Sonomicrometry-derived 3-dimensional geometry of the human tricuspid annulus

How well do 3D-printed tissue mimics represent the complex mechanics of biological soft tissues? An example study with Stratasys' cardiovascular TissueMatrix materials

Tissue mimicking materials are designed to represent real tissue in applications such as medical device testing and surgical training. Thanks to progress in 3D-printing technology, tissue mimics can now be easily cast into arbitrary geometries and manufactured with adjustable material properties to mimic a wide variety of tissue types. However, it is unclear how well 3D-printable mimics represent real tissues and their mechanics. The objective of this work is to fill this knowledge gap using the Stratasys Digital Anatomy 3D-Printer as an example. To this end, we created mimics of biological tissues we previously tested in our laboratory: blood clots, myocardium, and tricuspid valve leaflets. We printed each tissue mimic to have the identical geometry to its biological counterpart and tested the samples using identical protocols. In our evaluation, we focused on the stiffness of the tissues and their fracture toughness in the case of blood clots. We found that the mechanical behavior of the tissue mimics often differed substantially from the biological tissues they aim to represent. Qualitatively, tissue mimics failed to replicate the traditional strain-stiffening behavior of soft tissues. Quantitatively, tissue mimics were stiffer than their biological counterparts, especially at small strains, in some cases by orders of magnitude. In those materials in which we tested toughness, we found that tissue mimicking materials were also much tougher than their biological counterparts. Thus, our work highlights limitations of at least one 3D-printing technology in its ability to mimic the mechanical properties of biological tissues. Therefore, care should be taken when using this technology, especially where tissue mimicking materials are expected to represent soft tissue properties quantitatively. Whether other technologies fare better remains to be seen.

Ovine tricuspid annular dynamics and three-dimensional geometry during acute atrial fibrillation

OBJECTIVES

Long-standing atrial fibrillation (AF) may lead to tricuspid regurgitation (TR) and right ventricular dysfunction. However, the effect of acute AF on tricuspid annular (TA) dynamics and three-dimensional geometry is unknown.

METHODS

In eight adult sheep, sonomicrometry crystals were implanted around the tricuspid annulus and right ventricular free wall. Pressure transducers were placed in the right ventricle, left ventricle, and right atrium. After weaning from cardiopulmonary bypass and a period of hemodynamic stabilization, simultaneous sonomicrometry and hemodynamic data were collected in sinus rhythm (SR) and during experimental AF (400b/min right atrial pacing). Annular area, perimeter, dimensions, height, global and regional annular contraction, and strain were calculated based on cubic spline fits to crystal 3D locations.

RESULTS

Maximal TA area increased from 1084.9±273.9mm2 in SR to 1207.5±322.1mm2 during AF (p = 0.002). Anteroposterior diameter increased from 36.5±5.0mm to 38.4±5.5mm (p = 0.05). TA contraction decreased from 7±2% in SR to 2±1% in AF (p = 0.001). Anterior, posterior, and septal regional annular contraction decreased from 10±4%, 8±3% and 6±2% to 4±2%, 3±1% and 2±1% for SR and AF, respectively (p<0.05). AF perturbed systolic global annular strain (from -6.52±1.74% to -2.78±1.79%; p = 0.003) and caused annular stretch. Annular height marginally decreased with AF from 5.8±1.9mm to 5.7±2.0mm; p = 0.039.

CONCLUSIONS

Acute experimental AF in healthy sheep was associated with TA dilation, flattening, and decreased total and regional annular contractility. These data may help elucidate the pathophysiology of functional TR associated with AF.

