Comparison of Biomechanical Properties and Microstructure of Trabeculae Carneae, Papillary Muscles, and Myocardium in the Human Heart.

Trabeculae carneae account for a significant portion of human ventricular mass, despite being considered embryologic remnants. Recent studies have found trabeculae hypertrophy and fibrosis in hypertrophied left ventricles with various pathological conditions. The objective of this study was to investigate the passive mechanical properties and microstructural characteristics of trabeculae carneae and papillary muscles compared to the myocardium in human hearts. Uniaxial tensile tests were performed on samples of trabeculae carneae and myocardium strips, while biaxial tensile tests were performed on samples of papillary muscles and myocardium sheets. The experimental data were fitted with a Fung-type strain energy function and material coefficients were determined. The secant moduli at given diastolic stress and strain levels were determined and compared among the tissues. Following the mechanical testing, histology examinations were performed to investigate the microstructural characteristics of the tissues. Our results demonstrated that the trabeculae carneae were significantly stiffer (Secant modulus SM2 = 80.06 ± 10.04 KPa) and had higher collagen content (16.10 ± 3.80%) than the myocardium (SM2 = 55.14 ± 20.49 KPa, collagen content = 10.06 ± 4.15%) in the left ventricle. The results of this study improve our understanding of the contribution of trabeculae carneae to left ventricular compliance and will be useful for building accurate computational models of the human heart.

Computational Modeling of Human Left Ventricle to Assess the Effects of Trabeculae Carneae on the Diastolic and Systolic Functions.

Trabeculae carneae are irregular structures that cover the endocardial surfaces of both ventricles and account for a significant portion of human ventricular mass. The role of trabeculae carneae in diastolic and systolic functions of the left ventricle (LV) is not well understood. Thus, the objective of this study was to investigate the functional role of trabeculae carneae in the LV. Finite element (FE) analyses of ventricular functions were conducted for three different models of human LV derived from high-resolution magnetic resonance imaging (MRI). The first model comprised trabeculae carneae and papillary muscles, while the second model had papillary muscles and partial trabeculae carneae, and the third model had a smooth endocardial surface. We customized these patient-specific models with myofiber architecture generated with a rule-based algorithm, diastolic material parameters of Fung strain energy function derived from biaxial tests and adjusted with the empirical Klotz relationship, and myocardial contractility constants optimized for average normal ejection fraction (EF) of the human LV. Results showed that the partial trabeculae cutting model had enlarged end-diastolic volume (EDV), reduced wall stiffness, and even increased end-systolic function, indicating that the absence of trabeculae carneae increased the compliance of the LV during diastole, while maintaining systolic function.

Effect of Axial Stretch on Lumen Collapse of Arteries.

The stability of the arteries under in vivo pressure and axial tension loads is essential to normal arterial function, and lumen collapse due to buckling can hinder the blood flow. The objective of this study was to develop the lumen buckling equation for nonlinear anisotropic thick-walled arteries to determine the effect of axial tension. The theoretical equation was developed using exponential Fung strain function, and the effects of axial tension and residual stress on the critical buckling pressure were illustrated for porcine coronary arteries. The buckling behavior was also simulated using finite-element analysis. Our results demonstrated that lumen collapse of arteries could occur when the transmural pressure is negative and exceeded a critical value. This value depends upon the axial stretch ratio and material properties of the arterial wall. Axial tensions show a biphasic effect on the critical buckling pressure. The lumen aspect ratio of arteries increases nonlinearly with increasing external pressure beyond the critical value as the lumen collapses. These results enhance our understanding of artery lumen collapse behavior.

Computational simulations of the helical buckling behavior of blood vessels.

