Left ventricular assist devices (LVAD) support patients who have reached end-stage heart failure. The surgically implanted LVAD, a battery-operated, mechanical pump, helps the left ventricle (main pumping chamber of the heart) pump blood to the rest of the body. Cardiac surgeons may use LVADs as a “bridge” to maintain cardiac output for patients waiting for a heart transplant. Some patients who are not candidates for heart transplants can also use an LVAD to prolong and improve their lives.
Current LVAD devices present several technology challenges that limit their effectiveness including size and battery life. At FineHeart, a French medical device company focused on creating technologies in the cardiovascular space, we are overcoming those challenges with our new patented rotary pump, ICOMS (Implantable Cardiac Output Management System). This novel, wireless fully implantable mechanical circulatory support device can optimize cardiac output while preserving the heart’s innate contractility. It is a game-changing therapy for long-term circulatory assistance in severe heart failure patients.
Turbine Gen 2 design components. The motor is separated from blood with a mechanical seal.
Unlike traditional LVADs that bypass the aortic valve via an external graft, the ICOMS is implanted into the apex of the left ventricle and is positioned to direct a flow of ventricular blood toward the aortic valve during systole and allows for normal, low-pressure filling of the ventricle during diastole. To maintain normal heart function, the ICOMS turbine employs an electrocardiogram (EKG) algorithm to control a rapid and accurately timed transition from diastole/left ventricle (LV) filling to systole/LV ejection and then back to diastole/LV filling.
How Simulation Improved the Pump Design and Implant Position
A major challenge of LVAD design is to optimize blood flow so that it avoids thrombus formation (causing blood clots). Research has found that a combination of shear stress over time tends to cause clots. Unfortunately, the pressure and the shear stress, including the exposure time, are not available in the actual test bench. Nevertheless, the flow and mechanical parameters can be compared using simulation. I used ANSYS Fluent computational fluid dynamics (CFD) simulations to determine the shear stress in the baseline Gen 1 design and then used the simulation results to optimize the configuration for Gen 2.
Geometry model of the Gen 1 design
Simulation of blood flow velocity in optimized configuration
The torque is calculated using the current passing through the brushed direct current (DC) motor that drives the turbine. The measured torque is greater than the calculated torque (table below). This may not mean an error of torque calculation: Indeed, while the ANSYS simulation calculated only the torque of the impeller, the measured torque includes not only the turbine but all of the related system. Measurements of torque were useful for estimating the needed mechanical power, which allows for optimizing the design of the batteries and their capacitance. Knowing that the ICOMS should work between 104 to 733 rad/s, a mechanical power between 3 and 5 watts at maximum would be enough.
Comparison between ANSYS simulation and experimental results on the test bench
The ICOMS Gen 1 design was the initial and most time-consuming stage of development and provided a reasonable starting point for design enhancements. The major difference between the Gen 1 and the Gen 2 is the straightener design and modification inducer. Both Gen 1 and Gen 2 hydraulically performed well per the CFD predictions.
The bench results confirm the simulated flow rate and velocity. Accordingly, we can confirm that the design fulfills all the flow rate specifications required for an intracardiac turbine. In addition, data from particle image velocimetry (PIV) analyses clearly demonstrate that the ejection flow is straight and well-directed to the aorta (image, right). This result is obtained with the help of the straightener that not only improves the velocity and thus the flow rate, but also the flow vectors direction as compared with the design with diffuser only (left).
Flow vector direction: without straightener (left); with straightener (right)
The CFD predictions using Fluent for Gen 2 predicted reasonable performance for flows in our planned operating range. Only two regions were found with irregular flow patterns, per the CFD analysis: The trailing edge of the impeller blades showed a possible point of stagnation and a region with potential recirculation between the flow straightener blades at the trailing edge of the diffuser region. The hydraulic efficiency of the Gen 2 pump or best efficiencies ranged from 22 percent to 33 percent — typical values for blood pumps. The axial fluid forces ranged between approximately 0.5 N and 0.7 N, and the radial fluid forces varied between 0.1 N and 1.5 N for centered impeller conditions. At design, the blood damage analysis of the Gen 2 revealed a maximum fluid stress of 158 Pa at the leading edge of the impeller blades covering only approximately 2 percent of the surface area of the rotor’s hub. This blood damage analysis further predicted a mean fluid maximum residence time of 0.014 s.
What’s Next for FineHeart?
We are currently developing the final product but still have a way to go before we reach the marketing phase. The first in vitro studies provided promising results. Indeed, we could observe normalization of cardiac output when the pump is switched on, once placed within the left ventricle. We are still looking for optimization of reduce shear stress improvement thanks to the ANSYS simulation software by modifying the shape of specific parts of the pump. More studies will start quite soon and expect a first human implant for the end of 2019.
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