Leaflet remodeling reduces tricuspid valve function in a computational model

Tricuspid valve leaflets have historically been considered "passive flaps". However, we have recently shown that tricuspid leaflets actively remodel in sheep with functional tricuspid regurgitation. We hypothesize that these remodeling-induced changes reduce leaflet coaptation and, therefore, contribute to valvular dysfunction. To test this, we simulated the impact of remodeling-induced changes on valve mechanics in a reverse-engineered computer model of the human tricuspid valve. To this end, we combined right-heart pressures and tricuspid annular dynamics recorded in an ex vivo beating heart, with subject-matched in vitro measurements of valve geometry and material properties, to build a subject-specific finite element model. Next, we modified the annular geometry and boundary conditions to mimic changes seen in patients with pulmonary hypertension. In this model, we then increased leaflet thickness and stiffness and reduced the stretch at which leaflets stiffen, which we call "transition-λ." Subsequently, we quantified mean leaflet stresses, leaflet systolic angles, and coaptation area as measures of valve function. We found that leaflet stresses, leaflet systolic angle, and coaptation area are sensitive to independent changes in stiffness, thickness, and transition-λ. When combining thickening, stiffening, and changes in transition-λ, we found that anterior and posterior leaflet stresses decreased by 26% and 28%, respectively. Furthermore, systolic angles increased by 43%, and coaptation area decreased by 66%; thereby impeding valve function. While only a computational study, we provide the first evidence that remodeling-induced leaflet thickening and stiffening may contribute to valvular dysfunction. Targeted suppression of such changes in diseased valves could restore normal valve mechanics and promote leaflet coaptation.

Septal annular dilation in chronic ovine functional tricuspid regurgitation

INTRODUCTION

Annular reduction with prosthetic rings represents the current surgical treatment of functional tricuspid regurgitation (FTR). However, alterations of annular geometry and dynamics associated with FTR are not well characterized.

METHODS

FTR was induced in 29 adult sheep with either 8 weeks of pulmonary artery banding (PAB, n = 15) or 3 weeks of tachycardia-induced cardiomyopathy (TIC, n = 14). Eight healthy sheep served as controls (CTL). At the terminal procedure, all animals underwent sternotomy, epicardial echocardiography, and implantation of sonomicrometry crystals on the tricuspid annulus (TA) and right ventricular free wall while on cardiopulmonary bypass. Simultaneous hemodynamic, sonomicrometry, and echocardiographic data were acquired after weaning from cardiopulmonary bypass and stabilization. Annular geometry and dynamics were calculated from 3-dimensional crystal coordinates.

RESULTS

Mean FTR grade (0-4) was 3.2 ± 1.2 and 3.2 ± 0.5 for PAB and TIC, respectively, with both models of FTR associated with similar degree of right ventricular dysfunction (right ventricular fractional area contraction 38 ± 7% and 37 ± 9% for PAB and TIC, respectively). Left ventricular ejection fraction was significantly reduced in TIC versus baseline (33 ± 9%, vs 58 ± 4%, P = .0001). TA area was 651 ± 109 mm2, 881 ± 242 mm2, and 995 ± 232 mm2 for CTL, FTR, and TIC, respectively (P = .006) with TA area contraction of 16.6 ± 4.2%, 11.5 ± 8.0%, and 6.0 ± 4.0%, respectively (P = .003). Septal annulus increased from 33.8 ± 3.1 mm to 39.7 ± 6.4 mm and 43.1 ± 3.2 mm for CTL, PAB, and TIC, respectively (P < .0001).

CONCLUSIONS

Ovine FTR was associated with annular dilation and reduced annular area contraction. Significant dilation of septal annulus was observed in both models of FTR. As tricuspid rings do not completely stabilize the septal annulus, continued remodeling may contribute to recurrent FTR after repair.

Comparison between multimodality imaging approaches for measurement of the tricuspid annulus in severe tricuspid regurgitation

BACKGROUND

Tricuspid valve (TV) sizing is crucial for surgical or interventional procedures planning. Imaging TV is frequently challenging and often requires multimodal imaging techniques. Computed tomography (CT) is the gold standard for sizing. The authors compared measurements of the tricuspid annulus (TA) acquired using echocardiography and CT.

METHODS

Thirty-six patients with severe symptomatic tricuspid regurgitation were included in this retrospective analysis. During mid-diastole, the maximal two-dimensional (2D) TA diameter was directly measured in multiple views using transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE). Three-dimensional (3D) TA size was assessed using cross-sectional long-axis and short-axis diameters, areas, and perimeters measured in the projected plane. The TA diameter was quantified by the perimeter measured on the CT images (CT imaging_indirect) and compared with echocardiography measurements. Tenting height and tenting area were also measured using TTE at mid systole.

