Physiological Control of Blood Pumps Using Intrinsic Pump Parameters: A Computer Simulation Study

A sensorless, physiologic feedback control strategy to increase vascular pulsatility for rotary blood pumps

Continuous flow rotary blood pumps (RBP) operating clinically at constant rotational speeds cannot match cardiac demand during varying physical activities, are susceptible to suction, diminish vascular pulsatility, and have an increased risk of adverse events. A sensorless, physiologic feedback control strategy for RBP was developed to mitigate these limitations. The proposed algorithm used intrinsic pump speed to obtain differential pump speed (ΔRPM). The proposed gain-scheduled proportional-integral controller, switching of setpoints between a higher pump speed differential setpoint (ΔRPM Hr ) and a lower pump speed differential setpoint (ΔRPM Lr ), generated pulsatility and physiologic perfusion, while avoiding suction. The switching between ΔRPM Hr and ΔRPM Lr setpoints occurred when the measured ΔRPM reached the pump differential reference setpoint. In-silico tests were implemented to assess the proposed algorithm during rest, exercise, a rapid 3-fold pulmonary vascular resistance increase, rapid change from exercise to rest, and compared with maintaining a constant pump speed setpoint. The proposed control algorithm augmented aortic pressure pulsatility to over 35 mmHg during rest and around 30 mmHg during exercise. Significantly, ventricular suction was avoided, and adequate cardiac output was maintained under all simulated conditions. The performance of the sensorless algorithm using estimation was similar to the performance of sensor-based method. This study demonstrated that augmentation of vascular pulsatility was feasible while avoiding ventricular suction and providing physiological pump outflows. Augmentation of vascular pulsatility can minimize adverse events that have been associated with diminished pulsatility. Mock circulation and animal studies would be conducted to validate these results.

A Feasible Method to Control Left Ventricular Assist Devices for Heart Failure Patients: A Numerical Study

Installing and developing a sophisticated control system to optimize left ventricular assist device (LVAD) pump speed to meet changes in metabolic demand is essential for advancing LVAD technology. This paper aims to design and implement a physiological control method for LVAD pumps to provide optimal cardiac output. The method is designed to adjust the pump speed by regulating the pump flow based on a predefined set point (operating point). The Frank–Starling mechanism technique was adopted to control the set point within a safe operating zone (green square), and it mimics the physiological demand of the patient. This zone is predefined by preload control lines, which are known as preload lines. A proportional–integral (PI) controller was utilized to control the operating point within safe limits to prevent suction or overperfusion. In addition, a PI type 1 fuzzy logic controller was designed and implemented to drive the LVAD pump. To evaluate the design method, rest, moderate, and exercise scenarios of heart failure (HF) were simulated by varying the hemodynamic parameters in one cardiac cycle. This evaluation was conducted using a lumped parameter model of the cardiovascular system (CVS). The results demonstrated that the proposed control method efficiently drives an LVAD pump under accepted clinical conditions. In both scenarios, the left ventricle pressure recorded 112 mmHg for rest and 55 mmHg for exercise, and the systematic flow recorded 5.5 L/min for rest and 1.75 L/min for exercise.

A Flow Sensor-Based Suction-Index Control Strategy for Rotary Left Ventricular Assist Devices

Rotary left ventricular assist devices (LVAD) have emerged as a long-term treatment option for patients with advanced heart failure. LVADs need to maintain sufficient physiological perfusion while avoiding left ventricular myocardial damage due to suction at the LVAD inlet. To achieve these objectives, a control algorithm that utilizes a calculated suction index from measured pump flow (SIMPF) is proposed. This algorithm maintained a reference, user-defined SIMPF value, and was evaluated using an in silico model of the human circulatory system coupled to an axial or mixed flow LVAD with 5–10% uniformly distributed measurement noise added to flow sensors. Efficacy of the SIMPF algorithm was compared to a constant pump speed control strategy currently used clinically, and control algorithms proposed in the literature including differential pump speed control, left ventricular end-diastolic pressure control, mean aortic pressure control, and differential pressure control during (1) rest and exercise states; (2) rapid, eight-fold augmentation of pulmonary vascular resistance for (1); and (3) rapid change in physiologic states between rest and exercise. Maintaining SIMPF simultaneously provided sufficient physiological perfusion and avoided ventricular suction. Performance of the SIMPF algorithm was superior to the compared control strategies for both types of LVAD, demonstrating pump independence of the SIMPF algorithm.

