Full length articleQuantification and comparison of the mechanical properties of four human cardiac valves
Graphical abstract
Introduction
The four heart valves share the same function of directing blood flow, but exhibit different mechanical and structural characteristics. The semilunar valves, the aortic valve (AV) and pulmonary valve (PV), have nearly symmetrical tri-leaflet configurations and are positioned in the outflow tracts of the left and right ventricles to the elastic arteries. The atrio-ventricular valves, the mitral valve (MV) and tricuspid valve (TV), on the other hand, are far more complicated in structure and function due to their asymmetrical structure and the presence of tethering chordae tendineae and papillary muscles. These four valves function primarily as passive structures, i.e., they open and close due to the differential blood pressure on each side of the valve leaflets [1]. Under normotensive conditions, the peak differential pressures across the closed AV and MV are about 100 mmHg and 120 mmHg respectively, much higher than their right heart counterparts (i.e., 13 mmHg to 35 mmHg for PV and TV) [2]. These differences in physiological conditions result in differing structural and mechanical properties among the valves.
Over the past two decades, major advances have been made in the diagnosis and treatment of valvular heart disease [3]. The increase in the development of new surgical or percutaneous valve repair and replacement techniques and devices highlights the need for a deeper understanding of the mechanical and microstructural properties of the native, aged human heart valves. Unfortunately, most biomechanical studies of heart valves have focused on the individual MV and AV valves, partially due to the prevalence of their treatment compared to those for the TV and PV valves [3]. The recent explosive use of transcatheter valve techniques for AV [4], [5], [6], [7], [8] and MV [9], [10] diseases also brings the hope that the largely untreated patient population of TV [11], [12] and PV [13] patients can be treated by transcatheter approaches. An in-depth understanding of the biomechanical differences between the four valves can facilitate the transition of the transcatheter AV and MV experience to the PV and TV valve space. However, due to the scarce availability of fresh human heart valves, wide variations in tissue preparation, testing protocols, and determination of mechanical parameters, a cohesive consensus regarding the comparative mechanical properties of the four human heart valves has not been established.
The objective of this study is to therefore characterize the planar biaxial material properties of the MV, AV, TV, and PV from the same aged human patients. Utilizing planar biaxial testing, the mechanical properties of the valve cusps were characterized for each of the four valves and their microstructural properties were examined through histological analysis. Constitutive modeling with the Fung-type elastic model was utilized to describe the biaxial mechanical response. Results were compared both by valve and by age in order to understand the comparative age-dependent changes in mechanical properties in the four heart valves as well.
Section snippets
Sample procurement
Twelve human cadaver hearts were obtained from the National Disease Research Interchange (NDRI, Philadelphia, PA). The hearts were selected based on cause of death, wherein the patients were defined as having a cause of death unrelated to cardiovascular disease. Research on human cadaver tissues was approved by Biological Materials Safeguards Committee (BMSC) at Georgia Institute of Technology. The hearts were obtained fresh within a post-mortem recovery interval (15.32 ± 6.51 h). Upon arrival at
Valve morphology
Thickness measurements of all leaflets are shown in. The PML leaflets were significantly thicker than the AML leaflets (p = 0.045). For all the tri-leaflet valves (i.e., AV, PV and TV), no statistically significant difference in intra-valve leaflet thicknesses were found. The averaged leaflet thicknesses of the MV, TV, AV and PV were 1.59 ± 0.15, 0.76 ± 0.06, 1.06 ± 0.07, and 0.56 ± 0.04 mm, respectively (see Fig. 3).
The TV leaflets were found to be significantly thinner than all other valve leaflets (p <
Discussion
In this study, a total of 114 human valve leaflets were mechanically tested and analyzed. To our knowledge, this is currently the largest biaxial mechanical study of human heart valve leaflets and marks the first comparative study of mechanical properties of all four human heart valves from the same aged patient population in relatively good cardiovascular health. The mechanical and microstructural properties presented in this study can be consistently compared among valves and patients, due to
Conclusions
In this study, we performed mechanical testing and histological analysis to quantify and compare the mechanical properties of the four valves from the same human patient population. The Fung-type elastic model was used to describe the biaxial mechanical responses of the leaflets. From the results, we found that the four valves differed substantially in thickness, degree of anisotropy, and stiffness, with the valves in the left heart having significant higher values than their right heart
Conflict of interest
The authors declare that they do not have any conflict of interest and financial and personal relationships with other people or organizations that could inappropriately influence their work.
Acknowledgments
This research was funded in part by NIH HL 104080 and HL 127570 grants. We would also like to thank Camille Johnson for her experimental and data analysis support.
References (52)
- et al.
2014 AHA/ACC guideline for the management of patients with valvular heart disease: executive summary: a report of the American college of cardiology/American heart association task force on practice guidelines
J. Am. Coll. Cardiol.
(2014) - et al.
Transcatheter therapies for treating tricuspid regurgitation
J. Am. Coll. Cardiol.
(2016) - et al.
Percutaneous pulmonary valve implantation: present status and evolving future
J. Am. Coll. Cardiol.
(2015) - et al.
Mechanics of fresh, refrigerated, and frozen arterial tissue
J. Surg. Res.
(2007) - et al.
Retained structural integrity of collagen and elastin within cryopreserved human heart valve tissue as detected by two-photon laser scanning confocal microscopy
Cryobiology
(2009) - et al.
ACC/AHA 2006 guidelines for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (writing Committee to Revise the 1998 guidelines for the management of patients with valvular heart disease) developed in collaboration with the Society of Cardiovascular Anesthesiologists endorsed by the Society for Cardiovascular Angiography and Interventions and the Society of Thoracic Surgeons
J. Am. Coll. Cardiol.
(2006) - et al.
Hydrodynamic function of polyurethane prosthetic heart valves: influences of Young's modulus and leaflet thickness
Biomaterials
(2002) - et al.
The important roles of tissue anisotropy and tissue-to-tissue contact on the dynamical behavior of a symmetric tri-leaflet valve during multiple cardiac pressure cycles
Med. Eng. Phys.
(2013) - et al.
Age-dependent changes of stress and strain in the human heart valve and their relation with collagen remodeling
Acta Biomater.
(2016) - et al.
The effects of sterilization and storage treatments on the stress strain behavior of aortic valve leaflets
Ann. Thorac. Surg.
(1976)