RESULTS

The long-axis dimensions measured using 3DTEE (3DTEE_direct) best correlated with the TA diameter (CT imaging_indirect) (R = 0.851, P = 0.0001) and the least discrepancies (difference 1.2 ± 2.4 mm, P = 0.012). The TA diameters quantified by the perimeters measured using 3DTEE (3DTEE_indirect) were smaller than the CT values (difference 2.5 ± 2.5 mm, P = 0.0001). The maximal dimensions directly measured by 2DTEE (2DTEE_direct) were modestly correlated with the CT values. Overall, the maximal dimensions by TTE_direct were less reliable than those by CT. TA eccentricity index correlated with the maximal tenting height and area.

CONCLUSION

The patients with severe tricuspid regurgitation had a dilated and circular annulus. The long-axis TA dimensions (3DTEE_direct) were similar to the diameters (CT imaging_indirect).

Texas TriValve 1.0 : a reverse‑engineered, open model of the human tricuspid valve

Nearly 1.6 million Americans suffer from a leaking tricuspid heart valve. To make matters worse, current valve repair options are far from optimal leading to recurrence of leakage in up to 30% of patients. We submit that a critical step toward improving outcomes is to better understand the "forgotten" valve. High-fidelity computer models may help in this endeavour. However, the existing models are limited by averaged or idealized geometries, material properties, and boundary conditions. In our current work, we overcome the limitations of existing models by (reverse) engineering the tricuspid valve from a beating human heart in an organ preservation system. The resulting finite-element model faithfully captures the kinematics and kinetics of the native tricuspid valve as validated against echocardiographic data and others' previous work. To showcase the value of our model, we also use it to simulate disease-induced and repair-induced changes to valve geometry and mechanics. Specifically, we simulate and compare the effectiveness of tricuspid valve repair via surgical annuloplasty and via transcatheter edge-to-edge repair. Importantly, our model is openly available for others to use. Thus, our model will allow us and others to perform virtual experiments on the healthy, diseased, and repaired tricuspid valve to better understand the valve itself and to optimize tricuspid valve repair for better patient outcomes.

A pilot investigation of the tricuspid valve annulus in newborns with hypoplastic left heart syndrome

Objective

Hypoplastic left heart syndrome (HLHS) is a congenital disease characterized by an underdevelopment of the anatomical components inside the left heart. Approximately 30% of newborns with HLHS will develop tricuspid regurgitation, and it is currently unknown how the valve annulus mechanics and geometry are associated with regurgitation. Thus, we present an engineering mechanics-based analysis approach to quantify the mechanics and geometry of the HLHS-afflicted tricuspid valve (TV), using 4-dimensional echocardiograms.

Methods

Infants born with HLHS (n = 8) and healthy newborns (n = 4) had their TVs imaged, and the data were imported to 3D Slicer. The annular curves were defined at 5 points in the cardiac cycle. The geometry and deformation (strain) of the TV annulus were calculated to elucidate the mechanics of this critical structure and to compare them between neonates with and without HLHS.

Results

For the annular geometry, HLHS-afflicted newborns had significantly larger annular circumferences (20%-30%) and anteroposterior diameters (35%-45%) than the healthy patients. From a biomechanics' perspective, the HLHS patients had significantly smaller strains in the anterior segments (–0.1 ± 2.6%) during end-diastolic and end-isovolumetric relaxation (1.7 ± 3.0%) compared with the healthy counterparts (–13.3 ± 2.9% and 6.8 ± 0.9%, respectively).

Conclusions

The image-based analysis presented in this study may provide novel insights into the geometric and mechanistic differences in the TV annulus between the healthy and HLHS newborns. Future longitudinal studies of the biomechanics of TV annulus and other subvalvular structures may inform our understanding of the initiation and development of tricuspid regurgitation and the design of optimal repairs in this challenging population.

Tricuspid leaflet kinematics after annular size reduction in ovine functional tricuspid regurgitation

OBJECTIVE

Tricuspid annular size reduction with annuloplasty rings represents the foundation of surgical repair of functional tricuspid regurgitation. However, the precise effect of annular size reduction on leaflet motion and geometry remains unknown.