Non-parametric dynamical estimation of blood flow rate, pressure difference and viscosity for a miniaturized blood pump

Blood pumps are becoming increasingly important for medical devices. They are used to assist and control the blood flow and blood pressure in the patient's body. To accurately control blood pumps, information about important hydrodynamic parameters such as blood flow rate, pressure difference and viscosity is needed. These parameters are difficult to measure online. Therefore, an accurate estimation of these parameters is crucial for the effective operation of implantable blood pumps. In this study, in vitro tests with bovine blood were conducted to collect data about the non-linear dependency of blood flow rate, flow resistance (pressure difference) and whole blood viscosity on motor current and rotation speed of a prototype blood pump. Gaussian process regression models are then used to model the non-linear mappings from motor current and rotation speed to the hydrodynamic variables of interest. The performance of the estimation is evaluated for all three variables and shows very high accuracy. For blood flow rate - correlation coefficient (r2) = 1, root mean squared error (RMSE) = 0.31 ml min^-1, maximal error (ERRmax) = 9.31 ml min^-1; for pressure r2 = 1, RMSE = 0.09 mmHg, ERRmax = 8.34 mmHg; and for viscosity r2 = 1,RMSE = 0.09 mPa.s, ERRmax = 0.31 mPa⋅s. The current findings suggest that this method can be employed for highly accurate online estimation of essential hydrodynamic parameters for implantable blood pumps.

Correlation between Myocardial Function and Electric Current Pulsatility of the Sputnik Left Ventricular Assist Device: In-Vitro Study

This study assesses the electric current parameters and reports on the analysis of the associated degree of myocardial function during left ventricular assist device (LVAD) support. An assumption is made that there is a correlation between cardiac output and the pulsatility index of the pump electric current. The experimental study is carried out using the ViVitro Pulse Duplicator System with Sputnik LVAD connected. Cardiac output and cardiac power output are used as a measure of myocardial function. Different heart rates (59, 73, 86 bpm) and pump speeds (7600–8400 rpm in 200 rpm steps) are investigated. In our methodology, ventricular stroke volumes in the range of 30–80 mL for each heart rate at a certain pump speed were used to simulate different levels of contractility. The correlation of the two measures of myocardial function and proposed pulsatility index was confirmed using different correlation coefficients (values ≥ 0.91). Linear and quadratic models for cardiac output and cardiac power output versus pulsatility index were obtained using regression analysis of measured data. Coefficients of determination for CO and CPO models were in the ranges of 0.914–0.982 and 0.817–0.993, respectively. Study findings suggest that appropriate interpretation of parameters could potentially serve as a valuable clinical tool to assess myocardial therapy using LVAD infrastructure.

In vitro evaluation of multi-objective physiological control of the centrifugal blood pump

Left ventricular assist devices (LVADs) have been used as a bridge to transplantation or as destination therapy to treat patients with heart failure (HF). The inability of control strategy to respond automatically to changes in hemodynamic conditions can impact the patients' quality of life. The developed control system/algorithm consists of a control system that harmoniously adjusts pump speed without additional sensors, considering the patient's clinical condition and his physical activity. The control system consists of three layers: (a) Actuator speed control; (b) LVAD flow control (FwC); and (c) Fuzzy control system (FzC), with the input variables: heart rate (HR), mean arterial pressure (MAP), minimum pump flow, level of physical activity (data from patient), and clinical condition (data from physician, INTERMACS profile). FzC output is the set point for the second LVAD control schemer (FwC) which in turn adjusts the speed. Pump flow, MAP, and HR are estimated from actuator drive parameters (speed and power). Evaluation of control was performed using a centrifugal blood pump in a hybrid cardiovascular simulator, where the left heart function is the mechanical model and right heart function is the computational model. The control system was able to maintain MAP and cardiac output in the physiological level, even under variation of EF. Apart from this, also the rotational pump speed is adjusted following the simulated clinical condition. No backflow from the aorta in the ventricle occurred through LVAD during tests. The control algorithm results were considered satisfactory for simulations, but it still should be confirmed during in vivo tests.

