A three-dimensional biomodel of type A aortic dissection for endovascular interventions.

Thoracic endovascular aortic repair is widely used for type B aortic dissection. However, there is no favorable stent-graft for type A aortic dissection. A significant limitation for device development is the lack of an experimental model for type A aortic dissection. We developed a novel three-dimensional biomodel of type A aortic dissection for endovascular interventions. Based on Digital Imaging and Communication in Medicine data from the computed tomography image of a patient with a type A aortic dissection, a three-dimensional biomodel with a true lumen, a false lumen, and an entry tear located at the ascending aorta was created using laser stereolithography and subsequent vacuum casting. The biomodel was connected to a pulsatile mock circuit. We conducted four tests: an endurance test for clinical hemodynamics, wire insertion into the biomodel, rapid pacing, and simulation of stent-graft placement. The biomodel successfully simulated clinical hemodynamics; the target blood pressure and cardiac output were achieved. The guidewire crossed both true and false lumens via the entry tear. The pressure and flow dropped upon rapid pacing and recovered after it was stopped. This simulation biomodel detected decreased false luminal flow by stent-graft placement and detected residual leak. The three-dimensional biomodel of type A aortic dissection with a pulsatile mock circuit achieved target clinical hemodynamics, demonstrated feasibility for future use during the simulated endovascular procedure, and evaluated changes in the hemodynamics.

Development of a flow rate monitoring method for the wearable ventricular assist device driver.

Our research institute has been working on the development of a compact wearable drive unit for an extracorporeal ventricular assist device (VAD) with a pneumatically driven pump. A method for checking the pump blood flow on the side of the drive unit without modifying the existing blood pump and impairing the portability of it will be useful. In this study, to calculate the pump flow rate indirectly from measuring the flow rate of the driving air of the VAD air chamber, we conducted experiments using a mock circuit to investigate the correlation between the air flow rate and the pump flow rate as well as its accuracy and error factors. The pump flow rate was measured using an ultrasonic flow meter at the inflow and outflow tube, and the air flow was measured using a thermal mass flow meter at the driveline. Similarity in the instantaneous waveform was confirmed between the air flow rate in the driveline and the pump flow rate. Some limitations of this technique were indicated by consideration of the error factors. A significant correlation was found between the average pump flow rate in the ejecting direction and the average air flow rate in the ejecting direction (R2 = 0.704-0.856), and the air flow rate in the filling direction (R2 = 0.947-0.971). It was demonstrated that the average pump flow rate was estimated exactly in a wide range of drive conditions using the air flow of the filling phase.

Application of a search algorithm using stochastic behaviors to autonomous control of a ventricular assist device.

A ventricular assist device (VAD) is a device with mechanical pumps implanted adjacent to the patient's native heart to support the blood flow. Mechanical circulatory support using VADs has been an essential therapeutic tool for patients with severe heart failure waiting for a heart transplant in clinical site. Adaptive control of VADs that automatically adjust the pump output with changes in a patient state is one of the important approaches for enhanced therapeutic efficacy, prevention of complications and quality of life improvement. However adaptively controlling a VAD in the realistic situation would be difficult because it is necessary to model the whole including the VAD and the cardiovascular dynamics. To solve this problem, we propose an application of attractor selection algorithm using stochastic behavior to a VAD control system. In this study, we sought to investigate whether this proposed method can be used to adaptively control of a VAD in the simple case of a continuous flow VAD. The flow rate control algorithm was constructed on the basis of a stochastically searching algorithm as one example of application. The validity of the constructed control algorithm was examined in a mock circuit. As a result, in response to a low-flow state with the different causes, the flow rate of the pump reached a target value with self adaptive behavior without designing the detailed control rule based on the experience or the model of the control target.

Development and evaluation of endurance test system for ventricular assist devices.

