An In Vitro Fluid Dynamic Study of a Continuous Flow Device Outlet in Relation to Acquired von Willebrand Syndrome

Open Access
Author:
Lani, Ryan Anthony
Area of Honors:
Bioengineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Keefe B Manning, Thesis Supervisor
  • Peter J Butler, Honors Advisor
  • Steven Deutsch, Faculty Reader
Keywords:
  • laser Doppler velocimetry (LDV)
  • HeartMate II
  • von Willebrand factor (vWF)
  • ventricular assist devices (VAD)
  • fluid dynamics
Abstract:
Recently, there has been a significant increase in reports of patients diagnosed with acquired von Willebrand Syndrome with implanted continuous flow ventricular assist devices (VAD). This disease is characterized by uncontrollable bleeding episodes due to a decrease in the blood concentration of von Willebrand factor (vWF), a protein in the clotting cascade. Pump fluid dynamics are suspected to cause the decrease in vWF. To analyze the fluid dynamics of a pump, laser Doppler velocimetry (LDV) was used to measure the velocity in an acrylic model at the outlet of the HeartMate II® VAD. The pump was run at 8800 RPM with a pressure drop across it of 80 mmHg, yielding a flow rate of 5.0 ± 0.2 L/min. The downstream velocity profiles initially alternated between reversed and characteristic Dean flow profiles, before fully developing. To explain this finding, an analysis of the pump outlet was performed. Velocities were collected at the outlet and within the cannula bend. These velocities were then used to calculate the turbulence intensities and Reynolds stresses. Maximum Reynolds normal and shear stresses (RSS) were roughly -6429 and 2875 dyne/cm2, respectively. When normalized by the average velocity (0.690 m/s) through the outlet, maximum turbulence intensities were as high as 50% at some locations. These data were then used to determine regions above the proteolytic cleavage threshold for vWF. It was found that most RSS values were above the threshold magnitude but not the exposure duration, meaning that the potential proteolytic cleavage of vWF remains unresolved.