METHODS

Ten sheep underwent surgical implantation of a pacemaker with an epicardial lead and were paced 200-240 beats/min to achieve biventricular dysfunction and functional tricuspid regurgitation. Subsequently, sonomicrometry crystals were implanted on the right ventricle, the tricuspid annulus, and on the belly of anterior, posterior, and septal tricuspid leaflets. Double-layer polypropylene suture was placed around the tricuspid annulus and externalized to a tourniquet. Simultaneous echocardiographic, hemodynamic, and sonomicrometry data were acquired with functional tricuspid regurgitation and during 5 consecutive annular reduction steps. Annular area, tenting height, and volume, together with each leaflet strain, radial length, and angles, were calculated from crystal coordinates.

RESULTS

Rapid pacing reduced both left ventricle and right ventricle function and induced functional tricuspid regurgitation (0-3+) in all animals (from 0 ± 0 to 2.4 ± 0.7, P = .002), whereas tricuspid annulus diameter increased from 2.6 ± 0.3 cm to 3.3 ± 0.3 cm (P = .001). Tricuspid annular size reduction 1 to 5 resulted in 16% ± 7%, 37% ± 11%, 55% ± 11%, 66% ± 10%, and 76% ± 8% tricuspid annulus area reduction, respectively, and successively decreased tricuspid regurgitation. Tricuspid annular size reduction 2 to 5 induced anterior and posterior leaflet restricted motion and lower diastolic motion velocities. Tricuspid annular size reduction 5 perturbed septal leaflet range of motion but preserved its angle velocities. Tricuspid annular size reduction 3-5 generated compressive strains in all leaflets.

CONCLUSIONS

Tricuspid annular area reduction of 55% perturbed anterior and posterior leaflet motion while maintaining normal septal leaflet movement. More extreme reduction triggered profound changes in anterior and posterior leaflet motion, suggesting that aggressive undersizing impairs leaflet kinematics.

Tricuspid chordae tendineae mechanics: Insertion site, leaflet, and size-specific analysis and constitutive modelling

Background

Tricuspid valve chordae tendineae play a vital role in our cardiovascular system. They function as "parachute cords" to the tricuspid leaflets to prevent prolapse during systole. However, in contrast to the tricuspid annulus and leaflets, the tricuspid chordae tendineae have received little attention. Few previous studies have described their mechanics and their structure-function relationship.

Objective

In this study, we aimed to quantify the mechanics of tricuspid chordae tendineae based on their leaflet of origin, insertion site, and size.

Methods

Specifically, we uniaxially stretched 53 tricuspid chordae tendineae from sheep and recorded their stress-strain behavior. We also analyzed the microstructure of the tricuspid chordae tendineae based on two-photon microscopy and histology. Finally, we compared eight different hyperelastic constitutive models and their ability to fit our data.

Results

We found that tricuspid chordae tendineae are highly organized collageneous tissues, which are populated with cells throughout their thickness. In uniaxial stretching, this microstructure causes the classic J-shaped nonlinear stress-strain response known from other collageneous tissues. We found differences in stiffness between tricuspid chordae tendineae from the anterior, posterior, or septal leaflets only at small strains. Similarly, we found significant differences based on their insertion site or size also only at small strains. Of the models we fit to our data, we recommend the Ogden two-parameter model. This model fit the data excellently and required a minimal number of parameters. For future use, we identified and reported the Ogden material parameters for an average data set.

Conclusion

The data presented in this study help to explain the mechanics and structure-function relationship of tricuspid chordae tendineae and provide a model recommendation (with parameters) for use in computational simulations of the tricuspid valve.

Tricuspid Annuloplasty Rings: A Quantitative Comparison of Size, Nonplanar Shape, and Stiffness

BACKGROUND

Functional tricuspid regurgitation due to annular and ventricular dilatation is increasingly recognized as a significant source of morbidity and mortality. To repair the annulus, surgeons implant one of many annuloplasty devices that differ in size, 3-dimensional (3D) shape, and stiffness. However, there have been no quantitative comparisons between various available devices.