We developed a novel endurance test system that can arbitrarily set various circulatory conditions and has durability and stability for long-term continuous evaluation of ventricular assist devices (VADs), and we evaluated its fundamental performance and prolonged durability and stability. The circulation circuit of the present endurance test system consisted of a pulsatile pump with a small closed chamber (SCC), a closed chamber, a reservoir and an electromagnetic proportional valve. Two duckbill valves were mounted in the inlet and outlet of the pulsatile pump. The features of the circulation circuit are as follows: (1) the components of the circulation circuit consist of optimized industrial devices, giving durability; (2) the pulsatile pump can change the heart rate and stroke length (SL), as well as its compliance using the SCC. Therefore, the endurance test system can quantitatively reproduce various circulatory conditions. The range of reproducible circulatory conditions in the endurance test circuit was examined in terms of fundamental performance. Additionally, continuous operation for 6 months was performed in order to evaluate the durability and stability. The circulation circuit was able to set up a wide range of pressure and total flow conditions using the SCC and adjusting the pulsatile pump SL. The long-term continuous operation test demonstrated that stable, continuous operation for 6 months was possible without leakage or industrial device failure. The newly developed endurance test system demonstrated a wide range of reproducible circulatory conditions, durability and stability, and is a promising approach for evaluating the basic characteristics of VADs.

Flow visualization for different port angles of a pulsatile ventricular assist device.

The "washout effect" inside a blood pump may depend in part on the configuration of the blood pump, including its "port angle." The port angle, which is primarily decided based on anatomical considerations, may also be important from the rheological viewpoint. In our department, a next-generation diaphragm-type blood pump is being developed. In this study, we examined the influence of the port angle on flow conditions inside our new blood pump. Acrylic resin mock pumps with three different port angles (0°, 30°, and 45°) were prepared for flow visualization. Mechanical monoleaflet valves were mounted on the inlet and outlet ports of the mock pumps. Flow conditions within the mock pumps were visualized by means of particle image velocimetry during a half stroke. As a result, a high flow velocity region was seen along the main circular flow from the inlet to the outlet port. This circular flow was almost uniform and parallel to the plane of the diaphragm-housing junction (DhJ) when viewed from the inlet and outlet sides. Moreover, the proportion of high flow velocity vectors in the plane in the vicinity of the DhJ decreased as the degree of the port angle increased. In conclusion, we found that the flow behavior in the plane in the vicinity of the DhJ changed with the port angle, and that a port angle of 0° may be suitable for our diaphragm-type blood pump in view of the washout effect.

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.

Observation of cavitation pits on a mechanical heart valve surface in an artificial heart used in in vivo testing.

The aim of this study was to observe mechanical heart valve (MHV) cavitation pits resulting from in vivo testing of an electrohydraulic total artificial heart (EHTAH). During in vivo testing with three sets of valves (one set used in two animals), the slope of the driving pressure (left and right driving pressure) was used as a factor for investigating cavitation intensity, and the occurrence of cavitation was determined by the observation of cavitation pits on the explanted valve surfaces. Medtronic Hall valves were installed at the inlet and outlet positions of the two blood pumps. The EHTAH was tested using calves weighing 69-80 kg. The cavitation pits on the valve surface of the inlet valves of the left and right blood pumps were examined by scanning electron micrography. The driving pressure slope 5 ms before valve closure exceeded the cavitation threshold during in vitro testing. On both inlet valves, many large pits formed when the driving pressure slope was high and the pump operating time was long. When estimating cavitation intensity during in vivo testing, both a high driving pressure slope and a long operating time are important factors. The cavitation pits observed on the valve surfaces resulting from in vivo testing will eventually lead to leaflet fracture.

Investigation of unifying transcutaneous transformer for transmission of energy and information.

