was dramatically reduced (2.5 +/- 0.6 vs. 0.9 +/- 0.6; P < 0.0001); ejection fraction increased from 43.8 +/- 11% preoperatively to 44.8 +/- 12% postoperatively (P = 0.007). At the time of follow-up, New York Heart Association (NYHA) functional class was significantly improved (mean preoperative NYHA class 2.04 +/- 0.9 vs. mean postoperative NYHA class 1.25 +/- 0.6; P < 0.0001). No patient experienced thromboembolic events and no late mitral valve reoperation occurred.

CONCLUSIONS

Early and mid-term mitral valve function is satisfactory with trigone-to-trigone semirigid band annuloplasty, with excellent repair durability immediately after the operation and at two years. Moreover, after annuloplasty repair, an improvement in clinical functional status is obtained. A wider use of this semirigid annuloplasty band can be recommended.

Effects of hemodynamic alterations on anterior mitral leaflet curvature during systole

OBJECTIVES

The application of repair techniques to treat mitral valve incompetence has increased progressively during the past 20 years. Unfortunately, recent reports have demonstrated the longevity of these repairs to be less than previously believed. Most repair failures are stress related. Computational models to optimize valve repair are in development, but to be brought to fruition, a better understanding of dynamic leaflet geometry is necessary. In this study, sonomicrometry was used in an ovine model to compute systolic leaflet curvature at varying afterloads and states of contractility.

METHODS

The anterior leaflet of 12 sheep was instrumented with 5 piezoelectric transducers in a cruciate array. Systolic blood pressure ranged from 90 to 200 mm Hg with increasing phenylephrine hydrochloride infusion. Epinephrine was used to vary contractile state. Leaflet curvature was calculated continuously (200 Hz) during systole.

RESULTS

Anterior leaflet curvature in the septolateral direction was double that in the intercommisural direction. There were also significant changes in leaflet curvature during systole. Curvature in neither direction was affected by afterload. Epinephrine augmented intercommisural curvature in a dose-independent fashion, whereas it had no effect on curvature in the septolateral direction.

CONCLUSIONS

Dynamic mitral anterior leaflet geometry was found to be amazingly constant over a wide range of hemodynamic conditions. These data provide information about leaflet geometry that will aid in the construction of realistic computational models. Such models may facilitate the design of annuloplasty rings and surgical techniques that minimize leaflet stress and increase mitral valve repair longevity.

Planar biaxial creep and stress relaxation of the mitral valve anterior leaflet

A fundamental assumption in mitral valve (MV) therapies is that a repaired or replaced valve should mimic the functionality of the native valve as closely as possible. Thus, improvements in valvular treatments are dependent on the establishment of a complete understanding of the function and mechanical properties of the native normal MV. In a recent study [Grashow et al. ABME 34(2), 2006] we demonstrated that the planar biaxial stress-strain relationship of the MV anterior leaflet (MVAL) exhibited minimal hysteresis and a stress-strain response independent of strain rate, suggesting that MVAL could be modeled as a "quasi-elastic" material. The objective of our current study was to expand these results to provide a more complete picture of the time-dependent mechanical properties of the MVAL. To accomplish this, biaxial stress-relaxation and creep studies were performed on porcine MVAL specimens. Our primary finding was that while the MVAL leaflet exhibited significant stress relaxation, it exhibited negligible creep over the 3-h test. These results furthered our assertion that the MVAL functionally behaves not as a linear or non-linear viscoelastic material, but as an anisotropic quasi-elastic material. These results appear to be unique in the soft tissue literature; suggesting that valvular tissues are unequalled in their ability to withstand significant loading without time-dependent material effects. Moreover, insight into these specialized characteristics can help guide and inform efforts directed toward surgical repair and engineered valvular tissue replacements.

