In a previous study, we evaluated the velocity distribution of an oxygenator using Computational Fluid Dynamics (CFD) and an in-vitro experiment with a commercially available oxygenator, MENOX EL-4000 (DAINIPPON INK & CHEMICALS, INC). In both, the low flow distribution areas in the CFD analysis and the thrombus formation areas of the in-vitro experiment were found to be the same; the velocity distribution was found to be less than 11mm/s in those area. Because of these results, we suggested that the housing geometry was influencing the velocity distribution. In this study, we extend our previous study and examine the relationship between housing geometry and the velocity distribution. An improved diagonal model was set up with an inlet and an outlet. The two types of oxygenator (the commercially type and the improved type) were simulated by CFD software STAR-LT (CD-adapco JAPAN). The velocity distribution was calculated based on Darcy's law (on the assumption that the hollow fibers behave as a porous media). When the velocity distribution was less than 11mm/s at an optical horizontal, the ratio of area was 17.8% in the commercial type and 1.2% in the improved type. In conclusion, we observed that, when comparing the commercial type and the improved type, the ratio of area at a velocity distribution of less than 11mm/s was reduced 16.6%. Therefore, in designing an artificial lung to minimize the thrombus formation region, simulation with CFD can be considered an important tool.
A prototype of implantable artificial lung without pressure loading on the right ventricle (RV) has been developed and improved in our institute. We reported the results of in-vivo test for the previous prototype in 47th ASAIO meeting. We have developed the new version and evaluated its performance. Method: The membrane oxygenator, MENOX 6000(Dainippon Ink and Chemicals, INC, Tokyo, Japan) was used as a basic model. It has larger surface membrane area (1.3m2) and the priming volume (200ml) compared with those of the MENOX 4000(0.8m2, 110ml). The density of the hollow-fibers was reduced from 40% to 25%, and the housing configuration was re-designed for the purpose of implantation. The new prototype was evaluated for the gas transfer capacity and the pressure drop through the device (Pinlet-outlet) in in-vitro study using the single pass method with fresh bovine blood (n=3). They were measured under the condition of the blood flow rate: Q=1,2,3,5 and the gas flow/blood flow ratio: V/Q=1,2,3. Result: Oxygen transfer rate of the new prototype was 122ml/min (blood flow=3L/min, V/Q=2), and CO2 transfer rate was 87.6ml/min (blood flow =3L/min, V/Q=2). Pinlet-outlet of the new prototype was 6.70mmHg (blood flow =3L/min). The performance was improved and the results were similar to those of the computer-aided simulation. Conclusion: This prototype showed reasonable gas exchange capacity with sufficiently low blood flow resistance for implantation in the pulmonary circulation.
Purpose: The recent theoretical simulation using the computational fluid dynamics (CFD) method together with in-vivo tests clarified relation between the flow field inside the oxygenator and possible area of thrombus formation. The purpose of this study was to detect onset of thrombus formation in the hollow fiber oxygenators with an optical sensor. The intensity of backscattered light from thrombus was modeled using the first order scattering theory (FOS). Methods: Using rabbits (n=3), A-V bypass model was constructed. The model consisted of a flow chamber, a rabbit and a roller pump. Blood flow rate was maintained at40mVmin with the roller pump. ACT was adjusted to 170±10s with heparin. Hematocrit was adjusted at 35±5% with PBS solution. The onset of thrombus formation or aggregation of RBC in the flow chamber was detected using a pair of optical fibers, one for illumination at 630nm wavelength and the other for detection of backscattered light, separated by 250pm. Results: The intensity of backscattered light from the flow chamber started to decrease when the blood continued to flow more than 10 minutes. The autopsy finding revealed the thrombus with the thickness of more than 1mm in the flow chamber. Discussions: The decrease of the backscattered light intensity correlated with increase in RBC concentration or thrombus formation. The FOS simulation revealed the similar phenomenon. The two fibers back scattering measurement system together with FOS prediction is applicable to detect onset of thrombus formation in the hollow fiber oxygenators.