METHODS

Three-dimensional scanning, micro-computed tomography imaging, analytical methods, and mechanical tests were used to compare 3 Edwards Lifesciences (Irvine, CA) and 3 Medtronic (Minneapolis, MN) annuloplasty devices of all available sizes. We measured in-plane metrics of maximum diameter, perimeter, area, height, as well as elevation and curvature profiles. Furthermore, we computed bending stiffness as well as the maximum and minimum axes of the bending stiffness.

RESULTS

Most annular prostheses differed little in their in-plane geometries but varied significantly in height. In-plane properties deviated significantly from measurements of healthy human tricuspid annuli. Height of the Edwards' MC3 and Medtronic's Contour 3D resembled healthy human tricuspid valve annuli, whereas the Edwards' Physio and Classic, and Medtronic's TriAd, did not. Additionally, the elevation profiles of the MC3 and Contour 3D and curvature profiles between all devices were consistent and matched those of healthy human annuli. The tested devices also differed in their bending stiffness, both in terms of absolute values and their maximum and minimum axes.

CONCLUSIONS

Contoured devices, such as Edwards' MC3 and Medtronic's Contour 3D, most accurately resembled the healthy human tricuspid annulus but differed significantly in bending stiffness. To what extent prosthesis properties and shape affect tricuspid valve function remains to be determined.

Mid-term functional recovery after tricuspid annuloplasty concomitant with left-sided valve surgery

BACKGROUND

To elucidate the impact of tricuspid annuloplasty concomitant with left-sided valve surgery on the right ventricular (RV) function in patients with mild or more tricuspid regurgitation (TR).

METHODS

We enrolled 136 patients with mild or more TR who underwent left-sided valve surgery. Seventy-three patients underwent left-sided valve surgery alone (group non-T) and 63 underwent concomitant tricuspid annuloplasty (group T). The echocardiographic data at the latest follow-up (mean 1019 days) were compared using multiple regression analysis to adjust cofounding factors. Propensity score was calculated and included in the analysis as a covariate. In addition, propensity score matching was used for sensitive analysis (12 pairs).

RESULTS

In group non-T, there were more aortic valve surgeries, and fewer mitral valve surgeries. At baseline, body surface area, New York Heart Association class, and prevalence of atrial fibrillation were significantly different between groups. On preoperative echocardiography, left and right atrial diameter, RV diameter, and tricuspid annular diameter were larger in group T, whereas there was no significant difference in RV fractional area change. In multiple regression analyses, RV diameter in diastole was significantly lower and RV fractional area change was significantly higher at the follow-up period in group T. These results were not attenuated even in subgroup analysis in patients with only mild TR or mitral valve surgery alone.

CONCLUSION

Among patients with mild or more TR, RV dimensional and functional recovery was not obtained with left-sided valve surgery alone. Adding tricuspid annuloplasty may potentially achieve both outcomes.

Mechanical and Structural Evaluation of Tricuspid Bicuspidization in a Porcine Model

INTRODUCTION

Tricuspid regurgitation (TR) affects approximately 1.6 million Americans and is associated with just a 63.9% 1-year survival rate in its moderate to severe forms due to its asymptomatic nature and late diagnosis and surgical referral. As a result, industrial fervor has begun to broach this topic, with several percutaneous treatment devices currently under development. As much remains unknown about the tricuspid apparatus, the mechanics of these procedures remain unquantified. In this study, a testing apparatus and technique for the evaluation of percutaneous tricuspid valve (TV) bicuspidization were developed for the evaluation of these parameters in twelve porcine hearts.

METHODS

The passive relaxed myocardial state and the active contracted state were each induced in six porcine hearts and the bicuspidization experiment was run twice, the second time after induction of TR. TV annular area, cinching force, static leakage through the TV annulus, and annular ellipticity were quantified and compared among the groups.

RESULTS

The use of phenol was effective to induce functional TR by increased annular area. Cinching force was not found to differ between any of the testing states, but the bicuspidization experiment was able to reduce the TR annular area to that of its healthy counterpart in addition to reducing static leakage through the TV annulus. Despite appropriately reducing the area, bicuspidization was found to induce a more circular TV annular shape.

CONCLUSION

Taken together, these results provide a first mechanical analysis of the TV bicuspidization mechanism and may serve as a point of reference for future clinical animal studies.

