Flow stasis in an artificial heart may provide a situation where thrombus develops. Should part, or all, of the clot dislodge, a thromboembolism may lead to stroke(s), neurologic deficits, or even death. In an effort to determine if the regime of low shear or stasis exists, a two-dimensional particle image velocimetry (PIV) system was implemented to measure the velocity field within the 50 cc Penn State Artificial Heart. The velocity measurements were decomposed nearest the wall to obtain wall shear rates along the bottom of the chamber. The PIV measurements were made in three image planes across the depth of the chamber to reconstruct a surface distribution of the wall shear rates at the bottom over the entire heart cycle. The wall shear rate is shown to be spatially nonuniform, with persistently low wall shear rates. An area near the front edge of the chamber at the bottom showed wall shear rates not exceeding 250 s(-1). This was an area of clot formation seen in vivo, suggesting a link may exist between the low wall shear rate zone and thrombus formation.

Particle image velocimetry (PIV) has been gaining acceptance as a routine tool to evaluate the flow fields associated with fluid mechanical devices. We have developed algorithms to investigate the wall shear-rates within the 50cc Penn State artificial heart using low magnification, conventional particle image velocimetry (PIV). Wall shear has been implicated in clot formation, a major post-implant problem with artificial hearts. To address the issues of wall scattering and incomplete measurement volumes, associated with near wall measurements, we have introduced a zero masking and a fluid centroid shifting technique. Simulations using different velocity fields were conducted with the techniques to assess their viability. Subsequently, the techniques were applied to the experimental data collected. The results indicate that the size of the interrogation region should be chosen to be as small as possible to maximize resolution while large enough to ensure an adequate number of particles per region. In the current study, a 16 x 16 interrogation window performed well with good spatial resolution and particle density for the estimation of wall shear rate. The techniques developed with PIV allow wall shear-rate estimates to be obtained from a large number of sites at one time. Because a planar image of a flow field can be determined relatively rapidly, PIV may prove useful in any preliminary design procedure.

In order to bridge the gap of existing artificial heart technology to the diverse needs of the patient population, we have been investigating the viability of a scaled-down design of the current 70 cc Penn State artificial heart. The issues of clot formation and hemolysis may become magnified within a 50 cc chamber compared to the existing 70 cc one. Particle image velocimetry (PIV) was employed to map the entire 50 cc Penn State artificial heart chamber. Flow fields constructed from PIV data indicate a rotational flow pattern that provides washout during diastole. In addition, shear rate maps were constructed for the inner walls of the heart chamber The lateral walls of the mitral and aortic ports experience high shear rates while the upper and bottom walls undergo low shear rates, with sufficiently long exposure times to potentially induce platelet activation or thrombus formation. In this study, we have demonstrated that PIV may adequately map the flow fields accurately in a reasonable amount of time. Therefore, the potential exists of employing PIV as a design tool.

In the sac-driven artificial heart, the flow characteristics are coupled to the dynamics of the sac motion. The opening dynamics of the sac wall can, for example, strongly affect the chamber flow characteristics during diastole by directing or impeding the inflow. Poor sac motion can reduce the volume output of the pump and may increase the potential for thrombus formation within the ventricular chamber. It is particularly important for laboratory studies of the flow fields in artificial hearts that the diaphragm motion properly simulates the sac motion observed in vivo. In the present study, flow visualization was performed to investigate the relationship between the chamber flow characteristics of a Penn State artificial heart and the motion of the diaphragm during the filling phase during in vitro experimentation. The chamber flow pattern and diaphragm motion were recorded as a function of time, using high-speed videography. Experiments were conducted to determine the influence of diaphragm motion on the flow characteristics by altering the filling pressure, diaphragm thickness, and fluid density. Diaphragm motion was quantified by tracking the position of three surface points over the cardiac cycle. The alignment of these three surface trajectories can be used to quantify the uniformity of diaphragm motion. As a result, diaphragm motion was determined to be nonuniform under most operating conditions with the diaphragm opening in a wave-like pattern starting at the bottom of the chamber and propagating toward the inflow/outflow ports. This opening pattern simulates the opening pattern observed in an in vitro study of the clinical blood sac used in the Lionheart LVAD.

No abstract