Mock Ventricle Research Articles

Effect of cannulation site on emboli travel during cardiac surgery

BackgroundDuring cardiac surgery, micro-air emboli regularly enter the blood stream and can cause cognitive impairment or stroke. It is not clearly understood whether the most threatening air emboli are generated by the heart-lung machine (HLM) or by the blood-air contact when opening the heart. We performed an in vitro study to assess, for the two sources, air emboli distribution in the arterial tree, especially in the brain region, during cardiac surgery with different cannulation sites.MethodsA model of the arterial tree was 3D printed and included in a hydraulic circuit, divided such that flow going to the brain was separated from the rest of the circuit. Air micro-emboli were injected either in the HLM (“ECC Bubbles”) or in the mock left ventricle (“Heart Bubbles”) to simulate the two sources. Emboli distribution was measured with an ultrasonic bubble counter. Five repetitions were performed for each combination of injection site and cannulation site, where air bubble counts and volumes were recorded. Air bubbles were separated in three categories based on size.ResultsFor both injection sites, it was possible to identify statistically significant differences between cannulation sites. For ECC Bubbles, axillary cannulation led to a higher amount of air bubbles in the brain with medium-sized bubbles. For Heart Bubbles, aortic cannulation showed a significantly bigger embolic load in the brain with large bubbles.ConclusionsThese preliminary in vitro findings showed that air embolic load in the brain may be dependent on the cannulation site, which deserves further in vivo exploration.

Development of a cardiac loading device to monitor cardiac function during ex vivo graft perfusion.

BackgroundEx vivo heart perfusion systems, allowing continuous perfusion of the coronary vasculature, have recently been introduced to limit ischemic time of donor hearts prior to transplantation. Hearts are, however, perfused in an unloaded manner (via the aorta) and therefore, cardiac contractile function cannot be reliably evaluated.ObjectivesWe aim to develop a ventricular loading device that enables monitoring of myocardial function in an ex vivo perfusion system. In this initial study, was to develop a prototype for rat experimentation.MethodsWe designed a device consisting of a ventricular balloon and a reservoir balloon, connected through an electronic check valve, which opens and closes in coordination with changes in ventricular pressure. All balloons were produced in our laboratory and their properties, particularly pressure-volume relationships, were characterized. We developed a mock ventricle in vitro test system to evaluate the device, which was ultimately tested in ex vivo perfused rat hearts.ResultsBalloon production was consistent and balloon properties were maintained over time and with use on the device. Results from in vitro and ex vivo experiments show that the device functions appropriately; hemodynamic function can be measured and compares well to measurements made in an isolated, working (loaded) rat heart preparation.ConclusionsOur cardiac loading device appears to reliably allow measurement of several left ventricular hemodynamic parameters and provides the opportunity to control ventricular load.

Nonlinear Adaptive Control of a Piston Pump Mock Ventricle

High-performance control system for a piston pump mock ventricle is developed in this research to accurately reproduce the native ventricle's pumping dynamics and to secure the physiological feasibility of the cardiovascular circulatory simulator (CCS). The challenges are that a piston pump mock ventricle has inherent parameter uncertainties of check valve resistances, high bandwidth pressure dynamics, and hard nonlinearity due to check valves. To overcome these difficulties, a nonlinear adaptive control scheme is presented in this paper. A hardware CCS is constructed based on the electrical-analog reference model of the cardiovascular circulatory system and the mathematical model of the hardware was derived using fluidic element modeling and the parameter identification experiments. The adaptive controller was designed based on the derived model. Experimental results show that the adaptive control achieved outstanding control performance in spite of the parameter uncertainties and yielded a good match with the reference model behavior. As a result, high quality elastance curve over the whole cardiac cycle was made possible at the level that has never been achieved.

Feedback Linearization Control of a Cardiovascular Circulatory Simulator

In this brief, a nonlinear model-based feedback linearization (FBL) control is proposed for a high-performance cardiovascular circulatory simulator (CCS). The challenges are that the piston pump used for a mock ventricle in CCS has high-bandwidth pressure dynamics and hard nonlinearity due to check valves. Limited control performance in the previous researches due to these difficulties even raises the question of the physiological feasibility of the developed CCS. To overcome this problem, FBL theory based on the Lie algebra is applied in this research for the piston pump mock ventricle control. Dynamic model of the piston pump was derived, and parameter values of the model were identified experimentally for the controller design. The experimental results confirmed good performance of the proposed controller for various physiological scenarios. Good match with the reference model behavior was verified as well, and physiological feasibility of the CCS was secured thereby thanks to the proposed high-performance controller.

