J.R. Boston

Estimation of systemic vascular bed parameters for artificial heart control

An extended Kalman filter estimator for the identification of systemic circulation model parameters during cardiac ejection and cardiac filling is described. The estimator has been developed for use in the control of a cardiac ventricular assist device. A lumped element circuit with a time-varying capacitor was used to represent the systemic circulation and the left ventricle. Since the haemodynamic variables that are measurable in patients with impaired cardiac function vary dramatically as the patients move through different levels of care, the estimator was designed so that it can be used with different sets of blood pressure and flow measurements. Preliminary evaluation of the performance of the estimator using data from a computer simulation and from a patient during open-heart surgery is presented. The robustness of the estimator to variations in parameter initialization is also described.

Elastance-Based Control of a Mock Circulatory System

AbstractA new control strategy for a mock circulatory system (MCS) has been developed to mimic the Starling response of the natural heart. The control scheme is based on Sugas elastance model, which is implemented using nested elastance and pressure feedback control systems. The elastance control loop calculates the desired chamber pressure using a time-varying elastance function and the ventricular chamber volume signal. The pressure control loop regulates the chamber pressure according to this reference signal. Simulations and tests on MCS hardware showed that the elastance-based controller responds to changes in preload, afterload, and contractility in a manner similar to the natural heart. Since the elastance function is an arbitrary function of time, the controller allows modification of ventricular chamber contractility, giving researchers a new tool to mimic various pathological conditions which can be used in the evaluation of cardiac devices such as ventricular assist devices.

A Control System for Rotary Blood Pumps Based on Suction Detection

A control system for rotary ventricular assist devices was developed to automatically regulate the pumping speed of the device to avoid ventricular suction. The control system comprises a suction detector and a fuzzy logic controller (FLC). The suction detector can correctly classify pump flow patterns, using a discriminant analysis (DA) model that combines several indices derived from the pump flow signal, to classify the pump status as one of the following: no suction (NS), moderate suction (MS), and severe suction (SS). The discriminant scores, which are the output of the suction detector, were used as inputs to the FLC. Based on this information, the controller updates pump speed, providing adequate flow and pressure perfusion to the patient. The performance of the control system was tested in simulations over a wide range of physiological conditions, including hypertension, exercise, and strenuous exercising for healthy, sick, and very sick hearts, using a lumped parameter model of the circulatory system coupled with a left ventricular assist device. The controller was able to maintain cardiac output and mean arterial pressure within acceptable physiologic ranges, while avoiding suction, demonstrating the feasibility of the proposed control system.

Modeling and identification of an axial flow blood pump

This paper presents a model for identification of an axial pump. It aims to provide flow and pressure difference estimates of the axial pump as well as the parameters of the pump characteristics without using flow and pressure sensors.

A Nonlinear State-Space Model of a Combined Cardiovascular System and a Rotary Pump

A nonlinear lumped parameter model of the cardiovascular system coupled with a rotary blood pump is presented. The blood pump is a mechanical assist device typically interposed from the left ventricle to the aorta in patients with heart failure. The cardiovascular part of the model (consisting of the left heart atrium and ventricle), and the systemic circulatory system are implemented as an RLC circuit with two diodes. The diodes represent the mitral and aortic valves in the left heart. The cardiovascular model has been validated with clinical data from a patient suffering from cardiomyopathy. The pump model is a first order differential equation relating pressure difference across the pump to flow rate and pump speed. The combined cardiovascular-pump model has been represented as a fifth order nonlinear dynamical system in state space form with pump speed as the control variable. This model was used to simulate the hemodynamic variables to different values of afterload and a linearly increasing (ramp) pump speed. Because of its small dimensionality, the model is suitable for both parameter identification and the application of modern control theory.

