The Spiral Vortex (SV) ventricular assist device (VAD) was investigated by 2-component laser Doppler anemometry (LDA) while pumping a refractive index-matched blood analogue fluid. The VAD was operated under physiological conditions corresponding to 75% assist (4 litres/minute) or weaning from assist (2 litres/minute). Data were sampled on a 5-mm grid throughout most of the interior of the blood chamber, using two orthogonal LDA configurations from which 3D velocity data were synthesised. Data were subjected to statistical analysis of quasistatic time intervals and approximation by Fourier series. The velocity vector fields were explored statically (via 2D plots) and dynamically (using 3D animations of the reduced data). Reynolds stresses were computed and visualised in 2D. Fluid pathlines were simulated and plotted in 3D. The flow was found to be dominated by an irrotational vortex that accelerated and precessed in phase with the pumping diaphragm. Two unexpected flow structures, a rising, swirling near-wall layer in diastole and a reflection of the outflow vortex upon valve closure, enhanced washing of the walls. The thickness of the boundary layer was estimated to be 2 mm. Fluid velocities were generally lower than those reported in steady-flow studies on the SV VAD, although turbulence was comparable. Under the weaning mode, the coherence of the main vortex was degraded and flow recirculation was observed distal to the inflow port; this operating mode must be regarded as an indication for anticoagulation. In both pumping modes, turbulence was elevated in association with asymmetric buckling of the pneumatically driven diaphragm. Suboptimal orientation of the tilting-disc inlet valve gave rise to augmented turbulence production and skewing of the main vortex; similar results were obtained for an axisymmetric polymer (Jellyfish) valve, despite its advantageous haemodynamics. Flow stagnation was apparent where the inflow stream impinged on the wall, opposite the inflow port. The overall design of the SV VAD appears to almost ideal, in the context of current technology. However, elimination of recirculation/stagnation zones, especially in the weaning mode, remains a priority for the ultimate optimisation of haemocompatibility. Pulsatile VADs will probably never be entirely free of flow recirculation or stagnation, and published claims to the contrary probably reflect study limitations.
| Identifer | oai:union.ndltd.org:ADTP/187688 |
| Date | January 2005 |
| Creators | Nugent, Allen Harold, Biomedical Engineering, UNSW |
| Publisher | Awarded by:University of New South Wales. Biomedical Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Allen Harold Nugent, http://unsworks.unsw.edu.au/copyright |
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