Introduction:: Pulmonary arterial hypertension (PAH) occurs when arteries in the lung become thickened and narrowed, obstructing blood flow to the lungs. These alterations increase the heart’s workload, causing structural changes such as right-ventricular hypertrophy (RVH). As a result of these structural changes, which lead to functional changes, individuals with PAH are vulnerable to the progression of RV failure and death. For this reason, studying the right-ventricular function may be a good indicator of hypertensive heart disease. In particular, the mechanical properties of these tissues are assessed via planar biaxial testing. Planar biaxial testing is a mechanical test commonly performed on planar industrial materials. Its contemporary application for biomaterials was pioneered by Fung given the key (theoretical) principle that testing biological material should not differ from testing industrial materials.
Previous works have utilized planar biaxial testing to evaluate the mechanical characteristics of right-ventricular myocardium (RV MYO) in male rats induced with PAH. However, women exhibit an increased risk of developing PAH relative to men. Additionally, females statistically exhibit more favorable treatment outcomes compared to men. Thus, the objective of this work was to observe the sex-dependence of RV MYO mechanical properties by inducing PAH in female rats. Planar biaxial testing was performed on the sample providing loading and displacement data. With this data, it was possible to compute relevant quantities such as stress and strain. Relating these measures to one another, the material properties of the tested right ventricle (RV) samples were then evaluated.
Materials and Methods:: The SuHx animal model was selected for the treated rat groups, and PAH was verified in these animals by pulmonary arterial pressure measures. To obtain samples for testing, the heart of a rat was harvested. An approximately square sample was extracted from the isolated RV with one length aligned with the apex-to-outflow (AOT) while the other was aligned with the circumferential axis – x1 and x2 respectively. Having excised the sample, its edge lengths and thickness were recorded.
Each sample was attached to the Bose Electro-Force planar biaxial testing device via hooks looped to pulleys – allowing rotation as the tissue stretched. The samples evaluated in this work were subjected to 10% stretch at a frequency of 0.5 Hz with the biaxial ratios (x1:x2): 10:5, 10:2.5, 10:10, 5:10, and 2.5:10. To replicate in vivo conditions, the tissue was suspended in a phosphate-buffered saline solution heated to 37℃. As the tissue stretched, a camera tracked the motion of tissue markers, allowing for the isoparametric mapping from pixel positions. From the resulting displacement data, the deformation gradient tensor (F) and the Right Cauchy Green Strain tensor (E) were computed:
E = 1/2(FTF - I)
To relate stress with strain, the applied stress was computed from load measurements as the sample stretched. The First Piola-Kirchhoff (PK) Stress (P) was determined by normalizing force due to gravitational acceleration with the respective undeformed face area. Then, the Second PK Stress (S) was computed:
S = PF-T
Results, Conclusions, and Discussions:: The maximum equibiaxial stresses were determined for each sample in the control and treated groups. For the control group, it was found that the maximum equibiaxial stresses for strains between 8% – 9% ranged from 18.41kPa – 19.32kPa and 15.87kPa – 16.07kPa along the AOT and circumferential axes respectively. For the SuHx group, this range was between 55.53kPa – 67.99kPa and 71.43 kPa – 75.46kPa along the AOT and circumferential axes (sample plots provided for the control and SuHx groups). Pursell et al. found maximum equibiaxial stresses to be around 120 kPa at similar circumferential strains in male SuHx models. In the future, we will use a Fung-type exponential strain-energy model to fit the data and create averaged stress-strain relations for each group.
As predicted, the SuHx treatment group shows a considerable increase in stress relative to both controls for the female animals in this study and male controls from previous studies. Additionally, like previous studies, it was found that tissue still exhibited typical soft tissue properties like nonlinearity and anisotropy. Furthermore, the stress-strain relations exhibit a greater stiffness along the circumferential axis in the female SuHx samples (similar to male SuHx). Despite these similarities, there is a noticeable decrease in the magnitude of maximum equibiaxial stresses in the female SuHx relative to the male SuHx models adapted by previous works.
These results highlight the decreased stiffness of RV MYO in females compared to males induced with PAH. Furthermore, they potentially validate the phenomenon of increased favorable treatment outcomes for women by exhibiting less dire functional changes of the RV MYO in contrast to males with PAH. Most importantly, this work bridges gaps in literature focused on the mechanical impact of PAH by observing the effects on the tissue level in RV MYO for females with the disease. The results from this study further motivate future works to test hypotheses on why the treatment outcomes tend to be more favorable for women. One possible direction would be to examine the effect of female sex hormones on functional changes induced by PAH in female rat models.
Acknowledgements (Optional): :
References (Optional): : Vélez-Rendón, D., Pursell, E. R., Shieh, J., & Valdez-Jasso, D. (2019). Relative Contributions of Matrix and Myocytes to Biaxial Mechanics of the Right Ventricle in Pulmonary Arterial Hypertension. Journal of biomechanical engineering, 141(9), 091011. https://doi.org/10.1115/1.4044225