Comparison of Flow Estimators for Rotary Blood Pumps: An In Vitro and In Vivo Study

Various approaches for estimating the flow rate of a rotary blood pump have been proposed for monitoring and control purposes. They have been evaluated under different test conditions and, therefore, a direct comparison among them is difficult. Furthermore, a limited performance has been reported for the areas where the pump flow and motor current present a non-monotonic relationship. In this regard, we selected most approaches that have been presented in literature and added a modified one, resulting in four estimators, which are either non-invasive or invasive, i.e., inlet and outlet pump pressure sensors are used. Data from in vitro and in vivo studies with the Deltastream pump DP2 were used to compare the estimators under the same test conditions. These data included both constant and varying pre- and afterload, contractility, viscosity, as well as pump speed settings. Bland-Altman plots were used to evaluate the performance of the estimators. The mean error of the overall estimated flow in vitro ranged from 0.002 to 0.38 L/min and the limits of agreement (LoA) between ± 2 L/min. During negative flows the mean error decreased by about 25% when the pump inlet pressure was added as an input. In vivo, the mean errors increased, while the LoA remained in the same range. An estimator based on pump pressure difference improves the reliability in areas where flow and current relationship is not monotonic. A trade-off between estimation accuracy and number of sensors was identified. The estimation objective and the potential errors should be considered when selecting an estimation approach and designing the pump systems.

Evaluation of an advanced model reference sliding mode control method for cardiac assist device using a numerical model

In this study, the physiological control algorithm using sliding mode control method is implemented to track the reference input signal. The controller is developed using feed‐forward part, reference model, and steady‐state flow estimator. The proposed control method is evaluated using a dynamic heart‐pump interaction model incorporating descriptions of the cardiovascular system – rotary blood pump. The immediate response of the controller to preload as well as afterload was studied. Stability and feasibility of the control system were demonstrated through the tests. The results showed that the present controller, which allows the left ventricular to automatically adjust to the right ventricular output, reduces the risk of suction.

Clinical Implications of Physiologic Flow Adjustment in Continuous-Flow Left Ventricular Assist Devices

There is increasing evidence for successful management of end-stage heart failure with continuous-flow left ventricular assist device (CF-LVAD) technology. However, passive flow adjustment at fixed CF-LVAD speed is susceptible to flow balancing issues as well as adverse hemodynamic effects relating to the diminished arterial pulse pressure and flow. With current therapy, flow cannot be adjusted with changes in venous return, which can vary significantly with volume status. This limits the performance and safety of CF-LVAD. Active flow adjustment strategies have been proposed to improve the synchrony between the pump and the native cardiovascular system, mimicking the Frank-Starling mechanism of the heart. These flow adjustment strategies include modulation by CF-LVAD pump speed by synchrony and maintenance of constant flow or constant pressure head, or a combination of these variables. However, none of these adjustment strategies have evolved sufficiently to gain widespread attention. Herein we review the current challenges and future directions of CF-LVAD therapy and sensor technology focusing on the development of a physiologic, long-term active flow adjustment strategy for CF-LVADs.

Sensor-Based Physiologic Control Strategy for Biventricular Support with Rotary Blood Pumps

