Abstract
Despite the evolution of Ventricular Assist Devices (VADs), VAD patients stillsuffer from complications due to the damage to blood by fluid dynamic stress.
Measurements and simulations of cell and protein deformation show normal and
shear stresses deform, and potentially damage, cells and proteins differently.
The thesis explores numerical models for blood damage which are generally based on the Scalar Shear Stress (SSS), a scalar invariant calculated from the tensor components in analogy with the von Mises stress for solids and first introduced
by Bludszuweit. Relationships between damage to the blood components and
the SSS are then used in predictions of blood damage from the whole device. In
addition, flow conditions such as constant flow rate applied at the inlet boundary
do not reflect a pulsating flow of a beating heart. Combining flow dynamics of
the HVAD with flow data from a pulsatile mock circulation loop, the influence
of flow pulsatility on device-induced blood trauma was investigated. The calculations showed normal stresses do occur in rotary VADs: the fluid
volumes experiencing normal stress above 10 Pa were 0.011 ml (0.092%) and 0.027
ml (0.39%) for the HeartWare HVAD and HeartMate II, and normal stresses
over 100 Pa were present. Considering thresholds for red blood cells and von
Willebrand factor deformation by normal and shear stresses, the fluid volumes
causing deformation by normal stress were between 2.5 and 5 times the size of
those causing deformation by shear stress. In pulsatile flow, the normal stress
volumes were higher above the 10 Pa threshold which was 0.024 ml (0.2 %).
There is an increase in 2 orders of magnitude of normal stress compared with
steady flow at peak flow, which existed in the clearance between the top housing
and rotating impeller. Larger variations were observed in the flow profile and
stress field over time, but the time-averaged values were indiscernible compared
to fixed-rate (steady flow). It was found the results clearly show that while blood
within rotary VADs experiences more shear stress at much higher magnitudes as
compared with normal stress, there is sufficient normal stress present to cause
deformation of, and potentially damage to, the blood components
| Date of Award | 24 May 2023 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Katharine Fraser (Supervisor), Andrew Cookson (Supervisor) & Richie Gill (Supervisor) |