Abdominal Aortic Wall Mechanics - Stress, Strain and Stiffness in A Medical Perspective : An Experimental Study in Man

Abstract: Background:  The stiffness of the abdominal aorta is considered a significant factor affecting the morbidity and mortality of cardiovascular disease. Estimating vascular stiffness is an integral part in cardiovascular risk assessment. Wall stress of the abdominal aorta appears to be a crucial factor in the remodeling of the arterial wall and the growth of aneurysms. Consequently, arterial mechanics plays a vital role in the function of the cardiovascular system. Therefore, there is a need for comprehensive studies of mechanical forces in the vessel wall to better understand the mechanisms behind normal and pathological changes that are significant for hypertension, atherosclerosis, and the development of arterial aneurysms. The aim of this study was to explore the blood pressure-induced forces in the aortic wall using a computational mechanical model, with particular attention to the effects of age, sex, and blood pressure on the remodeling process of the vessel wall.  Methods:  A computational model, comprising a solid mechanical model and a parameter identification process known as the Parameter Identification Method for Mechanical Parameters (PIMMP), was used to investigate the mechanical properties of the abdominal aortic vessel wall. Data for the model were obtained from the human abdominal aorta of volunteers: 30 healthy individuals, females (n=15) and males, divided into three age groups with an equal number of females and males (n=5 in each age group). Invasive blood pressure, measured via catheter, and diameter variation in the abdominal aorta, measured via ultrasound, were acquired to be used as input data for PIMMP. This dataset was utilized in Papers I, III, and IV. In Paper II, 24 datasets were generated, based on model parameters presented in the scientific literature.   Results:  Paper I reveals that elderly males exhibit both higher aortic wall stress and higher isotropic stress component, than females. With age, males show an increase in isotropic load-bearing fraction and a decrease in anisotropic load-bearing fraction, a trend not observed in females.  Paper II validates an in silico aortic model against a computerized membrane model of an abdominal aorta. The membrane model accurately predicts stress states as well as the load-bearing fraction of anisotropic material across all blood pressure levels, independent of the transmural stress gradient. However, the model’s accuracy is limited due to insufficient in vivo axial loading information.  Paper III demonstrates that changes in circumferential stretch have a more pronounced effect on longitudinal stress than the other way around. Both circumferential and longitudinal stiffnesses increase with age, irrespective of sex. However, sex-based differences in stiffness are observed when comparing younger and older groups.  Paper IV investigates pulse wave velocity (PWV) calculations using the Moens-Korteweg equation and the Bramwell-Hill equation. PWV shows a positive association with both isotropic and anisotropic material properties, with a transition zone observed between diastolic and systolic blood pressures, to a positive association with anisotropic properties at systolic blood pressure. Furthermore, an increase in PWV with age, with no significant difference between sexes, is observed.  The Extra Material suggests a deficiency in age-related wall stress regulation in males, potentially due to insufficient stiffness of anisotropic materials such as collagen. In contrast, females show an age-related increase in abdominal aortic wall thickness and anisotropic material stiffness, indicating adequate wall stress regulation.  Conclusions:  This doctoral dissertation focused on the effects of age and sex on the abdominal aortic wall. Overall, the findings suggest potential alterations in the collagen and elastin content during the remodeling of the abdominal aorta, which may differ between sexes. These alterations could be induced chemically or mechanically. The model has shown potential in identifying healthy individuals within a population. These insights may contribute to the understanding of cardiovascular health and disease progression.