The Impact of System Parameters on a Mock Aorta in an Ex Vivo Heart Perfusion Pump Loop

  • Author / Creator
    Li, Sining
  • Ex Vivo Heart Perfusion (EVHP) preserves a donor heart’s natural beating function outside of the body. The purpose of this work was to study the influence of mock aorta properties on pump performance in a mechanical EVHP system. Previous work has shown potential benefits on pump workload by including the compliant nature of the aorta at the pulsatile pump exit. The EVHP system can be assumed analogous to a mechanical reciprocating pump loop.
    An experimental test loop was developed to model the EVHP cardiac systemic circulation flow loop for which all system parameters could be defined or measured. The developed mechanical test loop was modular and could be expanded to include more components as needed in future studies. The physiological pulsatile flow is generated using a programmable diaphragm pump. At the pump outlet, a mock aorta section simulates aortic elastic response. To predict system parameters, the flow loop was modeled using a Bernoulli analysis. Silicone mock aortas of variable length and compliance were investigated to understand the flow effects of the compliant section. The silicones tested are two-part silicones from Smooth-On Inc, Ecoflex and Dragon Skin. To test the effect of compliance, a design strategy was developed to cast the compliant aorta. This design strategy can be applied to other compliant components as EVHP testing requires.
    A parametric study of system parameters on pump performance with the effect of aortic compliance was conducted. System backpressure, aorta chamber pressure, pump flow rate, pump stroke volume, aorta length, and mock aorta material were varied. Fluid pressure was measured at the pump inlet, pump outlet, aorta inlet, and aorta outlet. Aorta distension was captured using a high-speed camera. Pump energy was calculated using measured fluid pressure. The energy decrease at the aorta outlet from the aorta inlet was found. Maximum aorta percent distension is found through processing high-speed camera images using custom edge detection code. Pump energy, energy decrease, and aorta distension are compared against system parameters to determine overall effect on pump performance.
    In general, for the Dragon Skin aorta, increasing backpressure led to an increase in pump workload. Increasing chamber-pressure, which led to a corresponding increase in aortic distension, led to an increase in pump workload. Increasing aorta length led to reduced pump workload. Pulse frequency and stroke volume are considered as one parameter together due to their connected nature during pump operation. Increasing pulse frequency/decreasing stroke volume at first led to an increase in pump workload up to approximately 80-100 bpm/down to approximately 66.5-72.5 mL/beat. From 100 bpm upwards, or 66.5 mL/beat downwards, pump workload decreased. The same trends were reflected in the scaled change in tube distention, (ΔD/D), the maximum aorta percent distension results. Backpressure effects on ΔD/D are inconclusive due to insufficient and conflicting data. There appeared to be an inverse relationship between E and ΔE, where E is the pump energy, and ΔE is the energy decrease from aorta inlet to aorta outlet. Parameters which increase E will decrease ΔE. This agrees with the relationships found for ΔD/D when plotted against E and ΔE. Parameters which increase tube distension will increase pump workload and decrease ΔE. The effects of tube distension on system energy are more pronounced at lower backpressure and lower pulse frequency/higher stroke volume conditions. Due to the limiting size of the pressure chamber containing the aorta, many Ecoflex aorta data cases were removed. This led to insufficient data points for a full analysis of Ecoflex aorta samples.
    Overall, a low system backpressure with low aorta distension due to low chamber-pressure and long aorta length will have a positive impact on pump performance. This applies for all pulse frequency/stroke volume conditions. Aortic parameters which lead to a low distension, such as high aorta length and low chamber pressure, result in both a reduced pump workload and a greater decrease in pump energy downstream of the aorta compliance section. These positive effects are greater when pump operating conditions are at the lower end of the pulse frequency, and correspondingly the higher end of stroke volume. Lowering the backpressure conditions acting on the pump will increase the positive effects of compliance parameters.

  • Subjects / Keywords
  • Graduation date
    Fall 2020
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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