Sliding mode control for the Frank–Starling response of a piston pump mock ventricle

Accurate reproduction of native ventricular pumping dynamicsFrank–Starling responseis key for reliable in vitro cardiovascular circulatory simulators. However, due to high-bandwidth pressure dynamics, nonlinearity and model uncertainty of the mock ventricle often built with piston pump, it has been difficult to achieve accurate realization of Frank–Starling response. To overcome such difficulties, a sliding mode controller that is effective for the systems with fast dynamics and uncertainties is proposed in this research. Stability of the proposed controller is proved by Barbalat's lemma, and its performance is confirmed through experiments. Parameters of the controller is identified by fluidic elements modeling of the experimental system. The proposed controller is experimentally verified to be effective for various physiologic scenarios. Thanks to the high control performance, cardiovascular circulatory simulator yield a good match with the reference model behavior, and thereby physiological validity is secured.

Model-referenced cardiovascular circulatory simulator: construction and control.

Physiological feasibility is the most important requirement for cardiovascular circulatory simulators (CCSs). However, previous simulators have been validated by a comparison with specific human data sets, which are valid only for very limited conditions, and so it is difficult to validate the fidelity of a CCS for various body conditions. To overcome this critical limitation, we propose a model-referenced CCS that reproduces the behavior of an electrical-analog model of the cardiovascular circulatory system, for which physiological fidelity is well established over a wide range. In this study, the electrical-analog reference model was realized in the hardware simulator using fluidic element modeling and by the feedback control of a mock ventricle. The proposed simulator showed a good match with the reference model behavior, and its physiological validity was thereby verified. The proposed simulator is able to show responsiveness to various body conditions as well. To the best of the author's knowledge, this is the first report of an in vitro CCS verified to be consistent with reference model behavior.

Control system design for the mock ventricle with aortic and mitral valve resistance uncertainty

Reproducing the native left ventricle’s contractile behavior is the key ingredient for the mock cardiovascular circulatory system. It can be achieved by controlling either mock ventricle’s pressure or volume so that it could follow the reference signal calculated by the Elastance waveform. However, due to inherent uncertainties of the parameter values of the mock ventricle, such as check valve resistance, it is difficult to achieve high quality control performance. In this paper, we present an adaptive control scheme to overcome this parameter value uncertainty and to achieve high quality control performance. To the best knowledge of the author, it is the first report that reproduces the mock ventricle pumping dynamics precisely considering the resistance of the aortic/mitral check valves and overcoming the uncertainty of them. In the paper, along with the detailed design of the controller, rigorous proof of the stability and the convergence is presented as well. Computer simulation was performed using an electrical-analog cardiovascular circulatory system model and a piston pump mock ventricle model. Results confirmed that adaptive control achieved almost perfect tracking performance in spite of large parameter uncertainty and high bandwidth dynamic characteristics of the mock ventricle. Performance comparison verified the superiority of the adaptive controller over PID controllers.

Bioprosthetic mitral valve effective orifice area by phase-contrast CMR. An in vitro comparison with Doppler echocardiography

Background Introduction: Current guidelines for the functional evaluation of bioprosthetic heart valves recommend the effective orifice area (EOA) as the product of the transvalvular stroke volume divided by Doppler derived diastolic time velocity integral (TVI). Phase contrast CMR may offer an alternative imaging modality to assess bioprosthetic valve EOA when Doppler methods are technically limited or unreliable. Methods Our circulatory loop includes a mock ventricle and a heart valve imaging chamber that has been fabricated using MRI-compatible components. In this study 3 different sized stented porcine mitral valve bioprostheses were evaluated (27 mm, 29 mm, 31 mm) replicating three different hemodynamic conditions with forward stroke volume of 70 ml, 90 ml and 110 ml respectively at a beat rate of 70 bpm. Imaging was performed with a

A Fast Building and Effective Hydraulic Pediatric Mock Circulatory System for the Evaluation of a Left Ventricular Assist Device