Rotary biventricular assist devices (BiVAD) are becoming a clinically accepted treatment option for end-stage biventricular failure. To improve BiVAD efficacy and safety, we propose a control algorithm to achieve the clinical objectives of maintaining left-right-sided balance, restoring physiologic flows, and preventing ventricular suction. The control algorithm consists of two proportional-integral (PI) controllers for left and right ventricular assist devices (LVAD and RVAD) to maintain differential pump pressure across LVAD (ΔPL) and RVAD (ΔPR) to provide left-right balance and physiologic flow. To prevent ventricular suction, LVAD and RVAD pump speed differentials (ΔRPML, ΔRPMR) were maintained above user-defined thresholds. Efficacy and robustness of the proposed algorithm were tested in silico for axial and centrifugal flow BiVAD using 1) normal and excessive ΔPL and/or ΔPR setpoints, 2) rapid threefold increase in pulmonary vascular or vena caval resistances, 3) transient responses from exercise to rest, and 4) ventricular fibrillation. The study successfully demonstrated that the proposed BiVAD algorithm achieved the clinical objectives but required pressure sensors to continuously measure ΔPL and ΔPR. The proposed control algorithm is device independent, should not require any modifications to the pump or inflow/outflow cannulae/grafts, and may be directly applied to current rotary blood pumps for biventricular support.

The Effect of Compliant Inflow Cannulae on the Hemocompatibility of Rotary Blood Pump Circuits in an In Vitro Model

Rotary blood pumps (RBPs) are used for mechanical circulatory support in heart failure patients but exhibit a reduced response to preload changes, which can lead to ventricular suction events. A passive control system, in the form of a compliant inflow cannula (IC), has been developed to mitigate suction, although this device may cause significant hemolysis. This study compared the incidence of mechanically induced hemolysis of two compliant IC designs (strutted and nonstrutted) with a rigid IC (control) in a blood circulation loop over 90 min. The nonstrutted compliant IC introduced high frequency and high amplitude oscillations in RBP inlet pressure and RBP IC resistance. These oscillations were correlated with a significant increase in plasma-free hemoglobin (pfHb) and hemolysis: pfHb increased to 2.005 ± 0.665 g/L, while normalized index of hemolysis (NIH) and modified index of hemolysis (MIH) increased to 0.04945 ± 0.01276 g/100 L and 4.0505 ± 0.6589 after 90 min (P < 0.05). In contrast, the strutted compliant IC performed similar to the clinically utilized rigid IC and did not increase pfHb (0.300 ± 0.090 and 0.320 ± 0.171 g/L, respectively) and rate of hemolysis (NIH 0.00435 ± 0.00155 and 0.00543 ± 0.00371 g/100 L; MIH 0.3896 ± 0.1749 and 0.4261 ± 0.2792, respectively) within the RBP circuit. These data indicated that strutted, compliant ICs meet the hemocompatibility of clinically used rigid ICs while also offering a potential solution to prevent ventricular suction events.

In Vivo Evaluation of Active and Passive Physiological Control Systems for Rotary Left and Right Ventricular Assist Devices

Preventing ventricular suction and venous congestion through balancing flow rates and circulatory volumes with dual rotary ventricular assist devices (VADs) configured for biventricular support is clinically challenging due to their low preload and high afterload sensitivities relative to the natural heart. This study presents the in vivo evaluation of several physiological control systems, which aim to prevent ventricular suction and venous congestion. The control systems included a sensor-based, master/slave (MS) controller that altered left and right VAD speed based on pressure and flow; a sensor-less compliant inflow cannula (IC), which altered inlet resistance and, therefore, pump flow based on preload; a sensor-less compliant outflow cannula (OC) on the right VAD, which altered outlet resistance and thus pump flow based on afterload; and a combined controller, which incorporated the MS controller, compliant IC, and compliant OC. Each control system was evaluated in vivo under step increases in systemic (SVR ∼1400-2400 dyne/s/cm(5) ) and pulmonary (PVR ∼200-1000 dyne/s/cm(5) ) vascular resistances in four sheep supported by dual rotary VADs in a biventricular assist configuration. Constant speed support was also evaluated for comparison and resulted in suction events during all resistance increases and pulmonary congestion during SVR increases. The MS controller reduced suction events and prevented congestion through an initial sharp reduction in pump flow followed by a gradual return to baseline (5.0 L/min). The compliant IC prevented suction events; however, reduced pump flows and pulmonary congestion were noted during the SVR increase. The compliant OC maintained pump flow close to baseline (5.0 L/min) and prevented suction and congestion during PVR increases. The combined controller responded similarly to the MS controller to prevent suction and congestion events in all cases while providing a backup system in the event of single controller failure.