When patients are fitted with a totally implantable artificial heart (TAH), they need to be implanted with two additional devices: one for the transmission of energy and one for information. However, this is a cumbersome process that affects the quality of life of the recipient. Therefore, we investigated the use of electromagnetic coupling for the transmission of energy and information and the possibility of unifying two transcutaneous transformers for the simultaneous transmission of energy and information. While unifying the transformers, it is important to suppress the electromagnetic coupling between energy and information transmission. Therefore, we ensured that the electromagnetic fields generated from the transformer windings for the transmissions of information and energy intersected perpendicularly. If the fields are perpendicular, the electromagnetic coupling between the energy and information transmissions will be suppressed significantly. The characteristics of the simultaneous transmission of information and energy using the unified transcutaneous transformer, developed experimentally, were evaluated by changing the number of windings used for the transmission of information. The electromagnetic coupling between the energy and information transmissions was suppressed by determining the direction of the magnetic field. Moreover, the optimum number of transformer windings required for the simultaneous transmission of energy and information was determined. We concluded that the externally coupled transcutaneous transformer unified for the simultaneous transmission of energy and information performed with good transmission characteristics.

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 x 8.5 x 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 x 13 mmHg, 3.9 x 0.9 l/min, and 12 x 1 W, and those for the second calf were maintained at 88 x 13 mmHg, 5.0 x 0.5 l/min, and 16 x 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.

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 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 x 25 x 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.

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.

Estimation of mechanical heart valve cavitation in a pneumatic ventricular assist device.

In this study, we investigated the possibility of estimating the mechanical heart valve (MHV) cavitation intensity using the slope of the driving pressure (DP) just before valve closure in a pneumatic ventricular assist device. We installed a 23-mm Medtronic Hall valve at the inlet of our pneumatic ventricular assist device (VAD). Tests were conducted under physiologic pressures at heart rates ranging from 60 to 90 beats/min and cardiac outputs ranging from 4.5 to 6.7 l/min. The valve-closing velocity was measured with a CCD laster displacement sensor, and the images of MHV cavitation were recorded using a high-speed video camera. The cavitation cycle time (equal to the observed duration of the cavitation bubbles) was used as the MHV cavitation intensity. The valve-closing velocity increased as the heart rate increased. Most of the cavitation bubbles were observed near the valve stop, and the cavitation intensity increased as the heart rate increased. The slope of the DP at 20 ms before valve closure was used as an index of the cavitation intensity. There were differences in the slope of the DP between low and high heart rates, but the slope of the DP had a tendency to linearly increase with increasing valve-closing velocity.

Hydrodynamic characteristics of bileaflet mechanical heart valves in an artificial heart: cavitation and closing velocity.

The aim of this study was to investigate the possibility of using the bileaflet valves in an electrohydraulic total artificial heart (EHTAH). Three kinds of bileaflet valves, namely the ATS valve (ATS Medical Inc., Minneapolis, MN, USA), the St. Jude valve (St. Jude Medical Inc., St. Paul, MN, USA), and the Sorin Bicarbon valve (Sorin Biomedica, Vercelli, Italy), were mounted in the mitral position on an inclined 45 degrees plane in an EHTAH. The pressure waves near the valve surface, the valve-closing velocity, and a high-speed camera were employed to investigate the mechanism for bileaflet valve cavitation. The cavitation bubbles in the bileaflet valves were concentrated along the leaflet tip. The cavitation intensity increased with an increase in the valve-closing velocity. It was established that squeeze flow holds the key to bileaflet valve cavitation. At lower heart rates, the delay time of the asynchronous closure motion between the two leaflets of the Sorin Bicarbon valve was greater than that of the other bileaflet valves. At higher heart rates, no significant difference was observed among the bileaflet valves.

Pulsatile blood pump with a linear drive actuator.

The main purpose of this study was to develop an implantable direct-electromagnetic left ventricular assist system driven by a linear actuator (linear LVAS). The linear LVAS is a pulsatile pump with a pusher plate that is driven directly by a linear oscillatory actuator (LOA) without any movement converters. This prototype pump unit with a LOA was 100 mm in diameter, 50 mm in thickness, and weighed 740 g. The full-fill/full-eject driving method was applied to the control algorithm. In addition, a mechanism to detect and release sucking was realized to overcome this problem that accompanies the active-filling type of VAS. The performance of the linear LVAS was evaluated in a long-term animal experiment using a goat (56 kg). The goat survived for 42 days. The reason why we terminated this experiment was that thrombus was found in the pump. There was no frictional debris found around the LOA. The linear LVAS did not exhibit electrical or mechanical problems during the first animal experiment.