A mock circulatory system (MCS) has been proven a useful tool in the development of a ventricular assist device. Nowadays a MCS aimed at the evaluation of pediatric blood pumps, which require many different considerations compared with that of adults, has become an urgent need. This article presents the details on how the dynamic process of the left ventricle, which is described in terms of the pressure-volume loop (P-V loop), and the properties of the circulation such as compliance and resistance are simulated by hydraulic elements. A simple control method is introduced to reproduce the physiological afterload and preload sensitivities of the mock ventricle for the first time. Hemodynamic performance of the system is obtained by medical sensors to validate the similarity of the device to the native cardiovascular system. The actual sensitivities of the mock ventricle are obtained intuitively from the changes of the P-V loops. The aortic input impedance of the MCS is also obtained and compared with the data from previous medical reports. At last a pediatric left ventricular assist device (LVAD) prototype is introduced for testing to further verify the effectiveness of the MCS. The experimental results indicate that this pediatric MCS is capable of reproducing basic hemodynamic characteristics of a child in both normal and pathological conditions and it is sufficient for testing a pediatric LVAD. Besides, most components constituting the main hydraulic part of this MCS are inexpensive off-the-shelf products, making the MCS easy and fast to build.

In Vitro Pulsatility Analysis of Axial-Flow and Centrifugal-Flow Left Ventricular Assist Devices

Recently, continuous-flow ventricular assist devices (CF-VADs) have supplanted older, pulsatile-flow pumps, for treating patients with advanced heart failure. Despite the excellent results of the newer generation devices, the effects of long-term loss of pulsatility remain unknown. The aim of this study is to compare the ability of both axial and centrifugal continuous-flow pumps to intrinsically modify pulsatility when placed under physiologically diverse conditions. Four VADs, two axial- and two centrifugal-flow, were evaluated on a mock circulatory flow system. Each VAD was operated at a constant impeller speed over three hypothetical cardiac conditions: normo-tensive, hypertensive, and hypotensive. Pulsatility index (PI) was compared for each device under each condition. Centrifugal-flow devices had a higher PI than that of axial-flow pumps. Under normo-tension, flow PI was 0.98 ± 0.03 and 1.50 ± 0.02 for the axial and centrifugal groups, respectively (p < 0.01). Under hypertension, flow PI was 1.90 ± 0.16 and 4.21 ± 0.29 for the axial and centrifugal pumps, respectively (p = 0.01). Under hypotension, PI was 0.73 ± 0.02 and 0.78 ± 0.02 for the axial and centrifugal groups, respectively (p = 0.13). All tested CF-VADs were capable of maintaining some pulsatile-flow when connected in parallel with our mock ventricle. We conclude that centrifugal-flow devices outperform the axial pumps from the basis of PI under tested conditions.

A new pulse duplicator with a passive fill ventricle for analysis of cardiac dynamics

A new pulse duplicator was designed for evaluation of the performance of ventricular assist devices through pressure-volume (P-V) diagrams of the native heart. A linear drive system in combination with a pusher-plate mechanism was designed as a drive system to implement the passive fill mechanism during diastole of the mock ventricle. The compliances of the native heart during both diastole and systole were simulated by placing a ventricle sack made of soft latex rubber in a sealed chamber and by varying the air-to-fluid volume ratio inside the chamber. The ratio of the capacities of the systemic venous and pulmonary circuits was adjusted to properly reflect the effects of volume shift between them. As the air-to-fluid volume ratio was varied from 1:12.3 to 1:1.58, the contractility of the ventricle expressed by E (max) varied from 1.75 to 0.56mmHg/ml with the mean V (0) of 4.58ml closely mimicking those of native hearts (p<0.05). Because the E (max) value of the normal human heart ranges from 1.3 to 1.6, with a value below 1.0 indicating heart failure, the mock ventricle is applicable in simulating the dynamics of the normal heart and the sick heart. The P-V diagram changes seen with rotary blood pump assistance revealed changes similar to those reported by other workers. The effects of the ventricular assist device, either pulsatile or continuous flow, on cardiac dynamics can be easily simulated with this system to derive design criteria for clinical circulatory assist devices.

Flow Visualization Techniques in a Mock Ventricle Supported by a Nonpulsatile Left Ventricular Assist Device

Little is known about flow patterns in ventricles supported by continuous flow left ventricular assist devices (LVADs), and valuable information can be obtained with simple flow visualization experiments. We describe the application of several experimental techniques for the in vitro study of ventricular flow patterns (e.g., unsteadiness, vortical motions, stagnation regions) in the presence of a continuous flow LVAD. We used dye streaks, particle paths, and hydrogen bubble techniques to visualize fluid flow in an idealized, static, transparent mock ventricle attached to a Jarvik 2000 continuous flow LVAD. We recorded ventricular flow behavior at various pump speeds while independently adjusting pump flow (by varying the afterload) to emulate in vivo pump flow at various phases of the cardiac cycle. Changes in ventricular flow behavior at different pump flow rates may be of clinical relevance, because continuous flow pumps are extremely sensitive to inflow and outflow pressures and instantaneous pump flow varies significantly at different points throughout the cardiac cycle. Further work is needed to quantitatively compare the flow behavior of different continuous flow devices in a pulsatile ventricular model.

