Description

Stiffening of blood vessels is an early indicator of cardiovascular disease and is accelerated in patients under Left Ventricular Assist Device (LVAD) support. The normal aorta serves as an elastic mechanical element that absorbs energy during the native heart contraction and uses the recoil to maintain blood pressure when the heart is filling. LVADs are rotary pumps attached to the heart that increase blood flow to tissues but dilute the pulsatility of blood pressure and flow resulting in areas of stasis with a propensity for blood clots and stroke. In this project, the feedback between reduced pulsatility and tissue compliance is evaluated using benchtop experiments and mechanical modeling. Two length/diameter tubular models of the human aorta with varying thicknesses (1.8mm and 3.175mm) were fabricated from SortaClear-40 silicone to resemble the range of native tissue stiffness (400kPa and 800 kPa). A pressure-volume apparatus was assembled to obtain the stress-strain relationship for each construct by applying a range of pressures and measuring the resulting construct expansion, allowing for quantification of each model’s elasticity. These models were placed in the ascending aorta position of a benchtop simulator of the cardiovascular system and pressure and flow were measured for a range of cardiac and LVAD conditions.The results showed that when the LVAD speed was 10krpm, approximately 80% of the flow exits the heart through the LVAD, and 20% through the aortic valve. At this speed, the thick and thin constructs exhibited values for aortic pressure of 98.9mmHg and 96.2mmHg and values for systemic flow of 4.22L/min and 4.30L/min. The energy absorbed in the system was characterized by integrating the dynamic pressure-flow responses. The change in total energy per cycle with respect to each LVAD speed was 65mmHg·L/krev and 137mmHg·L/krev for the thick and thin construct, respectively.These results show that as stiffness increases, the total energy absorbed in the system decreases. This suggests that when LVADs are implanted, the energy absorbed in the aorta is reduced. If the tissue stiffness increases in response, it is accompanied by a decrease in the energy absorbed suggesting a feedback loop designed to reestablish arterial compliance.