Mock Circulatory System Research Articles

Optimal Pressure Regulation of the Pneumatic Ventricular Assist Device With Bellows‐Type Driver

The bellows-type pneumatic ventricular assist device (VAD) generates pneumatic pressure with compression of bellows instead of using an air compressor. This VAD driver has a small volume that is suitable for portable devices. However, improper pneumatic pressure setup can not only cause a lack of adequate flow generation, but also cause durability problems. In this study, a pneumatic pressure regulation system for optimal operation of the bellows-type VAD has been developed. The optimal pneumatic pressure conditions according to various afterload conditions aiming for optimal flow rates were investigated, and an afterload estimation algorithm was developed. The developed regulation system, which consists of a pressure sensor and a two-way solenoid valve, estimates the current afterload and regulates the pneumatic pressure to the optimal point for the current afterload condition. Experiments were performed in a mock circulation system. The afterload estimation algorithm showed sufficient performance with the standard deviation of error, 8.8 mm Hg. The flow rate could be stably regulated with a developed system under various afterload conditions. The shortcoming of a bellows-type VAD could be handled with this simple pressure regulation system.

Simple In Vitro Testing Method for Antithrombogenic Evaluation of Centrifugal Blood Pumps

We developed a simple in vitro antithrombogenic testing method using a mock circulation system as used in the hemolysis tests to evaluate the antithrombogenicity of centrifugal blood pumps. This method was not designed to substitute for animal experiments but was intended to be a screening test method for selecting pumps robust enough to operate properly during animal experiments. In this study, we were able to maintain an almost constant activated clotting time for test blood for 10 hours by using both trisodium citrate and calcium chloride. We carried out the in vitro antithrombogenic testing of monopivot type centrifugal blood pumps (models DD3 and DD6) and hydrodynamic bearing pumps (models HH2 and HH7), which were developed at Advanced Industrial Science and Technology. Thrombus formation was not observed in the DD3 or DD6 pumps but occurred in the HH2 and HH7 pumps. The HH2 pump generated thrombi during a 1.5-hour ex vivo test, and the test was terminated. We expect this in vitro testing method to be useful for undertaking evaluations before animal experiments.

Effects of Thrombosed Vena Cava Filters on Blood Flow: Flow Visualization and Numerical Modeling

Inferior vena cava (IVC) filters are used to prevent pulmonary embolism (PE) in patients with deep vein thrombosis for whom anticoagulation is contraindicated. IVC filters have been shown to be effective in trapping embolized clots and preventing PE; however, among the commercially available designs, the optimal balance of clot capture efficiency, clot dissolution, and prevention of to vena cava occlusion is unknown. Clot capture efficiency has been quantified in numerous in vitro studies, in which model clots are released into a mock circulation system, with the relative capture efficiency of various IVC filters analyzed statistically. In general, two-stage filters have been found to be more efficient than one-stage filters. However, other factors may play a role in the ultimate dissolution of clots and in the overall effect of the resulting blood flow on caval vasculature. Clot dissolution has been shown to increase with increasing wall shear stress, while low and oscillating wall shear stresses are known to have a deleterious effect on vessel walls, causing intimal hyperplasia. This paper describes the effect of IVC filters on blood flow, velocity patterns, and wall shear stress by flow visualization and computational fluid dynamics.

The Influence of Mock Circulation Input Impedance on Valve Acceleration During In Vitro Cardiac Device Testing

For a mechanical heart valve, a strong spike in pressure during closing is associated with valve wear and erythrocyte damage; thus, for valid in vitro testing, the mock circulation system should replicate the conditions, including pressure spikes, expected in vivo. To address this issue, a study was performed to investigate how mock circulation input impedance affects valve closure dynamics. A left ventricular model with polyurethane trileaflet inflow valve and tilting disc outflow valve was connected to a Louisville mock circulation system, which incorporates 2 adjustable flow resistors and 2 compliances. In the study, 116 cases matched zero frequency modulus well (982-1147 dyn x s/cm), but higher harmonics were purposely varied. Acceleration measured at the outflow valve ring (42.4-89.4 milli-Gs) was uncorrelated with impedance error (74.1-237 dyn x s/cm relative to target impedance), but was correlated with end-systolic impedance (1082-1319 dyn x s/cm) for cases with high zero frequency modulus, which exhibited just less than full ejection. These differences demonstrate that mock circulation response affects the magnitude of the closing spike, indicating that control of this parameter is necessary for authentic testing of valves. Correlation of acceleration to end-systolic impedance was weak for low zero frequency modulus, which tended toward full or hyperejection, reinforcing common laboratory observations that valve closing also depends on ventricular operating conditions.

