Yoshinori Mitamura

An Optimally Controlled Respirator

An optimally controlled respirator was developed. It has three main features: 1) ventilation is controlled by the patients metabolic rate from continuously measured C0 2 output, 2) physiologic dead space approximated as a linear function of tidal volume is used to estimate alveolar ventilation, and 3) respiratory rate is computed to minimize ventilatory work.

Development of an implantable motor-driven assist pump system

A motor-driven artificial pump and its transcutaneous energy transmission (TET) system are discussed. The artificial pump consists of a high-speed DC brushless motor driving a ball screw and magnetic coupling mechanism between the blood pump and ball screw. The ball screw transfers high-speed rotary motion into low-speed rectilinear motion by a single component. Magnetic coupling enables active blood filling without applying an excess negative pressure to the pump. The transcutaneous transformer is formed from a pair of concave/convex ferrite cores. This design minimizes lateral motion of the external core. Information on motor voltage is transmitted through the skin by infrared pulses. The motor voltage is regulated by controlling the duty ratio of the square pulse supplied to the primary coil. Pump flow of 5.6 l/min was obtained with a mean outlet pressure of 100 mm Hg at a drive rate of 100 b.p.m. under preload of 15 mm Hg.<<ETX>>

Development of a Ceramic Heart Valve

A durable and thromboresistant ceramic heart valve comprised of a single crystal alumina disk and titanium nitride (TiN) valve ring has been developed. Blood compatiblity was examined by scanning electron microscopy (SEM) examinations of the valves implanted in sheep for 35 (#1), 26 (#2), 20 (#3), 23 (#4), and 26 (#5) days. The single crystal alumina and TiN surfaces were free of platelet aggregation or fibrin networks, except for some depositions of fibrin and platelets on the outflow TiN ring in #3, and isolated red cells on the outflow TiN ring in #5. Durability testing under high pressure (1750 mmHg = 233 KPa) pulsatile conditions showed that the safety factor of the ceramic valve was more than seven times greater than anticipated. The ceramic valve is promising as an artificial heart valve.

Development of a bidirectional transcutaneous optical data transmission system for artificial hearts allowing long-distance data communication with low electric power consumption

We have developed a wavelength division bidirectional transcutaneous optical data transmission system using amplitude shift keying (ASK) modulation. The bidirectional optical data transmission system consists of two kinds of light emitting diodes (LEDs) having different wavelengths and an ASK modulator and demodulator. Two narrow directional visible LEDs with a peak output wavelength of 590 nm were used to transmit data from inside the body to outside the body, and a narrow directional near-infrared LED with a peak output wavelength of 940 nm was used for transmission from outside the body to inside the body. The ASK modulator employs a carrier pulse signal (50 kHz) to support a maximum data transmission rate of 9600 bps. An in vitro experiment showed that the maximum tissue thickness of near-infrared optical data transmission without error was 45 mm; the figure was 20 mm for visible optical data transmission. There was no interference between the signals under full-duplex data transmission. Electric power consumption for the data transmission links was 122 mW for near-infrared light and 162 mW (81 mW × 2) for visible light. From the above results, a bidirectional transcutaneous optical data transmission system promises adequate performance for monitoring and control of an artificial heart.

Optimum selection of an implantable secondary battery for an artificial heart by examination of the cycle life test

An implantable secondary battery is one of the key components in a total artificial heart system. Because a 2 year cycle life is required, the cycle life of the secondary battery as well as its charge and discharge properties are important parameters for selection of an appropriate battery. We carried out cycle life tests on four kinds of rechargeable batteries (a Ni-MH secondary battery, a Ni-Cd secondary battery, a Li-ion battery with a graphite anode, and a Li-ion battery with a nongraphitizable carbon electrode) to determine their suitability as implanted back-up batteries. Each of the batteries was charge/discharge cycled at 37°C to 39°C using a charge current of 1 C ampere, and they were each fully discharged under either pulsatile discharge loads, which mimicked pulsatile operation, or a nonpulsatile load equivalent to the average of the pulsatile loads. The two Li-ion batteries made by different manufacturers both met the minimum requirement of cycle life of more than 1,500 cycles, considering safety coefficient regardless of the discharge pattern. In addition, the temperature increase of these Li-ion batteries (3°C) was lower than that of Ni-Cd and Ni-MH batteries (15–25°C) . Out of these four batteries, the two Li-ion batteries are the most suitable for use in a totally implantable artificial heart system.

Computer assisted design for the implantable left ventricular assist device (LVAD) blood pump using computational fluid dynamics (CFD) and computer-aided design and manufacturing (CAD/CAM)

