Yoshiyuki Taenaka

Characterization and optimization of the flow pattern inside a diaphragm blood pump based on flow visualization techniques.

We applied two different flow visualization techniques to obtain detailed information on the inside flow of the diaphragm blood pump of our electrohydraulic total artificial heart system to determine the optimum washout effect that would result in better antithrombogenicity. Major orifice directions of the inflow and outflow Bjork-Shiley valves of the left blood pump were independently changed to create 17 varied patterns. The character and velocity of the main flow at the diaphragm-housing junction were acquired using a laser light sheet method with polyethylene tracers. Wall shear flow, a major factor governing washout in the blood pump, was estimated by a newly developed paint erosion method. In this method, quantitative evaluation for an index of washout effect was made by calculating the residual ratio of the paint on the blood pump inner surface at 30 sec of pumping. When a single circular flow was consistently observed by the laser light sheet method, the paint residual ratio become low, indicating washout was relatively good. At the lowest paint residual ratio, the center of the circular flow observed by the laser light sheet method was located at the geometric center of the blood chamber. In conclusion, the flow pattern inside the blood pump could be characterized by combined use of these two flow visualization techniques, and the significant role of circular flow in better washout was clarified.

A transcutaneous energy transmission system with rechargeable internal back-up battery for a totally implantable total artificial heart

We have been developing an externally coupled transcutaneous energy transmission system (ECTETS) for a totally implantable total artificial heart (TITAH). When the ECTETS is unable to supply the energy to drive the TITAH from outside the body, a rechargeable internal back-up battery (RIBB) implanted inside the body is used as a back-up to supply the required energy. This paper reports on the performance characteristics of our ECTETS with an RIBB. In this study, a lithium-ion (Li+) secondary battery was used as the RIBB. The transcutaneous energy transmission and the charging control characteristics of the ECTETS, while simultaneously supplying energy to the TITAH and the RIBB, were evaluated in an in vitro experiment. The output power and transmission efficiency of the ECTETS operating in this mode were found to vary from 20 W to 34 W and from 84% to 71%, respectively. It was also found that a sufficient power of more than 20 W could be supplied to the TITAH. The time needed to fully charge the RIBB was 117 minutes, and a fully charged RIBB could drive the TITAH, consuming 20 W for 62 minutes. It may, therefore, reasonably be concluded that the ECTETS with the RIBB is sufficient to drive the TITAH.

Transcutaneous optical telemetry system with infrared laser diode

A transcutaneous telemetry system is indispensable when monitoring and controlling the operation of an artificial heart totally implanted inside the body. A telemetry system using light is more useful than radio waves from the viewpoint of electromagnetic interference and power consumption. In this report, a transcutaneous optical coupler consisting of an infrared laser diode (LD) and a PIN photodiode (PINPD) was evaluated, and the transcutaneous optical coupling and information transmission characteristics were evaluated in in vitro experiments. The wavelength and directional angle of the LD used were 830 nm and 9.5 degrees, respectively. With regard to the directional angle of PINPD, the authors found that a PINPD with a larger directional angle allowed for more deviation between the axes optical axes of the LD and the PINPD. It was also found that the transcutaneous coupler had an optimum distance for the permissible deviation to be maximized. The information signals modulated by the phase shift keying (PSK) were transmitted at a rate of 9,600 bps through goat skin 4 mm thick, and demodulated by the phase locked loop (PLL) on the receiving side. As a result, the information signals were demodulated without any errors in deviation within 10.5 mm at a distance of 11 mm. In conclusion, the transcutaneous optical telemetry system using an infrared LD has sufficient characteristics to monitor and control the operation of an artificial heart totally implanted inside the body.

Influence of pulsatile and nonpulsatile left heart bypass on the hormonal circadian rhythm.

We investigated endocrine circadian rhythm (CR) during pulsatile and nonpulsatile left heart bypass (PLHB and NLHB). A ventricular assist device (VAD) was installed in five goats. After 2 weeks of PLHB, NLHB was subsequently conducted for 4 weeks. The levels of melatonin, cortisol, and renin activity were measured every 2 hours for 24 hours on the last day of the second PLHB week and the second and fourth NLHB weeks. Acquired data were compared with those of the normal control goats (Cont) to determine the presence or absence of CR, as well as the timing, acrophase, amplitude, and mesor of CR. Variations in melatonin, CR were consistently detected at all sampling points. In contrast, CR of cortisol during PLHB and NLHB was observed in a few cases, although considerable CR was noted in most of the Cont cases. The variations in renin activity indicated CR in most of the Cont cases and in all cases at all sampling points during PLHB and NLHB, whereas their acrophases were diversified. In conclusion, the CR of melatonin, reflecting the fundamental circadian clock, was maintained during both PLHB and NLHB, while it could be modified for stress sensitive cortisol and hemodynamic responsive renin during these bypass modalities.

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.

Progress of an electrohydraulic total artificial heart system with a separate energy converter

We have been developing an electrohydraulic total artificial heart (EHTAH) system. The system consists of diaphragm blood pumps, an abdominally placed energy converter, an internal controller, a transcutaneous energy transfer (TET) system, a transcutaneous optical information transfer system, and internal and external lithium-ion (Li-ion) batteries. The energy converter was optimized to obtain better oil transfer. Maximum cardiac output and efficiency of the EHTAH were increased from 8 L/min to 10 L/min and from 10% to 12%, respectively. The volume of the energy converter was reduced from 280 to 210 ml. The pumping unit was successfully implanted in 68-85 kg calves without anatomic problems, and the calves survived up to 10 days with good circulatory results. The maximum temperature rise of the implanted energy converter was only 1 degrees C. Stable performance of the TET system was confirmed in goats for more than 1 month. DC-DC energy transfer efficiency with 20 W of energy transmission remained within the range of 80% to 85%, and no significant temperature rise was observed in the implanted circuit. The internal Li-ion battery was also evaluated in a goat, and the maximum temperature rise during the charging period was 1.5 degrees C, while the charging and discharging times were 72 and 58 min, respectively. We conclude that our system has progressed in its development as a practical implantable system.