Continuous Flow Total Artificial Heart: Modeling and Feedback Control in a Mock Circulatory System

We developed a mock circulatory loop and used mathematical modeling to test the in vitro performance of a physiologic flow control system for a total artificial heart (TAH). The TAH was constructed from two continuous flow pumps. The objective of the control system was to maintain loop flow constant in response to changes in outflow resistance of either pump. Baseline outflow resistances of the right (pulmonary vascular resistance) and the left (systemic vascular resistance) pumps were set at 2 and 18 Wood units, respectively. The corresponding circuit flow was 4 L/min. The control system consisted of two digital integral controllers, each regulating the voltage, hence, the rotational speed of one of the pumps. The in vitro performance of the flow control system was validated by increasing systemic and pulmonary vascular resistances in the mock loop by 4 and 8 Wood units (simulating systemic and pulmonary hypertension conditions), respectively. For these simulated hypertensive states, the flow controllers regulated circuit flow back to 4 L/min within seconds by automatically adjusting the rotational speed of either or both pumps. We conclude that this multivariable feedback mechanism may constitute an adequate supplement to the inherent pressure sensitivity of rotary blood pumps for the automatic flow control and left-right flow balance of a dual continuous flow pump TAH system.

A New Method of Providing Pulsatile Flow in a Centrifugal Pump: Assessment of Pulsatility Using a Mock Circulatory System

Previous studies have demonstrated the potential advantages of pulsatile flow as compared with continuous flow. However, to date, physiologic pumps have been technically complex and their application has therefore remained in the experimental field. We have developed a new type of centrifugal pump, which can provide pulsatile as well as continuous flow. The inner wall of a centrifugal pump is pulsed by means of a flexible membrane, which can be accurately controlled by means of either a hydraulic or pneumatic driver. The aim of this study was to assess the hydraulic behavior of the new pump in terms of surplus hemodynamic energy (SHE). We conducted experiments using a mock circulatory system including a membrane oxygenator. No differences were found in the pressure-flow characteristics between the new pump and a conventional centrifugal pump, suggesting that the inclusion of the flexible membrane does not alter hydraulic performance. The value of SHE rose when systolic volume was increased. However, SHE dropped when the percentage of ejection time was reduced and also when the continuous flow (programmed by the centrifugal console) increased. Mean flow matched well with the continuous flow set by the centrifugal console, that is, the pulsatile component of the flow was exclusively controlled by the pulsatile console, and was therefore independent of the continuous flow programmed by the centrifugal console. The pulsatility of the new pump was approximately 25% of that created with a truly pulsatile pump.

Mock Circulatory System for the Evaluation of Left Ventricular Assist Devices, Endoluminal Prostheses, and Vascular Diseases

A new digital computer mock circulatory system has been developed in order to replicate the physiologic and pathophysiologic characteristics of the human cardiovascular system. The computer performs the acquisition of pressure, flow, and temperature in an open loop system. A computer program has been developed in Labview programming environment to evaluate all these physical parameters. The acquisition system was composed of pressure, flow, and temperature sensors and also signal conditioning modules. In this study, some results of flow, cardiac frequencies, pressures, and temperature were evaluated according to physiologic ventricular states. The results were compared with literature data. In further works, performance investigations will be conducted on a ventricular assist device and endoprosthesis. Also, this device should allow for evaluation of several kinds of vascular diseases.