Intelligent control design for heart assist devices

Heart assist devices are blood pumps used to augment the cardiac output of patients with left ventricular failure. A new generation of devices being evaluated for human use is based on turbo-hydrodynamic methods of pumping, which offer several advantages over the reciprocating, pulsatile methods used in current devices. However, the new devices pose a more difficult control problem because of their sensitivity to circulatory load and other patient cardiovascular parameters. The paper describes the design of a control structure to regulate the operation of these devices. The controller has three different types of algorithm available: a model-based patient-adaptive algorithm; two heuristic algorithms that rely only on the device characteristics; and a default algorithm. The patient-adaptive algorithm uses a model of the patients systemic circulation to determine the required cardiac output for a given level of activity. The heuristic algorithms use the known operating characteristics of the device to adjust the cardiac output to changes in demand without knowledge of patient-specific conditions. The default algorithm provides a fixed speed operation to be used in case of system or sensor failure. An intelligent supervisor determines the cardiac output required from the assist device and selects the control algorithm to use, based on a multidimensional measure of the patients level of activity, available estimates of hemodynamic variables, reliability of the patient model, and the past history of the patient.

Control of rotary heart assist devices

Rotary assist devices are state of the art blood pumps that are used in therapies to treat weak hearts, usually associated with myocardial diseases. We develop three indices to detect the occurrence of suction in these pumps: pulsatility index, diminishing returns index and harmonic index. Experimental studies in calves show that these indices can detect suction at approximately the same speed. Control of ventricular assist devices based only on suction detection may not satisfy other requirements for some hemodynamic variables. A multi-objective optimization scheme has been developed to consider other hemodynamic variables. We demonstrate the feasibility of this approach using a Physbe simulation of AOP and LAP. A non-inferior set of pump speeds was found that includes speeds that optimize performance with respect to these two variables.

Dynamic systemic vascular resistance in a sheep supported with a Nimbus AxiPump.

Changes in systemic vascular resistance (SVR) in response to diminished pulse perfusion were analyzed over a dynamic range of flow conditions. An axial flow LVAD (Nimbus AxiPump, Rancho Cordova, CA) was implanted in a sheep for 28 days, during which time SVR was determined over several conditions of posture and excitability. Total arterial resistance (TR) was calculated dynamically as an index of SVR by analysis of pump flow in diastole, and systemic pressure estimated from the characteristic pressure-flow-speed relation of the AxiPump. TR was evaluated over a range of flow rates, including maximum flow--for which the pressures and flows were essentially nonpulsatile. Throughout the course of support, and independent of pulsatility, TR dropped when the sheep stood and was significantly lower than that in the sitting position (P < 0.01). Response to excitement followed the same trend: TR was significantly higher during agitation than during normal temper (P < 0.01). In spite of changes in pulse pressure and flow rate, SVR changes occurred according to expected physiologic responses for pulsatile perfusion. Because pump flow and pressure are sensitive to afterload, the results of these studies suggest that pump speed control must compensate for changes in SVR to maintain acceptable perfusion.

A rule-based controller based on suction detection for rotary blood pumps

A rule-based controller for rotary ventricular assist devices was developed to automatically regulate the pumping speed of the device without introducing suction in the ventricle. The control approach is based on a discriminant analysis function that detects the occurrence of suction, providing the input for the rule-based controller. This controller has been tested in simulations showing the ability to autonomously adjust pump flow according to the patients level of activity, while sustaining adequate perfusion pressures. The performance of the system (suction detector and controller) was tested for several levels of activity and contractility state of the left ventricle, using a lumped parameter model of the circulatory system coupled with a left ventricular assist device. In all cases, the controller kept cardiac output and mean arterial pressure within acceptable physiologic ranges.

Control issues in rotary heart assist devices

Heart assist devices are mechanical pumps used in patients with cardiovascular diseases who are awaiting heart transplantation. With increasing clinical success, these devices are being used for longer periods of time, and automatic control has become an important requirement. The principal control requirement is to mimic the normal response of the heart to changes in demand for cardiac output, and control therefore provides many challenging problems for control engineers involved in this important multidisciplinary area of research. A team of engineers and physicians at the University of Pittsburgh is developing a hierarchical structure for a control system to support a rotary-type heart assist device. In this paper, we review various types of devices, the control algorithms used, and the problems that must be solved.