Effect of prolonged nonpulsatile left heart bypass on vascular control status

We investigated the vascular control status, including vasoactive hormones, systemic vascular resistance (SVR), and baroreceptor sensitivity, in prolonged nonpulsatile left heart bypass (NLHB). Nine goats underwent pulsatile left heart bypass (PLHB) with a ventricular assist device (VAD). Two weeks postoperatively, the VAD was replaced with a centrifugal pump and NLHB was subsequently conducted for 4 weeks. Thirteen healthy goats were also evaluated to obtain normal control data. The aortic pulse pressure on average was 37 mmHg for controls; 36mmHg at the end of PLHB; and 11, 9, 12, and 12mmHg at the end of the first, second, third, and fourth NLHB weeks, respectively. Plasma norepinephrine (NE), vaso-pressin (VP), renin activity (RA), and endothelin-I (ET) levels were measured for controls and at each point during PLHB and NLHB. The baseline SVR and the minimum SVR values after nitroglycerin injection were determined at each point during PLHB and NLHB. Baroreceptor sensitivity was calculated as the regression slope between R-R intervals and mean aortic pressure for controls and at each point during PLHB and NLHB. The plasma levels of NE, VP, RA, and EN did not change significantly during the entire course of the experiments. The baseline and minimum SVR values after nitroglycerin injection remained unchanged during PLHB and NLHB. Furthermore, the baroreceptor sensitivity did not change significantly during the entire course of the experiments. These results indicate that prolonged NLHB does not affect vascular control status, including the major vasoactive hormone levels, SVR, and baroreceptor sensitivity.

Mixed venous oxygen saturation as a promising parameter for physiologic control of total artificial heart.

Mixed venous oxygen saturation (SvO2) has been proposed as one of the suitable parameters for physiologic control of a total artificial heart (TAH). To establish the practical application of SvO2, we investigated the response of cardiac output (CO) and SvO2 to step-loaded exercise. A normal calf was surgically equipped with an ultrasonic flowmeter probe and an oximetry catheter in the pulmonary artery to measure CO and SvO2, respectively. Three stage step treadmill exercise tests (1, 2, and 4 km/h) were performed three times. While CO increased from 8.9 L/min at preexercise level to 9.7, 10.2 and 11.4 L/min at 1, 2, and 4 km/h, respectively, SvO2 decreased from 59.6% to 56.8, 55.3, and 52.2%, respectively. There existed a linear correlation between the magnitude of changes in CO and SvO2. CO and SvO2 exhibited a similar course of change, expressing an inverted exponential curve. The time constant of SvO2 was from 19 to 35 seconds, whereas that of CO was from 21 to 39 seconds. We conclude that SvO2 changes in close association with CO during exercise and has good potential to be a parameter for physiologic control of a TAH, by reflecting the recipient’s CO demand without conspicuous time delay.

Aortic reaction to prolonged nonpulsatile left heart bypass

We investigated the reaction of the vascular system to prolonged nonpulsatile left heart bypass (LHB) using adult goats that underwent chronic nonpulsatile LHB. On morphological examination of the aorta, we found a decrease in wall thickness, a decrease in constituent volume ratio of smooth muscle, and an increase in the ratio of elastin. The atrophic change was further confirmed by an increase in smooth muscle cell density and an increase in smooth muscle cells, which exhibited atrophy-specific changes on electron microscopic observation. The changes in mechanical properties accompanying these morphological changes were evaluated according to stiffness parameter β and pulse wave velocity. No significant changes in those parameters were caused by prolonged nonpulsatile LHB. The change in vascular contractility was evaluated by the response of systemic vascular resistance to norepinephrine and nitroglycerin administration. Vascular contractility was markedly decreased by prolonged nonpulsatile LHB. These results indicated that the significant morphological changes, including aortic structure and smooth muscle atrophy, were caused by prolonged nonpulsatile LHB. However, these changes did not accompany the changes in mechanical properties. In contrast, this distinctive LHB modality caused a decrease in vascular contractility.

Control of a total artificial heart using mixed venous oxygen saturation.

We developed a control of the total artificial heart (TAH) using mixed venous oxygen saturation (SvO2). With this method, the TAH output was controlled by automatically adjusting drive parameters in response to changes in measured SvO2. The feasibility and validity of this method were evaluated in a series of treadmill exercise tests using a calf implanted with a pneumatic TAH. The cardiac output (CO) and SvO2 were measured with an ultrasonic flowmeter and an oximetric catheter, respectively. The calf performed graded exercise under a fixed drive control mode (fixed mode), in which the drive parameters were unchanged throughout the treadmill test, and under the SvO2 based control mode (SvO2 mode). The calf could go to Stage 6 in both modes, but the maximal oxygen consumption in the SvO2 mode was 1.5 times as large as that of the fixed mode. Compared with the fixed mode, CO was increased effectively in the SvO2 mode, and the capacity for exercise was augmented. When considering the relationship between oxygen consumption and oxygen delivery, it was revealed that the CO in the SvO2 mode responded more adequately to the recipients oxygen demand than that in the fixed mode. We conclude that this SvO2 based control method is feasible and physiologically effective.