Akihiko Homma

Up to 151 days of continuous animal perfusion with trivial heparin infusion by the application of a long-term durable antithrombogenic coating to a combination of a seal-less centrifugal pump and a diffusion membrane oxygenator

We developed a new coating material (Toyobo-National Cardiovascular Center coating) for medical devices that delivers high antithrombogenicity and long-term durability. We applied this coating to an extracorporeal membrane oxygenation (ECMO) system, including the circuit tube, cannulae, a seal-less centrifugal pump, and a diffusion membrane oxygenator, to realize prolonged cardiopulmonary support with trivial anticoagulant infusion. The oxygenator consisted of a hollow-fiber membrane made of polymethylpentene, which allows the transfer of gas by diffusion through the membrane. The centrifugal pump was free of seals and had a pivot bearing. We performed a venoarterial bypass in a goat using this ECMO system, and the system was driven for 151 days with trivial anticoagulant infusion. Plasma leakage from the oxygenator did not occur and sufficient gas-exchange performance was well maintained. In the oxygenator, thrombus formation was present around the top and the distributor of the inlet portion and was very slight in the outlet portion. In the centrifugal blood pump, there was some wear in the female pivot region and quite small amounts of thrombus formation on the edge of the shroud; the pivot wear seemed to be the cause of the hemolysis observed after 20 weeks of perfusion and which resulted in the termination of the perfusion. However, no significant amounts of thrombus were observed in other parts of the system. This ECMO system showed potential for long-term cardiopulmonary support with minimal use of systemic anticoagulants.

Effects of mechanical valve orifice direction on the flow pattern in a ventricular assist device

We have been developing a pneumatic ventricular assist device (PVAD) system consisting of a diaphragm-type blood pump. The objective of the present study was to evaluate the flow pattern inside the PVAD, which may greatly affect thrombus formation, with respect to the inflow valve-mount orientation. To analyze the change of flow behavior caused by the orifice direction (OD) of the valve, the flow pattern in this pump was visualized. Particle image velocimetry was used as a measurement technique to visualize the flow dynamics. A monoleaflet mechanical valve was mounted in the inlet and outlet ports of the PVAD, which was connected to a mock circulatory loop tester. The OD of the inlet valve was set at six different angles (OD = 0°, 45°, 90°, 135°, 180°, and 270°, where the OD opening toward the diaphragm was defined as 0°) and the pump rate was fixed at 80 bpm to create a 5.0 l/min flow rate. The main circular flow in the blood pump was affected by the OD of the inlet valve. The observed regional flow velocity was relatively low in the area between the inlet and outlet port roots, and was lowest at an OD of 90°. In contrast, the regional flow velocity in this area was highest at an OD of 135°. The OD is an important factor in optimizing the flow condition in our PVAD in terms of preventing flow stagnation, and the best flow behavior was realized at an OD of 135°.

The National Cardiovascular Center electrohydraulic total artificial heart and ventricular assist device systems: current status of development.

Electrohydraulic total artificial heart (EHTAH) and electrohydraulic ventricular assist device (EHVAD) systems have been developed in our institute. The EHTAH system comprises a pumping unit consisting of blood pumps and an actuator, as well as an electronic unit consisting of an internal controller, internal and external batteries, and transcutaneous energy transfer (TET) and optical telemetry (TOT) subunits. The actuator, placed outside the pericardial space, reciprocates and delivers hydraulic silicone oil to the alternate blood pumps through a pair of flexible oil conduits. The pumping unit with an external controller was implanted in 10 calves as small as 55 kg. Two animals survived for more than 12 weeks in a good general condition. The assumed cardiac output ranged between 6 and 10 L/min, the power consumption was 12–18 W, and the energy efficiency was estimated to be 9–11%. Initial implantation of subtotal system including electronic units was further conducted in another calf weighing 73 kg. It survived for 3 days with a completely tether free system. The EHVAD system is developed by using the left blood pump and the actuator of the EHTAH, which were packaged in a compact metal casing with a compliance chamber. In vitro testing demonstrated maximum output more than 9 L/min and more than 13% maximum efficiency. The initial animal testing lasted for 25 days. These results indicate that our EHTAH and EHVAD have the potential to be totally implantable systems.

Development of a compact portable driver for a pneumatic ventricular assist device.