A Modified Elastance Model to Control Mock Ventricles in Real-Time: Numerical and Experimental Validation

This article describes an elastance-based mock ventricle able to reproduce the correct ventricular pressure-volume relationship and its correct interaction with the hydraulic circuit connected to it. A real-time control of the mock ventricle was obtained by a new left ventricular mathematical model including resistive and inductive terms added to the classical Suga-Sagawa elastance model throughout the whole cardiac cycle. A valved piston pump was used to mimic the left ventricle. The pressure measured into the pump chamber was fed back into the mathematical model and the calculated reference left ventricular volume was used to drive the piston. Results show that the classical model is very sensitive to pressure disturbances, especially during the filling phase, while the modified model is able to filter out the oscillations thus eliminating their detrimental effects. The presented model is thus suitable to control mock ventricles in real-time, where sudden pressure disturbances represent a key issue and are not negligible. This real-time controlled mock ventricle is able to reproduce the elastance mechanism of a natural ventricle by mimicking its preload (mean atrial pressure) and afterload (mean aortic pressure) sensitivity, i.e., the Starling law. Therefore, it can be used for designing and testing cardiovascular prostheses due to its capability to reproduce the correct ventricle-vascular system interaction.

FLOW VISUALIZATION IN A MOCK VENTRICLE SUPPORTED WITH A LEFT VENTRICULAR ASSIST DEVICE

Purpose: Flow patterns were studied in a rigid transparent mock ventricle equipped with different left ventricular assist devices (LVADs) to detect vortical patterns, stagnation zones, and turbulence generated in the heart by clinically used LVADs. Methods: LVADs (Jarvik 2000 or HeartMate II) were connected to the apex of a rigid mock ventricle. Preload was kept constant at the level of the mitral orifice, through which dye was injected into the mock ventricle. Flow patterns were recorded with a digital video camera. Experiments were conducted at a range of Reynolds numbers (100 Q<5 L/min). The Reynolds number at pump inlet is defined as 4Q/πvd, where Q is measured pump flow rate, v is fluid kinematic viscosity, and d is pump inlet diameter. Results: At all Reynolds numbers studied, the Jarvik 2000 produced more mixing in the mock ventricle when compared to the HeartMate II. When a HeartMate II inlet cannula was placed upstream of the Jarvik 2000, ventricular flow patterns resembled those of the HeartMate II. Conclusions: The basic flow patterns created by different LVADs can be studied in a static ventricle. Flow visualization in a dynamic mock ventricle is in progress.Figure

LONG-TERM ENDURANCE OF GYRO CENTRIFUGAL BLOOD PUMP SYSTEM

Purpose: Our group is developing a centrifugal blood pump as a long-term (>2-year) ventricular assist device. The pump consists of a ceramic shaft with ultra-high molecular weight polyethylene bearings located at the top and bottom of the pump housing. The pump impeller is magnetically-coupled to a DC motor actuator. Pump/actuator control is via an implantable control unit with transcutaneous energy transmission (TET) or an external wearable control unit. Methods: Eight pump/actuators are being operated in a LVAD mode. Each test station consists of a mock ventricle, an arterial compliance chamber, arterial resistor, and atrial filling chamber. Pumps are operated at 5 L/min with a outlet pressure of 100 mmHg. Six pump/actuators are controlled by three implantable units with TET and two pump/actuators are controlled by external controllers. The pump fluid is normal saline at 37o C. Results: As of January 17, 2006, the eight pump/actuators have test periods of 692 days (n=4), 683 days (n=1), 674 days (n=1), 573 days (n=1), and 434 days (n=1). Measured pump flows are 5.02 + 0.25 L/min at outflow pressure of 98 + 4 mmHg. Pump speed has averaged 1853 + 46 RPM with a power consumption of 0.28 + 0.03 amps and a system efficiency of 19.2 + 1.7%. Three implantable controllers with TET have test periods of 384, 354, and 344 days. The two external controllers have test periods of 180 days each. Conclusion: Thus far, the eight Gyro centrifugal blood pumps and actuators have accumulated 14.1 cumulative years of continuous in vitro testing without failure.

