Jean-Pierre Rabbah

Mechanics of healthy and functionally diseased mitral valves: a critical review.

The mitral valve is a complex apparatus with multiple constituents that work cohesively to ensure unidirectional flow between the left atrium and ventricle. Disruption to any or all of the components-the annulus, leaflets, chordae, and papillary muscles-can lead to backflow of blood, or regurgitation, into the left atrium, which deleteriously effects patient health. Through the years, a myriad of surgical repairs have been proposed; however, a careful appreciation for the underlying structural mechanics can help optimize long-term repair durability and inform medical device design. In this review, we aim to present the experimental methods and significant results that have shaped the current understanding of mitral valve mechanics. Data will be presented for all components of the mitral valve apparatus in control, pathological, and repaired conditions from human, animal, and in vitro studies. Finally, current strategies of patient specific and noninvasive surgical planning will be critically outlined.

Peak Mechanical Loads Induced in the In Vitro Edge-to-Edge Repair of Posterior Leaflet Flail

BACKGROUND Percutaneous edge-to-edge mitral valve (MV) repair is a potential therapeutic option for patients presenting with mitral regurgitation, who may not be suitable for surgery. We characterized the edge-to-edge repair forces in a posterior leaflet flail MV model to identify potential modes of mechanical failure. METHODS Porcine MVs were evaluated in two different sizes (Physio II 32 and 40) in a left-side heart simulator under physiologic hemodynamic conditions. Edge-to-edge repair was simulated by suturing miniature force transducers near the free edge of the anterior and posterior leaflets, on the ventricular side, resulting in a double orifice MV. Posterior leaflet flail was created by selective chordal cutting. RESULTS Chordal cutting resulted in posterior leaflet flail and mitral regurgitation; all valves coapted normally before chordal cutting. Peak systolic control forces (size 32, 0.098 ± 0.058 N; size 40, 0.236 ± 0.149 N) were not significantly different from systolic flail forces (size 32, 0.136 ± 0.107 N; size 40, 0.220 ± 0.128 N) for either MV size. No correlation was observed between force magnitude and flail height or width. Peak systolic force was greater (p = 0.08) for the larger MVs (size 40 compared with size 32). Finally, peak diastolic force was significantly smaller (p = 0.04) than peak systolic force regardless of valve size. CONCLUSIONS For the first time, forces imparted on an edge-to-edge MV repair were quantified for a posterior leaflet flail model. Force magnitude was not significantly altered with flail compared with control; it was greatest during peak systole and increased with valve size.

Effects of Targeted Papillary Muscle Relocation on Mitral Leaflet Tenting and Coaptation

BACKGROUND Ischemic mitral valve (MV) repair for patients with severe left ventricular dilation remains challenging. The objective of this study was to investigate the efficacy of papillary muscle (PM) relocation to restore physiologic MV function. METHODS Fresh ovine MVs (n = 6) were studied in a left-heart simulator under physiologic hemodynamics. Ischemic MV disease was simulated by annular dilation and PM displacement. Initial valvular repair was performed with mitral annuloplasty; further PM displacement simulated progressive left ventricular dilation. Basal PM repositioning (Kron procedure), performed to alleviate leaflet tethering, consisted of relocating (1) both PMs toward the commissures; (2) both PMs toward the trigones; (3) the posteromedial PM toward the ipsilateral commissure; and (4) the posteromedial PM toward the ipsilateral trigone. Coaptation length and tenting area were measured using three-dimensional echocardiography as surrogates of MV function. RESULTS Papillary muscle relocation as an adjunct to mitral annuloplasty statistically improved coaptation length and tenting area compared with the disease condition. No statistical differences in coaptation length and tenting area were observed between final repaired conditions and control conditions. No statistical differences were observed between commissural and trigonal repairs at any incremental repair step. Coaptation length and tenting area were plotted against PM distance; the data were fit to linear regressions. CONCLUSIONS In a realistic in vitro model of ischemic left ventricular dilation, apical-basal PM relocation, as an adjunct procedure to mitral annuloplasty, restored optimal MV closure. Trigonal or commissural traction suture location did not significantly affect the degree of restored coaptation. Linear relationships between PM positions and leaflet variables were established, which could be used to inform surgical repairs.