Thrombus formation and hemolysis are critical issues in the design of a long-term implantable LVAS (left ventricular assist system). The fluid dynamic characteristics of the blood flow are one of the main factors that cause thrombus formation and hemolysis. In this study, we optimized blood chamber geometry, port design, and fluid dynamics in our implantable LVAS to ensure minimization of shear-stress-related blood damage. A blood pump chamber (stroke volume, 65 ml) and an inflow and outflow port were designed with three-dimensional CAD (computer-aided-design) software (Pro-Engineering version 20) and estimated by FEM (fine-element method) computational fluid dynamic (CFD) analysis (Ansys version 5.5). We adopted three-dimensional distribution of CFD results for qualitative evaluation, and we also tried to estimate the normalized index of hemolysis (NIH) and time-series change of hematocrit from the results of CFD analysis as quantitative index of optimization for geometry of the blood pump chamber. With the use of this design, the blood pump geometry was optimized as the decrease of NIH from 2.72 g/1001 in the first model to 0.098 g/1001 in the second model, corresponding to the decrease in shear stress. The hematocrit also improved from 0.7% in the first model to 11.5% in the second model 2 years after implantation of the pump. Areas where flow stagnation was observed in the first model were free of stagnation in the second model. The results show that computer-aided design of the blood pump contributes to optimizing a blood pump chamber for reducing thrombus formation and hemolysis, and also contributes to reducing cost and time in developing the implantable LVAS.

Surface pitting of heart valve disks tested in an accelerated fatigue tester

There are various reports on the fracture of mechanical heart valves implanted in humans or animals and it has been pointed out that fractures are induced by erosion of the disk surface due to cavitation bubbles. Cavitation erosion on mechanical heart valves was studied using our originally designed accelerated fatigue tester. Several valve housings with different compliance values were used. The number and position of pits on the valve disk were measured using an optical microscope. Disk-closing velocity was measured and cavitation bubbles were monitored by a high-speed video camera. It was found that disk-closing velocity increased and cavitation erosion was enhanced with an increase in compliance of the valve holder. Therefore, careful attention should be paid to the compliance of an accelerated fatigue tester.

A remote monitoring system for patients with implantable ventricular assist devices with a personal handy phone system.

The usefulness of a remote monitoring system that uses a personal handy phone for artificial heart implanted patients was investigated. The type of handy phone used in this study was a personal handy phone system (PHS), which is a system developed in Japan that uses the NTT (Nippon Telephone and Telegraph, Inc.) telephone network service. The PHS has several advantages: high-speed data transmission, low power output, little electromagnetic interference with medical devices, and easy locating of patients. In our system, patients have a mobile computer (Toshiba, Libretto 50, Kawasaki, Japan) for data transmission control between an implanted controller and a host computer (NEC, PC-9821V16) in the hospital. Information on the motor rotational angle (8 bits) and motor current (8 bits) of the implanted motor driven heart is fed into the mobile computer from the implanted controller (Hitachi, H8/532, Yokohama, Japan) according to 32-bit command codes from the host computer. Motor current and motor rotational angle data from inside the body are framed together by a control code (frame number and parity) for data error checking and correcting at the receiving site, and the data are sent through the PHS connection to the mobile computer. The host computer calculates pump outflow and arterial pressure from the motor rotational angle and motor current values and displays the data in real-time waveforms. The results of this study showed that accurate data on motor rotational angle and current could be transmitted from the subjects while they were walking or driving a car to the host computer at a data transmission rate of 9600 bps. This system is useful for remote monitoring of patients with an implanted artificial heart.

Design of a miniature implantable left ventricular assist device using CAD/CAM technology.

In this study, we developed a new miniature motor-driven pulsatile left ventricular assist device (LVAD) for implantation into a Japanese patient of average build by means of computer-aided design and manufacturing (CAD/CAM) technology. A specially designed miniature ball-screw and a high-performance brushless DC motor were used in an artificial heart actuator to allow miniaturization. A blood pump chamber (stroke volume 55 ml) and an inflow and outflow port were designed by computational fluid dynamics (CFD) analysis. The geometry of the blood pump was evaluated using the value of index of pump geometry (IPG) = (Reynolds shear stress) × (occupied volume) as a quantitative index for optimization. The calculated value of IPG varied from 20.6 Nm to 49.1 Nm, depending on small variations in pump geometry. We determined the optimum pump geometry based on the results of quantitative evaluation using IPG and qualitative evaluation using the flow velocity distribution with blood flow tracking. The geometry of the blood pump that gave lower shear stress had more optimum spiral flow around the diaphragm-housing (D-H) junction. The volume and weight of the new LVAD, made of epoxy resin, is 309 ml and 378 g, but further miniaturization will be possible by improving the geometry of both the blood pump and the back casing. Our results show that our new design method for an implantable LVAD using CAD/CAM promises to improve blood compatibility with greater miniaturization.

Role of physical properties of resistance vessel wall in regulating peripheral blood flow—II. Structural and mechanical behavior

In order to examine the structural and mechanical properties of the vessel wall resistance when subjected to autoregulatory flow control, a mechanical model for the vascular wall was derived from a mathematical model. The mechanical model was an analogue model which connected in series the Maxwell model (elastic modulus: K3) with the parallel elements of Hills model (elastic modules: K2) and Hookes elastic model (elastic modulus: K1); it was also mathematically equivalent to the Spring model (see part I). The structural and mechanical properties of the resistance vessel wall were characterized by the three elastic moduli (K1, alpha K2 and K3) [mmHg]. The parameter alpha was a modification factor of the elastic modulus K2 given by the myogenic mechanism. After a numerical analysis of the experimental data given by the mechanical model, we confirmed that the arterial pressure range for autoregulatory flow controls shifted to the upper region with an increase of the elastic modulus K1 and the flow regulation was reduced.