A Pilot Study on Linking Tissue Mechanics with Load-Dependent Collagen Microstructures in Porcine Tricuspid Valve Leaflets

The tricuspid valve (TV) is composed of three leaflets that coapt during systole to prevent deoxygenated blood from re-entering the right atrium. The connection between the TV leaflets’ microstructure and the tissue-level mechanical responses has yet to be fully understood in the TV biomechanics society. This pilot study sought to examine the load-dependent collagen fiber architecture of the three TV leaflets, by employing a multiscale, combined experimental approach that utilizes tissue-level biaxial mechanical characterizations, micro-level collagen fiber quantification, and histological analysis. Our results showed that the three TV leaflets displayed greater extensibility in the tissues’ radial direction than in the circumferential direction, consistently under different applied biaxial tensions. Additionally, collagen fibers reoriented towards the direction of the larger applied load, with the largest changes in the alignment of the collagen fibers under radially-dominant loading. Moreover, collagen fibers in the belly region of the TV leaflets were found to experience greater reorientations compared to the tissue region closer to the TV annulus. Furthermore, histological examinations of the TV leaflets displayed significant regional variation in constituent mass fraction, highlighting the heterogeneous collagen microstructure. The combined experimental approach presented in this work enables the connection of tissue mechanics, collagen fiber microstructure, and morphology for the TV leaflets. This experimental methodology also provides a new research platform for future developments, such as multiscale models for the TVs, and the design of bioprosthetic heart valves that could better mimic the mechanical, microstructural, and morphological characteristics of the native tricuspid valve leaflets.

A detailed mechanical and microstructural analysis of ovine tricuspid valve leaflets

The tricuspid valve ensures unidirectional blood flow from the right atrium to the right ventricle. The three tricuspid leaflets operate within a dynamic stress environment of shear, bending, tensile, and compressive forces, which is cyclically repeated nearly three billion times in a lifetime. Ostensibly, the microstructural and mechanical properties of the tricuspid leaflets have mechanobiologically evolved to optimally support their function under those forces. Yet, how the tricuspid leaflet microstructure determines its mechanical properties and whether this relationship differs between the three leaflets is unknown. Here we perform a microstructural and mechanical analysis in matched ovine tricuspid leaflet samples. We found that the microstructure and mechanical properties vary among the three tricuspid leaflets in sheep. Specifically, we found that tricuspid leaflet composition, collagen orientation, and valve cell nuclear morphology are spatially heterogeneous and vary across leaflet type. Furthermore, under biaxial tension, the leaflets' mechanical behaviors exhibited unequal degrees of mechanical anisotropy. Most importantly, we found that the septal leaflet was stiffer in the radial direction and not the circumferential direction as with the other two leaflets. The differences we observed in leaflet microstructure coincide with the varying biaxial mechanics among leaflets. Our results demonstrate the structure-function relationship for each leaflet in the tricuspid valve. We anticipate our results to be vital toward developing more accurate, leaflet-specific tricuspid valve computational models. Furthermore, our results may be clinically important, informing differential surgical treatments of the tricuspid valve leaflets. Finally, the identified structure-function relationships may provide insight into the homeostatic and remodeling potential of valvular cells in altered mechanical environments, such as in diseased or repaired tricuspid valves. STATEMENT OF SIGNIFICANCE: Our work is significant as we investigated the structure-function relationship of ovine tricuspid valve leaflets. This is important as tricuspid valves fail frequently and our current approach to repairing them is suboptimal. Specifically, we related the distribution of structural and cellular elements, such as collagen, glycosaminoglycans, and cell nuclei, to each leaflet's mechanical properties. We found that leaflets have different structures and that their mechanics differ. This may, in the future, inform leaflet-specific treatment strategies and help optimize surgical outcomes.

A Comparison of Transesophageal to Transthoracic Echocardiographic Measures of Right Ventricular Function

OBJECTIVES

To assess the concordance between transesophageal echocardiographic (TEE) and transthoracic echocardiograpic (TTE) measures of right ventricular (RV) function using standard 2-dimensional and Doppler methods. The authors hypothesized that there would be significant disagreement in tricuspid annular plane systolic excursion (TAPSE), fractional area change, right-sided index of myocardial performance, and tricuspid annular systolic velocity (S').

