Deformational Dynamics of the Aortic Root: Modes and Physiologic Determinants

Multiscale structure and function of the aortic valve apparatus

Whereas studying the aortic valve in isolation has facilitated the development of life-saving procedures and technologies, the dynamic interplay of the aortic valve and its surrounding structures is vital to preserving their function across the wide range of conditions encountered in an active lifestyle. Our view is that these structures should be viewed as an integrated functional unit, here referred to as the aortic valve apparatus (AVA). The coupling of the aortic valve and root, left ventricular outflow tract, and blood circulation is crucial for AVA's functions: unidirectional flow out of the left ventricle, coronary perfusion, reservoir function, and support of left ventricular function. In this review, we explore the multiscale biological and physical phenomena that underlie the simultaneous fulfillment of these functions. A brief overview of the tools used to investigate the AVA, such as medical imaging modalities, experimental methods, and computational modeling, specifically fluid-structure interaction (FSI) simulations, is included. Some pathologies affecting the AVA are explored, and insights are provided on treatments and interventions that aim to maintain quality of life. The concepts explained in this article support the idea of AVA being an integrated functional unit and help identify unanswered research questions. Incorporating phenomena through the molecular, micro, meso, and whole tissue scales is crucial for understanding the sophisticated normal functions and diseases of the AVA.

Strain estimation in aortic roots from 4D echocardiographic images using medial modeling and deformable registration

Even though the central role of mechanics in the cardiovascular system is widely recognized, estimating mechanical deformation and strains in-vivo remains an ongoing practical challenge. Herein, we present a semi-automated framework to estimate strains from four-dimensional (4D) echocardiographic images and apply it to the aortic roots of patients with normal trileaflet aortic valves (TAV) and congenital bicuspid aortic valves (BAV). The method is based on fully nonlinear shell-based kinematics, which divides the strains into in-plane (shear and dilatational) and out-of-plane components. The results indicate that, even for size-matched non-aneurysmal aortic roots, BAV patients experience larger regional shear strains in their aortic roots. This elevated strains might be a contributing factor to the higher risk of aneurysm development in BAV patients. The proposed framework is openly available and applicable to any tubular structures.

Biomechanics of Pulmonary Autograft as Living Tissue: A Systematic Review

Introduction: The choice of valve substitute for aortic valve surgery is tailored to the patient with specific indications and contraindications to consider. The use of an autologous pulmonary artery (PA) with a simultaneous homograft in the pulmonary position is called a Ross procedure. It permits somatic growth and the avoidance of lifelong anticoagulation. Concerns remain on the functionality of a pulmonary autograft in the aortic position when exposed to systemic pressure. Methods: A literature review was performed incorporating the following databases: Pub Med (1996 to present), Ovid Medline (1958 to present), and Ovid Embase (1982 to present), which was run on 1 January 2022 with the following targeted words: biomechanics of pulmonary autograft, biomechanics of Ross operation, aortic valve replacement and pulmonary autograph, aortic valve replacement and Ross procedure. To address the issues with heterogeneity, studies involving the pediatric cohort were also analyzed separately. The outcomes measured were early- and late-graft failure alongside mortality. Results: a total of 8468 patients were included based on 40 studies (7796 in pediatric cohort and young adult series and 672 in pediatric series). There was considerable experience accumulated by various institutions around the world. Late rates of biomechanical failure and mortality were low and comparable to the general population. The biomechanical properties of the PA were superior to other valve substitutes. Mathematical and finite element analysis studies have shown the potential stress-shielding effects of the PA root. Conclusion: The Ross procedure has excellent durability and longevity in clinical and biomechanical studies. The use of external reinforcements such as semi-resorbable scaffolds may further extend their longevity.

A SOX17-PDGFB signaling axis regulates aortic root development

Developmental etiologies causing complex congenital aortic root abnormalities are unknown. Here we show that deletion of Sox17 in aortic root endothelium in mice causes underdeveloped aortic root leading to a bicuspid aortic valve due to the absence of non-coronary leaflet and mispositioned left coronary ostium. The respective defects are associated with reduced proliferation of non-coronary leaflet mesenchyme and aortic root smooth muscle derived from the second heart field cardiomyocytes. Mechanistically, SOX17 occupies a Pdgfb transcriptional enhancer to promote its transcription and Sox17 deletion inhibits the endothelial Pdgfb transcription and PDGFB growth signaling to the non-coronary leaflet mesenchyme. Restoration of PDGFB in aortic root endothelium rescues the non-coronary leaflet and left coronary ostium defects in Sox17 nulls. These data support a SOX17-PDGFB axis underlying aortic root development that is critical for aortic valve and coronary ostium patterning, thereby informing a potential shared disease mechanism for concurrent anomalous aortic valve and coronary arteries.

