Computer Modeling of Fluid-Stress-Induced Blood Damage in a Mechanical Ventricular Assist Device

Congestive heart failure results the heart is unable to pump the required amount of blood to maintain the systemic circulation. World-wide, millions of patients are diagnosed with congestive heart failure every year, many of which ultimately become candidates for heart transplants. The limited number of available donor hearts, however, has resulted in a tremendous demand for alternative, supplemental circulatory support in the form of artificial heart pumps to serve as a “Bridge-to-Transplant”. The prospect of artificial heart pumps used for long-term support of congestive heart failure patients is directly dependent upon excellent blood compatibility. High fluid stress levels may arise due to high rotational speeds and narrow clearances between the stationary and rotating parts of the pump. Thus, fluid stress may result in damage to red blood cells and activation of platelets, contributing to thrombus formation. Therefore, it is essential to evaluate levels of blood trauma for successful design of a mechanical Ventricular Assist Device. Estimating the fluid stress levels that occur in a blood pump during the design phase also provides valuable information for optimization considerations. This study describes the CFD evaluation of blood damage in a magnetically suspended axial pump that occurs due to fluid stress. Using CFD, a blood damage index, reflecting the percentage of damaged red blood cells, was numerically estimated based on the scalar fluid stress values and exposure time to such stresses. A number of particles, with no mass and reactive properties, was injected at the inflow of the computational domain and traveled along their corresponding streamlines. A Lagrangian particle tracking technique was employed to obtain the stress history of each particle along its streamline, making it possible to consider the damage history of each particle. Maximum scalar stresses of approximately 430 Pa were estimated to occur along the tip surface of the impeller blades, more precisely at the leading edge of the impeller blades. The maximum time required for the vast majority of particles to pass through the pump was approximately 0.085sec. A small number of particles (approximately 5%), which traveled through the narrow gap between the stationary and rotating part of the pump, exited the computational domain in approximately 0.2 sec. The mean value of blood damage index was found to be 0.15% with a maximum value of approximately 0.47%. These values are one order of magnitude lower than the approximated damage indices published in the literature for other Ventricular Assist Devices. The low blood damage index indicates that red blood cells traveling along the streamlines considered are not likely to be ruptured, mainly due to the very small time of exposure to high stress.