DESIGN

Prospective observational.

SETTING

Cardiac operating room at a single academic medical center.

PARTICIPANTS

All adult patients undergoing elective cardiac surgery at a single tertiary care academic medical center over 6 months.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The fractional area change, S', TAPSE, right-sided index of myocardial performance, and tricuspid annular diameter were measured with TEE and TTE to assess for concordance using the concordance correlation coefficient and paired t tests, including 95% confidence limits. The study demonstrated that quantitative measures of RV function by TEE correlate poorly with TTE measurements in close temporal proximity under similar hemodynamic conditions.

CONCLUSIONS

When performing an assessment of RV function, transesophageal echocardiographers should exercise caution when extrapolating data validated by TTE to TEE studies. Measures of RV function by TEE tend to have fair agreement to TTE measurements obtained in close temporal proximity under similar hemodynamic conditions. Most importantly, the present study showed that TAPSE and S' values obtained from the modified transgastric RV inflow view tend to have lower values than those measured with TTE. Given the propensity for underestimating measurements from the modified transgastric RV inflow view, the authors conclude that values equal to or greater than established norms for tricuspid annular motion may be used to establish normal-but not abnormal-RV function.

Biomechanics of the Tricuspid Annulus: A Review of the Annulus' In Vivo Dynamics With Emphasis on Ovine Data

The tricuspid annulus forms the boundary between the tricuspid valve leaflets and their surrounding perivalvular tissue of the right atrioventricular junction. Its shape changes throughout the cardiac cycle in response to the forces from the contracting right heart myocardium and the blood-valve interaction. Alterations to annular shape and dynamics in disease lead to valvular dysfunctions such as tricuspid regurgitation from which millions of patients suffer. Successful treatment of such dysfunction requires an in-depth understanding of the normal shape and dynamics of the tricuspid annulus and of the changes following disease and subsequent repair. In this manuscript we review what we know about the shape and dynamics of the normal tricuspid annulus and about the effects of both disease and repair based on non-invasive imaging studies and invasive fiduciary marker-based studies. We further show, by means of ovine data, that detailed engineering analyses of the tricuspid annulus provide regionally-resolved insight into the kinematics of the annulus which would remain hidden if limiting analyses to simple geometric metrics.

Mechanical Response Changes in Porcine Tricuspid Valve Anterior Leaflet Under Osmotic-Induced Swelling

Since many soft tissues function in an isotonic in-vivo environment, it is expected that physiological osmolarity will be maintained when conducting experiments on these tissues ex-vivo. In this study, we aimed to examine how not adhering to such a practice may alter the mechanical response of the tricuspid valve (TV) anterior leaflet. Tissue specimens were immersed in deionized (DI) water prior to quantification of the stress-strain responses using an in-plane biaxial mechanical testing device. Following a two-hour immersion in DI water, the tissue thickness increased an average of 107.3% in the DI water group compared to only 6.8% in the control group, in which the tissue samples were submerged in an isotonic phosphate buffered saline solution for the same period of time. Tissue strains evaluated at 85 kPa revealed a significant reduction in the radial direction, from 34.8% to 20%, following immersion in DI water. However, no significant change was observed in the control group. Our study demonstrated the impact of a hypo-osmotic environment on the mechanical response of TV anterior leaflet. The imbalance in ions leads to water absorption in the valvular tissue that can alter its mechanical response. As such, in ex-vivo experiments for which the native mechanical response of the valves is important, using an isotonic buffer solution is essential.

Mechanics of the Tricuspid Valve-From Clinical Diagnosis/Treatment, In-Vivo and In-Vitro Investigations, to Patient-Specific Biomechanical Modeling