Isolated effect of geometry on mitral valve function for in silico model development

Computational models for the hearts mitral valve (MV) exhibit several uncertainties that may be reduced by further developing these models using ground-truth data-sets. This study generated a ground-truth data-set by quantifying the effects of isolated mitral annular flattening, symmetric annular dilatation, symmetric papillary muscle (PM) displacement and asymmetric PM displacement on leaflet coaptation, mitral regurgitation (MR) and anterior leaflet strain. MVs were mounted in an in vitro left heart simulator and tested under pulsatile haemodynamics. Mitral leaflet coaptation length, coaptation depth, tenting area, MR volume, MR jet direction and anterior leaflet strain in the radial and circumferential directions were successfully quantified at increasing levels of geometric distortion. From these data, increase in the levels of isolated PM displacement resulted in the greatest mean change in coaptation depth (70% increase), tenting area (150% increase) and radial leaflet strain (37% increase) while annular dilatation resulted in the largest mean change in coaptation length (50% decrease) and regurgitation volume (134% increase). Regurgitant jets were centrally located for symmetric annular dilatation and symmetric PM displacement. Asymmetric PM displacement resulted in asymmetrically directed jets. Peak changes in anterior leaflet strain in the circumferential direction were smaller and exhibited non-significant differences across the tested conditions. When used together, this ground-truth data-set may be used to parametrically evaluate and develop modelling assumptions for both the MV leaflets and subvalvular apparatus. This novel data may improve MV computational models and provide a platform for the development of future surgical planning tools.

Developing an Experimental Database for Mitral Valve Computational Modeling, Surgical Repair, and Device Evaluation

Numerical models of the heart’s mitral valve are being used to study valve biomechanics, facilitate predictive surgical planning, and are used in the design and development of repair devices. These models have evolved from simple two-dimensional approximations to complex three-dimensional fully coupled fluid structure interaction models. However, to date these models lack direct one-to-one experimental validation. Moreover, as computational solvers vary considerably based on researcher implementation, experimental benchmark data are critically important to ensure model accuracy. To this end, a multi-modality in-vitro pulsatile left heart simulator was used to establish a database of geometric and hemodynamic boundary conditions coupled with resultant valvular and fluid mechanics.© 2013 ASME

Experimental Platforms for Validation of Computational Approaches to Simulating Cardiovascular Flows

This paper describes three different versions of left heart simulators that have been developed at the Cardiovascular Fluid Mechanics Laboratory at Georgia Institute of Technology, specifically designed to provide high fidelity experimental datasets necessary for rigorous validation of computational tools. These systems are capable of simulating physiological and pathological flow, pressure and geometric conditions, and can be investigated using a variety of experimental tools to measure relevant biomechanical quantities. The development of such robust simulators is a critical step in ensuring applicability of patient specific computational tools.Copyright

Letter regarding the article by Vismara et al published in Int J Artif Organs 2011; 34 (4): 383-391

We are writing to you regarding a recently published article in the International Journal of Artificial Organs titled “A Pulsatile Simulator for the In Vitro Analysis of the Mitral Valve with Tri-axial Papillary Muscle Displacement,” by Vismara et al (1). In this article, the authors describe the development of a pulsatile, in vitro flow loop with the ability to 3-dimensionally position the papillary muscles (PMs) of freshly excised mitral valves (MVs), measure the force exerted by the chordae tendineae on the PMs, and possibly visually inspect the MVs for kinematic analysis. The motivation of this study is very relevant to develop an in vitro flow loop to study MV mechanics but the originality and novelty of the study are in question. In particular, the authors fail to cite more recent work from our group detailing such a simulator. Since 1996, we have developed, refined, and extensively validated a left heart simulator to study MV mechanics, and have generated over 20 referred publications in reputed clinical and engineering journals, of which a few recent ones are provided here (2-6). Using what is referred to as the Georgia Tech Left Heart Simulator (GTLHS), we have successfully evaluated the effects of MV geometry on mitral regurgitation, chordal forces, PM forces, leaflet coaptation characteristics using realtime 3D echocardiography, leaflet strain, chordal strain, and hemodynamic measurements. The current version of the GTLHS is capable of mimicking physiological and pathological left heart hemodynamics, as well as geometric alterations of the MV, both at the annular and subvalvular levels. Further, the GTLHS has also been utilized for assessing repair outcomes of annular and subvalvular repair techniques. Multiple levels of annular dilatation (equivalent to size 24-40 annuloplasty rings), mitral annulus saddle (0-30% anterior height to commissural width ratio) as well as PM positions (0-10 mm displacement in apical, lateral, and posterior directions) can be simulated in the Letter to the editor

Stereoscopic Particle Image Velocimetry of Native Ovine Mitral Valves in a Pulsatile Left Heart Simulator

Patient specific mitral valve computational models are being actively developed to facilitate surgical planning. These numerical models increasingly employ more realistic geometries, kinematics, and mechanical properties, which in turn requires rigorous experimental validation [1]. However, to date, native mitral flow dynamics have not been accurately and comprehensively characterized. In this study, we used Stereoscopic Particle Image Velocimetry (SPIV) to characterize the ventricular flow field proximal to a native mitral valve in a pulsatile experimental flow loop.Copyright