The Toyobo-National Cardiovascular Center pneumatic ventricular assist device (Toyobo-NCVC VAD) is widely used in Japan; however, the current pneumatic drivers have some drawbacks, including their large size, heavy weight, and high power consumption. These issues cause difficulty with mobility and contribute to an unsatisfactory quality of life for patients. Because it is urgently necessary to improve patients’ safety and quality of life, we have developed a compact, low-noise, portable VAD driver by utilizing an electrohydraulic actuator consisting of a brushless DC motor and a regenerative pump. This unit can be actuated for as long as 2 h with two rechargeable lightweight batteries as well as with external AC power. It is compact in size (33 × 25 × 43 cm) and light in weight (13 kg), and the unit is carried on a mobile wheeled cart. In vitro testing with a Toyobo-NCVC VAD demonstrated a sufficient pumping capacity of up to 8 l/min. We conclude that this newly-developed compact portable driver can provide a better quality of life and improved safety for patients using protracted pneumatic VAD support.

Observation of cavitation bubbles in monoleaflet mechanical heart valves.

Recently, cavitation on the surface of mechanical heart valves (MHVs) has been studied as a cause of fractures occurring in implanted MHVs. In the present study, we investigated the mechanism of MHV cavitation associated with the Björk–Shiley valve and the Medtronic Hall valve in an electrohydraulic total artificial heart (EHTAH). The valves were mounted in the mitral position in the EHTAH. The valve closing motion, pressure drop measurements, and cavitation capture were employed to investigate the mechanisms for cavitation in the MHV. There are no differences in valve closing velocity between the two valves, and its value ranged from 0.53 to 1.96 m/s. The magnitude of negative pressure increased with an increase in the heart rate, and the negative pressure in the Medtronic Hall valve was greater than that in the Björk–Shiley valve. Cavitation bubbles were concentrated at the edge of the valve stop; the major cause of these cavitation bubbles was determined to be the squeeze flow. The formation of cavitation bubbles depended on the valve closing velocity and the valve leaflet geometry. From the viewpoint of squeeze flow, the Björk–Shiley valve was less likely to cause blood cell damage than the Medtronic Hall valve in our EHTAH.

Observation of cavitation pits on mechanical heart valve surfaces in an artificial heart used in in vitro testing

Our group has developed an electrohydraulic total artificial heart (EHTAH) with two diaphragm-type blood pumps. Cavitation in a mechanical heart valve (MHV) causes valve surface damage. The objective of this study was to investigate the possibility of estimating the MHV cavitation intensity using the slope of the driving pressure just before valve closure in this artificial heart. Twenty-five and twenty-three-millimeter Medtronic Hall valves were mounted at the inlet and outlet ports, respectively, of both pumps. The EHTAH was connected to the experimental endurance tester developed by our group, and tested under physiological pressure conditions. Cavitation pits could be seen on the inlet valve surface and on the outlet valve surface of the right and left blood pumps. The pits on the inlet valves were more severe than those on the outlet valves in both blood pumps, and the cavitation pits on the inlet valve of the left blood pump were more severe than those on the inlet valve of the right blood pump. The longer the pump running time, the more severe the cavitation pits on the valve surfaces. Cavitation pits were concentrated near the contact area with the valve stop. The major cause of these pits was the squeeze flow between the leaflet and valve stop.

Current status of development and in vivo evaluation of the National Cardiovascular Center electrohydraulic total artificial heart system

We have been developing an electrohydraulic total artificial heart system. The system has a pumping unit, consisting of diaphragm-type blood pumps and an energy converter, and an electronics unit, consisting of an internal controller, an internal battery, and transcutaneous energy transfer and optical telemetry subunits. The energy converter, designed to be placed outside the pericardial space, reciprocates and delivers hydraulic silicone oil to the alternate blood pumps through a pair of flexible oil conduits. The left-right output balance is achieved with an interatrial shunt made in the composite atrial cuff. In vivo performance of the pumping unit has been evaluated by chronic implantation of 16 calves weighing 54–88 kg. Five calves survived for more than a week, and the longest-surviving animal lived for over 12 weeks until its accidental death. In this animal, a cardiac output of 6–81/min was maintained by the device with power consumption of 13.5±0.9W and 9%–11% efficiency. The left and right atrial pressures were 16±4 and 14±4 mm Hg, respectively, and the left-right output difference was adequately balanced with the interatrial shunt. The mixed venous oxygen saturation was 65±6% and the serum lactate level was 5±1 mg/dl, representing favorable oxygen metabolic conditions. The temperatures of the energy converter and the blood pump surfaces were 39.4±0.7° and 38.8±1.5°C, respectively, indicating that heat generation and dissipation were acceptable. The serum and tissue silicon levels were within normal (<1 μg/ml or <1 μg/g), indicating that permeation of silicone oil through the blood pump diaphragm was inconsequential and unlikely to be detrimental. We conclude that the system has the potential to be a totally implantable cardiac replacement.

