Date Published: February 23, 2018
Publisher: Public Library of Science
Author(s): Chia Min Leong, Gary B. Nackman, Timothy Wei, Roi Gurka.
The shape of a bypass graft plays an important role on its efficacy. Here, we investigated flow through two vascular graft designs–with and without cuff at the anastomosis. We conducted Digital Particle Image Velocimetry (DPIV) measurements to obtain the flow field information through these vascular grafts. Two pulsatile flow waveforms corresponding to cardiac cycles during the rest and the excitation states, with 10% and without retrograde flow out the proximal end of the native artery were examined. In the absence of retrograde flow, the straight end-to-side graft showed recirculation and stagnation regions that lasted throughout the full cardiac cycle with the stagnation region more pronounced in the excitation state. The contoured end-to-side graft had stagnation region that lasted only for a portion of the cardiac cycle and was less pronounced. With 10% retrograde flow, extended stagnation regions under both rest and excitation states for both bypass grafts were eliminated. Our results show that bypass graft designers need to consider both the type of flow waveform and presence of retrograde flow when sculpting an optimal bypass graft geometry.
The efficacy of bypass grafts depends on its shape . Numerous bypass grafts with cuff and patch technologies at the anastomosis like Linton patch , Miller cuff , Tyrell vein collar  and Taylor patch  had been designed to satisfy this goal. Cuffs and patches could be harvested from autologous veins or pre-formed during the manufacturing of bypass grafts. These designs having different geometries and sizes at the anastomosis would have an implication on hemodynamics in that region.
We conducted a series of Digital Particle Image Velocimetry (DPIV) experiments to visualize the differences between flow patterns through a non-cuffed versus a cuffed vascular graft model. In all cases, we conducted experiments using transparent cast models of graft geometries. A simplified sketch of the experiment is shown in Fig 1.
For all DPIV data presented in this section, velocity vectors are superimposed on outline of the graft models. The magnitude and direction of the flow at any point in the field of view is indicated by the length and orientation of the velocity vectors, respectively. In addition, vector colors indicate local vorticity at each point of the flow. While vorticity is a measure of fluid rotation, at the graft boundaries, vorticity is also proportional to wall shear stress. In light of this, red velocity vectors correspond to regions of high counter-clockwise shear. Blue vectors correspond to regions of high clockwise shear. Regions of zero shear appear with green vectors. Note that it is possible to have high speed regions with low shear (e.g. uniform flow); color should not be confused with speed. Identical color spectra and velocity length scales were used for every vector field acquired in this study. It is therefore possible to directly compare color and vector length between the different cases presented in the following sections.
Spatially and temporally resolved measurements of flow in a straight end-to-side and contoured end-to-side model were made using Digital Particle Image Velocimetry. Four different cases were examined for each model, including two different cardiac cycles (rest and excitation) and two upstream conditions for the native artery (100-0/no retrograde flow and 90-10/retrograde flow [distal:proximal]). Measurements were made from the side (in the symmetry plane of the graft model) and from below. Summary observations and conclusions made from these measurements include: