Ventricular remodeling is one of the most important mechanisms of HF progression, independent of the patient’s hemodynamic and neurohormonal status.[4
] From the gross anatomical standpoint, alterations in LV geometry leads to four major pathophysiological consequences: LV chamber dilation, increased LV sphericity, LV wall thinning, and mitral valve incompetence.[18
] As a result, LV wall stress dramatically increases during ventricular remodeling and places higher oxygen demands on an already burdened and failing heart, which in turn contributes to more remodeling and creates a vicious cycle. Passive ventricular restraint devices constrain the dysfunctional ventricles externally, and attempt to break the cycle by preventing further ventricular dilation and preserving its native elliptical geometry.[10
Long-term results of the largest clinical trial of CSD, ACORN, have been encouraging especially in terms of major cardiac events and NYHA functional class.[19
] Although not effective on the overall mortality, CorCap™
device caused a significant reduction in LV end-diastolic volume as well as a small increase in sphericity index, defined as LV end-diastolic length/width, indicating return to a more physiologic ellipsoidal shape.[19
] Reverse remodeling has been shown in several studies as well.[21
] In fact, cardiac restraint therapy leads not only to a size reduction but also restores the ellipsoidal geometry of the ventricles.[23
Despite the promising preliminary data, concerns still remain regarding the technical details and related complications of CSD.[24
] Lee et al. showed in a study that CorCap™
affects the left and right ventricles differently, and the left ventricle can tolerate more restraint than the right ventricle.[25
] In order for the mesh to be beneficial for the left ventricle, it should be pre-stressed to the level that may be excessive for the right ventricle. In other words, if the fabric is too stiff or fabric pre-stress is too great, LV diastolic compliance and filling can be compromised, which eventually leads to a tamponade effect. On the other hand, in a porcine study Dixon and associates found that despite a normal steady-state hemodynamics, LV maximal coronary reserve was blunted after placement of CSD, while interestingly this adverse effect was not observed in the right ventricle.[8
Currently, the amount of pre-stress placed on the Acorn CSD fabric at the time of implementation is poorly defined, and applied pre-stress is largely at the discretion of the cardiac surgeon. Furthermore, it is currently unknown what the CSD material properties are in different fiber orientations as that can have significant impact on the degree of ventricular restraint and give options for optimizing clinical implantation. In this study, we investigated the material properties of CorCap™ CSD using a combination of biaxial testing methods and a theoretical framework.
We modeled the effect of bi-directional reinforcement of the CorCap™
jacket in silicone using an anisotropic elasticity framework. Use of a silicone matrix in the composites resulted in a more uniform distribution of the biaxial forces across the samples. The coefficient C1
in equations 5a
for the composite strain energy function represents the isotropic contribution of silicone. Using biaxial experiments, C1
was determined to be 445kPa, which was similar to that of other rubber-like materials. Although the stretch obtained in the silicone samples was lower than could be achieved using the composite materials, this did not affect our results as the silicone behavior was completely linear throughout the experiment. In other words, because the material properties of silicone are isotropic, we were able to use the value of C1
to extrapolate to higher stretches in future analyses. The relatively linear behavior of the CSD jacket in the stretch range tested was consistent with the Walsh’s study, which found that the multiaxial stiffness of the CorCap up to 12% strain is essentially linear.[10
] Although they subjected the CSD to a multiaxial stress-strain testing using ball burst technique, they did not report any material constants for formulation of the CorCap characteristics and directionality.
A key issue in determining the effects of restraining devices like the CSD mesh is the compliance of the devices used.[26
] In this study, we demonstrated that the stiffness of CSD is greater in the aligned fiber direction, i.e. if the sample is aligned with one of the fibers along the circumferential axis of the ventricle then the circumferential stiffness is greater than the longitudinal stiffness. However, if the CSD is aligned in the cross-fiber direction, i.e. off axis, it is less stiff. These results have clinical implications respect to orienting the fibers along the heart to have optimal restraint. However, it is currently unknown what degree of restraint is optimal and what stretch ratio is required to best maintain the pressure after weeks or months. This study demonstrates the importance of directionality of fibers which would be an important aspect to follow clinically. An understanding of the strain distributions in infarcted regions, neighboring border zone, and remote myocardium-all surrounded by the passive CSD mesh network-are important for future optimization of passive constraining devices. Moreover, at the microscopic level, fibrosis is one of the most prominent characteristics of the failing myocardium [18
], which can significantly change ventricular mechanical properties and should be taken into account.
The CSD mesh is comprised of open air pores when placed on the epicardial surface of the ventricles. Postmortem studies of the CSD mesh implanted in dogs have shown an average thickness of 0.59 ± 0.15 mm thick layer of collagen fibers encapsulating the mesh after three months.[27
] Using CSD mesh implanted in an ovine model with myocardial infarction, Blom and coworkers have shown an increased myofibroblast cell density within the CSD accompanied by a significant highly organized matrix accumulation.[28
] They hypothesize that the incorporation of contractile cells and synthesis of matrix components within the CSD may be contributing factors that help the function of the restraining device. These factors would also increase the stiffness of the composite material, comprised of the CSD jacket embedded in extracellular matrix, and shift the stress-strain curves towards the left in the long term.[29
] An interesting aspect for future study may be the changes in mechanical properties of the CSD after implantation given not only the fiber direction in vivo
but also the incorporation of matrix material within the open pores.
In summary, we determined material parameters of CSD fabric embedded in an isotropic matrix (CSD fabric/ silicone composite) by using the laws of continuum mechanics, which would otherwise be unsuitable for analyzing an open mesh. The curve fits produced by the theoretical modeling of the CSD and silicone composites show good agreement with experimental biaxial tests. The compliance of CSD fabric alone was then calculated by setting matrix material constant to a low value thereby making the matrix material very compliant. We demonstrated important differences in stiffness based on fiber directionality which would be important to follow clinically for optimization of degree of restraint. These results of mechanical behavior provide a fundamental understanding of the Acorn CSD device and may be used in future studies to optimize design and application of CSD and improve outcomes in patients with congestive heart failure.