In vitro flow investigations in the aortic arch during cardiopulmonary bypass with stereo-PIV
Introduction
During open heart surgery patients are supplied with the cardiopulmonary bypass (CPB). The CPB maintains blood pressure and cardiac flow in the cardiovascular system, while the heart is stopped by cardioplegia. The blood, collected from the venous system, is returned with a cannula, typically inserted into the aorta. The CPB is related to problems like stroke or hypoxia, which cause neurological injuries. One reason amongst others is the cannula jet. It leads to altered flow conditions in the aortic arch and thereby increases the risk of cerebral undersupply and plaques decollation from the wall, due to the sandblast effect (Kapetanakis et al., 2004, Scarborough et al., 2003, Verdonck et al., 1998). Different cannulae shapes and cannulation strategies exist. The potential risk and their performance has been aim of previous studies (Gerdes et al., 2002, Grooters, 2003, Joubert-Hübner et al., 1999)
As detailed in vivo flow data is hardly accessible (Markl et al., 2007), in silico and in vitro flow studies are an important tool to investigate the underlying mechanisms. Several in silico studies investigate the hemodynamics during CPB with computer fluid dynamics or lumped parameter modeling. (Fukuda et al., 2009a, Kaufmann et al., 2014, Tokuda et al., 2008). As these simulations are based on model assumptions, experimental validation is indispensable. Different in vitro studies investigated the effect of different cannula tips or positions and focus on flow measurements in the aortic arch. Applied techniques for the flow measurements are Doppler Ultra Sound (Doppler-US) (Verdonck et al., 1998, Muehrcke et al., 1995, Grooters, 2003), ink injection (Scharfschwerdt et al., 2004, White et al., 2009) or Phase Contrast MRI (PC-MRI) (White et al., 2009). These measurement techniques have different limitations. Doppler-US has a limited temporal and spatial resolution, PC-MRI requires access to an MR facility and ink injection is only capable to visualize flow patterns without any quantification. Another applied technique to investigate cannula flow is the Particle Image Velocimetry (PIV) (Laumen et al., 2010, Fukuda et al., 2009b). This optical flow measurement technique basically consists of a laser, illuminating a particle seeded flow in a plane and a camera, which captures the particle movement. The PIV is a common and established technique in the field of flow measurement, numerous publications related to biomedical flows can be found (Manning and Miller, 2002, Day and McDaniel, 2005, Stamatopoulos et al., 2011). Although PIV is not applicable in vivo, it offers a better temporal and spatial resolution, compared to Doppler-US or PC-MRI. However the PIV is only capable to measure the two in-plane velocity components (2C) in a measurement plane, thus the results are limited to 2D. Several modification of the PIV have been developed to overcome this limitation. Very cumbersome methods are holographic PIV (Schäfer and Schröder, 2011) or tomographic PIV (Buchmann et al., 2011). With these techniques measurement in a volume is affordable, but they require high experimental effort. Another modification is the stereoscopic PIV (Stereo-PIV). Here, a second camera is directed to the illuminated plane from another angle, allowing the reconstruction of the “out of plane” velocity component. This results in measuring all three velocity components (3C) in a plane (2D). If multiple planes are measured, a quasi 3D flow field can be assembled during post processing. Examples for the application of Stereo-PIV in biomedical flows can be found in different studies (Spence et al., 2011, Soodt et al., 2012, Yagi et al., 2013). A detailed theoretical description of the technique can be found in Raffel et al. (2007).
In this work we present a Stereo-PIV approach to measure cannula flow in the aortic arch. The measurements were carried out on two silicone models of the aortic arch. One aortic model was cannulated with a typical CPB cannula. The second model had an inflow tube in the aortic valve position to mimic the heart inflow. Parts of the flow data, acquired during this work, were used to validate a recent numerical study of our group (Kaufmann et al., 2014). In this previous study the hemodynamics and cerebral pressure distribution during CPB were investigated with computer fluid dynamics and lumped parameter modeling. As the work focused on the development of the numerical methods, the purpose of the present work is the presentation of the measurement technique especially addressing the three-dimensional flow.
Section snippets
Silicone models
As Stereo-PIV is an optical measurement technique, it is necessary to manufacture transparent models of the vessels. It is also important to realize plane windows for the camera view and the laser light, in order to avoid optical distortion. A common technique is to build a silicone cast of the vessels in form of a block which provides windows for cameras and laser, as it is described in Geoghegan et al. (2012) or Stoiber et al. (2013) exemplarily. A similar technique was used in this work, to
Results
The resulting 3D velocity fields in the cannulated and non-cannulated aortic arch have been analyzed regarding velocity distribution, streamlines and vorticity. Fig. 5 shows the 3D-streamlines in the aortic arch in both models. The velocity scale is identical to ease comparison, although the maximum velocity behind the cannula tip is much higher than the maximum value of the scale. The streamlines in case of CPB inflow show more vorticity than in case of aortic inflow. This is mainly caused by
Discussion
In this work, in vitro Stereo-PIV flow measurements in a cannulated and non-cannulated aortic silicone model are presented. Stereo-PIV accommodates the three-dimensional character of the flow and offers a high spatial resolution. Herewith, it overcomes limitations of standard PIV, Doppler-US, PC-MRI, or other techniques, which have been applied in recent CPB cannula flow studies (Verdonck et al., 1998, Scharfschwerdt et al., 2004, White et al., 2009, Laumen et al., 2010). However, data
Conflict of interest statement
No conflicts of interests.
Acknowledgment
This study was funded by the German Research Foundation (KA 3374/1-1).
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