Estimation of Left Ventricular Pressure with the Pump as “Sensor” in Patients with a Continuous Flow LVAD

Introduction

In long-term ventricular support of patients with LVADs, left ventricular pressure (plv is relevant for indicating the unloading level of the heart. Monitoring of plv over time might give more insight into the increase or decrease in native ventricular function. In this study, we aim to assess dynamic plv noninvasively, using the LVAD as a pressure sensor.

Methods

Pressure head (dplvad) was estimated from pump flow with a dynamic pump model ( 1 ). Estimated dplvad and measured aortic pressure were used to calculate left ventricular pressure. Moreover, parameters dp/dtmax and mean, minimum, and maximum plv were derived. The method was validated with a porcine ex vivo beating heart model by measurements conducted in 4 hearts supported with a Micromed DeBakey VAD and 3 hearts with a Heartmate II VAD. During each measurement, aortic and left ventricular pressure, pump flow, and pressure head were recorded for 30 s with a sampling frequency of 1 kHz.

Results

The estimation of left ventricular pressure appeared to be accurate for both pumps. The parameters mean and minimum pressure were estimated with high accuracy. The degree of accuracy of the estimated plv was proportional to the degree of accuracy of the dynamic pump model.

Conclusions

We proved that the LVAD model described in this paper can be used as a pressure indicator to determine LV pressure at any time based on noninvasive measurements of pump flow, aortic pressure, and the properties of the outlet graft.

A compliant, banded outflow cannula for decreased afterload sensitivity of rotary right ventricular assist devices

Biventricular support with dual rotary ventricular assist devices (VADs) has been implemented clinically with restriction of the right VAD (RVAD) outflow cannula to artificially increase afterload and, therefore, operate within recommended design speed ranges. However, the low preload and high afterload sensitivity of these devices increase the susceptibility of suction events. Active control systems are prone to sensor drift or inaccurate inferred (sensor-less) data, therefore an alternative solution may be of benefit. This study presents the in vitro evaluation of a compliant outflow cannula designed to passively decrease the afterload sensitivity of rotary RVADs and minimize left-sided suction events. A one-way fluid-structure interaction model was initially used to produce a design with suitable flow dynamics and radial deformation. The resultant geometry was cast with different initial cross-sectional restrictions and concentrations of a softening diluent before evaluation in a mock circulation loop. Pulmonary vascular resistance (PVR) was increased from 50 dyne s/cm(5) until left-sided suction events occurred with each compliant cannula and a rigid, 4.5 mm diameter outflow cannula for comparison. Early suction events (PVR ∼ 300 dyne s/cm(5) ) were observed with the rigid outflow cannula. Addition of the compliant section with an initial 3 mm diameter restriction and 10% diluent expanded the outflow restriction as PVR increased, thus increasing RVAD flow rate and preventing left-sided suction events at PVR levels beyond 1000 dyne s/cm(5) . Therefore, the compliant, restricted outflow cannula provided a passive control system to assist in the prevention of suction events with rotary biventricular support while maintaining pump speeds within normal ranges of operation.

Starling-like flow control of a left ventricular assist device: in vitro validation

The application of rotary left ventricular (LV) assist devices (LVADs) is expanding from bridge to transplant, to destination and bridge to recovery therapy. Conventional constant speed LVAD controllers do not regulate flow according to preload, and can cause over/underpumping, leading to harmful ventricular suction or pulmonary edema, respectively. We implemented a novel adaptive controller which maintains a linear relationship between mean flow and flow pulsatility to imitate native Starling-like flow regulation which requires only the measurement of VAD flow. In vitro controller evaluation was conducted and the flow sensitivity was compared during simulations of postural change, pulmonary hypertension, and the transition from sleep to wake. The Starling-like controller's flow sensitivity to preload was measured as 0.39 L/min/mm Hg, 10 times greater than constant speed control (0.04 L/min/mm Hg). Constant speed control induced LV suction after sudden simulated pulmonary hypertension, whereas Starling-like control reduced mean flow from 4.14 to 3.58 L/min, maintaining safe support. From simulated sleep to wake, Starling-like control increased flow 2.93 to 4.11 L/min as a response to the increased residual LV pulsatility. The proposed controller has the potential to better match device outflow to patient demand in comparison with conventional constant speed control.