Proper tricuspid valve (TV) function is essential to unidirectional blood flow through the right side of the heart. Alterations to the tricuspid valvular components, such as the TV annulus, may lead to functional tricuspid regurgitation (FTR), where the valve is unable to prevent undesired backflow of blood from the right ventricle into the right atrium during systole. Various treatment options are currently available for FTR; however, research for the tricuspid heart valve, functional tricuspid regurgitation, and the relevant treatment methodologies are limited due to the pervasive expectation among cardiac surgeons and cardiologists that FTR will naturally regress after repair of left-sided heart valve lesions. Recent studies have focused on (i) understanding the function of the TV and the initiation or progression of FTR using both in-vivo and in-vitro methods, (ii) quantifying the biomechanical properties of the tricuspid valve apparatus as well as its surrounding heart tissue, and (iii) performing computational modeling of the TV to provide new insight into its biomechanical and physiological function. This review paper focuses on these advances and summarizes recent research relevant to the TV within the scope of FTR. Moreover, this review also provides future perspectives and extensions critical to enhancing the current understanding of the functioning and remodeling tricuspid valve in both the healthy and pathophysiological states.

Tricuspid valve leaflet strains in the beating ovine heart

The tricuspid leaflets coapt during systole to facilitate proper valve function and, thus, ensure efficient transport of deoxygenated blood to the lungs. Between their open state and closed state, the leaflets undergo large deformations. Quantification of these deformations is important for our basic scientific understanding of tricuspid valve function and for diagnostic or prognostic purposes. To date, tricuspid valve leaflet strains have never been directly quantified in vivo. To fill this gap in our knowledge, we implanted four sonomicrometry crystals per tricuspid leaflet and six crystals along the tricuspid annulus in a total of five sheep. In the beating ovine hearts, we recorded crystal coordinates alongside hemodynamic data. Once recorded, we used a finite strain kinematic framework to compute the temporal evolutions of area strain, radial strain, and circumferential strain for each leaflet. We found that leaflet strains were larger in the anterior leaflet than the posterior and septal leaflets. Additionally, we found that radial strains were larger than circumferential strains. Area strains were as large as 97% in the anterior leaflet, 31% in the posterior leaflet, and 31% in the septal leaflet. These data suggest that tricuspid valve leaflet strains are significantly larger than those in the mitral valve. Should our findings be confirmed they could suggest either that the mechanobiological equilibrium of tricuspid valve resident cells is different than that of mitral valve resident cells or that the mechanotransductive apparatus between the two varies. Either phenomenon may have important implications for the development of tricuspid valve-specific surgical techniques and medical devices.

Development of a Computational Method for Simulating Tricuspid Valve Dynamics

Computational modeling can be used to improve understanding of tricuspid valve (TV) biomechanics and supplement knowledge gained from benchtop and large animal experiments. The aim of this study was to develop a computational model of the TV using high resolution micro-computed tomography (μCT) imaging and fluid-structure interaction simulations. A three-dimensional TV model, incorporating detailed leaflet and chordal geometries, was reconstructed from μCT images of an excised porcine TV obtained under diastolic conditions. The leaflets were described using non-linear stress-strain relations and chordal properties were iteratively adjusted until valve closure was obtained. The leaflet coaptation zone obtained from simulation of valve closure was validated against μCT images of the TV captured at peak systole. The computational model was then used to simulate a regurgitant TV morphology and investigate changes in closure dynamics. Overall, the mean stresses in the leaflet belly region and the chordae tendinae of the regurgitant TV were 7% and 3% higher than the same regions of the normal TV. The maximum principal strain in the leaflet belly of the regurgitant TV was also 9% higher than the same regions of the normal TV. It is anticipated that this computational model can be used in future studies for further understanding of TV biomechanics and associated percutaneous repairs.

Tricuspid Valve Annular Mechanics: Interactions with and Implications for Transcatheter Devices

In the interventional treatment of tricuspid valve regurgitation, the majority of prosthetic devices interact with or are implanted to the tricuspid valve annulus. For new transcatheter technologies, there exists a growing body of clinical experience, literature, and professional discourse related to the difficulties in delivering, securing, and sustaining the function of these devices within the dynamic tricuspid annulus. Many of the difficulties arise from circumstances not encountered in open-heart surgery, namely; a non-arrested heart, indirect visualization, and a reliance on non-suture-based methods. These challenges require the application of procedural techniques or system designs to account for tricuspid annular motion, forces, and underlying tissue strength. Improved knowledge in these interactions will support the goals of improving device systems, their procedures, and patient outcomes. This review aims to describe current concepts of tricuspid annular mechanics, key device and procedural implications, and highlight current knowledge gaps for future consideration.