135 閉回路を利用した遠心血液ポンプの抗血栓性評価(OS1-5:循環器系・血流の計測とメカニクス(5),OS1:循環器系・血流の計測とメカニクス)

This study is to establish an evaluation system utilizing in vitro antithrombogenic testing in order to judge whether centrifugal blood pumps have sufficient antithrombogenicity to withstand animal experiments. In vitro tests using a mock circulation system were carried out using two monopivot (DD) pumps and a hydrodynamic bearing (HH) pump, under the conditions of 100mmHg, 1.0-5.0L/min, and ACT 200 for 2 hours at 37℃. Sodium citrate and calcium chloride were used to control ACT. Thrombus was generated in the HH pump, as had previously been seen with large amounts in the animal experiments, whereas very small amount of thrombus was generated in the DD pump even though some thrombus had been found in the animal experiments. From the results, the establishment of an in vitro antithrombogenic testing method can be expected. Repeatability of this testing was 80%.

Quantification of Pulsatility as a Function of Vascular Input Impedance: An In Vitro Study

The physiological benefits of pulsatility generated by ventricular assist device (VAD) support continue to be heavily debated as application of VAD support has been expanded to include destination and recovery therapies. In this study, the relationship between input impedance (Zart) and vascular pulsatility during continuous flow (CF) or pulsatile flow (PF) VAD support was investigated. Hemodynamic waveforms were recorded at baseline failure and with 50%, 75%, and 100% CF or PF VAD support for nine different Zart test conditions (combination of three different resistance and compliance settings) in a mock circulatory system simulating left ventricular failure. High-fidelity hemodynamic pressure and flow waveforms were recorded to calculate mean arterial pressure (MAP), Zart, energy equivalent pressure (EEP), and surplus hemodynamic energy (SHE) as metrics for quantifying vascular pulsatility. MAP and EEP were elevated with increasing resistance whereas SHE was reduced with increasing compliance. Vascular pulsatility was restored with increasing PF VAD support, but diminished by up to 90% with increasing CF VAD support. The nonpulsatile energy component (MAP) of the pressure waveform is dependent on resistance whereas the pulsatile energy component (SHE) is dependent on compliance. The impact of Zart and vascular pulsatility on patient recovery with VAD support warrants further investigation.

In Vitro Study of Flow Regulation for Pulmonary Insufficiency

Given the tolerance of the right heart circulation to mild regurgitation and gradient, we study the potential of using motionless devices to regulate the pulmonary circulation. In addition, we document the flow performance of two mechanical valves. A motionless diode, a nozzle, a mechanical bileaflet valve, and a tilting disk valve were tested in a pulmonary mock circulatory system over the normal human range of pulmonary vascular resistance (PVR). For the mechanical valves, regurgitant fractions (RFs) and transvalvular pressure gradients were found to be weak functions of PVR. On the low end of normal PVR, the bileaflet and tilting disk valves fluttered and would not fully close. Despite this anomaly, the regurgitant fraction of either valve did not change significantly. The values for RF and transvalvular gradient measured varied from 4 to 7% and 4 to 7 mm Hg, respectively, at 5 lpm for all tests. The diode valve was able to regulate flow with mild regurgitant fraction and trivial gradient but with values higher than either mechanical valve tested. Regurgitant fraction ranged from 2 to 17% in tests extending from PVR values of 1 to 4.5 mm Hg/lpm at 5 lpm and with concomitant increases in gradient up to 17 mm Hg. The regurgitant fraction for the nozzle increased from 2 to 23% over the range of PVR with gradients increasing to 18 mm Hg. The significant findings were: (1) the mechanical valves controlled regurgitation at normal physiological cardiac output and PVR even though they failed to close at some normal values of PVR and showed leaflet flutter; and (2) it may be possible to regulate the pulmonary circulation to tolerable levels using a motionless pulmonary valve device.