ENDURANCE PERFORMANCE OF GYRO CENTRIFUGAL BLOOD PUMP

Purpose: Our group is developing a double-pivot bearing centrifugal pump for long-term ventricular assistance. Previous studies have estimated bearing life at 5 years for LVAD and 8 years for RVAD. Endurance testing for 1-year demonstrated that top female bearing wear was 3 times greater for LVAD than RVAD. Top bearing is critical because pump impeller is maintained at top contact position during operation. To improve bearing life, the top female bearing radius was increased, and magnet coupling force was increased. We report endurance test results of modified double-pivot bearing centrifugal pump. Methods: Eight Gyro pumps are being operated in LVAD mode. Test stations consist of mock ventricle, arterial compliance, arterial resistor, and atrial chamber. Pumps are operated at 5 L/min at peak outflow pressure of 100 mmHg. Mock ventricles are operated at 78 beats/min with stoke volume of ∼30 ml/beat. Pump fluid is normal saline at 37oC. Results: As of January 14, 2005, pumps have accumulated test periods of 331(n=4), 322 (n=1), 313 (n=1), 212 (n=1), and 73 days (n=1). Measured pump flows are 5.03 + 0.06 L/min at mean outflow pressure of 93 + 3 mmHg and mean filling pressure of 26 + 5 mmHg. Pump speed averaged 1810 + 50 RPM with power consumption of 0.29 + 0.03 amps and a system efficiency of 19.2 + 1.7%. There have been no pump/actuator malfunctions without noticeable vibration or noise. Conclusion: Eight centrifugal pumps have 6.1 cumulative years of in vitro testing without failure. Studies will be continued for another year to validate 2-year endurance of double-pivot bearing Gyro centrifugal blood pump.

HEMODYNAMIC RESPONSES TO CONTINUOUS AND PULSATILE VENTRICULAR ASSIST IN AN ADULT MOCK CIRCULATION

The objective of this study was to investigate hemodynamic responses to continuous (CF) and pulsatile flow (PF) ventricular assist (VAD) in an adult mock circulation. The in vitro preparation consisted of s mock ventricle, systemic vasculature, and coronary network that produce physiologically-equivalent pressures and flows. The mock circulation was instrumented with a ventricular pressure-volume conductance and atrial pressure catheters, and aortic mot, aortic distal, VAD output, and coronary flow probes. CF (BioMedicus, Medtronic) or PF (UVAD85, University of Utah) VAD were connected by ventricular apical and aortic return cannulae. Hemodynamic measurements were recorded for baseline, CF, and PF during simulated normal (CO = 5 L/m), failing (CO = 3 L/m), and partially recovered (CO = 4 L/min) ventricular function with full (VAD output = 5 L/m) and partial (VAD output=2.5 L/m) unloading. PF assist reduced ventricular diastolic elastance (Ed) by providing larger ejection fraction (EF) and reducing ventricular filling pressure (Pd). CF assist slightly increased Ed with d modest increase in EF and a small reduction in Pd. Continuous unloading of the ventricle may result in a smaller ventricular chamber thereby reducing wall stress, but at the possible expense of a stiffer ventricle and redistribution of regional myocardial perfusion. These results may provide important clinical insight into the consequences of assist technique on cardiac mechanics and suggests the potential for unfavorable endocardial perfusion with CF assist.

PRELOAD SENSITIVE MOCK VENTRICLE FOR ROTARY BLOOD PUMP CONTROLLER DEVELOPMENT

Rotary blood pump flow changes depending on both preload and afterload of the pump. Under development of rotary blood pump controller for a left ventricular assist device (LVAD). it is necessary to concern about the filling of the LVAD which is dependent on the coupling to the native left ventricle. In a traditional mock circulatory system, the mock ventricle operates on either fixed stoke volume and fixed driving pressure. It cannot mimic the load-sensitive hemodynamics. Newly developed ventricle has inlet and outlet ports with bileaflet prosthetic valves and the third port which is connected to a rotary pump as an LVAD. It is driven by pneumatic actuated piston pump. The stroke volume and driving pressure change by sensing the preload. This can exhibit the hemodynamic response while the left ventricle is unloaded by the rotary pump. The piston is covered with a thin diaphragm and this can provide a collapse of the ventricle due to an overdriving condition of LVAD. This new mock ventricle simulates the natural left ventricle and could be useful to develop an automatic control algorithms for the rotary blood pump LVAD.