Preliminary study of a novel nanofiber-based valve integrated tubular graft as an alternative for a pulmonary valved artery

A tri-leaflet valve integrated tubular scaffold was obtained using a 3D printing mold by TIPS. After testing its valuvalar performance via computational fluid dynamics, the biocompatibility of resultant valve scaffold was evaluated in vivo.

Aortic root dimension changes during systole and diastole: evaluation with ECG-gated multidetector row computed tomography

Cardiac pulsatility and aortic compliance may result in aortic area and diameter changes throughout the cardiac cycle in the entire aorta. Until this moment these dynamic changes could never be established in the aortic root (aortic annulus, sinuses of Valsalva and sinotubular junction). The aim of this study was to visualize and characterize the changes in aortic root dimensions during systole and diastole with ECG-gated multidetector row computed tomography (MDCT). MDCT scans of subjects without aortic root disease were analyzed. Retrospectively, ECG-gated reconstructions at each 10% of the cardiac cycle were made and analyzed during systole (30-40%) and diastole (70-75%). Axial planes were reconstructed at three different levels of the aortic root. At each level the maximal and its perpendicular luminal dimension were measured. The mean dimensions of the total study group (n = 108, mean age 56 ± 13 years) do not show any significant difference between systole and diastole. The individual dimensions vary up to 5 mm. However, the differences range between minus 5 mm (diastolic dimension is greater than systolic dimensions) and 5 mm (vice versa). This variability is independent of gender, age, height and weight. This study demonstrated a significant individual dynamic change in the dimensions of the aortic root. These results are highly unpredictable. Most of the healthy subjects have larger systolic dimensions, however, some do have larger diastolic dimensions.

The left ventricular outflow in hypertrophic cardiomyopathy: from structure to function

Left ventricular outflow tract obstruction (LVOTO) is one of the defining features of hypertrophic cardiomyopathy (HCM) and one of the main determinants of prognosis. Although the importance of obstruction was recognized since the original description by Teare and Brock, its exact cause and methods for its relief are still being hotly debated. We believe that a rational approach to solving these issues depends on thorough understanding of the specific structure and functions of the left ventricular outflow tract (LVOT) in health and disease. There is now compelling evidence that the LVOT performs a series of vital sophisticated functions which are mediated by the design characteristics, structure, and biological properties of its component parts and that dysregulation of one or more of these functions results in obstruction and other abnormalities. We here review the integrated functions of the LVOT, its structural and functional relationships, with particular reference to its component parts (the major players) and their role in HCM. This knowledge is essential to evolve tailored restorative techniques for treating HCM.

Comparison between aortic, mitral and tricuspid annular velocities measured with quantitative two-dimensional color Doppler tissue imaging in healthy subjects

OBJECTIVES

To compare the systolic, early and late diastolic velocities of the aortic, mitral and tricuspid annuli in healthy subjects and to study the intraobserver and interobserver reproducibility (IIOR) of measuring the velocities at the aortic annulus.

METHODS

Twenty healthy subjects with mean age 28 years were investigated with quantitative two-dimensional color Doppler tissue imaging and the systolic, early and late diastolic velocities were measured at the aortic, mitral and tricuspid annuli.

RESULTS

The mitral annulus had significant higher systolic velocity and early diastolic velocity than the aortic annulus. The late diastolic velocity was significant lower at the aortic annulus compared with the both other annuli. At the different sites of the annuli the highest systolic velocity and early diastolic velocity were measured at the lateral site of the mitral annulus, whereas the lowest systolic velocity was measured at the septal site of the same annulus. The lowest early diastolic velocity was found at the septal site of the aortic annulus. The highest late diastolic velocity was measured at the lateral site of the tricuspid annulus and the lowest at the lateral site of the aortic annulus.

CONCLUSIONS

The mitral annulus has statistical significant higher systolic and early diastolic velocities than the aortic annulus. There are significant differences in velocities between several of the different sites of the annuli. IIOR of measuring the systolic and early diastolic velocities of the aortic annulus is good.

Autografts, homografts, and xenografts: overview on stentless aortic valve surgery

Stentless valves, either human (autografts, homografts) or animal (porcine xenografts), were historically among the first substitutes to be used to replace the diseased aortic valve. Forty years after those pioneering days and 15 years after revival of such valves, stentless grafts have become a mainstay in aortic surgery. Although limitations associated with the use of autografts, homografts and xenografts remain, stentless valves have profoundly improved quality of life after aortic valve/root replacement. In addition, stentless surgery has greatly advanced the understanding of aortic root anatomy, physiology and pathology among surgeons.