Miniaturization of mechanical circulatory support systems

Heart failure (HF) is increasing worldwide and represents a major burden in terms of health care resources and costs. Despite advances in medical care, prognosis with HF remains poor, especially in advanced stages. The large patient population with advanced HF and the limited number of donor organs stimulated the development of mechanical circulatory support (MCS) devices as a bridge to transplant and for destination therapy. However, MCS devices require a major operative intervention, cardiopulmonary bypass, and blood component exposure, which have been associated with significant adverse event rates, and long recovery periods. Miniaturization of MCS devices and the development of an efficient and reliable transcutaneous energy transfer system may provide the vehicle to overcome these limitations and usher in a new clinical paradigm in heart failure therapy by enabling less invasive beating heart surgical procedures for implantation, reduce cost, and improve patient outcomes and quality of life. Further, it is anticipated that future ventricular assist device technology will allow for a much wider application of the therapy in the treatment of heart failure including its use for myocardial recovery and as a platform for support for cell therapy in addition to permanent long-term support.

Control strategies for afterload reduction with an artificial vasculature device

Ventricular assist devices (VADs) have been used successfully as a bridge to transplant in heart failure patients by unloading ventricular volume and restoring the circulation. An artificial vasculature device (AVD) is being developed that may better facilitate myocardial recovery than VAD by controlling the afterload experienced by the native heart and controlling the pulsatile energy entering into the arterial system from the device, potentially reconditioning the arterial system properties. The AVD is a valveless, 80 ml blood chamber with a servo-controlled pusher plate connected to the ascending aorta by a vascular graft. Control algorithms for the AVD were developed to maintain any user-defined systemic input impedance (IM) including resistance, elastance, and inertial components. Computer simulation and mock circulation models of the cardiovascular system were used to test the efficacy of two control strategies for the AVD: 1) average impedance position control (AIPC)-to maintain an average value of resistance during left ventricular (LV) systole and 2) instantaneous impedance force feedback (IIFF) and position control (IIPC)-to maintain a desired value or profile of resistance and compliance. Computer simulations and mock loop tests were performed to predict resulting cardiovascular pressures, volumes, flows, and the resistance and compliance experienced by the native LV during ejection for simulated normal, failing, and recovering LV. These results indicate that the LV volume and pressure decreased, and the LV stroke volume increased with decreasing IM, resulting in an increased ejection fraction. Although the AIPC algorithm is more stable and can tolerate higher levels of sensor errors and noise, the IIFF and IIPC control algorithms are better suited to maintain any instantaneous IM or an IM profile. The developed AVD impedance control algorithms may be implemented with current VADs to promote myocardial recovery and facilitate weaning.

Comparison of preload-sensitive pressure and flow controller strategies for a dual device biventricular support system

The use of rotary left ventricular assist devices (LVADs) has extended to destination and recovery therapy for end-stage heart failure. Incidence of right ventricular failure while on LVAD support requires a second device be implanted to support the failing right ventricle. Without a commercially available implantable rotary right ventricular assist device, rotary LVADs are cannulated into the right heart and operation modified to provide suitable support for the pulmonary system. While this approach can alleviate the demand for transplant through long-term biventricular support, it uncovers a new challenge with respect to controller strategies for these dual device support systems. This study compares the preload sensitivity of rotary, dual device biventricular assistance controllers in light of their ability to adjust the flow rate according to physiological demand. A Frank-Starling-like flow controller which requires both inlet pressure and flow sensors is compared to pressure controllers which maintain atrial or inlet cannula pressures through the use of a single pressure sensor. It was found that cannula selection and the location of a pressure controller's single pressure sensor can be tailored to adjust the preload sensitivity. When located within the atria, this sensitivity is effectively infinite. Moving the sensor to the base of a 450-mm cannula, however, decreased the sensitivity to 0.22 (L/min)/mm Hg. This indicates the potential for simple and reliable VAD controllers with increased preload sensitivity without the need for complex controllers requiring an array of hemodynamic sensors.