Performance of Dynamic and Isovolumetric Trained Skeletal Muscle Ventricles

Dynamic training and maintaining muscle tension are important factors during skeletal muscle ventricle (SMV) conditioning that may improve SMV performance. This study sought to determine the effects of dynamic muscle training and progressive SMV resting pressure expansion on SMV pumping capability. SMVs were constructed in 14 goats using the left latissimus dorsi muscle. SMVs were conditioned with a 40 ml constant-volume isovolumetric implant (n = 5, IsoVol group) or a compliant, pneumatic system that allowed dynamic shortening and direct exposure to resting pressures. Dynamic SMV resting pressure was either progressively increased from 40 to 100 to 120 mmHg (n = 5, HiP group) or maintained at 40 mmHg (n = 4, LowP group) during conditioning. After 8 to 10 weeks of electrical stimulation conditioning, SMVs were connected to a counterpulsation mock circulation system and SMV pumping performance evaluated across a range of pressures and stimulation parameters. SMV pumping performance was similar in each group. Stroke works generally increased with pressure and reached a plateau in all groups above 80 mmHg (120 msec contraction approximately 80 mJ/stroke; 480 msec contraction approximately 180 mJ/stroke). Stroke volumes decreased with pressure except at high stimulation levels where loading effects were observed. Chronic changes in SMV volume significantly effected pumping performance. These data suggest that acute pumping performance is not different between 8 to 10 weeks of dynamic or isovolumetric training if SMV volume is not constrained. A potentially improved SMV conditioning protocol is proposed that determines, positions, and maintains SMV volume near the initial volume for maximal isovolumetric pressure during conditioning.

ALGORITHM OF CO-PULSATION ESTIMATION FOR THE COUNTERPULSATION USING THE VAD OUTLET CANNULA PRESSURE

The ventricular assist device(VAD) helps to reduce the load against the patient’s native heart (NH). The pulsatile VAD generates pulsatile flow. If the pulsatile VAD pumps the blood simultaneously with the NH, the NH experiences elevated afterload and this could deteriorate the NH rather than help to recover. Thus, counterpulsation algorithms to avoid co-pulsation have been adopted by many VADs, however these utilize electrocardiography and arterial pressure signals which may have difficulties to acquire consistently for a long period. In this study, the co-pulsation estimation algorithm for the counterpulsation is developed using the VAD outlet cannula pressure signal. This signal is good to maintain for a long time. The pulse pressure by the NH is reflected to the VAD outlet pressure signal. Thus, the VAD outlet pressure signal was separated into pulse peaks by the VAD and the NH using Fast Fourier Transform, and the pulse rate and pulse peak time of each the NH and the VAD were calculated. From these results, the next co-pulsation time was estimated, and these are calculated using the MATLAB software. The experiments were performed for various pump rate conditions with mock circulation system and the performance was evaluated. For all conditions, the co-pulsation time was estimated well. This co-pulsation estimation algorithm could be used in the counterpulsation algorithm of the implantable VAD or the extracorporeal life support system.

Comparative In Vitro Study of Bileaflet and Tilting Disk Valve Behavior in the Pulmonary Position

A study of mechanical heart valve behavior in the pulmonary position as a function of pulmonary vascular resistance is reported for the St. Jude Medical bileaflet (SJMB) valve and the MedicalCV Omnicarbon (OTD) tilting disk valve. Tests were conducted in a pulmonic mock circulatory system and impedance was varied in terms of system pulmonary vascular resistance (PVR). An impedance spectrum was found using instantaneous pulmonary artery pressure and flow rate curves. Both valves fully opened and closed at and above a nominal PVR of 3.0 mmHg/L/min. The SJMB valve was prone to leaflet bounce at closure, but otherwise completely closed, at settings above and below this nominal setting. At PVR values at and below 2.0 mmHg/L/min, the SJMB valve exhibited two types of leaflet aberrant behavior: single leaflet only closure while the other leaflet fluttered, and incomplete closure where both leaflets flutter but neither remain fully closed. The OTD valve fully opened and closed to a PVR value of 1.6 mmHg/L/min. At lower values, the valve did not close. Valves designed for the left heart can show aberrant behavior under normal conditions as pulmonary valves.

원심형 혈액펌프의 최적화 수력설계 및 성능해석

This paper presents the hydrodynamic design and performance analysis method for a miniaturized centrifugal blood pump using three-dimensional computational fluid dynamics (CFD) code. In order to obtain the hydraulically high efficient configuration of a miniaturized centrifugal blood pump for cardiopulmonary circulation, a well-established commercial CFD code was incorporated considering detailed flow dynamic phenomena in the blood pump system. A prototype of centrifugal blood pump developed by the present design and analysis method has been tested in the mock circulatory system. Predicted results by the CFD code agree very well with in vitro hydraulic performance data for a centrifugal blood pump over the entire operating conditions. Preliminary in vivo animal testing has also been conducted to demonstrate the hemodynamic feasibility for use of centrifugal blood pump as a mechanical circulatory support. A miniaturized centrifugal blood pump developed by the hydraulic design optimization and performance prediction method presented herein shows the possibility of a good candidate for intra and extracorporeal cardiopulmonary circulation pump in the near future.

