Quantification and comparison of the mechanical properties of four human cardiac valves

Abstract

Objective

Although having the same ability to permit unidirectional flow within the heart, the four main valves—the mitral valve (MV), aortic (AV), tricuspid (TV) and pulmonary (PV) valves—experience different loading conditions; thus, they exhibit different structural integrity from one another. Most research on heart valve mechanics have been conducted mainly on MV and AV or an individual valve, but none quantify and compare the mechanical and structural properties among the four valves from the same aged patient population whose death was unrelated to cardiovascular disease.

Methods

A total of 114 valve leaflet samples were excised from 12 human cadavers whose death was unrelated to cardiovascular disease (70.1 ± 3.7 years old). Tissue mechanical and structural properties were characterized by planar biaxial mechanical testing and histological methods. The experimental data were then fitted with a Fung-type constitutive model.

Results

The four valves differed substantially in thickness, degree of anisotropy, and stiffness. The leaflets of the left heart (the AV leaflets and the anterior mitral leaflets, AML) were significantly stiffer and less compliant than their counterparts in the right heart. TV leaflets were the most extensible and isotropic, while AML and AV leaflets were the least extensible and the most anisotropic. Age plays a significant role in the reduction of leaflet stiffness and extensibility with nearly straightened collagen fibers observed in the leaflet samples from elderly groups (65 years and older).

Conclusions

Results from 114 human leaflet samples not only provided a baseline quantification of the mechanical properties of aged human cardiac valves, but also offered a better understanding of the age-dependent differences among the four valves. It is hoped that the experimental data collected and the associated constitutive models in this study can facilitate future studies of valve diseases, treatments and the development of interventional devices.

Statement of Significance

Most research on heart valve mechanics have been conducted mainly on mitral and aortic valves or an individual valve, but none quantify and compare the mechanical and structural properties among the four valves from the same relatively healthy elderly patient population. In this study, the mechanical and microstructural properties of 114 leaflets of aortic, mitral, pulmonary and tricuspid valves from 12 human cadaver hearts were mechanically tested, analyzed and compared. Our results not only provided a baseline quantification of the mechanical properties of aged human valves, but a age range between patients (51–87 years) also offers a better understanding of the age-dependent differences among the four valves. It is hoped that the obtained experimental data and associated constitutive parameters can facilitate studies of valve diseases, treatments and the development of interventional devices.

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.