Computational Parametric Studies Investigating the Global Hemodynamic Effects of Applied Apical Torsion for Cardiac Assist

A New Coding System for the Identification of Left Ventricular Rotation Patterns and Their Relevance to Myocardial Function

Rotational mechanics is a fundamental determinant of left ventricular ejection fraction (LVEF). The coding system currently employed in clinical practice does not distinguish between rotational patterns. We propose an alternative coding system that makes possible to identify the rotational pattern of the LV and relate it to myocardial function. Echocardiographic images were used to generate speckle tracking-derived transmural global longitudinal strain (tGLS) and rotational parameters. The existence of twist (basal and apical rotations in opposite directions) is expressed as a rotational gradient with a positive value that is the sum of the basal and apical rotation angles. Conversely, when there is rigid rotation (basal and apical rotations in the same direction) the resulting gradient is assigned a negative value that is the subtraction between the two rotation angles. The rotational patterns were evaluated in 87 healthy subjects and 248 patients with LV hypertrophy (LVH) and contrasted with their myocardial function. Our approach allowed us to distinguish between the different rotational patterns. Twist pattern was present in healthy controls and 104 patients with LVH and normal myocardial function (tGLS ≥ 17%, both). Among 144 patients with LVH and myocardial dysfunction (tGLS < 17%), twist was detected in 83.3% and rigid rotation in 16.7%. LVEF was < 50% in 34.7%, and all patients with rigid rotation had a LVEF < 50%. The gradient rotational values showed a close relationship with LVEF (r = 0.73; p < 0.001). The proposed coding system allows us to identify the rotational patterns of the LV and to relate their values with LVEF.

[Study on direct ventricular assist loading mode based on a finite element method]

To investigate the biomechanical effects of direct ventricular assistance and explore the optimal loading mode, this study established a left ventricular model of heart failure patients based on the finite element method. It proposed a loading mode that maintains peak pressure compression, and compared it with the traditional sinusoidal loading mode from both hemodynamic and biomechanical perspectives. The results showed that both modes significantly improved hemodynamic parameters, with ejection fraction increased from a baseline of 29.33% to 37.32% and 37.77%, respectively, while peak pressure, stroke volume, and stroke work parameters also increased. Additionally, both modes showed improvements in stress concentration and excessive fiber strain. Moreover, considering the phase error of the assist device's working cycle, the proposed assist mode in this study was less affected. Therefore, this research may provide theoretical support for the design and optimization of direct ventricular assist devices.

Computational methods for parametric evaluation of the biventricular mechanics of direct cardiac compression

Despite the increasing incidence of heart failure, advancements in mechanical circulatory support have become minimal. A new type of mechanical circulatory support, direct cardiac compression, is a novel support paradigm that involves a soft deformable cup around the ventricles, compressing it during systole. No group has yet investigated the biomechanical consequences of such an approach. This article uses a multiscale cardiac simulation software to create a patient-specific beating heart dilated cardiomyopathy model. Left and right ventricle (LV and RV) forces are applied parametrically, to a maximum of 2.9 and 0.46 kPa on each ventricle, respectively. Compression increased the ejection fraction in the left and right ventricles from 15.3% and 27.4% to 24.8% and 38.7%, respectively. During applied compression, the LV freewall thickening increased while the RV decreased; this was found to be due to a change in the balance of the preload and afterload in the freewalls. Principal strain renderings demonstrated strain concentrations on the anterior and posterior LV freewall. Strains in these regions were found to exponentially increase after 0.75 normalized LV force was applied. Component analysis of these strains illuminated a shift in the dominating strain from transmural to cross fiber once 0.75 normalized LV force is exceeded. An optimization plot was created by nondimensionalizing the stroke volume and maximum principal strain for each compression profile, selecting five potential compression schemes. This work demonstrates not only the importance of a computational approach to direct cardiac compression but a framework for tailoring compression profiles to patients.

Computational Studies on the Effects of Applied Apical Torsion for Cardiac Assist on Regional Wall Mechanics

OBJECTIVE

Here we report the results of parametric computational simulations evaluating the biomechanical effects of applied apical torsion (AAT) on a patient-specific bi-ventricular failing heart model.

METHODS

We examined the resulting effects on cardiac biomechanics with varying device coverage areas and applied rotation angles to determine the practical working limits of AAT on a dilated cardiomyopathy heart model.

RESULTS

The largest maximum principal stresses and strains observed in the heart failure model were 80.21 kPa (at the basal node of the left ventricular epicardium) and 0.56 (at the node of the device base of the left ventricular free wall). Results show that increasing levels of AAT beyond 45 degrees produce supra-physiologic levels of stress and strain in the myocardium.

CONCLUSION

Maximum principal stresses greater than 100 kPa were observed at multiple nodes along the epicardium and endocardium of the ventricular base and in the endocardium at the device base. Maximum principal strains greater than 0.60 were observed at multiple nodes along the epicardium and endocardium of the ventricular base.

SIGNIFICANCE

This suggests that while AAT has the potential to provide meaningful returns to hemodynamic function in failing hearts, the large deformations produced by this approach with the upper bounds of applied rotation angle realistically excludes supra-physiological rotations as a means for cardiac support. However, lower AAT angles - closer to that of the native left-ventricular torsion - coupled with another means of external cardiac compression may prove to be a viable method of cardiac assist.

Left ventricular simulation of cardiac compression: Hemodynamics and regional mechanics

Heart failure is a global epidemic. Left ventricular assist devices provide added cardiac output for severe cases but cause infection and thromboembolism. Proposed direct cardiac compression devices eliminate blood contacting surfaces, but no group has optimized the balance between hemodynamic benefit and excessive ventricular wall strains and stresses. Here, we use left ventricular simulations to apply compressions and analyze hemodynamics as well as regional wall mechanics. This axisymmetric model corresponds with current symmetric bench prototypes. At nominal pressures of 3.1 kPa applied over the epicardial compression zone, hemodynamics improved substantially. Ejection fraction changed from 17.6% at baseline to 30.3% with compression and stroke work nearly doubled. Parametric studies were conducted by increasing and decreasing applied pressures; ejection fraction, peak pressure, and stroke work increased linearly with changes in applied compression. End-systolic volume decreased substantially. Regional mechanics analysis showed principal stress increases at the endocardium, in the middle of the compression region. Principal strains remained unchanged or increased moderately with nominal compression. However, at maximum applied compression, stresses and strains increased substantially providing potential constraints on allowable compressions. These results demonstrate a framework for analysis and optimization of cardiac compression as a prelude to biventricular simulations and subsequent animal experiments.

Ventricle-specific epicardial pressures as a means to optimize direct cardiac compression for circulatory support: A pilot study

Direct cardiac compression (DCC) holds enormous potential as a safe and effective means to treat heart failure patients who require long-term, or even permanent, biventricular support. However, devices developed to date are not tuned to meet the individual compression requirements of the left and right ventricles, which can differ substantially. In this paper, a systematic study examining the relationship, range, and effect of independent pressures on the left and right epicardial surfaces of a passive human heart model was performed as a means to optimize cardiac output via DCC support. Hemodynamic and tissue deformation effects produced by varying epicardial compressions were examined using finite element analysis. Results indicate that 1) designing a direct cardiac compression pump that applies separate pressures to the left and right ventricles is critical to maintain equivalent stroke volume for both ventricles, and 2) left and right ventricular epicardial pressures of 340 mmHg and 44 mmHg, respectively, are required to induce normal ejection fractions in a passive heart. This pilot study provides fundamental insights and guidance towards the design of improved direct cardiac compression devices for long-term circulatory support.