1A1-A08 冠状動脈バイパス手術における吻合手技評価訓練用血管モデルの開発

To establish an integrated evaluation system for cardiac surgeons, our group has been proposed a new concept "Self-training evaluation system"; which includes an in-vitro mock circulatory system that provides various types of coronary disease as a training machine for cardiac surgeons. The "Self-training evaluation system" consists of three parts. 1)Training plate, 2)Pulsation Unit, and 3)Scoring system for self-evaluation. This paper describes the method of training with Pulsation Unit, and evaluation of the surgical technique quantitatively based on hydrodynamic analysis.

Hydrodynamic design and performance analysis of a centrifugal blood pump for cardiopulmonary circulation

This paper presents the hydrodynamic design and performance analysis method for a miniaturized centrifugal blood pump using three-dimensional computational fluid dynamics (CFD) code. In order to obtain the hydraulically high efficient configuration of a miniaturized centrifugal blood pump for cardiopulmonary circulation, well-established commercial CFD codes were incorporated considering detailed flow dynamic phenomena in the blood pump system. A prototype of centrifugal blood pump developed by the present design and analysis method has been tested in the mock circulatory system. Predicted results by the CFD code agree very well with in vitro hydraulic performance data for a centrifugal blood pump over the entire operating conditions. Preliminary in vivo animal testing has also been conducted to demonstrate the haemodynamic feasibility for use of centrifugal blood pump as a mechanical circulatory support. A miniaturized centrifugal blood pump developed by the hydraulic design optimization and performance prediction method, presented herein, shows the possibility of a good candidate for intra and extracorporeal cardiopulmonary circulation pump in the near future.

The Effect of Gravitational Acceleration on Cardiac Diastolic Function: A Biofluid Mechanical Perspective with Initial Results

Echocardiographic measurements of astronaut cardiac function have documented an initial increase, followed by a progressive reduction in both left ventricular end-diastolic volume index and stroke volume with entry into microgravity (micro-G). The investigators hypothesize that the observed reduction in cardiac filling may, in part, be due to the absence of a gravitational acceleration dependent, intraventricular hydrostatic pressure difference in micro-G that exists in the ventricle in normal gravity (1-G) due to its size and anatomic orientation. This acceleration-dependent pressure difference, DeltaP(LV), between the base and the apex of the heart for the upright posture can be estimated to be 6660 dynes/cm(2) ( approximately 5 mm Hg) on Earth. DeltaP(LV) promotes cardiac diastolic filling on Earth, but is absent in micro-G. If the proposed hypothesis is correct, cardiac pumping performance would be diminished in micro-G. To test this hypothesis, ventricular function experiments were conducted in the 1-G environment using an artificial ventricle pumping on a mock circulation system with the longitudinal axis anatomically oriented for the upright posture at 45 degrees to the horizon. Additional measurements were made with the ventricle horizontally oriented to null DeltaP(LV)along the apex-base axis of the heart as would be the case for the supine posture, but resulting in a lesser hydrostatic pressure difference along the minor (anterior-posterior) axis. Comparative experiments were also conducted in the micro-G environment of orbital space flight on board the Space Shuttle. This paper reviews the use of an automated cardiovascular simulator flown on STS-85 and STS-95 as a Get Away Special payload to test this hypothesis. The simulator consisted of a pneumatically actuated, artificial ventricle connected to a closed-loop, fluid circuit with adjustable compliance and resistance elements to create physiologic pressure and flow conditions. Ventricular instrumentation included pressure transducers in the apex and base as well as immediately upstream of the inflow valve and downstream of the outflow valve, and a flow probe downstream of the outflow valve. By varying the circulating fluid volume, ventricular function could be determined for varying preload pressures at a regulated, mean afterload pressure of 95 mm Hg. This variation in preload condition permitted the construction of a ventricular function curve for the micro-G environment for comparison to the same curve for the 1-G environment. Data were collected from both missions at the upper end of the ventricular function curve. Experiment operation in the 1-G, supine orientation or in the micro-G environment eliminated the DeltaP(LV) observed in the 1-G, upright orientation. Consistent with the hypothesis, additional atrial pressure was required in micro-G to obtain stroke volumes and flow rates similar to those measured in 1-G for the upright posture. The necessary increase in atrial pressure was approximately 5 mm Hg in these experiments. In the same range of flow rates and stroke volumes, similar flows were observed in the 1-G supine posture for atrial pressures intermediate to the 1-G upright and micro-G values, also consistent with the hypothesis. Additional experiments on board the Space Shuttle are in preparation to gather data across the rest of the normal physiologic range of the ventricular function curve.