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 degrees, 45 degrees, 90 degrees, 135 degrees, 180 degrees, and 270 degrees, where the OD opening toward the diaphragm was defined as 0 degrees ) 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 degrees. In contrast, the regional flow velocity in this area was highest at an OD of 135 degrees. 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 degrees.

Mechanism for cavitation of monoleaflet and bileaflet valves in an artificial heart.

It is possible that mechanical heart valves mounted in an artificial heart close much faster than those used for clinical valve replacement, resulting in the formation of cavitation bubbles. In this study, the mechanism for mechanical heart cavitation was investigated using the Medtronic Hall monoleaflet valve and the Sorin Bicarbon bileaflet valve mounted at the mitral position in an electrohydraulic total artificial heart. The valve-closing velocity was measured with a charge-coupled device (CCD) laser displacement sensor, and images of mechanical heart valve cavitation were recorded using a high-speed video camera. The valve-closing velocity of the Sorin Bicarbon bileaflet valve was lower than that of the Medtronic Hall monoleaflet valve. Most of the cavitation bubbles generated by the monoleaflet valve were observed near the valve stop; with the Sorin Bicarbon bileaflet valve, cavitation bubbles were concentrated along the leaflet tip. The cavitation density increased as the valve-closing velocity and the valve stop area increased. These results strongly indicate that squeeze flow holds the key to cavitation in the mechanical heart valve. From the perspective of squeeze flow, bileaflet valves with a low valve-closing velocity and a small valve stop area may cause less blood cell damage than monoleaflet valves.

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 alveolar fibrosis in a goat following venoarterial bypass for up to 5 months using extracorporeal membrane oxygenation.

Prolonged cardiopulmonary bypass such as venoarterial bypass with extracorporeal membrane oxygenation (VA-ECMO) is becoming a potent therapeutic option in treating patients with severe respiratory and circulatory failure. However the chronic effects of this bypass modality have not yet been fully clarified. Recently, we developed an extremely durable thrombo-resistant ECMO system, and were successful with more than 5 months of continuous heparinless VA-ECMO in an animal experiment. This article presents the pathological findings on the lungs of the animal.A goat underwent VA-ECMO for a scheduled period of 151 days. This animal demonstrated a good general condition during the course of the experiment. On autopsy, however, the lungs of the animal showed severe alveolar fibrosis with topical atelectasis. von Willebrand factor levels on the endothelial cells in the alveolar capillaries were increased compared with those of normal goats. The ultrastructure of these cells showed ischemia-induced endothelial swelling. The pathological findings indicated that the vascular endothelial phenotypes had changed from respiratory type to nutrient type. The results of this study indicated that prolonged VA-ECMO may cause pulmonary alveolar fibrosis as a result of ischemia of the lungs accompanying reduced pulmonary blood flow.

Observation of cavitation in a mechanical heart valve in a total artificial heart.

Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures occurring in implanted mechanical heart valves. The cause of cavitation in mechanical heart valves was investigated using the 25 mm Medtronic Hall valve and the 23 mm Omnicarbon valve. Closing of these valves in the mitral position was simulated in an electrohydraulic totally artificial heart. Tests were conducted under physiologic pressures at heart rates from 60 to 100 beats per minute with cardiac outputs from 4.8 to 7.7 L/min. The disk closing motion was measured by a laser displacement sensor. A high-speed video camera was used to observe the cavitation bubbles in the mechanical heart valves. The maximum closing velocity of the Omnicarbon valve was faster than that of the Medtronic Hall valve. In both valves, the closing velocity of the leaflet, used as the cavitation threshold, was approximately 1.3-1.5 m/s. In the case of the Medtronic Hall valve, cavitation bubbles were generated by the squeeze flow and by the effects of the venturi and the water hammer. With the Omnicarbon valve, the cavitation bubbles were generated by the squeeze flow and the water hammer. The mechanism leading to the development of cavitation bubbles depended on the valve closing velocity and the valve stop geometry. Most of the cavitation bubbles were observed around the valve stop and were generated by the squeeze flow.