Improvement in magnetic field immunity of externally-coupled transcutaneous energy transmission system for a totally implantable artificial heart

Transcutaneous energy transmission (TET) that uses electromagnetic induction between the external and internal coils of a transformer is the most promising method to supply driving energy to a totally implantable artificial heart without invasion. Induction-heating (IH) cookers generate magnetic flux, and if a cooker is operated near a transcutaneous transformer, the magnetic flux generated will link with the external and internal coils of the transcutaneous transformer. This will affect the performance of the TET and the artificial heart system. Hence, it is necessary to improve the magnetic field immunity of the TET system. During operation of the system, if the transcutaneous transformer is in close proximity to an IH cooker, the electric power generated by the cooker and coupled to the transformer can drive the artificial heart system. To prevent this coupling, the external coil was shielded with a conductive shield that had a slit in it. This reduces the coupling between the transformer and the magnetic field generated by the induction cooker. However, the temperature of the shield increased due to heating by eddy currents. The temperature of the shield can be reduced by separating the IH cooker and the shield.

Development of a compact wearable pneumatic drive unit for a ventricular assist device

The purpose of this study was to develop a compact wearable pneumatic drive unit for a ventricular assist device (VAD). This newly developed drive unit, 20 × 8.5 × 20 cm in size and weighing approximately 1.8 kg, consists of a brushless DC motor, noncircular gears, a crankshaft, a cylinder-piston, and air pressure regulation valves. The driving air pressure is generated by the reciprocating motion of the piston and is controlled by the air pressure regulation valves. The systolic ratio is determined by the noncircular gears, and so is fixed for a given configuration. As a result of an overflow-type mock circulation test, a drive unit with a 44% systolic ratio connected to a Toyobo VAD blood pump with a 70-ml stroke volume achieved a pump output of more than 7 l/min at 100 bpm against a 120 mmHg afterload. Long-term animal tests were also performed using drive units with systolic ratios of 45% and 53% in two Holstein calves weighing 62 kg and 74 kg; the tests were terminated on days 30 and 39, respectively, without any malfunction. The mean aortic pressure, bypass flow, and power consumption for the first calf were maintained at 90 × 13 mmHg, 3.9 × 0.9 l/min, and 12 × 1 W, and those for the second calf were maintained at 88 × 13 mmHg, 5.0 × 0.5 l/min, and 16 × 2 W, respectively. These results indicate that the newly developed drive unit may be used as a wearable pneumatic drive unit for the Toyobo VAD blood pump.

Transcutaneous Energy Transmission System for a Totally-Implantable Artificial Heart in case Using External Battery

We have been developing the externally-coupled transcutaneous energy transmission system (ECTETS) for a totally-implantable artificial heart (TIAH). The transcutaneous energy transmission (TET) system enables the TIAH to supply the driving energy without infectious disease and reduction of patient’s QOL (quality of life). Since the TET system uses the electromagnetic induction between the external (primary) and the internal (secondary) coils, it is necessary for the TET system to be compatible electromagnetically. In the practical use of artificial heart, such as a patient going out, the artificial heart is driven by the energy from the portable rechargeable battery outside of the body. In this paper, the drive performance and the electromagnetic compatibility of the TET system were investigated in case of using the external rechargeable battery. As a result, the ECTETS, driven by the external rechargeable battery consisting of the Lithium ion battery pack with a weight of 430 g, was able to drive the TIAH actuator for 4 hours and 9 minutes. The radiated emission from the ECTETS was suppressed within the regulation of class-B and group-1 in CISPR (international special committee on radio interference) Pub.11, and the ECTETS satisfied the electrostatic discharge immunity test based on IEC61000-4-2 (International Electrotechnical Commission), the radiation immunity test based on IEC61000-4-3, and the power supply frequency magnetic field immunity test based on IEC61000-4-8. Therefore, it was concluded that the ECTETS investigated had satisfactory characteristics in the drive performance and the electromagnetic compatibility.