A method for control of an implantable rotary blood pump for heart failure patients using noninvasive measurements

We propose a deadbeat controller for the control of pulsatile pump flow (Q(p) ) in an implantable rotary blood pump (IRBP). Noninvasive measurements of pump speed and current are used as inputs to a dynamical model of Q(p) estimation, previously developed and verified in our laboratory. The controller was tested using a lumped parameter model of the cardiovascular system (CVS), in combination with the stable dynamical models of Q(p) and differential pressure (head) estimation for the IRBP. The control algorithm was tested with both constant and sinusoidal reference Q(p) as input to the CVS model. Results showed that the controller was able to track the reference input with minimal error in the presence of model uncertainty. Furthermore, Q(p) was shown to settle to the desired reference value within a finite number of sampling periods. Our results also indicated that counterpulsation yields the minimum left ventricular stroke work, left ventricular end diastolic volume, and aortic pulse pressure, without significantly affecting mean cardiac output and aortic pressure.

A passively controlled biventricular support device

Clinical studies have reported the balancing of pump outputs to be a serious control issue for rotary biventricular support (BiVS) systems. Poor reliability of long-term, blood immersed pressure sensors encouraged the development of a new control strategy to improve their viability. A rotary BiVS device was designed and constructed with a mechanical passive controller to autoregulate pump outputs to emulate the native baroreceptor response. In vitro testing in a dual circuit, hydraulic mock circulation loop showed that the prototype was able to maintain arterial pressures when subjected to sudden induced hemodynamic destabilization. However, inlet suction was observed when sudden simulated hypertension briefly reduced venous return to the cannulated ventricle. The results have encouraged further development of the device as a means to create an inherently stable, fully passive biventricular support device.

Adaptive physiological speed/flow control of rotary blood pumps in permanent implantation using intrinsic pump parameters

An adaptive speed/flow controller was developed based on previous work using the intrinsic pump parameters. Those intrinsic parameters were measured by long-term reliable sensors. The adaptive controller was designed to track the varying total peripheral resistance and update the controller parameters correspondingly. The controller was studied in computer simulation on two different types of pumps, whose hydrodynamic characteristics are described by static and dynamic equation, respectively. The pump pressure rise of both pumps is accessible. With the designed adaptive controller, the abnormal hemodynamic values indicating congestive heart failure, including total blood flow, mean aortic pressure, left ventricular end-diastolic pressure, are all successfully restored to normal ranges. This good performance is consistent for both pumps in the variation of activities and left ventricular failure levels. The results show that the designed controller can be applicable for rotary blood pumps whose pump pressure rise can be measured or derived from pump intrinsic parameters.

A Computer Model of the Pediatric Circulatory System for Testing Pediatric Assist Devices

Lumped parameter computer models of the pediatric circulatory systems for 1- and 4-year-olds were developed to predict hemodynamic responses to mechanical circulatory support devices. Model parameters, including resistance, compliance and volume, were adjusted to match hemodynamic pressure and flow waveforms, pressure-volume loops, percent systole, and heart rate of pediatric patients (n = 6) with normal ventricles. Left ventricular failure was modeled by adjusting the time-varying compliance curve of the left heart to produce aortic pressures and cardiac outputs consistent with those observed clinically. Models of pediatric continuous flow (CF) and pulsatile flow (PF) ventricular assist devices (VAD) and intraaortic balloon pump (IABP) were developed and integrated into the heart failure pediatric circulatory system models. Computer simulations were conducted to predict acute hemodynamic responses to PF and CF VAD operating at 50%, 75% and 100% support and 2.5 and 5 ml IABP operating at 1:1 and 1:2 support modes. The computer model of the pediatric circulation matched the human pediatric hemodynamic waveform morphology to within 90% and cardiac function parameters with 95% accuracy. The computer model predicted PF VAD and IABP restore aortic pressure pulsatility and variation in end-systolic and end-diastolic volume, but diminish with increasing CF VAD support.