OPTIMIZATION OF PRESSURE TRANSMISSION LINE CONDITON IN ELECTRICAL PNEUMATIC VENTRICULAR ASSIST DEVICE

In most pneumatic pumps, pressure is transmitted through line which connects pressure generating system to blood sac. The previous studies have shown that characteristics of pressure transmission line are important to pump’s performance. The objective of the study is to find parameters that affect the characteristics of pressure transmission line and optimize the conditions of transmission line of electrical pneumatic VAD, HVAD which has been developed by Korea Artificial Organ Center. First of all, the following assumptions are made: (1)The transmission line is filled with compressible fluid. (2)The temperature variations are small. (3)Tube compliance is ignored. In order to optimize the condition of pressure transmission, various conditions were tested with mock circulation system. Test conditions are as follows: transmission line lengths of 0.5, 1, 2, and 4 m; transmission line diameters of 6 mm(outer diameter), 4 mm(inner diameter) and 9 mm(outer diameter), 6 mm(inner diameter); terminal volumes of 60 and 65 cc; and afterload pressures of 0,20,40,60, and 80 mmHg. Several parameters such as pump output, afterload pressure difference and pump efficiency were considered as criteria of optimization. The results have shown that the diameter and length of pressure transmission line have influences on the performance. And the performance was dependent on bellows volume and afterload conditions. From these results, optimal transmission line conditions were determined and the performance of HVAD could be enhanced by 10% with this optimization.

PUMP OUTPUT ESTIMATION FOR ELECTRO PNEUMATIC TYPE HYBRID VAD (H-VAD)

Electrical pneumatic type ventricular assist device, H-VAD was developed in Korea artificial organ center. This device has the unique feature that it is the hybrid driving mechanism combining pneumatic system and bellows mechanism. Pneumatic circuit of H-VAD is composed of bellows, drive line and blood pump. In this study, pump output estimation of H-VAD was investigated by using bellows inner pressure signals. This study was performed with mock circulation system. The pump output was dependent to afterload, preload, and pump rate. But pump output estimation was possible with bellows inner pressure and pump rate. This estimation algorithm can be applied to flow monitoring and control of electrical pneumatic ventricular assist device, H-VAD.

DIAPHRAGM MONITORING OF PNEUMATIC PULSATILE BLOOD PUMP USING ULTRASOUND

We have developed a new method for monitoring the pump diaphragm positions using ultrasound transducers and recognizing Full-Fill Full-Eject (FFFE) before the diaphragm overextends. One of the distance measurement methods using ultrasound is that the distance is calculated from the time difference between transmission and reception. Because the diaphragm changes the intricate shape, transmitted ultrasound may not be reflected in direction of receiver in case of the pneumatic pumps. We noted that the reverberations of the ultrasound changed depending on the diaphragm positions, and proposed the method of presuming the position of the diaphragm by integrating the receiving ultrasounds at constant intervals. Two ultrasound transducers were mounted to the air chamber side of the pump housing. The transducers were 10 mm in diameter, 7 mm in thickness and 40 kHz in resonance frequency. The ultrasounds at 40 kHz were transmitted 7 pulses at intervals of 50 Hz. After the ultrasounds were converted to the electrical signals by receiver, the integrated value of receiving signals was calculated by the personal computer at interval of 50 Hz. The pump was driven under various drive conditions utilizing a mock circulation system. The difference between FFFE and Over-Fill Over-Eject (OFOE) has been recognized depending on the proposal method. In the future, adapting this method to the control of the pumping will be effective for avoiding OFOE, and it will encourage the durability of a pump.