Tortuous vessels are often observed in vivo and could hinder or even disrupt blood flow to distal organs. Besides genetic and biological factors, the in vivo mechanical loading seems to play a role in the formation of tortuous vessels, but the mechanism for formation of helical vessel shape remains unclear. Accordingly, the aim of this study was to investigate the biomechanical loads that trigger the occurrence of helical buckling in blood vessels using finite element analysis. Porcine carotid arteries were modeled as thick-walled cylindrical tubes using generalized Fung and Holzapfel-Gasser-Ogden constitutive models. Physiological loadings, including axial tension, lumen pressure, and axial torque, were applied. Simulations of various geometric dimensions, different constitutive models and at various levels of axial stretch ratios, lumen pressures, and twist angles were performed to identify the mechanical factors that determine the helical stability. Our results demonstrated that axial torsion can cause wringing (twist buckling) that leads to kinking or helical coiling and even looping and winding. The specific buckling patterns depend on the combination of lumen pressure, axial torque, axial tension, and the dimensions of the vessels. This study elucidates the mechanism of how blood vessels buckle under various mechanical loads and how complex mechanical loads yield helical buckling.

Trabecular cutting: a novel surgical therapy to increase diastolic compliance.

Heart failure with preserved ejection fraction (HFpEF) is a common cause of hospital admission in patients over 65 yr old and has high mortality. HFpEF is characterized by left ventricular (LV) hypertrophy that reduces compliance. Current HFpEF therapies control symptoms, but no existing medications or therapies can sustainably increase LV compliance. LV trabeculae develop hypertrophy and fibrosis that contribute to reduced LV compliance. This study expands our previous results in ex vivo human hearts to show that severing LV trabeculae increases diastolic compliance in an ex vivo working rabbit heart model. Trabecular cutting was performed in ex vivo rabbit hearts set up in a working heart perfusion system perfused with oxygenated Krebs-Henseleit buffer. A hook was inserted in the LV to cut trabeculae. End-systolic and end-diastolic pressure-volume relationships during transient preload reduction were recorded using an admittance catheter in the following three groups: control (no cutting; n = 9), mild cutting (15 cuts; n = 5), and aggressive cutting (30 cuts; n = 5). In a second experiment, each heart served as its own control. Hemodynamic data were recorded before and after trabecular cutting (n = 10) or sham cutting (n = 5) within the same heart. In the first experiments, trabecular cutting did not affect systolic function (P > 0.05) but significantly increased overall diastolic compliance (P = 0.009). Greater compliance was seen as trabecular cutting increased (P = 0.002, r2 = 0.435). In the second experiment, significant increases in systolic function (P = 0.048) and diastolic compliance (P = 0.002) were seen after trabecular cutting compared with baseline. In conclusion, trabecular cutting significantly increases diastolic compliance without reducing systolic function.NEW & NOTEWORTHY We postulate that, in mammalian hearts, free-running trabeculae carneae exist to provide tensile support to the left ventricle and minimize diastolic wall stress. Because of hypertrophy and fibrosis of trabeculae in patients with left ventricular hypertrophy, this supportive role can become pathologic, worsening diastolic compliance. We demonstrate a novel operation involving cutting trabeculae as a method to acutely increase diastolic compliance in patients presenting with heart failure and diastolic dysfunction to improve their left ventricle compliance.

Twist buckling of veins under torsional loading.

Veins are often subjected to torsion and twisted veins can hinder and disrupt normal blood flow but their mechanical behavior under torsion is poorly understood. The objective of this study was to investigate the twist deformation and buckling behavior of veins under torsion. Twist buckling tests were performed on porcine internal jugular veins (IJVs) and human great saphenous veins (GSVs) at various axial stretch ratio and lumen pressure conditions to determine their critical buckling torques and critical buckling twist angles. The mechanical behavior under torsion was characterized using a two-fiber strain energy density function and the buckling behavior was then simulated using finite element analysis. Our results demonstrated that twist buckling occurred in all veins under excessive torque characterized by a sudden kink formation. The critical buckling torque increased significantly with increasing lumen pressure for both porcine IJV and human GSV. But lumen pressure and axial stretch had little effect on the critical twist angle. The human GSVs are stiffer than the porcine IJVs. Finite element simulations captured the buckling behavior for individual veins under simultaneous extension, inflation, and torsion with strong correlation between predicted critical buckling torques and experimental data (R2=0.96). We conclude that veins can buckle under torsion loading and the lumen pressure significantly affects the critical buckling torque. These results improve our understanding of vein twist behavior and help identify key factors associated in the formation of twisted veins.