Turbulent flow generated by prosthetic devices at the bloodstream level may cause mechanical stress on blood particles. Measurement of the Reynolds stress tensor and/or some of its components is a mandatory step to evaluate the mechanical load on blood components exerted by fluid stresses, as well as possible consequent blood damage (hemolysis or platelet activation). Because of the three-dimensional nature of turbulence, in general, a three-component anemometer should be used to measure all components of the Reynolds stress tensor, but this is difficult, especially in vivo. The present study aimed to derive the maximum Reynolds shear stress (RSS) in three commercially available prosthetic heart valves (PHVs) of wide diffusion, starting with monodimensional data provided in vivo by echo Doppler.
Accurate measurement of PHV flow field was made using laser Doppler anemometry; this provided the principal turbulence quantities (mean velocity, root-mean-square value of velocity fluctuations, average value of cross-product of velocity fluctuations in orthogonal directions) needed to quantify the maximum turbulence-related shear stress.
The recorded data enabled determination of the relationship, the Reynolds stresses ratio (RSR) between maximum RSS and Reynolds normal stress in the main flow direction. The RSR was found to be dependent upon the local structure of the flow field.
The reported RSR profiles, which permit a simple calculation of maximum RSS, may prove valuable during the post-implantation phase, when an assessment of valve function is made echocardiographically. Hence, the risk of damage to blood constituents associated with bileaflet valve implantation may be accurately quantified in vivo.
[Show abstract][Hide abstract] ABSTRACT: Experimental and computational studies were performed to elucidate the role of turbulent stresses in mechanical blood damage (hemolysis). A suspension of bovine red blood cells (RBC) was driven through a closed circulating loop by a centrifugal pump. A small capillary tube (inner diameter 1 mm and length 70 mm) was incorporated into the circulating loop via tapered connectors. The suspension of RBCs was diluted with saline to achieve an asymptotic apparent viscosity of 2.0 +/- 0.1 cP at 23 degrees C to produce turbulent flow at nominal flow rate and pressure. To study laminar flow at the identical wall shear stresses in the same capillary tube, the apparent viscosity of the RBC suspension was increased to 6.3 +/- 0.1 cP (at 23 degrees C) by addition of Dextran-40. Using various combinations of driving pressure and Dextran mediated adjustments in dynamic viscosity Reynolds numbers ranging from 300-5,000 were generated, and rates of hemolysis were measured. Pilot studies were performed to verify that the suspension media did not affect mechanical fragility of the RBCs. The results of these bench studies demonstrated that, at the same wall shear stress in a capillary tube, the level of hemolysis was significantly greater (p < 0.05) for turbulent flow as compared with laminar flow. This confirmed that turbulent stresses contribute strongly to blood mechanical trauma. Numerical predictions of hemolysis obtained by computational fluid dynamic modeling were in good agreement with these experimental data.
[Show abstract][Hide abstract] ABSTRACT: Flow field of the pulmonary circulation has been investigated by in vitro pulsatile and steady flow visualization in simulation models. A couple of counter-rotating secondary flows were symmetric about the centerline in the normal valve. As the pulmonic valve became more stenotic, the two counter-rotating secondary flows in both the left pulmonary arteries (LPA) and right pulmonary arteries (RPA) were no longer symmetric. With a normal Hancock porcine aortic valve inside the extracardiac conduit, the flow of the proximal conduit was spiral, and that of the distal portion was axial. In stenosed Hancock porcine aortic valve loaded conduit, the flow was a continuous spiral. Studies on cavopulmonary connection models showed that energy savings were more evident at the 50:50 right / left pulmonary artery ratio, and the energy losses increased in proportion to total flow rates. A 60° to 90° anastomotic angle between the subclavian artery and the graft of Blalock-Taussig shunt could result in favorable pulmonary artery flow distribution and peak pressure. Simulations in the Norwood circulation model showed that larger shunts rendered an increased cardiac output to the lungs. In order to determine the idealistic cardiac surgical technical conditions, in vitro flow visualization study is a primarily useful tool in optimizing the flow and diminishing the energy losses.
The Kuwait medical journal: KMJ: the official journal of the Kuwait Medical Association 09/2013; 45(3):192-198.
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