In-Vivo Dynamic Deformation of the Mitral Valve Anterior Leaflet

BACKGROUND

Surgical techniques have been developed for mitral valve repair for a wide range of pathologies. However, excessive tissue stress and damage have been identified as etiologic factors limiting long-term durability. Before computational models to optimize valve repair can be realistically developed, in-vivo dynamic mitral valve leaflet strain data are required. However, these data do not presently exist. In the present study, a sheep model and sonomicrometry were used to compute the in-surface Eulerian strain tensor of the anterior leaflet over the cardiac cycle at varying afterloads.

METHODS

The anterior leaflet of nine Dorsett sheep (35 kg to 45 kg) was instrumented with nine 1-mm hemispherical piezoelectric transducers in a 15-mm square array. Three-dimensional crystal spatial positions were recorded at 250 Hz over several cardiac cycles, with peak left ventricular pressures varying from 90 mm Hg to 200 mm Hg. The in-surface Eulerian strain tensor was computed from the crystal displacements.

RESULTS

The mitral valve anterior leaflet experiences large anisotropic strains and peak strain rates of 400%/s, followed by an absolute cessation of any deformation during systole. Increasing left ventricular pressure also increased the effective leaflet stiffness but not the peak strains.

CONCLUSIONS

We report the first data on the dynamic in-vivo strain tensor of a functioning mitral valve anterior leaflet, which indicated large anisotropic strains and very high strain rates. Our observations also suggest that changes in left ventricular pressure and annular geometry result in altered effective leaflet stiffness, and may be an important factor in reducing leaflet stress and as such potentially affect mitral valve repair longevity.

Fracture of Colvin-Galloway Future Band Causing a Tear in the Anterior Mitral Leaflet

The first case of a fracture of a semi-rigid annuloplasty device is reported. A 56-year-old man presented with recurrence of severe mitral and tricuspid regurgitation 10 months after proper implantation of a Colvin-Galloway Future band (Medtronic Inc, Minneapolis, MN). During redo surgery we made the unexpected finding that its metal skeleton was fractured near the posterior segment. A tear in the anterior mitral leaflet was also found. It is speculated that the device fracture may have put additional strain on the native annulus.

Seven years’ experience with suture annuloplasty for mitral valve repair

OBJECTIVE

Our early experience with the mural annulus shortening suture procedure for mitral valve repair showed superior hemodynamic performance over ring annuloplasty. The aim of this study was to assess the durability of the mural annulus shortening suture procedure and evaluate our 7-year experience regarding valve function, hemodynamic performance, and clinical outcome.

METHODS

Between 1996 and 2003, 222 elective consecutive patients (58.1% males; age, 59 +/- 14 years) underwent simple or complex mitral valve repair. Minimal invasive reconstruction was performed in 150 patients. For correction of annular dilatation, we used double-running 2-0 polytetrafluoroethylene sutures to reinforce the posterior circumference of the annulus. Patients were investigated prospectively by means of transthoracic echocardiography before discharge and 1 and 5 years after the operation. The mean follow up was 32 +/- 21 months (range 1-77 months).

RESULTS

The operative mortality was 3.1%. Hemodynamic performance at 1 and 5 years showed low mean transvalvular gradients (2.1 +/- 0.9 and 2.0 +/- 0.8 mm Hg, respectively) and a calculated mitral valve orifice area of 3.3 +/- 0.9 cm2 and 3.1 +/- 0.6 cm2, respectively, with progressive annular dilatation from 31.2 +/- 3 mm to 33.9 +/- 4 mm at 1 year and 35.7 +/- 4 mm at 5 years (P < .01). Clinical status improved from New York Heart Association class 3.0 +/- 0.4 to 0.6 +/- 0.8 at 1 year and 0.8 +/- 0.8 at 5 years. Freedom form nontrivial residual mitral regurgitation was 82.3%, freedom from reoperation was 95.1% and actuarial survival was 87.2%, all at 77 months.

CONCLUSIONS

The midterm results show satisfactory hemodynamic performance and clinical improvement. Valve competence and reoperation rates are comparable with those of other reports. Durability of the mural annulus shortening suture procedure for mitral valve repair is questioned because progressive annular redilatation occurs.